Magnetar giant flares are rare explosive events releasing up to 10SUP47SUP erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 10SUP1516SUP G refs. SUP12SUP. Only three such flares have been seen from magnetars in our GalaxySUP34SUP and in the Large Magellanic CloudSUP5SUP in roughly 50 years. This small sample can be enlarged by the discovery of extragalactic events as for a fraction of a second giant flares reach luminosities above 10SUP46SUP erg sSUP1SUP which makes them visible up to a few tens of megaparsecs. However at these distances they are difficult to distinguish from short gammaray bursts GRBs much more distant and energetic 10SUP5053SUP erg events originating in compact binary mergersSUP6SUP. A few short GRBs have been proposedSUP711SUP with different amounts of confidence as candidate giant magnetar flares in nearby galaxies. Here we report observations of GRB 231115A positionally coincident with the starburst galaxy M82 ref. SUP12SUP. Its spectral properties along with the length of the burst the limits on its Xray and optical counterparts obtained within a few hours and the lack of a gravitational wave signal unambiguously qualify this burst as a giant flare from a magnetar in M82.
['Astrophysics - High Energy Astrophysical Phenomena']
A magnetar giant flare in the nearby starburst galaxy M82
{'A magnetar giant flare in the nearby starburst galaxy M82': "Sandro Mereghetti 1 , ∗ , Michela Rigoselli 1 , Ruben Salvaterra 1 , Dominik Patryk Pacholski 1 , 2 , James Craig Rodi 3 , Diego Gotz 4 , Edoardo Arrigoni 5 , 1 , Paolo D'Avanzo 6 , Christophe Adami 7 , Angela Bazzano 3 , Enrico Bozzo 8 , 6 , Riccardo Brivio 9 , 6 , Sergio Campana 6 , Enrico Cappellaro 10 , Jerome Chenevez 11 , Fiore De Luise 12 , Lorenzo Ducci 13 , 8 , Paolo Esposito 14 , 1 , Carlo Ferrigno 8 , 6 , Matteo Ferro 9 , 6 , Gian Luca Israel 15 , Emeric Le Floc'h 4 , Antonio Martin-Carrillo 16 , Francesca Onori 12 , Nanda Rea 17 , 18 , Andrea Reguitti 6 , 10 , Volodymyr Savchenko 19 , Damya Souami 20 , Leonardo Tartaglia 12 , William Thuillot 21 , Andrea Tiengo 14 , 1 , Lina Tomasella 10 , Martin Topinka 22 , Damien Turpin 4 , Pietro Ubertini 3  \n- 3 INAF - Istituto di Astrofisica e Planetologia Spaziali di Roma, Via del Fosso del Cavaliere 100, 00133 Roma, Italy\n- 4 Universit'e Paris-Saclay, Universit'e Paris Cit'e, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France", 'Ireland': 'Jules Janssen, F-92195 Meudon, France  \n- 21 Institut de mecanique celeste et de calcul des ephemerides (IMCCE) UMR 8028 du CNRS - Observatoire de Paris 77 Av. Denfert Rochereau 75014 Paris, France\n- 22 INAF - Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy  \nGiant flares, short explosive events releasing up to 10 47 erg of energy in the gamma-ray band in less than one second, are the most spectacular manifestation of magnetars, young neutron stars powered by a very strong magnetic field, 10 14 -15 G in the magnetosphere and possibly higher in the star interior 1, 2 . The rate of occurrence of these rare flares is poorly constrained, as only three have been seen from three different magnetars in the Milky Way 3, 4 and in the Large Magellanic Cloud 5 in ∼ 50 years since the beginning of gamma-ray astronomy. This sample can be enlarged by the discovery of extragalactic events, since for a fraction of a second giant flares reach peak luminosities above 10 46 erg s -1 , which makes them visible by current instruments up to a few tens of Mpc. However, at these distances they appear similar to, and difficult to distinguish from, regular short gamma-ray bursts (GRBs). The latter are much more energetic events, 10 50 -53 erg, produced by compact binary mergers and originating at much larger distances 6 . Indeed, only a few short GRBs have been proposed 7-11 , with different levels of confidence, as magnetar giant flare candidates in nearby galaxies. Here we report the discovery of a short GRB positionally coincident with the central region of the starburst galaxy M82 12 . Its spectral and timing properties, together with the limits on its X-ray and optical counterparts obtained a few hours after the event and the lack of an associated gravitational wave signal, qualify with high confidence this event as a giant flare from a magnetar in M82.  \nInstruments on board the INTEGRAL satellite detected GRB 231115A on 2023 November 15 at 15:36:20.7 UT 13 . This short burst (duration T 90 = 93 ms) was seen also by other satellites 14-18 , but only INTEGRAL could promptly associate it to M82 thanks to a localization at few arcmin level, publicly distributed by the INTEGRAL Burst Alert System (IBAS 19 ) only 13 s after the burst detection. The burst occurred in the field of view of the IBIS coded mask instrument 20 , at coordinates R.A.=149.0205 deg, Dec.=+69.6719 deg (J2000, 2 arcmin 90% c.l. radius). This position is consistent with, and supersedes, the one derived using the preliminary satellite attitude available in near real time 13 . The position of GRB 231115A coincides with the nearby starburst  \ngalaxy M82 12 (see Figure 1). Notably, the central region of the galaxy, where most star formation activity occurred, is inside the INTEGRAL /IBIS error region. Considering that the total angular size of all the galaxies with apparent luminosity brighter than M82 (excluding the Magellanic Clouds and M31) is ∼ 6000 arcmin 2 , the chance alignment with GRB 231115A is 4 × 10 -5 . A more elaborate analysis 21 indicates that the Bayes factor favouring a giant flare in M82 with respect to a chance alignment of a short GRB in the background is a factor 30 larger than that of the previous best candidate (GRB 200415A possibly associated to NGC 253 9 ), making GRB 231115A by far the most compelling case for a magnetar giant flare outside the local group of galaxies.  \nThe light curves plotted in Figure 2 show that GRB 231115A was clearly visible in the two IBIS detectors, which operate in different energy ranges: ISGRI 22 (20 keV - 1 MeV) and PICsIT 23 (175 keV - 15 MeV). The ISGRI and PICsIT spectra of the burst, extracted from the time interval with a significant detection in both detectors (from T 0 +0.687 s to T 0 +0.748 s, with T 0 =15:36:20.0 UT), have been jointly fit and are well described by an exponentially cut-off power law with photon index α = 0 . 04 +0 . 27 -0 . 24 , peak energy E p = 551 +81 -59 keV, and flux F 30 -2600 = (7 . 2 +0 . 6 -0 . 7 ) × 10 -6 erg cm -2 s -1 in the 30 keV - 2.6 MeV range (see Methods sect. 1). Assuming this spectral shape for the whole duration of the burst, the fluence in the same energy range is (6 . 3 ± 0 . 5) × 10 -7 erg cm -2 .  \nM82 was in the field of view of IBIS starting from 9.2 hours before up to 11.8 minutes after the burst, as well as during a target of opportunity (ToO) observation performed in the following three days, but no other bursting or persistent emission besides GRB 231115A was detected. Integrating all the data of the ToO observation, we obtained an upper limit of 2 . 6 × 10 -11 erg cm -2 s -1 on the 30-100 keV persistent emission at the position of GRB 231115A. Also, no other bursts from M82 were detected in all the available ISGRI archival data obtained since 2003, which provided a total exposure of more than 16 Ms (see Methods sect. 1).  \nAt the distance of M82 (3.6 Mpc 24 ), the GRB 231115A fluence measured with IBIS implies an emitted isotropic energy E iso = 10 45 erg (1 keV - 10 MeV), well below the typical value for a short GRB, but consistent with the one of the initial pulses of the three giant flares securely associated with magnetars 3-5 . The short duration and hard spectrum of GRB 231115A are also in agreement with the properties seen in the initial pulses of magnetar giant flares. In the three giant flares securely associated with magnetars, the initial short and hard pulse was followed by a softer  \ntail, with a duration of a few minutes and a maximum luminosity of ∼ 10 42 erg s -1 , characterized by a periodic modulation induced by the neutron star rotation. A similar feature would be too faint to be visible by IBIS at the distance of M82. This is shown in Figure 3, where we compare the limits obtained for GRB 231115A with the light curve of the most energetic Galactic giant flare ever observed (the one emitted by SGR 1806-20 in 2004 4 ) rescaled to a distance of 3.6 Mpc.  \nThe field of GRB 231115A was promptly observed with the Neil Gehrels Swift Observatory starting 9 ks after the burst 25 and a deeper X-ray observation was carried out with the XMM-Newton satellite, starting 0.7 days after the burst 26 (see Methods sect. 2). The comparison of the Swift and XMM-Newton images with those obtained in previous observations of M82 does not show evidence for new X-ray sources inside the GRB 231115A error region. Due to the presence of unresolved X-ray emission from the central part of the galaxy, the corresponding upper limit is position-dependent. The limit of 4 × 10 -14 erg cm -2 s -1 derived from the XMM-Newton data, which applies to 60% of the error region, is inconsistent with the flux at T 0 +1 day of the large majority of X-ray afterglows of short GRBs detected with the Swift /XRT instrument (see Methods Figure 7). Although we can not exclude a rapid afterglow drop, as observed in the class of so-called short lived short GRBs 27 , this X-ray upper limit disfavors the GRB interpretation. It is instead consistent with the quiescent X-ray emission of a magnetar in M82. In fact the X-ray luminosity of known magnetars, also considering the peak values reached during outbursts 28 , never exceeds 10 36 erg s -1 , corresponding to 6 × 10 -16 erg cm -2 s -1 at the M82 distance.  \nHolding true the association of GRB 231115A with M82 on the basis of the remarkable spatial coincidence, the low value of E iso could be reconciled with a short GRB origin only if the jet axis is pointed away from our direction, similar to the case of the gravitational wave event GRB 170817A 29 , which produced the bright kilonova AT2017gfo in the optical/NIR band. However, the deep upper limits of m > 20.0 - 24.0 mag (depending on the assumed position in the INTEGRAL error circle) on the optical counterpart that we obtained starting at T 0 +0.2 days (see Methods sect. 3) exclude this possibility, both for an AT2017gfo-like event and for even fainter kilonovae (see Methods sect. 3). We finally note that the binary merger of two compact objects at 3.6 Mpc would have produced a strong signal in gravitational waves, at variance with the non-detection reported by the LIGO/Virgo/KAGRA Collaboration 30 .  \nThe discovery of a young, active magnetar in M82, a starburst galaxy characterized by a  \nhigh star formation rate 31 is consistent with the origin of magnetars in core collapse supernova explosions 32 . The volumetric rate of giant flares with E iso > 4 × 10 44 erg has been recently estimated 10 as (3 . 8 +4 . 0 -3 . 1 ) × 10 5 Gpc -3 yr -1 . Assuming a direct link with the young stellar population of core collapse progenitors and considering the total star formation rate of 4000 M ⊙ yr -1 within 50 Mpc, this corresponds to a rate of R ( E iso > 4 × 10 44 ) = 0 . 05 +0 . 05 -0 . 04 yr -1 ( M ⊙ yr -1 ) -1 . Given the star formation rate of 7.1 M ⊙ yr -1 in M82 31 and assuming a power-law distribution of the giant flares energies with slope 1.7, the expected rate of giant flares with E iso > 10 45 erg in M82 is R M82 ( E iso > 10 45 ) = 0 . 19 +0 . 19 -0 . 15 yr -1 . Recent calculations based on relativistic hydrodynamical simulations 33 show that such ejecta are efficient sources for the nucleosynthesis of heavy elements through the r-process. Thus the giant flares with E iso > 10 46 erg (each producing up to 10 26 g of ejecta) can give a yield of ∼ 2 × 10 -9 M ⊙ yr -1 of r-process elements in M82 through this channel.  \nGRB 051103 is another short GRB that has been proposed as a possible magnetar giant flare in the M81 group of galaxies 7 . According to a detailed statistical analysis 10 , M82 is the most likely host of GRB 051103, despite being slightly outside its 260 arcmin 2 localization region. The rate of giant flares derived above is fully consistent with the detection of two events from M82 in 20 years. Therefore, starburst galaxies such as M82, which produce a significant population of young magnetars, appear as promising targets to constrain the energy distribution function of giant flares by means of long dedicated observations with future high sensitivity instruments.', 'References': '- 36. Goldwurm, A., et al. The INTEGRAL/IBIS scientific data analysis. Astron. Astrophys. , 411 , L223 (2003).\n- 37. Arnaud, K. A. XSPEC: The First Ten Years. Astronomical Data Analysis Software and Systems V , 101 , 17 (1996).\n- 38. Mereghetti, S., Topinka, M., Rigoselli, M., & Gotz, D. INTEGRAL Limits on Past High-energy Activity from FRB 20200120E in M81. Astrophys. J. Lett. , 921 , L3 (2021).\n- 39. HI4PI Collaboration, et al. HI4PI: A full-sky H I survey based on EBHIS and GASS. Astron. Astrophys. , 594 , A116 (2016).\n- 40. Struder, L., et al. The European Photon Imaging Camera on XMM-Newton: The pn-CCD camera. Astron. Astrophys. , 365 , L18 (2001).\n- 41. Turner, M. J. L., et al. The European Photon Imaging Camera on XMM-Newton: The MOS cameras. Astron. Astrophys. , 365 , L27 (2001).\n- 42. Gabriel, C., et al. The XMM-Newton SAS - Distributed Development and Maintenance of a Large Science Analysis System: A Critical Analysis. Astronomical Data Analysis Software and Systems (ADASS) XIII , 314 , 759 (2004).\n- 43. Becker, A. HOTPANTS: High Order Transform of PSF ANd Template Subtraction. Astrophysics Source Code Library , ascl:1504.004 (2015).\n- 44. Stetson, P. B. DAOPHOT: A Computer Program for Crowded-Field Stellar Photometry. Pub. Ast. Soc. Pac. , 99 , 191 (1987).  \n- 45. Karpov, S. STDPipe: Simple Transient Detection Pipeline. arXiv e-prints , arXiv:2112.006 (2021).\n- 46. Bradley, L., Sip"ocz, B. Robitaille, T., et al. astropy/photutils: 1.1.0, Zenodo , 10.5281/zenodo.4624996 (2021).\n- 47. Rossi, A., et al. A comparison between short GRB afterglows and kilonova AT2017gfo: shedding light on kilonovae properties. Mon. Not. R. Astron. Soc. , 493 , 3379 (2020).\n- 48. Bulla, M. POSSIS: predicting spectra, light curves, and polarization for multidimensional models of supernovae and kilonovae. Mon. Not. R. Astron. Soc. , 489 , 5037 (2019).\n- 49. Ferro, M., et al. A search for the afterglows, kilonovae, and host galaxies of two short GRBs: GRB 211106A and GRB 211227A. Astron. Astrophys. , 678 , A142 (2023).  \nAcknowledgements Wethank the ESA Mission Scientists Jan-Uwe Ness and Norbert Schartel for approving and quickly implementing the INTEGRAL and XMM-Newton ToO observations. This work is based on observations with INTEGRAL and XMM-Newton , ESA missions with instruments and science data centres funded by ESA member states, and with the participation of the Russian Federation and the USA. It is also based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundaci\'on Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This paper includes optical data taken with the Schmidt 67/92 telescope operated by INAF Osservatorio Astronomico di Padova (Mt. Ekar, Asiago). This work received financial support from INAF through the Magnetars Large Program Grant (PI S.Mereghetti) and from the GRAWITA Large Program Grant (PI P. D\'Avanzo). JCR, AB, SM and PU acknowledge financial support from ASI under contract n. 2019-35-HH.0. FO acknowledges support from MIUR, PRIN 2020 (grant 2020KB33TP) \'Multimessenger astronomy in the Einstein Telescope Era (METE).  \nAuthors contributions All authors reviewed the manuscript and contributed to the source interpretation. SM coordinated the work and the interpretation of the results, contributed to the analysis of the INTEGRAL and XMM-Newton data, and wrote most of the manuscript. RS and EA contributed to write the main part of the paper. DP and JCR carried out most of the INTEGRAL data analysis. DG, CF, EB, LD and VS routinely contribute to the operation of the IBAS software and participated to the near real time INTEGRAL analysis. PDA coordinated the analysis of the optical data from Italian telescopes. MR analysed the XMM-Newton data and contributed to the INTEGRAL spectral analysis. SC analysed the Swift data. MT contributed to the  \nsoftware for the burst search in archival data. DT, WT and CA coordinated the observation and the analysis of the optical data taken at OHP. LT analysed the optical data taken with the Schmidt 67/92 telescope in Asiago under the Large Program \'Search and characterisation of optical counterparts of GW triggers\' (P.I. Tomasella). AR and EC triggered, reduced and analysed the observations at the Asiago Schmidt telescope. RB and MF provided the short GRB afterglows and kilonovae observed and simulated optical light curves.  \nCompeting Interests The authors declare no competing interests.  \nCorrespondence Correspondence and requests for materials should be addressed to Sandro Mereghetti (email: sandro.mereghetti@inaf.it).  \nData Availability The data of the INTEGRAL, XMM-Newton and Swift satellites are publicly available in  \nthe respective online archives  \n(https://www.isdc.unige.ch/integral/archive,  \nhttps://www.cosmos.esa.int/web/xmm-newton/xsa,  \nhttps://swift.gsfc.nasa.gov/archive/ )  \n- . Optical data are available upon reasonable request.  \nCode Availability The software used for the data analysis is public and can be retrieved at  \nhttps://www.cosmos.esa.int/web/xmm-newton/sas,  \nhttps://www.isdc.unige.ch/integral/analysis Software,  \nhttps://heasarc.gsfc.nasa.gov/xanadu/xspec/  \nFigure 7: X-ray light curves of short GRB afterglows. The Swift /XRT (black square) and XMM-Newton /EPIC (black diamond) 3σ upper limits of GRB 231115A are indicated.  \n<!-- image -->  \nFigure 8: Optical light curves of kilonovae. The r-band light curves of AT2017gro and of the faintest red kilonova (simulated with the POSSIS code) are shown with dashed and dashed-dotted lines, respectively, assuming the M82 distance (3.6 Mpc). The magnitude 3σ upper limits obtained for a position inside (outside) the M82 galaxy are shown as blue (red) arrows.  \n<!-- image -->  \na  \n<!-- image -->  \nFigure 9: Optical light curves of short GRB afterglows. The observed light curves are shown in a , while b shows the light curves of those GRBs which have a measure of redshift, rescaled to the M82 distance (3.6 Mpc). The 3σ upper limits obtained for a position inside (outside) the M82 galaxy are shown as blue (red) arrows.  \n<!-- image -->', '1 INTEGRAL data analysis and results': 'IBIS is a γ -ray telescope based on two position-sensitive detection planes working in different overlapping energy ranges: ISGRI 22 (20 keV - 1 MeV) and PICsIT 23 (175 keV - 15 MeV). A coded mask placed about 3.2 m above ISGRI provides imaging capabilities over a field of view of ∼ 30 × 30 deg 2 . GRB231115A was detected at an off-axis angle of 4 · , inside the fully coded field of view. For the data reduction and analysis, we used version 11.2 of the Offline Science Analysis (OSA) software 36 , with the most recent calibration files, that properly account for the time evolution of the instrument response.  \nBurst spectrum We extracted an ISGRI spectrum, in 14 logarithmically spaced energy channels between 30 -500 keV, for the time interval from 15:36:20.687 to 15:36:20.748 UT. The PICsIT spectrum for the same time interval was extracted from the spectral-timing data mode in 8 predefined energy channels spanning 212 -2600 keV. Data in spectral-timing mode (counts integrated over the whole detector in 3.9 ms time bins) do not provide imaging information (contrary to the ISGRI data). Therefore, the background count-rate in each PICsIT energy channel was calculated as the median count rate during the INTEGRAL pointing (5013 s duration) containing the burst. A proper correction factor was applied to the PICsIT effective area to account for the angular distance between the IBIS pointing direction and GRB 231115A.  \nWe added 5% systematic uncertainties to both spectra. The ISGRI and PICsIT spectra were fitted simultaneously using XSPEC 37 version 12.12.1. A fit with a power law was unacceptable ( χ 2 = 59 . 4 for 17 degrees of freedom), while a power law with exponential cut-off, defined as F ( E ) = KE α exp( -E (2 + α ) /E p ) , gave a good fit with photon index α = 0 . 04 +0 . 27 -0 . 24 , peak energy E p = 551 +81 -59 keV, and 30 -2600 keV flux F = (7 . 2 ± 0 . 6) × 10 -6 erg cm -2 s -1 .  \nThe ISGRI spectrum for the time interval corresponding to the whole GRB, from 15:36:20.687 UT to 15:36:20.833 UT, is less well constrained, but the best fit parameters are within the errors consistent with those of the ISGRI plus PICsIT spectrum. All the fit parameters are summarized in Table 1, where we give also the results obtained with a blackbody model.  \nFollow up observation. Immediately after the discovery of GRB 231115A we requested an INTEGRAL Target of Opportunity (ToO) observation that started only 21 hours after the burst and continued  \nin the next satellite revolution. The first part of the ToO was done between 12:38 UT and 19:46 UT of November 16. The next one from 07:21 UT of November 17 to 23:09 UT of November 18, resulting in a total exposure of 162 ks divided in 57 pointings. Using the OSA 11.2 software, we produced the mosaics of the ISGRI images in the 30 -100 keV energy range for three periods: November 16, November 17-18, and November 16 to 18. No sources were detected at the position of GRB231115A. The 3σ upper limits on the 30 -100 keV flux, derived assuming a thermal bremsstrahlung spectrum with temperature kT = 30 keV, are 6 . 5 × 10 -11 erg cm -2 s -1 for November 16, 2 . 9 × 10 -11 erg cm -2 s -1 for November 17-18, and 2 . 6 × 10 -11 erg cm -2 s -1 for the sum of the two periods. We also derived an upper limit of 3 . 8 × 10 -10 erg cm -2 s -1 on the flux in the time interval from T 0 + 1.7 s to T 0 + 11.8 min, during which the GRB 231115A position was in the IBIS field of view. All these upper limits are plotted in Figure 3, where it can be seen that, even a giant flare as energetic as the one emitted from SGR 1806-20, at the distance of M82 would have been undetectable by IBIS after the initial bright pulse.  \nSearch for past activity from M82. The region of M82 has been repeatedly observed with INTEGRAL , starting in November 2003. After eliminating the time intervals with strong and variable background (typically due to solar activity or to particles trapped in the Earth radiation belts when the satellite is close to perigee) we obtained about 16 millions of seconds of useful time with M82 in the IBIS field of view. We extracted ISGRI lightcurves in the 30 -150 keV energy range, using only detector pixels illuminated for more than 50% from a source at the GRB 231115A position and searched for excess counts in the light curves over eight logarithmically spaced timescales between 0.01 and 1.28 s (see 38 for more details). All the burst candidates found in the light curves were then examined by making the sky images of the corresponding time intervals. None of them showed the presence of a source at the M82 position. Through simulations we found that bursts down to a factor ∼ 5 fainter than GRB 231115A would have been detected in these data, assuming the same time profile and spectrum.', '2 X-ray observations': "Swift pointed at M82, starting to collect data 9.0 ks after the burst occurrence 25 . Swift /XRT observations were carried out in Photon Counting mode in the 9 . 0 -39 . 2 ks time interval, collecting 4.4 ks of data. No new X-ray sources were detected within the INTEGRAL error region of GRB231115A. The 3σ upper limit is position-dependent due to the diffuse X-ray emission from the M82 galaxy. Outside the galaxy, the upper limit on the count rate is ∼ 2 -3 × 10 -3 counts  \ns -1 . Assuming a power-law spectrum with photon index Γ = 2 and the Galactic column density of 6 . 5 × 10 20 cm -2 (the total Galactic absorption column in the line of sight of M82 39 ) this corresponds to a 0 . 3 -10 keV unabsorbed flux of ∼ 10 -13 erg cm -2 s -1 .  \nA 47 ks long ToO observation of GRB 231115A was carried out with the XMM-Newton satellite, starting on 2023 November 16 at 08:29:16 UT, about 16.9 hours after the burst (Obs.ID 093239) 26 . The EPIC-pn 40 and the two EPIC-MOS 41 cameras were operated in Full window mode, with the thin optical-blocking filter. We processed the data with the EPPROC and EMPROC pipelines of version 18 of the Science Analysis System (SAS) 42 and the most recent calibration files and selected the EPIC events with standard filtering expressions. Time intervals of high background were removed with the ESPFILT task with standard parameters, thus yielding net exposure times of 7.56 ks (pn), 18.78 ks (MOS1) and 23.41 ks (MOS2). We selected single- and multiple-pixel events (PATTERN ≤ 4 for the pn and ≤ 12 for the MOS cameras); out-of-time events were also removed following the standard procedure a . 46:00.0 44:00.0 42:00.0  \na  \nb  \nFigure 4: EPIC-pn images of M82 . The exposure-corrected images refer to the 0 . 3 -2 keV ( a ) and 2 -10 keV ( b ) energy ranges. The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.  \n<!-- image -->  \nFigure 4 shows the pn exposure-corrected images in the soft ( 0 . 3 -2 keV) and hard ( 2 -10 keV) energy ranges. The images are dominated by the diffuse X-ray emission of the M82 galaxy, especially in the soft X-ray band. We compared the images obtained in this observation with those of all the previous pointings on M82 available in the XMM-Newton archive (Obs.ID 011229, 020608, 056059, 065780, 087094, 089106). We did not find any evidence for the appearance of a new source. In our observation, the total emission between 2 -10 keV from a circular region of radius 1.5 arcmin at the center of the galaxy (coordinates R.A. = 09 h 55 m 50 s . 9 , Dec. = +69 · 40 ' 47 '' ; J2000) can be described by an absorbed power law with N H = (1 . 0 ± 0 . 1) × 10 22 cm -2 , photon index Γ = 2 . 01 ± 0 . 04 and unabsorbed flux F 2 -10 keV = (2 . 88 ± 0 . 04) × 10 -11 erg cm -2 s -1 . The flux measured in previous XMM-Newton observations from the same extraction region and the same spectral model varied between ∼ 1 . 2 × 10 -11 erg cm -2 s -1 and ∼ 5 . 4 × 10 -11 erg cm -2 s -1 , indicating the presence of one or more variable sources that cannot be individually resolved with the XMM-Newton spatial resolution.  \nTo compute the upper limit for a new point source in the GRB 231115A error box, we applied the EUPPER task to the 2 -10 keV images. We used a circle of radius 15 '' for the source extraction and a concentric annulus of radii 22 . 5 '' and 30 '' for the background. The derived upper limits have been corrected for the fraction of source counts falling outside the extraction circle. Figure 5 shows the 3σ upper limits obtained by applying this procedure on a grid of positions with a step of 5 '' .  \nWe used the appropriate EPIC response matrices to convert the count rates of each camera to fluxes in the 0 . 3 -10 keV and 2 -10 keV energy ranges, with the assumption of an absorbed power-law spectrum with photon index Γ = 2 and N H = 6 . 5 × 10 20 cm -2 Finally we combined the upper limits of the three cameras to obtain an EPIC upper limit for each energy band. The results are shown in Figure 6. About 60% of the error circle has a 3σ upper limit of F 2 -10 keV = 1 . 2 × 10 -14 erg cm -2 s -1 and F 0 . 3 -10 keV = 4 . 0 × 10 -14 erg cm -2 s -1 . In less than 10% of the error circle, the upper limit is worse than 7 × 10 -14 erg cm -2 s -1 ( 2 -10 keV) and 1 . 6 × 10 -13 erg cm -2 s -1 ( 0 . 3 -10 keV). At the distance of M82, these fluxes correspond to luminosities of 1 . 9 × 10 36 and 6 . 2 × 10 36 erg s -1 , respectively.  \nFigure 7 shows the comparison between the X-ray light curves of short GRBs (defined such as T 90 ≤ 2 s), obtained from the Swift GRB catalog b and the 3σ upper limits we derived from the Swift and XMM-Newton observations of GRB 231115A.  \n41:00.0  \n30.0  \n69:40:00.0  \nFigure 5: Maps of count rate upper limits. The figure gives 3σ upper limits on the 2 -10 keV count rates of the EPIC-MOS1 ( a ), EPIC-MOS2 ( b ) and EPIC-pn ( c ) cameras. The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.  \n<!-- image -->", '3 Optical observations': "Multi-filter follow-up optical observations of GRB 231115A were carried out with the wide-field Schmidt telescopes sited in INAF observatories of Padova (Asiago, Italy), Abruzzo (Campo Imperatore, Italy) and with the 3.6-m Telescopio Nazionale Galileo (TNG, Canary Islands, Spain) between about 5 and 12 hours from the event T 0 . We also took additional VRI images about 7 hours after T 0 with the 120 cm Newton telescope located at the Observatoire de Haute Provence (OHP, France). The log of the observations is reported in Table 2. Image reduction was carried out following the standard procedures: subtraction of an averaged bias frame and division by a normalized flat frame. Astrometry was performed using the Pan-STARRS c catalogue. We carried out image subtraction with respect to SDSS d and telescope archival templates using the HOTPANTS (High Order Transform of Psf ANd Template Subtraction code 43 ) package to find and pinpoint variable sources within the INTEGRAL error circle. Aperture and PSF-matched photometry were performed  \n30.0  \n41:00.0  \nFigure 6: Maps of flux upper limits. The figure gives 3σ upper limits on the fluxes in the 2 -10 keV ( a ) and 0 . 3 -10 keV ( b ) energy range, obtained by combining the three maps of ?? . The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.  \n<!-- image -->  \nusing the DAOPHOT package 44 and the STDPIPE 45 and the PHOTUTILS e 46 packages for OHP/T120 images. To minimize any systematic effect, we performed differential photometry with respect to a selection of local isolated and non-saturated reference stars from the Pan-STARRS f . The presence of the M82 galaxy makes the background highly non-uniform within the INTEGRAL error circle. Magnitude upper limits have been computed for two regions (see Table 2): one (most conservative) corresponding to the centre of the M82 galaxy (R.A. = 09 h 55 m 53 s . 75 , Dec. = +69 · 40 ' 53 '' . 9 ; J2000) and another outside the core of the galaxy light (R.A. = 09 h 56 m 11 s . 31 , Dec. = +69 · 39 ' 09 '' . 6 ; J2000).  \nOur upper limits in the optical bands (Table 2) exclude the majority of cosmological short GRB afterglows. If occurred at the distance of M82, in a position located inside (outside) the galaxy, a short GRB would have produced an optical afterglow more than 12 (16) magnitudes  \nTable 1: Results of spectral fits of GRB 231115A. Errors are at 1 σ , fluxes are in the 30 -2600 keV energy range. Time intervals are in seconds after T 0 = 15:36:20 UT.Table 2: Log of optical observations of GRB 231115A. Magnitudes are in the AB system, not corrected for Galactic extinction. Upper limits are given at 3σ confidence level for a source within (outside) the M82 galaxy.  \nbrighter than our limits (see Fig. 9). These upper limits are about 9 (13) magnitudes fainter than the expected brightness of a kilonova like AT2017gfo occurring at the distance of M82 inside (outside) the galaxy. Since kilonovae associated to short GRBs can display significant differences in their luminosity, spectral properties, and temporal evolution 47 , we simulated a set of light curves with the POSSIS code 48 using the parameter range reported in 49 . We found that even the faintest event, placed at the distance of M82, is still about 5 (9) magnitudes brighter than our upper limits."}