# Quiet, but not silent: Uncovering quiescent state properties of two
transient High Mass X-ray binaries

Gayathri Raman,1 Varun,2 Pragati Pradhan,3 Jamie Kennea1
1Department of Astronomy and Astrophysics, The Pennsylvania State University,
525 Davey Lab, University Park, PA 16802, USA
2Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak,
Nainital-263001, Uttarakhand, India
3Embry-Riddle Aeronautical University, Department of Physics and Astronomy,
3700 Willow Creek Road, Prescott, AZ 86301, USA E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present the first set of broadband spectral and timing studies of two
transient X-ray pulsars, MXB 0656-072 and MAXI J1409-619 using NuSTAR
observations conducted during quiescence. Despite being captured at one of
their lowest luminosity states, both these targets show signs of ongoing low-
level accretion. Results from the time-averaged spectral analysis indicate for
the first time, the presence of a strong soft power law component along with
thermal emission from the neutron star hot spots. For both targets, the
quiescent thermal X-ray emission is consistent with the deep crustal heating
model. In MXB 0656-072, we do not detect any pulsations or indications of a
cyclotron line during quiescence. However, in MAXI J1409-619 we detect strong
pulsations at 502 s with a pulsed fraction of $\sim$66%, which adds this
pulsar to the list of a handful of quiescent-state pulsating systems.

###### keywords:

pulsars: individual, accretion, accretion discs, X-rays: binaries

††pagerange: Quiet, but not silent: Uncovering quiescent state properties of
two transient High Mass X-ray binaries–Quiet, but not silent: Uncovering
quiescent state properties of two transient High Mass X-ray binaries

##  1 Introduction

Accretion powered X-ray pulsars exhibit a broad dynamical range of X-ray
luminosities ranging from 1032-34 ergs s-1 during the quiescent phase all the
way up to 1037-38 ergs s-1 during outbursts, making them ideal laboratories to
probe accretion flows in various accretion regimes. While there have been
numerous studies of these systems during their highest luminosity states (see
for example, Bachetti et al. 2014; Doroshenko et al. 2020; Raman et al. 2021),
it is only with the advent of more sensitive missions like XMM-Newton, Chandra
and NuSTAR, that has it become possible to explore the low accretion rate
regime of these sources in greater detail (for example, Campana et al. 2002;
Rothschild et al. 2013; Elshamouty et al. 2016; Tsygankov et al. 2017a; Rouco
Escorial et al. 2020). The advantage of quiescent state observations is that
they offer a direct view of the emission from the neutron star (NS) atmosphere
and hotspots without having to deal with additional complexities that come
with increased accretion such as the presence of an accretion column and
interactions of X-ray photons with matter in the vicinity of these columns.
Moreover, transient pulsars spend a considerable amount of time in quiescence
during which we anticipate the observation of cooling effects in the neutron
star (NS) crust. These effects are directly influenced by the pulsar’s
outburst history. Such cooling processes can significantly contribute to X-ray
emission and/or variability, making it crucial to enhance our understanding of
these low accretion states through careful observation.

As the post-outburst luminosity decreases, the NS spin (Pspin) and the surface
magnetic field become crucial parameters in determining the system evolution
(Rouco Escorial et al., 2020). In particular, for fast spinning pulsars at
lower accretion states, their magnetospheric radius expands to beyond their
co-rotation radius. This effect centrifugally inhibits the infall of incoming
material and further expels material outwards. This is most commonly referred
to as the ‘propeller effect’ (Illarionov & Sunyaev, 1975). More recent works
have suggested some improvement in the standard ‘centrifugal barrier’ picture
(D’Angelo & Spruit, 2011, 2012). According to these works, the mass
transferred from a binary companion onto a spinning magnetosphere might be
accreted cyclically rather than being propelled out. This takes place via the
formation of a ‘Dead disc’ which is coupled with the NS magnetosphere.
Observations have indicated that residual X-ray emission is observed in
several Be X-ray pulsars at very low luminosities (Tsygankov et al., 2017a).
There has also been evidence of pulsations being detected in several objects
such as A0535+262 (Rothschild et al., 2013; Doroshenko et al., 2014; Tsygankov
et al., 2017a), 4U 1145-619 (Rutledge et al., 2007), 1A 1118-615 (Rutledge et
al., 2007), among others, indicating that matter might still be reaching the
NS surface (Rouco Escorial et al., 2020).
The powering mechanism of quiescent state (below or close to the propeller
luminosity) X-ray emission is still highly uncertain. Campana et al. (2001)
proposed that there could be matter leaking via the magnetospheric centrifugal
barrier and contributing to the observed emission. In particular, for slow
rotating systems (P${}_{\rm{spin}}\sim$ 10s of seconds), it has been shown
that the required matter leakage can be achieved via quasi-stable accretion
from a ‘cold’ recombined disk (Tsygankov et al., 2017b). Rapidly rotating
systems will be unable to achieve a cold disk because of the onset of the
propeller regime at fairly higher mass accretion rates. Another proposed
origin for residual X-ray emission from quiescent pulsars is the ‘deep crustal
heating’ model (Brown et al., 1998). Freshly accreted matter compresses the NS
crust thus inducing nuclear reactions that power the thermal emission observed
during quiescence. This mechanism has been successful in explaining the
quiescent state emission for several X-ray binaries hosting NS with low
magnetic field strengths (for eg. LMXBs, Wijnands et al. 2017). In particular,
for systems with no recent outburst episodes or other signatures of increased
activity, it can be safely assumed that the neutron star crust and core are at
thermal equilibrium (Wijnands et al., 2017). In such cases, we can probe the
core temperatures and in turn, examine NS core physics by using the NS surface
temperature as a proxy. Additionally, if the NS is highly magnetized, we can
further study the effect of the magnetic field on the heating and cooling of
NS crusts.

The Be X-ray binary transient, MXB 0656-072, was first discovered by SAS-3 in
1975 at a flux density of 80 mCrab (Clark et al., 1975). It was followed by
two more observations at 50 and 70 mCrab in 1976 (Kaluzienski, 1976) and was
initially classified to be a low mass X-ray binary (Liu et al., 2001). After
several decades of remaining in quiescence, the source went into an outburst
in 2003 for a duration of about 2 months (Remillard & Marshall, 2003). The
source was identified as a pulsating object with periodicity at 160.7 s
(Morgan et al., 2003). The optical counterpart was identified as an O9.7 Ve
spectral type star using ROSAT PSPS observations (Pakull et al., 2003) (later
refined and re-classified as a O9.5 Ve star, Nespoli et al. 2012). The 2003
outburst was classified as a type-II outburst using RXTE observations.
Preliminary RXTE results during that outburst revealed the presence of a
cyclotron resonant scattering feature at$\sim$33 keV (Heindl et al., 2003).
Further detailed studies of the outburst characteristics were carried out by
McBride et al. (2006), where they found that the width of the Cyclotron
Resonant Scattering Feature (CRSF) increased during the decline, while no
changes in CRSF properties were observed as a function of pulse phase. With an
average pulse period of 160.4$\pm$0.4 s, a spin-up of 0.45 s was observed
across the outburst (McBride et al., 2006). Using optical observations, the
source was estimated to be located at a distance of 3.9$\pm$0.1 kpc (Pakull et
al., 2003; McBride et al., 2006). The next set of outbursts took place between
November 2007 to November 2008 that was monitored using INTEGRAL (Kreykenbohm
et al., 2007), RXTE  (Pottschmidt et al., 2007) and Swift-BAT (Kennea et al.,
2007). Optical observations conducted between 2006-2009, indicated a steadily
strengthening H$\alpha$ line, whose equivalent width was strongest just prior
to the onset of the 2007 outburst (Yan et al., 2007). This was attributed to
an extended circumstellar disk. Moreover, correspondingly fainter UBV
magnitudes, during the same 2007 outburst, were indicative of inner disk
dilution (Yan et al., 2007). Long term X-ray observations using Swiftand RXTE
during the period 2006 to 2009 indicated an orbital period of 101.2 days, for
the first time (Yan et al., 2012). The HEXTE spectrum obtained during outburst
was described using a cutoff power law with a low energy absorption, along
with a 6.4 keV Fe line and a CRSF at 30 keV (Yan et al., 2012). Additional
X-ray/optical variability studies identified a correlation between the
aperiodic variability and spectral parameters, similar to 1A 1118-615 (Nespoli
et al., 2012).

MAXI J1409-619 is a transient first detected on board MAXI during outburst in
October 2010 (Yamaoka et al., 2010). This was localized to a position
corresponding to RA=14:08:02.56 and Dec=-61:59:00.3 using Swift-XRT (Kennea et
al., 2010b). The lack of an optical counterpart and the presence of a nearby
IR source suggested a possible High Mass X-ray Binary nature for the transient
(Orlandini et al., 2012). In November 2010, MAXI J1409-619 went into its
second outburst that was 7 times brighter than its previous event. This
triggered the Swift Burst Alert Telescope (BAT). Pointed XRT observations of
the source in the PC mode revealed the presence of a 503$\pm$10 s periodicity
with a 42% pulsed fraction, which was associated with the spin period of the
pulsar in the system (Kennea et al., 2010a). Fermi-GBM observations from
December 2010, indicated a refined spin period of 506.95 s with the pulse
profile having double peaked shape (Camero-Arranz et al., 2010).
Interestingly, Quasi Periodic Oscillations (QPOs) were detected in this object
at 0.192$\pm$0.006 Hz along with two higher harmonics using RXTE observations
(Kaur et al., 2010). More recently, Dönmez et al. (2020) also reported
correlations between the source flux state and the presence and strength of
the QPOs and their harmonics. This source and 4U 0115+63 are the only two
known accreting X-ray pulsars to exhibit QPO harmonics (Dönmez et al., 2020).
The source continuum spectrum studied using RXTE observations during outburst
was well described by a cutoff power law with index 1.3 along with either a
partially covered absorption model or a reflection model (Dönmez et al.,
2020). The spectrum also showed a strong 6.5 keV Fe line with no indication of
a cyclotron feature (Yamamoto et al., 2010). The source was re-discovered with
archival BeppoSAX observations from 2000 during its low state. The broadband
BeppoSAX spectrum in the 1.8–100 keV band was best described using an absorbed
power law ($\Gamma\sim$0.8) (Orlandini et al., 2012). A cyclotron absorption
feature with a fundamental at 44 keV (along with two higher harmonics at 73
keV and 128 keV) was also detected in the quiescent state that allowed for the
measurement of the neutron star surface magnetic field of 3.8$\times$1012 G
(Orlandini et al., 2012).

The two X-ray pulsars analyzed as part of this work have been in their
quiescent accretion state for more than a decade. A dedicated observing
campaign was conducted using Chandra, XMM-Newton and Swift to explore the
quiescent state of X-ray pulsars (Wijnands, 2003). As part of that observing
campaign in 2012, MXB 0656-072 was observed using Chandra, where it displayed
a soft thermal spectrum (Tsygankov et al., 2017a). The source was observed at
a flux level of 2$\times$10-12 erg cm-2s-1, which corresponds to a luminosity
of 6.2$\times$1033 erg s-1 (assuming a distance of 5.1 kpc obtained using
recent Gaia measurements Arnason et al. 2021). MAXI J1409-619 has been in
quiescence since its last outburst in 2010 and has never been studied using
current instruments since. In this paper, we study the broadband spectral
properties of these two transient X-ray pulsars using NuSTAR observations. In
Section 2, we present the methods and observation details, followed by the
analysis and results in Section 3. We further summarize our results and
discussions in Section 4.

##  2 Observations and data reduction

The Nuclear Spectroscopic Telescope Array (NuSTAR) is a space-based high
energy mission capable of carrying out sensitive X-ray imaging and
spectroscopy in the 3–79 keV band. It comprises of two co-aligned identical
X-ray focal plane modules FPMA and FPMB, each with a FOV of
13${}^{\prime}\times$13′ (Harrison et al., 2013). It provides a moderate
spectral resolution of 400 eV around 10 keV. With its broadband spectral
sensitivity, NuSTAR is ideally suited to carry out cyclotron line studies and
broadband spectroscopy. The NuSTAR observations of MXB 0656-072 and MAXI
J1409-619 were carried out in November 2021 and September 2022 with an
exposure time of 50 ks and 56 ks, respectively. Details of the observations
are given in Table 1. Both sources were observed during the ongoing low
accretion states. Figure 2 shows the long-term Swift-BAT 15-50 keV light curve
marking the previous outbursts and the location of the NuSTAR observations
used for this work.

Figure 1: NuSTAR images are shown for MXB 0656-072 (top) and MAXI J1409-619 (bottom) in the two detector modules with the source and background regions marked.  Target | Obs ID | Tstart (UTC) | MJD | Exp time | Count rate
---|---|---|---|---|---
|  |  |  | (ks) | (s-1)
MXB 0656-072 | 30701016002 | 2021-11-10 11:02:24 | 59528.46 | 50 | 0.04
MAXI J1409-619 | 30801023002 | 2022-09-06 22:56:09 | 59828.95 | 56 | 0.06
Table 1: NuSTAR observations of MXB 0656-072 and MAXI J1409-619 used in this
work.

Figure 2:  Figure shown the long term Swift-BAT (15–50 keV) light curves for
MXB 0656-072 (top panel) and MAXI J1409-619 (bottom panel) with the NuSTAR and
Chandra quiescent state observations marked. Both targets have remained in
quiescence since their previous major outbursts.

The raw data were processed using the standard NuSTAR data analysis software
nustardas version 2.1.1 along with the CALDB version 20210210. Source photons
were extracted from a circular region with radius 50 arc seconds for both
targets. The S/N was optimized by collecting background photons from a nearby
source-free region, using the standard background extraction procedure. We
used the routine NUPIPELINE to generate the calibrated level 2 event files.
The photon arrival times in the source event file were corrected to the solar
center barycenter using the target coordinates (in J2000): RA=104.57 degrees
and Dec=-7.209 degrees for MXB 0656-072, and RA=212.01 degrees and
Dec=-61.98342 degrees for MAXI J1409-619. The light curves, spectra, and other
auxiliary response files were extracted using the task nuproducts for both
FPMA and FPMB instruments.

##  3 Analysis & Results

###  3.1 Timing analysis

In order to examine their low-level accretion properties, we carried out a
pulsation search for the targets. The source and background light curves were
generated from the FPMA and FPMB modules. The background-subtracted light
curves have a mean count rate of $\sim$0.04 c s-1 and 0.06 c s-1, for MXB
0656-072 and MAXI J1409-619, respectively, in the 3–79 keV band. We further
carried out a pulsation search using the ftool efsearch around the previously
reported spin period values and also searched for any detectable QPO peaks in
the power spectrum using the FTOOLS task - powspec. For MXB 0656-072, we were
unable to detect any signatures of excess power at any frequency during this
observation. Our results are consistent with previous null reports of
pulsations during quiescence using Chandra (Tsygankov et al., 2017a). In MAXI
J1409-619, we were able to identify a clear peak in efsearch at 502$\pm$3 s
when the light curves from each of the detector modules were folded a
resolution of 0.1 s (Figure 3). The measured period is consistent with
previous measurements of 503$\pm$10 s during its 2010 outburst (Kennea et al.,
2010a). The uncertainty on the spin measurement is computed using a thousand
realizations of Gaussian randomized light curves (a method similar to what is
described in Boldin et al. 2013 and Varun et al. 2022). Since MAXI J1409-619
was known to exhibit Quasi Periodic Oscillations (QPOs) during its low
accretion state (Kaur et al., 2010; Dönmez et al., 2020), we carried out a
search for QPOs in the power density spectrum between 0.0001 Hz to 1 Hz using
a light curve binned at 0.5 s. The power density spectrum was generated by
averaging out 16 individual segments with 8192 s duration each. In Figure 5,
we show the PDS in the 0.0–4mHz frequency range, where we detect the pulse
peak at 1.99 mHz with an SNR of $\sim$5$\sigma$ (Figure 5). However, we do not
observe any features that resemble a QPO beyond 0.1 Hz (as observed in prior
works such as Dönmez et al. 2020) from both the detector modules in MAXI
J1409-619. The 3–79 keV light curve was folded at the observed pulse period of
502 s at an epoch of MJD 59828 and 32 folded phase bins. The pulse profile
consists of a double-peaked shape with a pulse fraction111PF=(Imax -
Imin)/(Imax + Imin), where Imax and Imin are the maximum and minimum
normalized count rates associated with the pulse profile. of 66% in the entire
NuSTAR band (Figure 4 last panel). We further extracted light curves in
different energy bands - 3–5 keV, 5–10 keV, and 10–15 keV. Due to high
background beyond 15 keV, we were unable to detect pulsations or measure
pulsed fractions. We folded the energy resolved light curves using the same
fold parameters to obtain energy resolved pulse profiles as shown in Figure 4.
We obtain pulsed fractions of 74%, 84% and 88% in the 3–5 keV, 5–10 keV, and
10–15 keV bands.

Figure 3: Period search results obtained using efsearch for MAXI J1409-619
shows a clear peak at 502 s in both detector modules.

.

Figure 4: Energy resolved pulse profiles for MAXI J1409-619 light curves
folded at the pulse period of 501.6 s.

.

Figure 5: The power density spectrum for MAXI J1409-619 obtained using light
curves from both detector modules showing the pulse peak at 1.99 mHz.

###  3.2 Spectroscopy

We further carried out a phase averaged spectral analysis for MXB 0656-072 and
MAXI J1409-619 in XSPEC v.12.12.0. For each individual pulsar, we carried out
a joint fit using the FPMA and FPMB modules. The spectra were grouped using
the tool grppha in such a way that they contain at least 20 counts per bin for
MXB 0656-072 and 30 counts per bin for MAXI J1409-619, respectively. A
constant normalization was included between the spectra from the two modules
to account for any variations in the effective areas or potential
uncertainties in their calibrations. The constant was kept fixed at 1.0 for
FPMA and was allowed to vary for FPMB. We used the Tuebingen-Boulder ISM
absorption model ‘TBabs’ model to describe the line-of-sight neutral Hydrogen
column density (NH) assuming WILMS abundances (Wilms et al., 2000) and VERN
cross sections (Verner et al., 1996).

For characterizing the continuum spectra of accretion powered X-ray pulsars
during the quiescence, their spectra have been traditionally described using
several combinations of one or more phenomenological models, which include the
following models: 1) a simple power law (with or without the addition of
thermal blackbody components), 2) a neutron star atmosphere (NSA) model, which
assumes emission from a magnetized neutron star (Pavlov et al., 1995) and 3) a
model comprising of two Comptonization components (Tsygankov et al., 2019),
among others. Chandra observations of quiescent pulsars such as GRO J1750-27
and V0332+53, have been well described using the NSA model (Rouco Escorial et
al., 2019; Elshamouty et al., 2016). The double compton model assumes
comptonization by hot and cold electrons and has been recently observed to fit
spectra for at least three quiescent pulsars including A0535+262, GX 304-1 and
X-Persei (Tsygankov et al., 2019) (see Sokolova-Lapa et al. 2021 for a
detailed theoretical interpretation of the compton double hump model for low
accretion level emission). We describe the fitting process adopted for each of
the pulsars using some of the models below.

####  3.2.1 MXB 0656-072

MXB 0656-072 has been in a low luminosity state since its last outburst in
2008. Its quiescent state spectra, obtained using Chandra, four years after
the outburst, had a thermal shape with a blackbody temperature around 1 keV
and a luminosity of $\sim$4$\times$1033 erg s-1 (Tsygankov et al., 2017a). We
use this as a starting point for the spectral analysis. Since the background
counts start dominating above 20 keV, we ignore the energy range beyond 20
keV. The NH was kept fixed at the galactic
value222https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl of
0.6$\times$1022 cm-2 (HI4PI Collaboration et al., 2016). We first fit the
3.0–20.0 keV spectrum using a simple absorbed power law model. This resulted
in a reasonable fit with a $\chi^{2}$/dof of 93.2/93. We additionally also
tried a combination of a blackbody and a powerlaw model that gave similar fit
results. A single blackbody model was insufficient to fit the entire broadband
spectrum, so we discard that model. We also alternatively attempted the fit
using the partial covering absorber model (pcfabs) instead of the blackbody
function. We found that the resultant blackbody fits were marginally better
(reduced $\chi^{2}\sim$0.91 for 93 dof) compared to the pcfabs (reduced
$\chi^{2}$$\sim$0.96 for 107 dof). In addition to these models, we carried out
a fit using a model comprising of two broad comptonized components as
prescribed by Tsygankov et al. (2019). We adopted the compTT model from XSPEC
that describes the comptonization process of soft photons in a hot plasma
(Titarchuk, 1994). We tied the seed photon temperatures of both the components
together and allowed the plasma temperature and plasma optical depth
parameters to vary. The fit was unable to constrain the plasma temperature for
both compton components. Alternatively, we tried modeling the possible two
comptonized components using two power law functions. Although the fit did not
seem to have any visible residuals, the resultant power law index values were
not getting constrained. We therefore disregard the double Comptonization
models for this paper. The fit parameters for all the models discussed above,
except the double comptonization models (comptt+comptt and po+po), are shown
in Table 2 and the spectral fit for the PL+bbodyrad model is shown in Figure 6
(left panel). The data statistics beyond 20 keV were poor to constrain the
CRSF line parameters.

In this current quiescent epoch, the source spectrum appears to be extremely
soft. We used the CFLUX model to derive an average source flux of
1.5$\times$10-12 ergs cm-2 s-1 in the 3–79 keV band. Assuming a source
distance of 5.1 kpc (Arnason et al., 2021), we obtain a luminosity of
4.7$\times$1033 erg s-1, which is lower by almost a factor of 2 from the
previous Chandra measurement.

Figure 6: Best fit spectrum using the PL+bbody model for MXB 0656-072 (left
panel) and for MAXI J1409-619 (right panel). The dotted lines indicate the
additive bbody and PL components. For MXB 0656-072, the PL component dominates
beyond $\sim$6 keV, while for MAXI J1409-619 the emission in the entire
spectral band is dominated by the PL component. For both targets, the addition
of the bbody model improved the fits.

####  3.2.2 MAXI J1409-619

We have carried out the spectral fitting for MAXI J1409-619 following the
spectral analysis from its previous low state observations using BeppoSAX
(Orlandini et al., 2012). We fix the line of sight column density (NH) to its
galactic value of 2.0$\times$1022 cm-2. The grouped spectra were jointly fit
for both the FPMA and the FPMB modules in XSPEC in the 3.0–30.0 keV band,
since statistics were poor beyond that. We first fit an absorbed power law
model which yielded a reasonable fit with a $\chi^{2}$/d.o.f of 107.4/86.
Since the spectrum is background dominated above 30 keV, we were unable to
identify signatures of the predicted CRSF line near 44 keV. To improve fit
further and address the low energy residuals, we added a thermal blackbody
component. This improved the fit ($\Delta\chi^{2}\sim$ 9 for 86 dof).
Alternatively, we also tried the fit using the partial covering absorber model
(pcfabs). The resultant fits were marginally better with the blackbody model
(reduced $\chi^{2}\sim$1.10 for 86 dof) compared to using pcfabs (reduced
$\chi^{2}$$\sim$1.26 for 90 dof). We again attempted to use a combination of
two comptonization models in the form of two comptt models and alternatively,
two PL models. In both these cases, the fitting was unable to constrain the
power law index or norm. Additionally, weird residuals at lower and higher
energies showed up. We therefore ignore these double Comptonization models for
further discussion. Details of the best fit parameters for the PL and
PL+bbodyrad models are shown in Table 3 and the spectral fit for the
PL+bbodyrad model is shown in Figure 6 (right panel). MAXI J1409-619 had a
source flux of 4.4$\times$10-12 ergs cm-2 s-1. Assuming a source distance of
14.5 kpc, we obtain a luminosity of 1$\times$1035 erg s-1.

Parameter/Model | PL | PL+BB
---|---|---
const FPMA | 1.0 (fixed) | 1.0 (fixed)
const FPMB | 0.97 | 0.97
NH ($\times$1022cm-2) | 0.6 (fixed) | 0.6 (fixed)
$\Gamma$ | 2.69$\pm$0.11 | 2.1${}^{+0.5}_{-0.7}$
norm ($\times$10-4) |  | 5.6
T${}_{\texttt{bbody}}$ (K) | – | 0.99${}^{+0.16}_{-0.11}$
R${}_{\texttt{bbody}}$ (km) | – | 0.11$\pm$0.04
Red. $\chi^{2}$(dof) | 1.01(93) | 0.91(93)
Unabsorbed flux (3–79 keV)1 | 1.5$\pm$0.15 | 1.82$\pm$0.2
Unabsorbed flux (3–20 keV)1 | 1.3$\pm$0.13 | 1.28$\pm$0.12
Table 2: Best fit spectral parameters for the joint FPMA+FPMB fits for MXB
0656-072 assuming two model combinations as detailed in the text (see Section
3.2). Errors are indicated at 90% confidence.
1 In units of 10-12 erg cm-2 s-1 Parameter/Model | PL | PL+BB
---|---|---
const FPMA | 1.0 (fixed) | 1.0 (fixed)
const FPMB | 1.1 | 1.1
NH ($\times$1022cm-2) | 2.0 (fixed) | 2.0 (fixed)
$\Gamma$ | 1.73$\pm$0.07 | 1.54$\pm$0.25
norm ($\times$10-4) | 4.2 | 2.8
T${}_{\texttt{bbody}}$ (K) |  | 1.75${}^{+0.8}_{-0.3}$
R${}_{\texttt{bbody}}$ (km) |  | 0.10$\pm$0.052
Red. $\chi^{2}$(dof) | 1.24(86) | 1.1 (86)
Unabsorbed flux (3–79 keV)1 | 4.41$\pm$0.04 | 4.50$\pm$0.11
Unabsorbed flux (3–30 keV)1 | 2.70$\pm$0.02 | 2.63$\pm$0.03
Table 3: Best fit spectral parameters for the combined (FPMA+FPMB) spectra for
MAXI J1409-619 assuming two model combinations as detailed in the text (see
Section 3.2). Errors are indicated at 90% confidence.
1 In units of 10-12 erg cm-2 s-1.
2 Assuming 10% uncertainty in distance measurement for MAXI J1409-619.

##  4 Discussion

In this paper, we present the results from the analysis of MXB 0656-072 and
MAXI J1409-619 during one of their lowest-ever observed luminosity states
using broadband NuSTAR observations conducted after a decade of quiescence.
For the first time in these two systems, we have uncovered a dominant non-
thermal contribution to the quiescent emission, whose origin we discuss below.
Our results also reveal the presence of thermal hotspots that are indicative
of ongoing accretion albeit at an extremely low level. In MAXI J1409-619, we
detect pulsed emission with a spin period of 502 s.

###  4.1 The propeller regime

In the ‘propeller’ regime (Illarionov & Sunyaev, 1975), a centrifugal barrier
is generated by the rotating NS magnetosphere, due to which matter infall is
halted. Further accretion will continue to remain inhibited if the velocity of
the magnetic field lines is greater than the local keplerian velocity. Only
when the magnetospheric radius shrinks below the corotation radius, which is
easily realized during high accretion rates, can matter penetrate into the
magnetosphere and can get accreted. Since the magnetospheric radius is a
function of the mass accretion rate, one can determine the limiting luminosity
for the onset of the propeller regime (Campana et al., 2002; Tsygankov et al.,
2016) by equating the magnetospheric radius and the corotating radius:

$L_{lim}(R)=\frac{GM{\dot{M}_{lim}}}{R}=4\times
10^{37}~{}k^{7/2}~{}B^{2}_{12}~{}P^{-7/3}~{}M^{-2/3}_{1.4}~{}R^{5}_{6}~{}\rm
erg~{}s^{-1}$ (1)

Here, $M_{1.4}$ is the neutron star mass in units of $1.4~{}M_{\odot}$,
$\dot{M}$ is the mass accretion rate, $R_{6}$ is the radius of the NS in units
of 106 cm, $B_{12}$ is the magnetic field in units of 1012 G and $P$ is the
spin period of the pulsar in seconds. The factor $k$ is defined such that it
relates the magnetospheric radius for disc accretion and the Alfvèn radius for
spherical accretion and is typically assumed to be 0.5 (Ghosh & Lamb, 1978;
Tsygankov et al., 2016). In several cases, accreting pulsars transitioning to
the propeller regime have been associated with sudden luminosity drops (for
example, 4U 0115+63 and V0332+53 Campana et al. 2001; Tsygankov et al. 2016).

MXB 0656-072 has been observed using NuSTARat a luminosity of 4.7$\times$1033
ergs s-1 which is lower from the previous quiescent state measurement using
Chandra (Tsygankov et al., 2017a). We assume canonical NS mass and radius
values of 1.4 M⊙ and 12 km (Nättilä et al., 2017), respectively, and further
input the measured spin period of 160.4 s (McBride et al., 2006) and a
magnetic field strength of 3.6 $\times$1012 G (Heindl et al., 2003). We find
the limiting luminosity for this source to be at 6.5$\times$1032 erg s-1,
which is down by a factor of 10 from the current source luminosity. For the
case of MAXI J1409-619, the observed luminosity during the recent NuSTAR
observations is 1$\times$1035 erg s-1. By using the spin period measured in
this work (501.6 s) and the magnetic field estimate of 3.8$\times$1012 G
(Orlandini et al., 2012), we obtain a propeller limiting luminosity of
5.1$\times$1031 erg s-1, which is well below the observed luminosity. Our
findings are therefore in line with the expectations that both targets are in
a low-accretion regime rather than a propeller regime.

###  4.2 Origin of low-level emission

Following the above discussion on the limiting luminosity for transition to
the propeller regime, we can safely assume that during the NuSTAR observations
for these two objects, accretion is not centrifugally inhibited in any way,
which suggests that the observed X-ray emission is indicative of ongoing
accretion. Here, we first discuss various quiescent emission mechanisms and
compute their predicted luminosity estimates. We will then discuss the
applicability of these models for the two targets.

####  4.2.1 The cold disk (CD) model

Recent observations of slowly rotating pulsars accreting at low levels have
demonstrated that not all pulsars undergo such transitions to the propeller
regime (Tsygankov et al., 2017b). It has been proposed that for slow pulsars,
a slowly rotating magnetosphere can further push down the limiting luminosity,
while maintaining temperatures $\sim$6500 K that would sustain recombination
of hydrogen. This model is known as the ‘cold’ accretion disk model (Tsygankov
et al., 2017b; Tsygankov et al., 2017a). Below a critical mass accretion rate
($\dot{M}_{cold}$), such recombinations are predicted to set in (Lasota, 1997;
Tsygankov et al., 2017b):

$\dot{M}_{cold}=3.5\times 10^{15}~{}r_{10}^{2.65}~{}M_{1.4}^{-0.88}\rm
g~{}s^{-1}$ (2)

where $r_{10}=r/10^{10}$ cm is the inner disc radius. The luminosity at which
the transition to a cold disc occurs will correspond to the following:

$L_{cold}=9\times
10^{33}~{}k^{1.5}~{}M_{1.4}^{0.28}~{}R_{6}^{1.57}~{}B_{12}^{0.86}~{}\rm
erg~{}s^{-1}$ (3)

This is $\sim$2$\times$1034 erg s-1 for typical pulsars with magnetic field
strengths of $\sim$1012 G. Here the value of $k$ is taken as 0.5 as commonly
considered for disc accretion (Ghosh & Lamb, 1978). If $L_{\rm cold}>L_{\rm
prop}$, then lower mass accretion rates can drive such systems to stably
accrete from a cold disc. In the standard accretion scenario, the
magnetospheric radius increases as the source gets dimmer. As opposed to that,
for pulsars that transition to the ‘cold’ phase, the inner disk radius
decreases at lower luminosity states. This ensures that these sources continue
to stably accrete from a cold disc before switching to the propeller regime at
even lower luminosities (Tsygankov et al., 2017a). Furthermore, according to
this model, the luminosity is expected to fade as $L\propto t^{-0.7}$ and
observations of sustained pulsed emission can be expected (Tsygankov et al.,
2017b). The transition luminosity to the cold disc regime and the
corresponding predicted luminosities (expected to be observed at any given
$t$) are shown in Table 4.

####  4.2.2 The deep crustal heating (DCH) model

An alternative explanation for the origin of quiescent X-ray emission in
pulsars is the deep crustal heating model. According to this model, the soft
thermal component is assumed to arise from a cooling neutron star which is
powered by nuclear reactions that occur during each outburst (Rouco Escorial
et al., 2020; Brown et al., 1998). Matter that is deposited on the NS surface
during every outburst compresses the deeper crustal layers, which then
triggers nuclear reactions and releases heat. This will drive the heated crust
out of equilibrium with the NS core. After the outburst is complete, and
quiescence sets in, thermal radiation is emitted in the form of cooling
radiation from the NS surface and eventually brings the crust and core back
into equilibrium (see Rouco Escorial et al. 2019, Wijnands et al. 2017 and
section 4.3 in Tsygankov et al. 2017a for a detailed review).

For a population of non-pulsating LMXBs and accreting ms pulsars (Wijnands et
al., 2017), it appears that enhanced cooling processes such as the Direct URCA
process (as against standard neutrino cooling via the modified URCA or
bremsstrahlung processes) may need to be invoked in order to explain the
observed crustal cooling curves. Wijnands et al. (2017) elaborate on the
various cooling mechanisms applicable for different particle compositions of
the NS core. In addition to the DCH model, a ‘shallow heating’ mechanism has
also been invoked in the past in order to explain crust cooling curves seen in
LMXBs (Rouco Escorial et al., 2019; Deibel et al., 2015). Be X-ray pulsars
being relatively young systems, may not possess a crust that is completely
made of accreted material. Instead, they can be expected to have as ‘hybrid’
crusts (Wijnands et al., 2017). It is unclear whether such hybrid crusts can
support all kinds of accretion induced nuclear reactions (Wijnands et al.,
2017) or whether some of the deep crustal reactions that generate the
necessary heating get inhibited (Rouco Escorial et al., 2019). Moreover, the
presence of ‘shallow heating’ effects remains largely explored for Be pulsars
(see Rouco Escorial et al. 2019 for the singular case of GRO J1750-27), since
the effect of strong magnetic fields on the heating and cooling of the NS
crusts is barely understood. For the Be systems that have shown signatures of
cooling of an accretion heated NS, the emission has been found to arise from a
small portion confined to the NS hotspots (Campana et al., 2002; Elshamouty et
al., 2016; Wijnands et al., 2017). 4U 0115+63 and V0332+53 are the only two Be
pulsars, for which signatures of crustal heating and cooling have been
identified (Wijnands & Degenaar, 2016; Rouco Escorial et al., 2017).
Additional searches for signatures of crust cooling were carried out in other
Be pulsars such as GRO 10750-27 (Rouco Escorial et al., 2019), with no
success.

In order to understand the thermal heating and crust cooling effects in all
categories of accreting NS systems, we construct a comprehensive plot of the
observed quiescent luminosities as a function of their long term mass
accretion rate (Figure 8). This plot includes compilations of measured
quiescent thermal luminosities of 1) LXMBs, accreting ms-pulsars and Soft
X-ray Transients (SXT) from Potekhin et al. (2019), and 2) of Be-X-ray pulsars
from Tsygankov et al. (2017a). Several key factors play into the estimation of
the quiescent thermal luminosity for accreting neutron stars. These include
the long term averaged mass aceretion rate, the composition of the crust, and
the most relevant cooling process. We note that several uncertainties exist
within these measurements. These include variations in the long term average
accretion rate (during different quiescent epochs), measurement methods of the
long term accretion rate, and distance estimates, to name a few. We therefore
treat these estimates as crude. This plot can serve as a reference to test
various crust cooling scenarios (as shown in Wijnands et al. 2017).

In order to derive the expected quiescent luminosities from deep crustal
heating, we adopt a procedure similar to what has been indicated in Tsygankov
et al. (2017a) and Elshamouty et al. (2016). We use the long term Swift-BAT
light curve count rates and convert it to flux using
WebPIMMS333https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl and
WebSPEC444https://heasarc.gsfc.nasa.gov/webspec/webspec.html by assuming a
simple power law model with an index 1.0 and a high energy cutoff at 15 keV,
which is a representative spectrum for typical X-ray pulsars in this energy
band. We use this flux to then estimate the average long term luminosities for
both targets. Assuming perfect accretion efficiency, we then compute the mass
accretion rate as follows:

$\langle\dot{M}\rangle=\frac{L_{avg}~{}R}{GM}$ (4)

where $G$ is the gravitational constant, $M$ is the mass and $R$ is the radius
of the neutron star. We assume canonical neutron star mass and radius values
of 1.4 M$\odot$ and 12 km, respectively. This gives us an accretion rate of
0.3$\times$10-10 M$\odot$ yr-1 and 1.1$\times$10-10 M$\odot$ yr-1, for MXB
0656-072 and MAXI J1409-619, respectively. We then estimate the expected
quiescent luminosity as described in Tsygankov et al. (2017a) and Brown et al.
(1998) as follows:

$L_{q,predicted,DCH}=\frac{\langle\dot{M}\rangle}{10^{-11}~{}M_{\odot}~{}\rm
yr^{-1}}\times 6\times 10^{32}~{}\rm erg~{}s^{-1}$ (5)

This gives us predicted quiescent state thermal luminosities of
2.0$\times$1033 erg s-1 and 1$\times$1034 erg s-1, for MXB 0656-072 and MAXI
J1409-619, respectively. The predicted luminosities are shown in Table 4.

Source | Period1 | Lprop | $\langle\dot{\rm M}\rangle$ | Lq,predicted,DCH | L${}_{\rm q,transition~{}to~{}CD}$ | Lq,predicted,CD | Lbb | LPL
---|---|---|---|---|---|---|---|---
| (s) | (1032 erg s-1) | (10-10 M$\odot$ yr-1) | (1033 erg s-1) | (1034 erg s-1) | (1031 erg s-1) | (erg s-1) | (erg s-1)
MXB 0656-072 | - | 6.5 | 0.3 | 2.0 | 1.2 | 0.5 | 1.6$\times$1033 | 4.1$\times$1033
MAXI J1409-619 | 502 | 0.5 | 1.1 | 10.0 | 1.3 | 1.9 | 1.1$\times$1034 | 10.4$\times$1034
Table 4: Parameters determined in this study for MXB 0656-072 and MAXI
J1409-619.
1 Period measured during quiescence.

####  4.2.3 Contribution from the companion star

Some fraction of the observed low-level X-ray emission can potentially be
expected to come from the companion star. The companion for MXB 0656-072 is a
Be star and that for MAXI J1409-619 has not been confirmed as yet. However,
detailed estimates carried out by Tsygankov et al. (2017a) indicate that the
X-ray luminosities of typical O or B type stars are of the order of $\sim$1031
ergs s-1, which is several orders of magnitude lower than our measured
luminosities for both MXB 0656-072 and MAXI J1409-619 as part of this work. We
therefore assume that the companion does not, in any significant way,
contribute to the observed X-ray emission for both sources.

####  4.2.4 Likely emission mechanism in MXB 0656-072 and MAXI J1409-619

The Be X-ray source MXB 0656-072 continues to exhibit thermal emission
consistent with the predicted emission expected from the cooling of accretion
heated neutron star during the current NuSTARobservation (this work and
Chandra observations analyzed in Tsygankov et al. 2017a). The radius of the
blackbody emitting region ($\sim$0.06 km) is consistent with that of a NS
hotspot near the poles. In addition, we have detected a strong power law
component, for the first time, dominating over the thermal flux (see table 4),
as against a purely thermal spectrum observed using Chandra (Tsygankov et al.,
2017a) in 2012. The origin of this power law component provides additional
incentive to better understand the processes occurring at low luminosities
(see Section 4.4 below for detailed discussion). The cold accretion disk model
(which supports continued accretion and therefore, pulsations) predicts a
quiescent luminosity of 1.3$\times$1031 erg s-1, which is two orders of
magnitudes lower than what we observe using NuSTAR. This makes it an unlikely
scenario for the current observations. Being a slowly rotating pulsar, it is
unlikely that it will ever descend into a propeller regime where accretion is
centrifugally inhibited. Our inability to detect pulsations for this pulsar
using NuSTAR possibly hints at very low pulse fraction during this low state
observation. On the other hand, we detect the presence of a thermal component
(which is preferred over a partial covering absorber) from our spectral fits,
which we can attribute to that arising from deep crustal heating. Our measured
quiescent luminosity obtained from the thermal component for MXB 0656-072
corresponds to 1.6$\times$1033 erg s-1. The predicted quiescent state
luminosities due to the deep crustal heating model is $\sim$2.0$\times$1033
erg s-1, which is of the order of the observed thermal luminosities and is
therefore likely to be the dominant source of quiescent emission in MXB
0656-072.

Figure 7: Variation of the spectral index as a function of luminosity. The red
data point indicates the NuSTAR measurement obtained from this work.

In MAXI J1409-619, the quiescent emission is observed to be extremely soft
($\Gamma\sim$1.6). Interestingly this source is among a few others that
exhibit positive flux-spectral index correlations beyond 10-9 erg cm-2 s-1
(for example EXO 2030+275, Epili et al. 2017) and anti-correlation below these
fluxes (Dönmez et al., 2020). Such correlations and anti-correlations are also
observed in sources that exhibit cyclotron features (Malacaria et al., 2015;
Dönmez et al., 2020). Our current low luminosity NuSTAR observations align
with this anti-correlation trend (Figure 7). We also detect strong pulsations
at 502 s which indicates ongoing low-level accretion. The decade-long
quiescence should have ensured that the NS crust and core are at thermal
equilibrium. The predicted luminosity for the transition to a cold disk is for
MAXI J1409-619 is estimated as $\sim$1034 erg s-1. This would mean that during
our current observations, MAXI J1409-619 could not have transitioned to this
state. Moreover, the predicted luminosity as would be observable during these
observations in September 2022 is of the order of 1.9$\times$1031 erg s-1
(assuming an L $\propto$ t-0.7 behavior, Tsygankov et al. 2017b). This is way
lower than the observed luminosity using NuSTAR. We disregard the cold
accretion disk model for MAXI J1409-619 as well. The observed residual thermal
emission likely arises from the NS hotspots which were heated during
outbursts. This is further supported by the size of the emission region, which
is 0.1 km (far less than the NS radius). Thermal emission from accretion-
heated hot spots has similarly been observed for other Be pulsars such as 4U
0115+63 (Rouco Escorial et al., 2017) and V0332+53 (Elshamouty et al., 2016).
Keeping in mind the uncertainties in the distance estimates for MAXI
J1409-619, the measured thermal luminosity from this work seems to prefer the
DCH model over the CD model.

###  4.3 Detection of pulsation in MAXI J1409-619 at low accretion states

At luminosities $\sim$1036 erg s-1, Orlandini et al. (2012) reported detection
of CRSFs in MAXI J1409-619 (the fundamental and its two harmonics) in the
BeppoSAX spectrum. However, no pulsations were reported from those
observations in 2000. Our results are indicative that pulsed emission can be
observed at low accretion levels as well. Indeed pulsations have been detected
in several low luminosity Be pulsars - for example, SAX J2103.5+4545 (Reig et
al., 2014), 4U 078-25, GRO J1008-57 (Tsygankov et al., 2017b), 2S 1417-624
(Tsygankov et al., 2017a), etc. In some highly magnetized systems, only the
hard X-ray component was found to be pulsed (see Cep X-4, McBride et al. 2007
and RX J0812.4–3114 Zhao et al. 2019). MAXI J1409-619 was observed at a
luminosity of $\sim$1035 erg s-1 during the current NuSTAR observations, which
happens to be one of its lowest to date, though not low enough to drive the
system to a propeller regime. A strong pulsation has been measured at 502 s.
Table 5 indicates all previously measured spin values using Swift(Kennea et
al., 2010b), RXTE (Dönmez et al., 2020) and the Fermi-GBM (Malacaria et al.,
2020) for MAXI J1409-619.

Mission | MJD | Pulse period (Hz)
---|---|---
Swift XRT1 | 55530 | 0.001988$\pm$3.9524e-5
Fermi-GBM2 | 55531.96 | 0.00197045$\pm$9.14e-8
Fermi-GBM2 | 55535.97 | 0.00197707$\pm$7.61e-8
Fermi-GBM2 | 55540.01 | 0.00198282$\pm$8.99e-8
RXTE-PCA3 | 55540.0150 | 0.00198287$\pm$0.0004
Fermi-GBM2 | 55544.02 | 0.00198792$\pm$1.40e-7
Fermi-GBM2 | 55548.09 | 0.001992423$\pm$1.34e-7
NuSTAR4 | 59828.95 | 0.00199203$\pm$1.19e-5
Table 5: Table of measured pulse period for MAXI J1409-619 since the time of
its discovery in 2010.
1Kennea et al. (2010b)
2 Malacaria et al. (2020)
3 Dönmez et al. (2020)
4 This work.

Several X-ray pulsars have exhibited a large pulsed fraction during
quiescence. For example, in RX J0812.4–3114, the hard component was observed
to be pulsed with a pulsed fraction of $\sim$88% (Zhao et al., 2019). The
majority of pulsars studied by Tsygankov et al. (2017a), which had detectable
pulsed emission (refer to Table 3 in Tsygankov et al. 2017a), displayed
significant pulsed fractions within the 50% to 70% range. High pulsed
fractions have been found to be common for X-ray pulsars at low accretion
states (Lutovinov & Tsygankov, 2009) (similar to this work for MAXI J1409-619)
as well as for pulsars in complete quiescence (Negueruela et al., 2000; Reig
et al., 2014). Variations in the behavior of pulsed fractions as a function of
energy as well as luminosity states have been characterized for several X-ray
pulsars in terms of a simple accretion model that is a function of the
emission region and accretion geometry (Lutovinov & Tsygankov, 2009).

It is interesting to note here that we do not yet know the nature of the
companion star in MAXI J1409-619. The source underwent an outburst in 2010 and
has been in quiescence since. Based on what appears to be the erratic outburst
of the source, it can be compared to a class of X-ray transients called the
Supergiant Fast X-ray Transients (SFXTs), like IGR J17544-2619, rather than
persistent supergiant X-ray binaries like Vela X-1. SFXTs exhibit similar
properties as classical systems, including supergiant companions and orbital
period distribution. However, SFXTs display greater dynamic variability than
classical systems, characterized by sporadic short X-ray outbursts and faint
flares with fast rise times (tens of minutes) and typical durations of a few
hours. On average, SFXTs have X-ray luminosities 2-3 orders of magnitude lower
than classical systems with similar orbital periods, outside of these outburst
events (Walter et al., 2015). There have been debates in the literature about
the nature of compact objects in SFXTs. Should MAXI J1409-619 be classified as
an SFXT, the presence of a pulsating NS would be in favor of a NS compact
object scenario. In addition, the presence of cyclotron line at 44 keV could
further indicate that magnetic field strengths of SFXTs can be $\sim$1012 G,
contradicting some leading theories about them being accreting magnetars
(Bozzo et al., 2008).

Figure 8: Plot showing the measured quiescent thermal luminosity as a function
of the long term averaged mass accretion rate $\langle\dot{M}\rangle$. The
quiescent luminosity data for LMXBs have been plotted from (Potekhin et al.,
2019) and those for Be X-ray pulsars are adopted from (Tsygankov et al.,
2017a). Results for MXB 0656-072 and MAXI J1409-619 are marked in red and
green, respectively. We assume a 10% uncertainty on the 14.5 kpc distance
measurement for MAXI J1409-619 since no previously reported uncertainty exists
in the literature.

###  4.4 Non-thermal emission at low luminosities

In literature, there are at least three Be XRPs (A0535+262, GX 304-1 and X
Persei) that have displayed an unusual spectrum at low luminosities - the
presence of non-thermal components like the two comptonization components
(Tsygankov et al., 2019). The detection of a CRSF during quiescence (for
example, A0535+262, Tsygankov et al. 2017a), also indicates that the broadband
spectrum of Be XRPs in quiescence can be modeled with non-thermal components
since CRSFs are generated through non-thermal origin (inverse Compton
scattering of electrons off photons). Motivated by these findings, we modeled
the broadband spectra of MXB 0656-072 and MAXI J1409-619 using two non-thermal
components (two comptonization and two powerlaw). Although we were not able to
constrain both the non-thermal components - given the faintness of both
sources compared against other bright Be XRPs like A0535+262 in quiescence -
we could constrain at least one non-thermal component. The detection of this
non-thermal component is crucial as it indicates ongoing accretion even in
quiescence. The power law component has been generally understood to arise due
to low-level accretion in LMXBs (Wijnands et al., 2015) and even in some X-ray
pulsars such as RX J0812.4-3114 (Zhao et al., 2019). In pulsars such as GS
0834-430, Swift J1626.6-5156, Cep X-4, etc., it was proposed that the non-
thermal emission arose as a result of low-level accretion from a cold disc
(Tsygankov et al., 2017a). For the case of MAXI J1409-619, the cold disc model
is not favored. Moreover, an accretion column is not expected to form at such
low luminosity states. Additionally, the lack of knowledge about the companion
star can indicate alternative sources of origin for the non-thermal emission.
Possible avenues include the accretion disk (wind-fed systems could still form
disks, Karino et al. 2019) or the companion star’s circumstellar disk.

It is worthwhile to note that the presence of cyclotron lines at 33 keV and 44
keV for MXB 0656-072 and MAXI J1409-619, respectively, outside quiescence,
also indicate the non-thermal origin of X-rays in these systems out of
quiescence. Although we were limited by statistics to constrain these features
in the quiescent spectrum, these features are tell-tale signatures of the non-
thermal origin of X-rays in these systems.

The study of such low luminosity HMXBs in quiescence can be greatly boosted
with instruments having a larger effective area in hard X-rays. One such
proposed probe class mission is the High-Energy X-ray Probe (HEX-P; Madsen et
al. 2019). HEX-P provides focused hard X-rays up to $\sim$ 150 keV using two
high-energy telescopes (HETs) and soft X-ray coverage with a low-energy
telescope (LET) with a much larger area than NuSTAR. HEX-P’s unique
capabilities would enable the study of accretion onto neutron stars across a
wide range of energies, including in low-luminosity regimes like the ones
studied in this paper.

##  5 Conclusions

In this work, we study the quiescent state properties of MXB 0656-072 and MAXI
J1409-619 using sensitive NuSTAR observations. We detect a strong pulsation at
502 s in MAXI J1409-619 with a 66% pulsed fraction in the folded pulse
profile. Our measured spin period is consistent with previous measurements.
The observed pulse profile is typical of X-ray pulsars accreting at low
levels. The pulsed fraction is seen to have an increasing trend as a function
of energy, which is also consistent with what is observed in other X-ray
pulsars. We do not detect QPOs in the power density spectra for MAXI
J1409-619.

The fluxes measured in this work indicate that neither of these two sources,
MXB 0656-072 and MAXI J1409-619, is in the propeller regime despite
observations being carried out luminosities of 2.6$\times$1033 erg s-1 and
1$\times$1035 erg s-1, respectively. We show that the broadband spectral fits
for both pulsars are best described using a combination of thermal and non-
thermal emission components. The non-thermal component, which is very soft, is
likely to emerge as a result of low-level accretion. From the measured radius
of the blackbody emitting region, we infer that the thermal emission likely
arises from the accretion heated hot spots on the surface of the NS. The
detection of pulsed emission along with non-thermal emission in MAXI J1409-619
is indicative of ongoing accretion during quiescence. Future sensitive
broadband observations of X-ray pulsars during quiescence will be useful to
further understand low luminosity accretion processes.

## Acknowledgements

This research has made use of data and software provided by the High Energy
Astrophysics Science Archive Research Center (HEASARC), which is a service of
the Astrophysics Science Division at NASA/GSFC and the High Energy
Astrophysics Division of the Smithsonian Astrophysical Observatory. The
analysis work has made use of the NuSTAR Data Analysis Software (NuSTARDAS)
jointly developed by the ASI Space Science Data Center (SSDC, Italy) and the
California Institute of Technology (Caltech, USA). G.R is supported by NASA
under award number 80NSSC22K1814. G.R is grateful to Deepto Chakraborty and
Ron Remillard for useful discussions.

## Data Availability

The observational data utilized in this work is publicly available through the
High Energy Astrophysics Science Archive Research Center (HEASARC). Any
additional information will be shared on reasonable request to the
corresponding author.

## References

  * Arnason et al. (2021) Arnason R. M., Papei H., Barmby P., Bahramian A., Gorski M. D., 2021, MNRAS, 502, 5455
  * Bachetti et al. (2014) Bachetti M., et al., 2014, Nature, 514, 202
  * Boldin et al. (2013) Boldin P. A., Tsygankov S. S., Lutovinov A. A., 2013, Astronomy Letters, 39, 375
  * Bozzo et al. (2008) Bozzo E., Falanga M., Stella L., 2008, ApJ, 683, 1031
  * Brown et al. (1998) Brown E. F., Bildsten L., Rutledge R. E., 1998, ApJ, 504, L95
  * Camero-Arranz et al. (2010) Camero-Arranz A., Finger M. H., Jenke P., 2010, The Astronomer’s Telegram, 3069, 1
  * Campana et al. (2001) Campana S., Gastaldello F., Stella L., Israel G. L., Colpi M., Pizzolato F., Orlandini M., Dal Fiume D., 2001, ApJ, 561, 924
  * Campana et al. (2002) Campana S., Stella L., Israel G. L., Moretti A., Parmar A. N., Orlandini M., 2002, ApJ, 580, 389
  * Clark et al. (1975) Clark G. W., Schmidt G. D., Angel J. R. P., 1975, IAU Circ., 2843, 1
  * D’Angelo & Spruit (2011) D’Angelo C. R., Spruit H. C., 2011, MNRAS, 416, 893
  * D’Angelo & Spruit (2012) D’Angelo C. R., Spruit H. C., 2012, MNRAS, 420, 416
  * Deibel et al. (2015) Deibel A., Cumming A., Brown E. F., Page D., 2015, ApJ, 809, L31
  * Dönmez et al. (2020) Dönmez Ç. K., Serim M. M., İnam S. Ç., Şahiner Ş., Serim D., Baykal A., 2020, MNRAS, 496, 1768
  * Doroshenko et al. (2014) Doroshenko V., Santangelo A., Doroshenko R., Caballero I., Tsygankov S., Rothschild R., 2014, A&A, 561, A96
  * Doroshenko et al. (2020) Doroshenko V., et al., 2020, MNRAS, 491, 1857
  * Elshamouty et al. (2016) Elshamouty K. G., Heinke C. O., Chouinard R., 2016, MNRAS, 463, 78
  * Epili et al. (2017) Epili P., Naik S., Jaisawal G. K., Gupta S., 2017, MNRAS, 472, 3455
  * Ghosh & Lamb (1978) Ghosh P., Lamb F. K., 1978, ApJ, 223, L83
  * HI4PI Collaboration et al. (2016) HI4PI Collaboration et al., 2016, A&A, 594, A116
  * Harrison et al. (2013) Harrison F. A., et al., 2013, ApJ, 770, 103
  * Heindl et al. (2003) Heindl W., Coburn W., Kreykenbohm I., Wilms J., 2003, The Astronomer’s Telegram, 200, 1
  * Illarionov & Sunyaev (1975) Illarionov A. F., Sunyaev R. A., 1975, A&A, 39, 185
  * Kaluzienski (1976) Kaluzienski L. J., 1976, IAU Circ., 2935, 5
  * Karino et al. (2019) Karino S., Nakamura K., Taani A., 2019, PASJ, 71, 58
  * Kaur et al. (2010) Kaur R., et al., 2010, The Astronomer’s Telegram, 3082, 1
  * Kennea et al. (2007) Kennea J. A., Romano P., Pottschmidt K., Wilms J., Cummings J., Evans P., Burrows D. N., 2007, The Astronomer’s Telegram, 1293, 1
  * Kennea et al. (2010a) Kennea J. A., Krimm H., Romano P., Mangano V., Curran P., Evans P., 2010a, The Astronomer’s Telegram, 2962, 1
  * Kennea et al. (2010b) Kennea J. A., Curran P., Krimm H., Romano P., Mangano V., Evans P. A., Yamaoka K., Burrows D. N., 2010b, The Astronomer’s Telegram, 3060, 1
  * Kreykenbohm et al. (2007) Kreykenbohm I., et al., 2007, The Astronomer’s Telegram, 1281, 1
  * Lasota (1997) Lasota J. P., 1997, in Wickramasinghe D. T., Bicknell G. V., Ferrario L., eds, Astronomical Society of the Pacific Conference Series Vol. 121, IAU Colloq. 163: Accretion Phenomena and Related Outflows. p. 351 (arXiv:astro-ph/9610068), doi:10.48550/arXiv.astro-ph/9610068
  * Liu et al. (2001) Liu Q. Z., van Paradijs J., van den Heuvel E. P. J., 2001, A&A, 368, 1021
  * Lutovinov & Tsygankov (2009) Lutovinov A. A., Tsygankov S. S., 2009, Astronomy Letters, 35, 433
  * Madsen et al. (2019) Madsen K., et al., 2019, Bulletin of the AAS, 51
  * Malacaria et al. (2015) Malacaria C., Klochkov D., Santangelo A., Staubert R., 2015, A&A, 581, A121
  * Malacaria et al. (2020) Malacaria C., Jenke P., Roberts O. J., Wilson-Hodge C. A., Cleveland W. H., Mailyan B., GBM Accreting Pulsars Program Team 2020, ApJ, 896, 90
  * McBride et al. (2006) McBride V. A., et al., 2006, A&A, 451, 267
  * McBride et al. (2007) McBride V. A., et al., 2007, A&A, 470, 1065
  * Morgan et al. (2003) Morgan E., Remillard R., Swank J., 2003, The Astronomer’s Telegram, 199, 1
  * Nättilä et al. (2017) Nättilä J., Miller M. C., Steiner A. W., Kajava J. J. E., Suleimanov V. F., Poutanen J., 2017, A&A, 608, A31
  * Negueruela et al. (2000) Negueruela I., Reig P., Finger M. H., Roche P., 2000, A&A, 356, 1003
  * Nespoli et al. (2012) Nespoli E., Reig P., Zezas A., 2012, A&A, 547, A103
  * Orlandini et al. (2012) Orlandini M., Frontera F., Masetti N., Sguera V., Sidoli L., 2012, ApJ, 748, 86
  * Pakull et al. (2003) Pakull M. W., Motch C., Negueruela I., 2003, The Astronomer’s Telegram, 202, 1
  * Pavlov et al. (1995) Pavlov G. G., Shibanov Y. A., Zavlin V. E., Meyer R. D., 1995, in Alpar M. A., Kiziloglu U., van Paradijs J., eds, NATO Advanced Study Institute (ASI) Series C Vol. 450, The Lives of the Neutron Stars. p. 71
  * Potekhin et al. (2019) Potekhin A. Y., Chugunov A. I., Chabrier G., 2019, A&A, 629, A88
  * Pottschmidt et al. (2007) Pottschmidt K., et al., 2007, The Astronomer’s Telegram, 1283, 1
  * Raman et al. (2021) Raman G., Varun Paul B., Bhattacharya D., 2021, MNRAS, 508, 5578
  * Reig et al. (2014) Reig P., Doroshenko V., Zezas A., 2014, MNRAS, 445, 1314
  * Remillard & Marshall (2003) Remillard R., Marshall F., 2003, The Astronomer’s Telegram, 197, 1
  * Rothschild et al. (2013) Rothschild R., et al., 2013, ApJ, 770, 19
  * Rouco Escorial et al. (2017) Rouco Escorial A., Bak Nielsen A. S., Wijnands R., Cavecchi Y., Degenaar N., Patruno A., 2017, MNRAS, 472, 1802
  * Rouco Escorial et al. (2019) Rouco Escorial A., Wijnands R., Ootes L. S., Degenaar N., Snelders M., Kaper L., Cackett E. M., Homan J., 2019, A&A, 630, A105
  * Rouco Escorial et al. (2020) Rouco Escorial A., Wijnands R., van den Eijnden J., Patruno A., Degenaar N., Parikh A., Ootes L. S., 2020, A&A, 638, A152
  * Rutledge et al. (2007) Rutledge R. E., Bildsten L., Brown E. F., Chakrabarty D., Pavlov G. G., Zavlin V. E., 2007, ApJ, 658, 514
  * Sokolova-Lapa et al. (2021) Sokolova-Lapa E., et al., 2021, A&A, 651, A12
  * Titarchuk (1994) Titarchuk L., 1994, ApJ, 434, 570
  * Tsygankov et al. (2016) Tsygankov S. S., Lutovinov A. A., Doroshenko V., Mushtukov A. A., Suleimanov V., Poutanen J., 2016, A&A, 593, A16
  * Tsygankov et al. (2017a) Tsygankov S. S., Wijnands R., Lutovinov A. A., Degenaar N., Poutanen J., 2017a, MNRAS, 470, 126
  * Tsygankov et al. (2017b) Tsygankov S. S., Mushtukov A. A., Suleimanov V. F., Doroshenko V., Abolmasov P. K., Lutovinov A. A., Poutanen J., 2017b, A&A, 608, A17
  * Tsygankov et al. (2019) Tsygankov S. S., Doroshenko V., Mushtukov A. A., Suleimanov V. F., Lutovinov A. A., Poutanen J., 2019, MNRAS, 487, L30
  * Varun et al. (2022) Varun Iyer N., Paul B., 2022, arXiv e-prints, p. arXiv:2209.13847
  * Verner et al. (1996) Verner D. A., Ferland G. J., Korista K. T., Yakovlev D. G., 1996, ApJ, 465, 487
  * Walter et al. (2015) Walter R., Lutovinov A. A., Bozzo E., Tsygankov S. S., 2015, A&ARv, 23, 2
  * Wijnands (2003) Wijnands R., 2003, Chandra observations of accretion-driven millisecond X-ray pulsars in quiescence, Chandra Proposal ID 05400417
  * Wijnands & Degenaar (2016) Wijnands R., Degenaar N., 2016, MNRAS, 463, L46
  * Wijnands et al. (2015) Wijnands R., Degenaar N., Armas Padilla M., Altamirano D., Cavecchi Y., Linares M., Bahramian A., Heinke C. O., 2015, MNRAS, 454, 1371
  * Wijnands et al. (2017) Wijnands R., Degenaar N., Page D., 2017, Journal of Astrophysics and Astronomy, 38, 49
  * Wilms et al. (2000) Wilms J., Allen A., McCray R., 2000, ApJ, 542, 914
  * Yamamoto et al. (2010) Yamamoto T., et al., 2010, The Astronomer’s Telegram, 3070, 1
  * Yamaoka et al. (2010) Yamaoka K., et al., 2010, The Astronomer’s Telegram, 2959, 1
  * Yan et al. (2007) Yan J., Liu Q., Li H., 2007, The Astronomer’s Telegram, 1303, 1
  * Yan et al. (2012) Yan J., Zurita Heras J. A., Chaty S., Li H., Liu Q., 2012, ApJ, 753, 73
  * Zhao et al. (2019) Zhao Y., Heinke C. O., Tsygankov S. S., Ho W. C. G., Potekhin A. Y., Shaw A. W., 2019, MNRAS, 488, 4427