Recent observations have revealed rare previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes and with periodicity of minutes to an hour 14. These transient radio signals must originate from sources in the Milky Way and from coherent emission processes in astrophysical plasma. They are theorised to be produced in the extreme and highly magnetised environments around white dwarfs or neutron stars 58. However the astrophysical origin of these signals remains contested and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source ILT J11015521 whose roughly minutelong pulses arrive with a periodicity of 125.5 minutes. We find that ILT J11015521 is an M dwarf  white dwarf binary system with an orbital period that matches the period of the radio pulses which are observed when the two stars are in conjunction. The binary nature of ILT J11015521 establishes that some longperiod radio transients originate from orbital motion modulating the observed emission as opposed to an isolated rotating star. We conclude that ILT J11015521 is likely a polar system where magnetic interaction has synchronised the rotational and orbital periods of the white dwarf 9. Magnetic interaction and plasma exchange between the two stars may generate the sporadic radio emission. Such mechanisms have been previously theorised 1013 but not observationally established.
['Astrophysics - High Energy Astrophysical Phenomena', 'Astrophysics - Solar and Stellar Astrophysics']
A white dwarf binary showing sporadic radio pulses at the orbital period
{'No Header': '5', 'A white dwarf binary showing sporadic radio pulses at the orbital period': "I. de Ruiter 1* , K.M. Rajwade 2 , C.G. Bassa 3 , A. Rowlinson 1,3 , R.A.M.J. Wijers 1 , C.D. Kilpatrick 4 , G. Stefansson 1 , J.R. Callingham 3,5 , J.W.T. Hessels 1,3,6,7 , T.E. Clarke 8 , W. Peters 8 , R.A.D. Wijnands 1 , T.W. Shimwell 3,5 , S. ter Veen 3 , V. Morello 10 , G.R. Zeimann 9 , S. Mahadevan 11,12  \n- 1* Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, 1098 XH, The Netherlands.\n- 2 Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK.  \n3  \nASTRON, Netherlands Institute for Radio Astronomy, Oude  \nHoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands.  \n4  \nCenter for Interdisciplinary Exploration and Research in  \nAstrophysics/Northwestern University, Evanston, 60201, USA.  \nLeiden Observatory, Leiden University, PO Box 9513, Leiden, 2300 RA,  \nThe Netherlands.  \n6 Trottier Space Institute, McGill University, 3550 rue University, Montr'eal, QC H3A 2A7, Canada.  \n7 Department of Physics, McGill University, 3600 rue University, Montr'eal, QC H3A 2T8, Canada.  \n8 U.S. Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington, DC 20375, USA.  \n9 Hobby Eberly Telescope, University of Texas, Austin, Austin, TX, 78712, USA.  \n10 SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield, SK11 9FT, UK.  \n- 11  \nDepartment of Astronomy & Astrophysics, 525 Davey Laboratory, Penn State, University Park, PA, 16802, USA. 12 Center for Exoplanets and Habitable Worlds, 525 Davey Laboratory, Penn State, University Park, PA, 16802, USA.  \n*Corresponding author(s). E-mail(s): i.deruiter@uva.nl; Contributing authors: kaustubh.rajwade@physics.ox.ac.uk;", 'Abstract': 'Recent observations have revealed rare, previously unknown flashes of cosmic radio waves lasting from milliseconds to minutes, and with periodicity of minutes to an hour [1-4]. These transient radio signals must originate from sources in the Milky Way, and from coherent emission processes in astrophysical plasma. They are theorised to be produced in the extreme and highly magnetised environments around white dwarfs or neutron stars [5-8]. However, the astrophysical origin of these signals remains contested, and multiple progenitor models may be needed to explain their diverse properties. Here we present the discovery of a transient radio source, ILT J1101 + 5521 , whose roughly minute-long pulses arrive with a periodicity of 125.5 minutes. We find that ILT J1101 + 5521 is an M dwarf white dwarf binary system with an orbital period that matches the period of the radio pulses, which are observed when the two stars are in conjunction. The binary nature of ILT J1101 + 5521 establishes that some long-period radio transients originate from orbital motion modulating the observed emission, as opposed to an isolated rotating star. We conclude that ILT J1101 + 5521 is likely a polar system where magnetic interaction has synchronised the rotational and orbital periods of the white dwarf [9]. Magnetic interaction and plasma exchange between the two stars may generate the sporadic radio emission. Such mechanisms have been previously theorised [10-13], but not observationally established.', '1 Main': "ILTJ110160 . 52+552119 . 62 (ILTJ1101+5521 hereafter) was discovered in a commensal transient search of Low-Frequency Array (LOFAR) all-sky survey data (LOFAR Two-Metre Sky Survey, LoTSS [14]), to detect radio transients on timescales of seconds to hours using radio images [15]. A single bright radio pulse from ILT J1101+5521 was detected in data from February 8th, 2015, using 8-second snapshot images. We detected six additional pulses in other archival LOFAR data. ILT J1101 + 5521 is localised to RA (ICRS) = 11 h 01 m 50 . s 5 ± 1 . '' 9 and Dec (ICRS) = +55 · 21 ' 19 . '' 6 ± 0 . '' 39, equivalently ( l , b )= (150 . 4551 · ± 0 . 0004 , 55 . 5200 · ± 0 . 0001) in Galactic coordinates. In total, we discovered seven radio pulses lasting between 30 to 90 seconds, with peak flux densities ranging from 41 ± 6 to 256 ± 10 mJy/beam in five LoTSS observations, lasting eight hours each and spanning the years 2015 to 2020. A new 16-hour LOFAR monitoring campaign at the end of 2023 yielded no additional detections. A summary of all LOFAR observations and detected pulses is given in Extended Data Table 1. The Very Large Array's VLITE archive was also searched for pulses from ILTJ1101 + 5521 (Methods), but no additional pulses were found.  \nFigure 1a shows the light curves from the five observations in which we detected radio pulses (see Extended Data Table 1). We use the times of arrival (ToAs) of the pulses to determine a phase-connected timing solution (Methods) with a period of  \n125 . 51950 ± 0 . 00004 minutes. Furthermore, we are able to obtain a 3 σ upper limit on the period derivative of 3.04 × 10 -11 s s -1 . Figure 1a shows both the pulses and the non-detections at times when we expect to find pulses based on the periodicity. The pulses have a 2% duty cycle, and the intermittency of the pulses combined with the non-detections in the 2023 follow-up observations (not shown in Figure 1a), indicate that the source is intrinsically highly variable in nature: we detect a pulse in 7 out of 26 observed periods.  \nThe pulses are visible across the observed range of radio frequencies (120 -168 MHz). For the brightest pulse in our sample, we find that the spectral index of the pulse is extraordinarily steep and we determine the spectral index to be α = -4 . 1 ± 1 . 1 (Methods), with S ν ∝ ν α , where S ν is the flux density and ν the observing frequency. Additionally, for the brightest pulse in our sample, we determine the linear polarization fraction to be 51 ± 6% for a Faraday rotation measure (RM) of 4 . 72 ± 0 . 14 rad m -2 . No circularly polarized emission is detected in the pulses, with an upper limit of < 1 . 6% on the circular polarization fraction for the brightest pulse. For the brightest pulse we reprocessed the raw imaging data with a 1-second time resolution (Figure 1b), which reveals multiple sub-components in the pulse. Additionally, the high-time-resolution data allows us to constrain the dispersion measure (DM) to 16 ± 6 pc cm -3 .  \nSimultaneous Swift X-ray Telescope (XRT) observations were carried out during the 2023 LOFAR observations, yielding an upper limit on the quiescent X-ray luminosity of L < (1 -1 . 6) · 10 30 · [ d 504 pc ] 2 erg s -1 (Methods).  \nA search in archival multi-wavelength datasets on the coordinates of ILT J1101 + 5521 resulted in a match with a star catalogued in the Sloan Digital Sky Survey, SDSSJ110150.52+552119.9 [16], with an r-band magnitude r = 20 . 86 ± 0 . 05, whose Gaia Data Release 3 (DR3) position [17] is 0 . '' 44 offset from ILT J1101 + 5521 , but within the astrometric uncertainty of the radio-derived position of ILT J1101 + 5521 (error in (RA,Dec)= ± (1 . 9 '' , 0 . 39 '' )). The probability of the optical source aligning with ILT J1101 + 5521 by chance is extremely small ( p < 0 . 00014; Methods) due to the low stellar density at high Galactic latitudes. The geometric distance to this star based on Gaia Early Data Release 3 (EDR3) data is 504 +148 -109 pc [18].  \nSpectroscopic follow-up determined the spectral type of the star to be M4.5V and showed that the star has a significant radial velocity variation of ∼ 200 kms -1 . The radial velocity as a function of time is shown in Figure 2. We fit two sinusoids to the data, one where all fit parameters are unbound and one where we fix the period to the 125-minute period of the radio pulses. A simple sinusoid describes the data well, indicating a close-to-circular binary orbit. There is no significant difference in goodness-of-fit between the two fits (Bayesian Information Criterion, BIC= 51.3 versus BIC=51.9), clearly showing that the period of the radio pulses is tied to the binary period. Figure 2 additionally shows the predicted radio pulse arrival time, according to the phase-connected timing solution. We find that the radio pulses are all emitted when the M dwarf is at superior conjunction with respect to its companion; that is, the M dwarf is seen to be behind and in line with the companion star from the perspective of an observer on Earth. Given the small chance alignment probability, the agreement between the dispersion measure of the radio pulses and the distance to the star (Methods), and the periodicity of the radio pulses being equal to the orbital  \n<!-- image -->  \nFig. 1 : a) Light curves (flux density versus time) of the LOFAR-detected pulses (blue) and the non-detections (grey) at times when we expect to find pulses based on their 125-minute periodicity. The y-axis shows the flux density in arbitrary units and the x-axis shows time in seconds (bottom) and duty cycle (top). The pulses are aligned according to the measured period, with period derivative assumed to be zero. The orange-shaded region indicates a 2% duty cycle window, where the pulses have been observed from 2015 to 2020. The relative flux scaling between pulses is consistent. b) Temporal profile (top) and the dynamic spectrum (bottom) of the brightest pulse in our sample, shown in dark blue in a. The time resolution is 1 second. Note that this dynamic spectrum is not dedispersed. 4  \n<!-- image -->  \nFig. 2 : Radial velocity of the M dwarf associated with ILT J1101+5521 , as calculated from MMT-Binospec (black points) and HET-LRS2-R (grey point) observations. The orange line shows the best fit to the data, leaving all parameters free (BIC=51.3). The blue line shows the best fit to the data if we fix the period to be the 125-minute period of the radio pulses (BIC=51.9). The red dashed vertical lines show the predicted pulse arrival time, according to the phase-connected timing solution.  \n<!-- image -->  \nperiod of this star, we conclude that ILT J1101 + 5521 is a binary system where one of the components is an M4.5V star.  \nThe SDSS ugriz [16] and Pan-STARSS PS1 griz magnitudes [19] yield color-color diagrams in which the candidate optical counterpart is consistently offset towards the blue with respect to the main locus of stars, suggesting that the binary companion to the optical star could be a white dwarf. We note that the blue excess could, in principle, alternatively originate from a star that is highly irradiated by a companion neutron star, as seen in some neutron star - M dwarf binary systems [20]. However, when compared to the spectrum of such systems, we do not see any evidence that would suggest significant irradiation of the M dwarf. Figure 3 shows a simple leastsquares fit to the broadband photometry with a model that allows for a main-sequence star only (top panel), and a model that allows for a main-sequence star and a white dwarf (bottom panel). The difference in Bayesian Information Criterion for both fits, BIC = 88 and BIC = 65, respectively, is very strong evidence that the latter model, including a main-sequence star and a white dwarf, is preferred. We constrain the M dwarf mass, white dwarf mass, temperature, and distance through a Markov-Chain Monte-Carlo likelihood analysis [21] (Methods). We show that an alternative scenario in which the binary consists of two M dwarfs is excluded by the lack of photometric variability (Methods) and conclude that the M dwarf companion is a white dwarf.  \nUsing the radial velocity amplitude measurement presented in Figure 2 we apply the binary mass function to constrain the mass of the white dwarf as a function of orbital inclination, as shown in Figure 4. Here we assume a mass of 0 . 188 M ⊙ for the M dwarf (which is a result of the broadband photometry fit, 0 . 188 ± 0 . 01 M ⊙ , see Figure 3), a period of 125.52 minutes, and a radial velocity amplitude of 103 ± 13 km / s.  \nFig. 3 : Broadband photometry (SDSS ugriz and 2MASS JHK s ) for ILT J1101 + 5521 fit to a single M dwarf model and a combination of an M dwarf and a white dwarf model. An M dwarf with a mass of M = 0 . 22 M ⊙ at a distance of d = 410pc produces the fit in the top panel. An M dwarf with a M = 0 . 18 M ⊙ and a white dwarf with a mass of M = 0 . 63 M ⊙ , and effective temperature of T eff = 5156K at a distance of d = 322pc produces the fit in the bottom panel. The Bayesian Information Criterion (BIC) for both fits is shown in the top-left of the plot. The residuals are defined as ( O i -C i ) 2 /σ 2 i , where O i is the observed value, C i is the model value, and σ i is the error on the observed value. The reduced chi-squared ( χ 2 ν ) is the sum of the residuals divided by the degrees of freedom (number of data points minus number of fit parameters).  \n<!-- image -->  \nWe find that even for a white dwarf companion as light as 0 . 2 M ⊙ , the inclination of the system has to be smaller than 40 · . In the known sample of white dwarf - M dwarf binaries, the white dwarf typically has a mass larger than 0 . 6 M ⊙ [25]. Extended Data Figure 8 is a more detailed version of Figure 4, showing that for white dwarf masses above 0 . 2 M ⊙ the point of gravitational equipotential between the two stars (Roche-Lobe radius) would equal the stellar radius of the M dwarf.  \nThe radio pulses are incompatible with typical low-frequency stellar radio emission from M dwarfs in terms of luminosity (by five orders of magnitude) and polarimetric properties [26, 27] (Methods). Therefore, we conclude that the radio emission originates from the white dwarf or the interaction between the white dwarf and the M dwarf. The high linear polarization fraction indicates the presence of strongly ordered  \nFig. 4 : Binary mass function, with the black line showing the allowed companion mass as a function of system inclination (the shaded region around the black line indicates the 1 σ error). A radial velocity amplitude of 103 ± 13 km/s and an M dwarf mass of 0 . 188 M ⊙ are assumed. In the background we show typical mass ranges of isolated white dwarfs in dark red (0 . 5 -0 . 7 M ⊙ [22]), and the full range of known white dwarf masses (0 . 17 -1 . 35 M ⊙ [23, 24]) in the lighter shade of red.  \n<!-- image -->  \nmagnetic fields, often found around white dwarfs [28]. Magnetic white dwarfs are the only systems next to neutron stars that are confirmed to emit coherent and highly linearly polarized radio pulses. AR Scorpii [29] and J1912 -4410 [30] are examples of white dwarf binaries that show periodic radio emission, with pulse periods on the order of minutes and orbital periods of about four hours (Methods).  \nThe origin and evolution of magnetic white dwarfs in close binary stars are described in detail in [9]. The high magnetic field of the white dwarf originates from a crystallization- and rotation-driven dynamo. A strong white dwarf magnetic field can connect with the field of the M dwarf and provide a synchronising torque on the white dwarf spin. AR Scorpii and J1912 -4410 are thought to be in the beginning stages of the synchronisation process. The shorter orbital period of ILT J1101 + 5521 indicates that the binary system is in the polar stage, where the synchronisation process is complete and the M dwarf fills its Roche-Lobe again. Additionally, the pulses from ILTJ1101 + 5521 show a periodicity that is consistent with the orbital period. This is in strong contrast to AR Scorpii and J1912 -4410, where the pulse and orbital periods differ by a factor of more than 40. For polars, the magnetic field strength of the white dwarf has increased to around B ∼ 1 MG. The formation of an accretion disk is suppressed, but accretion does occur directly onto the magnetic pole of the white dwarf.  \nPolars enter states with little to no accretion, and during these times the system appears as a practically detached white dwarf plus M dwarf system[25, 28]. No sustained accretion seems to occur for ILT J1101 + 5521 based on the lack of X-ray emission [31]. Additionally, the presence of radio pulses indicates little to no accretion,  \nwhich would likely disturb the creation of coherent radio emission, similar to statetransitioning millisecond pulsar systems (e.g. [32]). A 125-minute period with a M4.5V spectral type donor as observed for ILT J1101 + 5521 sits well within the population of polars; see Figure 7 in [33]. Typical polars have a white dwarf mass ( ∼ 0 . 6 M ⊙ ) and white dwarf effective temperatures of less than 11000 K [25]. The temperature of the white dwarf in ILT J1101 + 5521 is likely to be lower ( T eff between 4500 and 7500 K, see Extended Data Figure 9) indicating a more evolved system compared to the known sample from optical surveys, which are observationally biased to find hotter systems.  \nAlthough the energy budget is such that the observed radio luminosity could originate from the rotation of the white dwarf (Methods), we argue that the more likely scenario is that the radio emission comes from the interaction of the M dwarf with the white dwarf magnetic field, as magnetic coupling between the M dwarf and white dwarf is confirmed to occur for polar systems [9, 28]. The exact mechanism that produces the radio emission is unknown but, given the polar configuration, it seems most natural that we observe pulsed radio emission due to beaming effects. Here we observe the system in a certain geometry (superior conjunction, when the M dwarf is seen to be in line with and behind the white dwarf) once per orbital cycle, effectively looking down a beam of radio emission. In this case, the highly intermittent nature of ILT J1101 + 5521 (pulses are seen in one quarter of the observed orbits) could be explained by a strong variation in the brightness of the coherent radio emission, similar to what has been proposed for Rotating Radio Transients (RRATs) [34]. The steep spectral index for ILT J1101 + 5521 (brightest pulse α = -4 . 1 ± 1 . 1) seems consistent with what was found for and J1912 -4410 [30], where the in-band spectra suggest a steep negative spectral index of ∼ -3. These spectral indices are extreme and indicate that the spectrum could be similar to the drastic cutoff seen for electron cyclotron maser instability (ECMI) emission [35]. The radio emission could be triggered by the reconfiguration of the magnetic field (see e.g. [10, 11, 36-38] or in a more exotic scenario, the radio emission could come from accretion of material onto the magnetic pole of the white dwarf (at very low accretion rates), in a similar fashion to [12] or [13].  \nWe know of a small population of long-period transient radio sources (periods longer than 10 minutes) [1-4]. None of the previously detected long-period radio sources have a known binary companion and the periodicity of the pulses is argued to originate from the spin period of either a neutron star or a white dwarf. ILTJ1101+5521 is the only long-period radio source that is confirmed to be a binary and to feature a white dwarf. Furthermore, the radio pulses from ILT J1101+5521 have been shown to occur at the orbital period, and at the time of stellar conjunction. We provide a more detailed comparison of ILT J1101+5521 to the other long-period radio sources in the Methods. ILT J1101 + 5521 reveals that there are likely multiple progenitors that can produce long-period radio pulses. More speculatively, the existence of ILT J1101+5521 may provide an analogy for understanding periodically active fast radio burst (FRB) sources [39], which could originate from highly magnetised neutron stars interacting with a massive stellar companion [40]. Lastly, the high Galactic latitude of ILT J1101 + 5521 makes it easier to study via multi-wavelength observations. This will allow us to further study the exact geometry of this binary system, the properties of the two stars, as well as the detailed emission mechanisms at play.  \nAcknowledgements. We appreciate the input from Silvia Toonen's research group at the Anton Pannekoek Institute for Astronomy, and thank Manisha Caleb, Natasha Hurley-Walker, and Zorawar Wadiasingh for valuable discussions.  \nThis paper is based in part on data obtained with the International LOFAR Telescope (ILT) under project codes LC3 008 , LT14 003 and DDT20 005 . LOFAR [41] is the Low-Frequency Array designed and constructed by ASTRON. It has observing, data processing, and data storage facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Universit'e d'Orl'eans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland.  \nThis work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France 10.26093/cds/vizier . The original description of the VizieR service was published in [42].  \nThis work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/ consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.  \nThis work is based, in part, on observations obtained with the Hobby-Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. The Low Resolution Spectrograph 2 (LRS2) was developed and funded by the University of Texas at Austin McDonald Observatory and Department of Astronomy, and by Pennsylvania State University. We thank the Leibniz-Institut fur Astrophysik Potsdam (AIP) and the Institut fur Astrophysik Goettingen (IAG) for their contributions to the construction of the integral field units.", 'Declarations': "- · Funding I.d.R. acknowledges support through the project CORTEX (NWA.1160.18.316) of the research programme NWA-ORC which is (partly) financed by the Dutch Research Council (NWO). AR acknowledges funding from the NWO Aspasia grant (number: 015.016.033). J.W.T.H. and the AstroFlash research group at McGill University, University of Amsterdam, ASTRON, and JIVE are supported by: a Canada Excellence Research Chair in Transient Astrophysics (CERC-2022-00009); the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme ('EuroFlash'; Grant agreement No. 101098079); and an NWO-Vici grant ('AstroFlash'; VI.C.192.045).  \nBasic research in radio astronomy at the U.S. Naval Research Laboratory is supported by 6.1 Base Funding. Construction and installation of VLITE was supported by the NRL Sustainment Restoration and Maintenance fund. SM, GS, EF acknowledge funding from NASA XRP Grant 80NSSC24K0155  \n- · Conflict of interest/Competing interests The authors declare no competing interests.\n- · Ethics approval and consent to participate Not applicable\n- · Consent for publication Not applicable\n- · Data availability The data that support the findings of this study will be made available on Zenodo.\n- · Materials availability Not applicable\n- · Code availability The data that support the findings of this study will be made available on Zenodo.\n- · Author contribution I.d.R performed the transient search on LoTSS data and led the writing of the paper with suggestions from all co-authors. I.d.R. is the PI of the LOFAR follow-up observations and performed the analysis of the radio data and optical spectra. K.M.R. undertook the timing and dispersion measure analysis. C.B. developed the methods to fit the broadband photometry. A.R., R.A.M.J.W. and T.W.S. contributed to the development of the transient pipeline. A.R., R.A.M.J.W. and J.W.T.H. helped steer the project and plan follow-up observations.\n- C.B., K.M.R. and S.t.V. commissioned the simultaneous beamformed and interferometric observations for the LOFAR follow-up proposal. C.D.K. performed the MMT spectroscopic observations. G.S., S.M., E.F. acquired the HET LRS2 data and G.Z. performed the reductions. J.R.C. contributed to the analysis of the spectral data and writing of the sections on stellar activity. G.S. analysed the ZTF data. W.P. and T.E.C. led the search through archival VLITE data. R.A.D.W. performed the analysis of the UVOT data. V.M. allowed the use of the periodicity search code Altris .", 'LOFAR observations and pulse search analysis': "The Low-Frequency Array (LOFAR) [41] is a radio telescope that is comprised of many thousands of dipole antennas grouped into stations. The LOFAR Two-Metre Sky Survey (LoTSS; [14]) aims to image the whole northern sky using 3168 pointings. The survey has had two major data releases so far, Data Release 1 (DR1) [43] covering 58 pointings and Data Release 2 (DR2) [44] covering 814 pointings, spanning 5635 deg 2 . LoTSS observes between 120 and 168 MHz. The flux densities of detected sources are referenced to a central frequency of 144 MHz.  \nIn [15] a transient detection pipeline is described that looks for transient sources in 8-second to 1-hour image snapshots. The transient detection pipeline uses the Live Pulse Finder ( LPF ) [45]. Transient candidates are marked for further inspection if their position in the snapshot image does not correspond to the position of a known source from the LoTSS catalogue, indicating that the source is only visible for a small part of the full 8-hour integration. This transient detection pipeline was tested on a small subset of 58 LoTSS pointings, corresponding to the DR1 fields [15]. For each of  \nExtended Data Figure 1 : Gallery of ILT J1101+5521 pulse profiles. For each pulse both the light curve and the SNR as a function of time are shown. The crosses indicate the initial values for the flux and SNR that triggered the detection of this pulse. These detections are done with a quick source finder ( LPF ) on lower-quality (but 'quick') images. The blue light curves indicate the full pulse profile determined with a sophisticated source finder ( PySE ) using images that are of higher quality.  \n<!-- image -->  \nthese pointings, only the most sensitive part of the primary beam is considered (an inner circular region with a radius of 1.5 · ).  \nILTJ1101 + 5521 was detected in the P164+55 pointing in two 8-second snapshots and the 2-minute snapshot that encompasses this interval. Ref. [15] lists all the steps that were undertaken to test whether ILT J1101 + 5521 is a genuine astrophysical source or whether the signal is an imaging artefact. Once ILT J1101 + 5521 was firmly established as an astrophysical source, additional imaging steps were undertaken to characterise it. This included re-imaging the raw visibility data (without subtracting the sky model) with WSClean [46] with increased padding, and deconvolution cycles to obtain snapshot images with the lowest-possible rms noise. The most important imaging parameter settings are: -minuv-l 50.0 , -maxuv-l 60000 , -weight briggs -0.2 , -auto-mask 3 , -auto-thresholding 0.3 . Additionally, in these new images we use PySE , a more commonly used source finder that fully captures the properties of point sources in radio images [47]. The Extended Data Figure 1a shows the light curve corresponding to the pulse detected in the 2015 dataset. The two black crosses show the flux density (top panel) and signal-to-noise ratio (bottom  \nExtended Data Table 1 : LOFAR observations of ILT J1101 + 5521 . The final column shows the individual pulse peak flux entities for observations with one or more detections, whereas '-' indicates observations without detections. *: Peak flux density from the 1-second time slices is 431 mJy/beam.  \npanel) of the initial detection in the transient detection pipeline (with LPF ). It is clear that the improved imaging settings optimise the signal-to-noise ratio of the pulse.  \nAfter discovery of this initial pulse, we searched the LOFAR archive for data where the position of ILT J1101+5521 lies within the observed field-of-view. Four additional observations of the P164+55 field were found in the LOFAR Long Term Archive. Again, these observations had an averaged integration time of 8 seconds, the total observations were 8 hours, and the direction-independent calibrated data products were readily available. The same methods as in Ref. [15] were applied, and six additional pulses were found. Extended Data Figure 1 shows the flux and signal-to-noise ratio profiles for these pulses. The two pulses identified in the 2020-12-29 dataset are just slightly above the signal-to-noise ratio threshold of 6 used to identify candidate pulses. After the re-imaging procedure, one of the pulses turns out to be spurious. For all archival data, we tried pushing the signal-to-noise ratio down to 5 to see whether any additional pulses could be identified. The few candidates that we found turned out to disappear after re-imaging (similar to the first pulse detected on 2020-12-19, top-right of Extended Data Figure 1). An overview of the observations and detected pulses is shown in Extended Data Table 1. After establishing that we had robustly identified 7 bright pulses from this source, we obtained additional LOFAR observations through Director's Discretionary Time (project DDT20 005 ). Here we conducted simultaneous beamformed and imaging observations, in order to obtain both high spatial resolution and sub-second time resolution for any new pulses. Unfortunately, during these observations, no additional pulses were identified. The few candidates that were initially identified after dropping the detection threshold to a signal-to-noise of 5, turned out to disappear after properly re-imaging (see bottom column of Extended Data Figure 1).", 'Periodicity search and timing of the radio pulses': 'Despite the limited number of radio pulses, we performed a search for any potential underlying periodicity in their arrival times. First we computed the time of arrival (ToA) of each pulse. We then barycentred the ToAs before doing any further analysis. Typically, a template is used to compute ToAs but due to the dearth of pulses to  \nExtended Data Figure 2 : Number of matched time differences between pulse ToAs as a function trial period in minutes. The red star shows the preferred period of 125.52 minutes, while the cyan and green star show the (sub)harmonics of the preferred period.  \n<!-- image -->  \ncreate a stable radio profile for template creation, we decided to use every pulse for the ToA calculation individually by fitting a Gaussian to each pulse to compute the ToA based on the estimated peak of the Gaussian. To get a first estimate of the period, we run the barycentered ToAs through a simple folding algorithm 1 that is often used for highly intermittent sources, such as rotating radio transients (RRATs). For ILTJ1101 + 5521 we have seven ToAs, implying 21 unique time differences. Extended Data Figure 2 shows the number of time differences (between pulse ToAs) that are matched given a certain period, trialling periods from 1 second to 360 minutes with a timestep δt = 0 . 01 seconds and maximum 2% tolerance in phase to count as a match. We find that a period of 125 . 52 minutes, indicated by the red star, lines up all the pulse ToAs to within 2% of a full cycle, and we find that this period is preferred over its harmonics.  \nAdditionally, we used Altris ( 2 ) a brute-force search algorithm that fits the integer number of rotations within all the gaps between consecutive ToAs. The algorithm attempts to recursively discover the rotation counts under the assumption that a tentative solution for some n shortest gaps is known. Then, a phase model is fitted to the ToA gaps, from which a range of possibilities for the rotation count n + 1 is calculated. If no integer rotation counts are possible, the solution is discarded; otherwise, the algorithm attempts to further expand the set of new tentative solutions for the first n +1 gaps.Using this technique, we found the best period of 125.34 minutes  \nFigure 1a shows the light curves of the LOFAR detections folded with a period of 125.52 minutes. The smallest time window that fully captures all pulses implies a duty cycle of ∼ 2%, as well as a jitter in the pulse arrive times of ∼ 1% of the pulse period.  \n<!-- image -->  \nMJD  \nExtended Data Figure 3 : Timing residuals (the difference between the ToA and the ToA predicted by the timing model) divided by the period as a function of MJD.  \nwith a Q-factor of 225.6 where the Q-factor is the goodness of fit metric for the period (Q= 1 σ ph where σ ph is the rms of the offset of time of the peak of the pulse from the predicted arrival time of the peak). The Q-factor is a measure of the signal-to-noise ratio of the detected period. Then, we used TEMPO2 [48] to fit a simple timing model to the barycentred ToAs. We only used the spin frequency (F0), here also a proxy for the orbital period, as a fitting parameter, keeping the best-known position (RA, Dec) as fixed parameters. We first phase-connected the 6 ToAs that were close in time to each other (spanning 10 days) to obtain an rms of 23.476 seconds. Projecting that solution back to the first ToA about 4 years before, we obtain a 1σ timing rms of 46.577 seconds. Figure 3 shows the timing residuals after subtracting the best-fit phase-connected timing solution from the ToAs. By phase-connected we mean that we can predict the exact number of rotations/orbits across the entire time span over which the pulses were observed. Since the rms is less than 1% of the period, we believe this to be a phase-connected solution. We now use this rms to obtain an upper limit of 3.04 × 10 -11 s s -1 for the period derivative regardless of the origin of the period (spin or orbital origins).', 'Astrometry of the radio pulses': "Based on the 5 pulses that were identified with peak flux densities of > 68 mJy/beam, we perform an astrometric correction on the images to get the most accurate position possible. Using three bright ( > 200 mJy) point sources close to ILT J1101 + 5521 (within 10 ' ) we define the astrometric offset as the distance between the position of these bright point sources in the snapshot images and their LoTSS catalogue position. Subsequently, we shift ILT J1101 + 5521 in each snapshot according to this  \nsystematic offset. The position is then determined again in each snapshot and the final position is determined by averaging the positions weighted by the signal-to-noise ratio of ILT J1101 + 5521 in the specific snapshot. This yields (RA, Dec) (ICRS) = 165 . 4605 · ± 1 . 9 '' , 55 . 35545 · ± 0 . 39 '' , or RA (ICRS) = 11 h 01 m 50 . 5 s ± 1 . 9 '' and Dec (ICRS) = +55 d 21 ' 19 . 6 '' ± 0 . 39 '' . Additionally, we check our procedure by applying FitsWarp [49] to our images. This software is also designed to de-distort ionospheric effects in the image plane. We find a similar correction, that (within errors) matches the result from the aforementioned procedure, but with larger error bars ( ± 10 '' ) on the position.", 'Pulse polarization and Faraday rotation measure': 'We estimate the Faraday rotation measure (RM) using RM-Tools [50] from the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA). For the three brightest pulses we take the 8-second time slices with the highest total intensity and create full Stokes images with 12 frequency channels. Performing the QU-fit we find rotation measures of 4 . 503 ± 0 . 015 rad m -2 for the L801324 pulse, 4 . 72 ± 0 . 14 rad m -2 for the L801366 pulse, and 4 . 457 ± 0 . 033 rad m -2 for the L801380 pulse. We note that a large polarization position angle swing over the duration of the pulse would distort this result. The RM measurements have not been corrected for the time-variable ionospheric RM contribution, which is the most likely explanation for their apparent variability. These RMs are consistent with the contribution from the smoothed Galactic foreground [51], precluding the presence of significant local RM contribution from the corona or wind of a star in the system. We find peak linear polarization fractions of 51 +6 -6 % for the L801324 pulse, 13 +5 -3 % for the L801366 pulse, and 42 +7 -5 % for the L801380 pulse. No significant circular polarization is detected at the position of ILT J1101 + 5521 in the Stokes V images for any of the pulses in our sample. Performing a forced-flux extraction in the Stokes V images, we find circular polarization fractions of < 1 . 6% for the L801324 pulse, < 1 . 6% for the L801366 pulse, and < 4 . 2% for the L801380 pulse. These estimates are limited by the local noise in the Stokes V images.', 'Dynamic spectrum and dispersion measure': 'The average time resolution of 8 seconds does not allow for a sensitive dispersion analysis. We thus extract the original data products for the observations of the brightest pulse from the LOFAR Long Term Archive (project code LT14\\_004 ), and reprocess at the highest-possible time resolution of 1 second. Figure 1b shows the temporal profile and the dynamic spectrum of the reprocessed data for the bright L801324 pulse (see Extended Data Table 1). The higher time resolution reveals that the pulse consists of at least two components. The temporal profile was obtained by running the LPF source finder on the 1-second Stokes I images with a signal-to-noise ratio threshold of three. No clear dispersion track is visible in the dynamic spectrum. We used the LORDS software suite to estimate a DM value of 16 ± 6 pc cm -3 (see Extended Data Figure 4). The Galactic DM in the direction of ILT J1101+5521 to a distance of 504 pc  \nExtended Data Figure 4 : Frequency averaged, peak signal-to-noise ratio of the bright pulse presented in Figure 1b as a function of assumed dispersion measure in blue. The red dashed curve shows a Gaussian fit to the data.  \n<!-- image -->  \n(estimated distance to the M dwarf star) is ∼ 10 pc cm -3 based on the NE2001 electron density model [52, 53]. Within errors, the DM is consistent with the distance to the star. We note that the electron density models in directions well off of the Galactic plane ( b = 55 . 5200 · ± 0 . 0001 for ILT J1101 + 5521 ) are not constrained as well as directions closer to the Galactic plane.  \nThe best estimate for the spectral index is obtained from the in-band spectrum of the brightest pulse in L801324, as presented in Figure 1b. The best fit for the first component is α = -4 . 5 ± 1 . 0, and α = -4 . 8 ± 1 . 0 for the second component, assuming S ∝ ν α , where S ν is the flux density and ν the observing frequency. Averaging over the full pulse we obtain a very steep spectral index of α = -4 . 1 ± 1 . 1. However, we caution that the spectrum is not necessarily well described as a power law.', 'VLITE observations and analysis': "The VLA Low-band Ionosphere and Transient Experiment (VLITE 3 ) [54, 55] is a commensal system capable of continuously accessing 64 MHz of bandwidth (centered on 352 MHz) from the 236 -492 MHz Low-Band system deployed on the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array (NRAO, VLA). VLITE has been operational since November 2014 recording data from up to 18 antennas during nearly all regular VLA observations. Since VLITE accumulates a large amount of data, an automated calibration and imaging pipeline has been developed [54] that  \ncalibrates the visibilities and produces self-calibrated images. The VLITE Database Pipeline [56] then takes these images and creates a source database using the PyBDSF [57] source finding algorithm.  \nWe searched the VLITE Database at the position of ILT J1101+5521 , but found no catalogued source. We re-processed all archival VLITE observations within two degrees of ILT J1101+5521 where the VLA was in the high resolution A or B configuration. The images were broken into 60-second snapshots to match the pulse length of ILT J1101+ 5521 and maximise the VLITE sensitivity given the measured steep spectral index. The 225 resulting images were searched by PyBDSF for any source above 5 σ , and then visually inspected for emission at lower levels for the target position. In the same manner we searched all 103 VLITE Commensal Sky Survey [VCSS, 58] 28-second snapshot images from six separate days between 2017 and 2023 that cover the source position. Emission was not detected in either the VLITE or VCSS snapshots.  \nILTJ1101+5521 falls between 0.9 · and 1.9 · , with an average offset of 1.3 · , from the correlation centre of the archival VLITE data, resulting in significant sensitivity loss due to the instrumental response. The VCSS snapshots are observed in highly overlapping, continuous declination scans across the sky. Similar to the targeted images, the average source position is 1.2 · from the VCSS correlation centre, although it ranges from 0.5 · to 1.7 · . Due to the telescope movement during each of the 28 second VCSS frames, there is a slight additional loss of sensitivity. In Extended Data Figure 5, we plot the 3 σ upper limits from VLITE and VCSS snapshots together with the LOFAR detections and predicted 340-MHz flux based on the measured full pulse spectral index and its uncertainty. Finally, we re-imaged the VLITE data between November 2017 and September 2023 with snapshot cadences of 8 and 2 seconds to search for signatures of very bright pulses. No emission was detected, and we place 5 σ upper limits on bright pulses at 350 and 700 mJy/beam for the 8- and 2-second cadences, respectively. Archival VLITE and VCSS observations are not sensitive to pulses as steep as measured for the brightest ILT J1101 + 5521 burst ( α = -4 . 1 ± 1 . 1) as indicated by the lower panel in Extended Data Figure 5. VLITE and VCSS would be sensitive to flatter spectrum or brighter pulses, but unfortunately the temporal coverage in the archive is sparse for this position, making it difficult to rule out such possibilities.", 'Chance alignment probability with optical counterpart': "We performed a cone search through the available optical and near-infrared catalogues using Vizier [42] around the position of ILT J1101 + 5521 . This search yields an optical source at RA (ICRS) = 11 h 01 m 50 . '' 5698 and Dec (ICRS) = +55 · 21 ' 19 . '' 738, which is 0 . '' 44 offset from ILT J1101 + 5521 , but within the astrometric uncertainty of the radio-derived position of ILT J1101 + 5521 (error in (RA,Dec)= ± (1 . 9 '' , 0 . 39 '' )). To identify the probability of chance association we extract all the sources that lie within a radius of 3 degrees around the position of ILT J1101 + 5521 in Gaia EDR3 [59, 60]. We do not apply any other filters to the Gaia catalogue. Subsequently, 10 6 random positions within this patch are drawn and we determine the number of times one of these random positions falls within half an arcsecond of a Gaia source. We find that this only happens in 143 cases, and therefore conclude that the probability of chance alignment is p < 0 . 00014.  \nExtended Data Figure 5 : Top panel: 3 σ upper limits from VLITE 1-minute snapshot images (blue triangles) and the peak flux densities of the LOFAR detections (orange crosses) as a function of time. Bottom panel: distribution of 3 σ upper limits from VLITE 1-minute snapshot images (blue) and the extrapolated LOFAR peak flux densities at 340 MHz (VLITE frequency) for a spectral index of α = -4 . 1 (orange lines) and α = -3 . 0 (green dashed dotted lines).  \n<!-- image -->", 'Spectroscopic observations of the M dwarf': "We observed the M dwarf associated with ILT J1101+5521 using the Binospec optical spectrograph [61] on the 6.5-m Multiple Mirror Telescope (MMT) located at the MMT Observatory on Mt. Hopkins, Arizona. The observation occurred on 4 April 2024 starting at 02:42:49 UTC. We observed the M dwarf with 6 × 900 s exposures, using a 1 . 0 '' longslit, a 270 l/mm grating, set to a central wavelength of 6560 ˚ A, and using the LP3800 blocking filter. This resulted in spectral coverage from ≈ 3900-9300 ˚ A with an average spectral resolution of R ≈ 1530. Conditions were near photometric at the time of observation, with an average seeing of 1.2 '' , and the airmass ranged from ≈ 1 . 1-1.3 throughout the observation.  \nWe reduced all observations using pypeit [62], with dome flat and arc lamp calibration files obtained on the same night and instrumental configuration. We used standard pypeit parameters for Binospec, resulting in a signal-to-noise averaged over  \neach optimally extracted spectrum of ≈ 2-3. We then flux calibrated each spectrum with a sensitivity function derived from the standard star G191-B2B obtained in January 2024 and corrected the spectra for telluric absorption using an atmospheric model for the MMT Observatory.  \nAdditionally, we observed an optical R = 2 , 500 spectrum of the M dwarf associated with ILT J1101+5521 using the LRS2 [63] instrument on the Hobby Eberly Telescope [64, 65]. We used the LRS2-R unit on the 'red' channel from 6450 -8420 ˚ A. We reduced the LRS2 spectrum using the publicly available Panacea pipeline [66], which performs basic CCD reduction tasks, wavelength calibration, fiber extraction, sky subtraction, and flux calibration.  \nExtended Data Figure 6 shows the LRS2-R spectrum and a coadded spectrum of the 6 epochs obtained with Binospec. The highlighted regions are used for radial velocity fitting below. To determine the spectral type of the M dwarf we fit the obtained spectra to a range of spectral templates, including the ATLAS-T spectra [67] obtained with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, a suite of Keck LRIS spectra of late-M dwarfs [68], UVES/VLT high-resolution spectra of late type sub-dwarfs [69], and a complete M spectral type sequence for a sample of confirm young sources [70] 4 . We find that an M4.5V spectrum fits our data best, but we cannot confidently rule out an M4.0V star.  \nNext, we use the individual epochs to look for a radial velocity signature. In the following, we opt to use the [68] M4.5V model, as the resolution of 1.9 ˚ A is similar to the binned resolution of our observations, and the wavelength range overlaps our data. To obtain the Doppler shift of the individual epochs, we select a particular wavelength range. We avoid the noisy edges of the LRS2-R spectrum and disregard data above 8300 ˚ A (see Extended Data Figure 6). For consistency, we therefore also only consider the spectral range between 6500 and 8300 ˚ A for the Binospec data. Additionally, the signal-to-noise ratio (before binning) is below 3 for Binospec data below 6500 ˚ A. In this wavelength range the Binospec data has a resolution of 1 . 3 ˚ A. We bin the data by a factor of 2 and fit the template to the data while applying a Doppler shift.  \nWe note that excluding the H α line (6562 . 8 ˚ A) from the radial velocity analysis does not affect the outcome in a way that would alter our conclusions. The signalto-noise ratio in the blue part of the Binospec spectra is insufficient to observe the absorption lines we might expect from the white dwarf (e.g. [71]).", 'Swift X-ray Telescope observations and analysis': 'To search for any flaring activity of ILT J1101 + 5521 , we proposed for Swift observations during the L203* observing runs (see Extended Data Table 1). We obtained four individual visits using the X-ray Telescope (XRT; [72]) in December 2023 for a total time of 13.3 ks on source. We do not find any flares, as no photons are detected at the position of ILT J1101 + 5521 . The 3 σ upper limit that we find by stacking all observations is a count-rate limit of 1 . 43 · 10 -3 counts s -1 (0 . 3 -10 keV). Based on the position of ILT J1101 + 5521 we predict an absorbing Galactic column density of N H ≈ 8 . 7 · 10 19 cm -2 . This estimate was obtained using the web version of the nH tool [73] in HEASOFT [74]. To convert the count rate upper limit to unabsorbed flux  \nExtended Data Figure 6 : Spectrum of the M dwarf associated with ILT J1101 + 5521 . The top panel shows the LRS2-R spectra obtained with the HET. The bottom panel shows a coadded spectrum of the 6 epochs obtained by the Binospec instrument on the MMT. The highlighted parts of the spectrum are used for radial velocity fitting. Both spectra have been binned by a factor of two to increase signal-to-noise.  \n<!-- image -->  \nlimits we use WebPIMMS 5 and assume two spectral models for a pulsar/magnetar, following Ref. [6]. In the first scenario we assume thermal emission from a young pulsar, implying a blackbody spectrum with kT = 0 . 3 keV. In the second scenario, we assume non-thermal emission from an energetic pulsar, for which the corresponding spectral scenario is a power law with index Γ = 2. These two scenarios yield unabsorbed flux limits of 3 . 3 · 10 -14 erg cm -2 s -1 and 5 . 1 · 10 -14 erg cm -2 s -1 , respectively. This translates to L < (1 -1 . 6) · 10 30 · [ d 504 pc ] 2 erg s -1 .', 'Swift Ultra-violet Optical Telescope observations and analysis': "During the Swift XRT observations described above, ILT J1101+5521 was also simultaneously observed using the Swift UV/Optical Telescope (UVOT; [75]). We use several HEASOFT [74] tasks to obtain an upper limit on the UV flux by combining all observations. uvotsource was used to combine extensions within one visit and extract source and background information, uvotimsum was used to combine the visits, and uvotsource was used to determine the flux upper limit at the ILT J1101 + 5521 position. We use an extraction radius of 15 '' on the source location and to determine the background flux. The UVOT flux limit is < 1 . 0 · 10 -17 erg s -1 cm -2 ˚ A -1 in the uvm2 filter at 2246 ˚ A. We note that the data quality of these UVOT observations  \nis low. All point sources show up as double point sources which is caused by the attitude control problems of the Swift satellite during the period of early August 2023 until the beginning of April 2024 (due to problems with one of the three on-board gyroscopes; for more details see [76, 77]). Unfortunately, all our UVOT data (taken in December 2023) are affected by this issue. We take this into account by increasing our source extraction radius to 15 '' .", 'Zwicky Transient Facility photometry and analysis': 'Photometric variability might be expected from a close binary like ILT J1101 + 5521 , either at the orbital period due to irradiation of the M dwarf by the white dwarf [78] or at half the orbital period by ellipsoidal variations from the (close to) RocheLobe-filling M dwarf [79]. We analyse the z i , z r and z g photometry from the Zwicky Transient Facility (ZTF, [80]) for ILT J1101 + 5521 . Overall there are 592 datapoints publicly available in the three filters, where we rejected any points that were obtained in non-optimal conditions (non 0 quality flags) following the recommendations in the ZTF pipeline. This left a total of 464 photometric points with 37 in the z g filter, 337 in the z r filter, and 90 in the z i filter, which we used for our analysis. From the ZTF photometry, we find no evidence for modulations at the orbital period or half the orbital period in the periodograms of the ZTF photometry (Figure 7). We note that there are hints of two peaks at 70.3 min and 71.4 min seen in the z r and z i bands, respectively (highlighted in red Figure 7). However, although these two peaks do not correspond to any peaks in the window functions of the datasets, as the two periods are sufficiently different from each other and correspond to false alarm probabilities higher than 0.1%, we conclude that the data do not show significant detections of photometric variability.  \nTo probe the sensitivity of the ZTF data to detecting signatures of photometric variability, we performed injection tests where we injected sinusoidal variations with a period of 125.52 minutes with various amplitudes in the data. Doing so, we place an upper limit of 0.1 mag in the z r data and 0.05 mag in the z i data. The expected ellipsoidal variations using the equations in Ref. [81] for a white dwarf - M dwarf system are expected to have ellipsoidal variation amplitudes at or below the detectability threshold of the ZTF filters, in agreement with the non-detections in the ZTF filters.  \nAssuming an M dwarf radius of R MD = 0 . 217 R ⊙ and an orbital separation of a = 0 . 76 R ⊙ (for an M dwarf with mass 0 . 188 M ⊙ and a white dwarf with mass 0 . 6 M ⊙ ), we calculate the maximum inclination angle based on the fact that we do not see eclipses in the ZTF data. We find i < 74 · . Similarly, we estimate the irradiation luminosity from the white dwarf on the M dwarf. L irr = L WD · πR 2 MD 4 πa 2 = 0 . 02 L WD . We thus expect the white dwarf to at most contribute to the luminosity of the M dwarf at the level of ∼ 2%. This is consistent with the ZTF data where we do not find any evidence for photometric variability due to irradiation. Finally, we can rule out an M dwarf - M dwarf binary scenario from the lack of photometric variability in the ZTF data (at twice the orbital period). From the binary mass function, we know that a M4.5V dwarf with a sub M4.5V dwarf companion ( M < 0 . 188 M ⊙ ) needs to have an inclination i > 50 · to create the observed radial velocity amplitude. However, for short-period double M dwarf binaries with such inclinations strong photometric  \nExtended Data Figure 7 : ZTF photometry of ILT J1101 + 5521 . Data are shown for three filters: (a) z g , (b) z r , and (c) z i . Error bars denote 1 σ uncertainties. (d-f) Generalized Lomb-Scargle periodograms for each filter. The red vertical line denotes the P orb = 125 . 52min orbital period of the system, and the orange vertical line denotes half the orbital period ( P orb / 2) expected from ellipsoidal variations. There are no peaks above the 0.1% false alarm probability threshold (grey horizontal dotted lines) at the orbital period or half the orbital period. In the z r and z i filters the maximum peaks correspond to periods of 70.3min and 71.4min, but are below the 0.1% false alarm probability line. The power in the periodogram is normalized following [82].  \n<!-- image -->  \nvariability due to ellipsoidal variations from the (close to) Roche-Lobe-filling M dwarfs are expected and observed [79]. We expect the M dwarf to be close to Roche-Lobe filling for white dwarf masses M WD > 0 . 2 M ⊙ , see Extended Data Figure 8.', 'Broadband photometry fits': 'To constrain the parameters of a possible binary consisting of a white dwarf and a main-sequence star, we compare the observed ugriz magnitudes from the Sloan Digital Sky Survey (SDSS) Data Release 16 (DR16) [16] and the Two Micron All-Sky Survey (2MASS) JHK s [83] to absolute magnitude predictions in the same bands from white dwarf cooling models 6 [84-86] and stellar evolutionary tracks [87-89]. The models are interpolated over the stellar mass, white dwarf mass and white dwarf effective temperature, and the summed flux is corrected for the distance and absorption using an E g -r = 0 . 011 reddening value [90] and R V = 3 . 1 extinction values [91].  \nExtended Data Figure 8 : Constraints on the white dwarf mass given the observed mass function of the M-dwarf from optical spectroscopy. Constraints from orbital inclination are indicated in black with dotted, dashed, solid and dashed-dotted lines indicating an edge-on orbit ( i = 90 · ), and orbits at i = 60 · , i = 30 · and i = 20 · , respectively, with the light grey shaded area being excluded. The orange lines and shaded area indicate constraints provided by the Roche lobe around the M dwarf, indicated in fractions of the M dwarf radius of R 1 = 0 . 217 R ⊙ , where a Roche lobe radius of R L1 = 0 . 9 R 1 indicates Roche lobe overflow. The blue vertical line indicates the M dwarf mass of M 1 = 0 . 188 ± 0 . 010 M ⊙ determined from photometry.  \n<!-- image -->', 'White dwarf parameters via MCMC fitting': 'Through a Markov-Chain Monte-Carlo likelihood analysis [21] the observed magnitudes are well matched for a stellar mass of M MD = 0 . 188 ± 0 . 010 M ⊙ at a distance of d = 333 ± 25 pc (see Extended Data Figure 9). The white dwarf mass and the white dwarf effective temperature range are degenerate, with T eff ranging from 4500 to 7500 K for masses of M WD = 0 . 2 to 1.3 M ⊙ . The UVOT-upper limit ( < 1 . 0 · 10 -17 erg s -1 cm -2 ˚ A -1 at 2246 ˚ A, see Methods) is consistent with white dwarfs in this temperature and mass range. The distance following from the MarkovChain Monte-Carlo likelihood analysis is smaller than the Gaia geometric distance of 504 +148 -109 pc, but the distance estimates are consistent within 2 σ .  \nExtended Data Figure 9 shows the one and two-dimensional projections of the posterior probability distributions of the parameters used to fit the broadband photometry data presented in Figure 3. We use flat priors T eff ϵ { 3000 , 12000 } K, M WD ϵ { 0 . 2 , 1 . 3 } M ⊙ and M MS ϵ { 0 . 09 , 0 . 5 } M ⊙ and do not put a prior on the distance. The distance of d = 333 ± 25 pc is offset from the Gaia geometric distance estimate of 504 +148 -109 pc. We try setting a Gaussian prior on the distance ( µ = 504pc and σ = 109pc) but find  \nExtended Data Figure 9 : One and two-dimensional projections of the posterior probability distributions of the parameters used to fit the broadband photometry data presented in Figure 3. This Figure was created using [92].  \n<!-- image -->  \nthat the model quickly converges to a shorter distance again, with a preferred distance of d = 337 ± 25 pc.', 'Chromospheric activity from the M dwarf': 'We use the X-ray observations to investigate whether the M dwarf is chromospherically active. Usually, the X-ray luminosity of chromospherically active M dwarfs in the range 0 . 2 -2 keV varies between 2 · 10 26 -3 . 6 · 10 29 erg s -1 (see supplementary material in [27]).  \nFor ILT J1101 + 5521 , assuming a soft X-ray upper limit of ∼ 1 · 10 -14 erg cm -2 s -1 and a distance of 504 pc this translates to L < 3 · 10 29 erg s -1 . Based on the X-ray limits we cannot exclude chromospheric activity. Another indicator for chromospheric activity are the chromospheric emission lines Hα and CaII which are the result from magnetic heating of the stellar atmosphere. The spectra obtained in this work show a clear Hα emission line (see Extended Data Figure 6). This emission line follows the same radial velocity pattern as the M dwarf, thus the Hα line originates from the M dwarf. Following [93] we estimate the equivalent width (EW) of the Hα emission line in the HET LRS2-R spectrum, as the signal-to-noise and spectral resolution are higher than for the combined MMT spectrum. We find the EW of Hα to be -9 . 1, clearly indicating strong chromospheric activity when compared to the sample of [93] (see their Figure 2).', 'Radio emission from the M dwarf': 'Coherent radio emission from stellar systems can be produced by two mechanisms: plasma emission that occurs at the ambient plasma frequency and its harmonics, and cyclotron emission that occurs at the ambient cyclotron frequency and its harmonics [94]. Both plasma and cyclotron emission are expected to be highly circularly polarised [95].  \nWe therefore conclude that ILT J1101+5521 cannot originate from the M dwarf, as the circular polarisation fraction of the radio pulses is less than a few percent at most. In [27] it was found that even the weakest circularly polarised M dwarf still showed a circular polarisation fraction of 38%. Furthermore, the brightest radio luminosity (assuming the distance to the star to be 504 pc) is roughly L = 7 . 8 · 10 19 erg s -1 Hz -1 , which is 5 orders-of-magnitude brighter than the population of M dwarfs that has been observed at low radio frequencies [27]. Stellar flares with significant linear polarisation fractions have been observed [26, 96, 97], but these sources were always significantly circularly polarised as well. We do not know of any physical mechanism by which the M dwarf could produce the observed radio emission. Therefore, this radio emission must originate from the interaction between the companion and the M dwarf.', 'Chromospherically active binaries': 'Close stellar binaries, such as RS Canum Venaticorum (RS CVn) variables are the most luminous stellar radio systems and show heightened levels of chromospheric activities. The typical X-ray luminosity of chromospherically active binaries is > 10 30 ergs/s [98100]. However, RS CVn systems with X-ray luminosities as low as 10 29 ergs/s have also been found [99]. For ILT J1101 + 5521 , the X-ray limit is L < (1 -1 . 6) · 10 30 · [ d 504 pc ] 2 erg s -1 . A stronger argument against a RS CVn origin for ILT J1101+5521 is the relation between the X-ray and radio luminosity of such systems, the Gudel-Benz relation: L X = 9 . 48 · 10 18 L 0 . 73 ν,rad [101]. For the radio luminosities of the detected pulses ( L ν,rad = 1 . 4 -7 . 8 · 10 19 [ d 504 pc ] 2 erg s -1 Hz -1 ), this implies X-ray luminosities of L X = 0 . 9 -3 . 2 · 10 33 erg/s, which is three orders of magnitude above our X-ray  \nupper limit. Deviations from the Gudel-Benz relation have been observed [99], but no systems with over an order-of-magnitude discrepancy between X-ray and radio luminosities have been found. Additionally, the radio emission from RS CVn systems is thought to be produced by the electron cyclotron maser instability, which is expected to yield a circularly polarised radio signature [95, 98]. Finally, the RV signature and optical photometry imply a mass that is consistent with a compact object - not with a similar mass object. Therefore, we conclude that ILT J1101 + 5521 is not a close chromospherically active stellar binary.', 'Comparison to AR Scorpii and J1912 -4410': 'AR Scorpii [29] and J1912 -4410 [30] are examples of white dwarf binaries that show periodic radio emission. These systems are both in a binary with an M dwarf, with orbital periods of 3.56 and 4.03 hours, respectively. Both sources show radio pulses with periods of 1.97 and 5.3 minutes, respectively, which is the beat frequency between the spin period of the white dwarf and the orbital period. The pulse period in these systems is more than 23 times shorter than for ILT J1101 + 5521 . The pulse duration is only a fraction of the spin period, and therefore much shorter (roughly an order-ofmagnitude) than the pulses observed for ILT J1101 + 5521 . For AR Scorpii there are two emission components with different polarisation properties that contribute to the radio emission. These two components are likely to be the spin-down from the rapidly rotating, highly magnetised (up to 500 MG) white dwarf and the magnetic interactions between the M dwarf and the white dwarf [102]. Although both ILT J1101 + 5521 and the aforementioned systems are M dwarf - white dwarf binaries, ILT J1101 + 5521 is likely to be in a different evolutionary state than AR Scorpii and J1912 -4410 because of the shorter orbital period and because the radio pulses are seen at the orbital period instead of a beat period between the rotational and orbital period.', 'White dwarf spin-down luminosity': 'Spin-down luminosity is defined as the maximum luminosity that can be derived from the rotation of a magnetised white dwarf or neutron star. We consider whether the radio emission from ILT J1101 + 5521 can be powered by the spin-down of the white dwarf. Assuming that the spin period of the white dwarf has synchronised to the orbital period of the binary (as is the case for polars), we calculate the spin-down luminosity of a pulsar using L spin -down = 4 π 2 I ˙ P P 3 , where P = 7531 seconds and ˙ P ≤ 3 . 04 × 10 -11 . Assuming I ≈ 10 50 g cm 2 for a typical white dwarf we find that L spin -down < 2 . 8 · 10 29 erg s -1 . The peak radio luminosity (256 mJy at 144 MHz) is L ν,rad = 7 . 8 · 10 19 [ d 504 pc ] 2 erg s -1 Hz -1 . Integrating over the LOFAR bandwidth of 48 MHz and assuming a flat spectrum over this bandwidth, we find L rad = 3 . 7 · 10 27 [ d 504 pc ] 2 erg s -1 . This implies that for a radio efficiency of ξ ∼ 10 -2 , the spin-down luminosity could power the observed radio emission. For neutron stars radio efficiencies up to 10 -2 have been observed [103], but typical radio efficiencies for pulsars are much lower ( < 10 -2 ).', 'Comparison to other known long-period transient radio sources': "The first instance of a long-period radio source was the 'Galactic Centre Radio Transient' (GCRT), which initially showed pulses with a 10-minute duration on a period of 77 minutes [1]. Follow-up observations revealed fainter and shorter 2-minute pulses [104]. Two more recent discoveries are GLEAM-X J1627 -52 [2] and GPM J1839 -10 [3], which were found to have pulse periods of 18 and 21 minutes, respectively, with pulses that last for half a minute to 5 minutes. These aforementioned long-period sources are all thought to feature a neutron star or white dwarf, as a coherent emission mechanism is required to create radio emission with the observed brightness temperatures. Finally, ASKAP J1935+2148 was recently reported with a 54-minute period [4]. For ASKAP J1935+2148, two types of radio pulses are observed, one with extremely bright tens of seconds wide, highly linearly polarised pulses, and one with weaker pulses that last only hundreds of milliseconds and are highly circularly polarised. The different emission states are similar to changes in emission that have been observed for pulsars, and the authors argue that for ASKAP J1935+2148 a neutron star scenario is most likely. In contrast to ILT J1101 + 5521 , none of the aforementioned long-period radio sources are known to be in a binary system.  \nThe radio pulses from ILT J1101 + 5521 appear to be similar to the radio emission observed from other long-period sources in terms of timing, polarization and spectral properties. This is shown by the clustering of these sources in terms of radio luminosity and duration (see Extended Data Figure 10). Additionally, we find ILT J1101+5521 to be active for over 5 years, which again fits with the other long-period sources as GPM J1839 -10 was found in archival data spanning 30 years. Finally, the lack of Xray emission from ILT J1101+5521 ( L < (1 -1 . 6) · 10 30 · [ d 504 pc ] 2 erg s -1 ) is consistent with the lack of X-ray emission from GLEAM-X J1627 -52 and GPM J1839 -10 ( ≤ 10 32 -33 erg s -1 [2, 3]) and ASKAP J1935+2148 ( < 4 · 10 30 erg s -1 [4]).  \nThe four known long-period sources are all located towards the Galactic Plane (Galactic latitudes between -3 · and +1 · ), complicating follow-up at optical and infrared wavelengths. None of the previously known long-period sources has a confirmed optical counterpart. We note that there might be a bias to finding long-period radio sources towards the Galactic Plane because of the nature of the surveys that were used to find them. For GLEAM-X J1627 -52, the lack of an optical/infrared counterpart at the estimated distance of 1.3 kpc does not rule out a lower-mass, mainsequence companion [2], and an AR Scorpii-like system could pass unnoticed if it has a relatively large extinction. [6]. For GPM J1839 -10, the optical limits are not strong enough to rule out a binary companion given its larger distance ( d = 5 . 7 ± 2 . 9 kpc)[3], and a similar situation holds for ASKAP J1935+2148 [4] ( d = 4 . 85 kpc).  \nFast radio bursts (FRBs) are coherent radio flashes seen from other galaxies [106]. One repeating FRB source, FRB 20180916B, has a well-established 16.33-day activity period, where the observed bursts systematically occur at lower radio frequencies at later times during the activity window [39, 107]. Another repeating FRB source, FRB 20121102A, has a candidate activity period of about 160 days [108, 109]. The periodic activity of these FRB sources is theorised to originate from orbital motion, rotation, or precession (Ref. [40] and references therein). The discovery of  \nExtended Data Figure 10 : The luminosity of different types of radio transients as a function of their width (W) and frequency ( ν ), adapted from [4]. Diagonal lines represent constant brightness temperatures. The brightness temperature of 10 12 K roughly separates coherent emitters from the incoherent ones, with the shaded region (lower-right triangle) encapsulating the incoherent emitters. The long-period sources presented in [1, 3, 4, 105] shown in the legend appear to cluster together as indicated by the box, which is merely to highlight the sources and does not have a physical significance. The pulses from ILT J1101 + 5521 are indicated with the green squares. The green squares with a white face colour are the two sub-pulses as resolved in the higher time resolution data, see Figure 1b. The pulses from ILT J1101 + 5521 fit with the population of long-period sources.  \n<!-- image -->  \nILTJ1101+5521 presents a possible analogy for periodically active FRBs, which could originate from highly magnetised neutron stars (potentially 'magnetars') interacting with a massive stellar companion [40], and which would have greatly scaled-up energetics compared with a white dwarf system. FRB 20180916B is at a luminosity distance of about 150 Mpc, and its millisecond-duration bursts are at least trillions of times more luminous than ILT J1101 + 5521 [110]. The 16.33-day activity period of FRB 20180916B is comparable to the orbital periods of high-mass binaries known to harbour a neutron star. For instance, LS I +61 · 303 has an orbital period of about 26.5 days and has recently been shown to harbour a neutron star producing sporadic pulsations with a period of 269 ms [111]. In contrast to LS I +61 · 303, however, the periodic burst activity of FRB 20180916B could be related to the orbital period of the  \nsystem and share some of the viewing geometry and magnetic interaction effects we propose to explain ILT J1101 + 5521 .", 'References': '- [21] Foreman-Mackey, D., Hogg, D.W., Lang, D., Goodman, J.: emcee: the mcmc  \n- [32] Stappers, B., Archibald, A., Hessels, J., Bassa, C., Bogdanov, S., Janssen, G.,  \n[111] Weng, S.-S., Qian, L., Wang, B.-J., Torres, D.F., Papitto, A., Jiang, P., Xu, R., Li, J., Yan, J.-Z., Liu, Q.-Z., et al. : Radio pulsations from a neutron star within the gamma-ray binary ls i+ 61 303. Nature Astronomy 6 (6), 698-702 (2022)'}