A number of studies have revealed variability from neutron star low-mass X-ray binaries during quiescence. Such variability is not well characterised, or understood, but may be a common property that has been missed due to lack of multiple observations. One such source where variability has been observed is Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in quiescence, covering a period of approximately 2 years. There is clear variability between the epochs, with the most striking feature being a flare-like increase in the flux by a factor of 5. Spectral fitting is inconclusive as to whether the power-law and/or thermal component is variable. We suggest that the variability and flare-like behaviour during quiescence is due to accretion at low rates which might reach the neutron star surface.
The measurement of neutron star mass and radius is one of the most direct way to distinguish between various dense matter equations of state. The mass and radius of accreting neutron stars hosted in low mass X-ray binaries can be constrained by several methods, including photospheric radius expansion from type-I X-ray bursts and from quiescent spectra. In this paper, we apply for the first time these two methods simultaneously to constrain the mass and radius of Aql X-1, as a reliable distance estimation, high signal-to-noise ratio quiescent spectra from Chandra and XMM-Newton, and photospheri…
The Swift satellite monitored the quiescence of the low-mass X-ray binary transient Aql X-1 on a weekly basis during the March-November 2012 interval. A total of 42 observations were carried out in the soft X-ray (0.3-10 keV) band with the X-ray Telescope on board Swift. We investigated the X-ray variability properties of Aql X-1 during quiescence by tracking luminosity variations and characterising them with a detailed spectral analysis. The source is highly variable in this phase and two bright flares were detected, with peak luminosities of ~4x10^{34} erg s^{-1} (0.3-10 keV). Quiescent X-ra…
Aql X-1 is a prolific transient neutron star low-mass X-ray binary that exhibits an accretion outburst approximately once every year. Whether the thermal X-rays detected in intervening quiescent episodes are the result of cooling of the neutron star or due to continued low-level accretion remains unclear. In this work we use Swift data obtained after the long and bright 2011 and 2013 outbursts, as well as the short and faint 2015 outburst, to investigate the hypothesis that cooling of the accretion-heated neutron star crust dominates the quiescent thermal emission in Aql X-1. We demonstrate th…
Aql X-1 is the most prolific low mass X-ray binary transient hosting a neutron star. In this paper we focus on the return to quiescence following the 2010 outburst of the source. This decay was monitored thanks to 11 pointed observations taken with XMM-Newton, Chandra and Swift. The decay from outburst to quiescence is very fast, with an exponential decay characteristic time scale of ~2 d. Once in quiescence the X-ray flux of Aql X-1 remained constant, with no further signs of variability or decay. The comparison with the only other well-monitored outburst from Aql X-1 (1997) is tail-telling. …
Aql X-1 is the prototypical low mass X-ray binary transient. The Rossi X-ray Timing Explorer All Sky Monitor provided a ~16 yr coverage revealing 20 outbursts. This is by far the most extensive legacy of outbursts from the same source. We investigated the outbursts characteristics in terms of energetics, peak luminosities, durations, decays and recurrence times. We found that bright outbursts (peak luminosity >10^{37} erg s^-1) equal in number dimmer outbursts (<10^{36.6} erg s^-1). The peak luminosity does not correlate with outburst energetics, durations or quiescent times. We analysed the l…
Similar to black-hole X-ray binary (BHXRB) transients, hysteresis-like state transitions are also seen in some neutron-star X-ray binaries (NSXRBs). Using a method based on wavelets and lightcurves constructed from archival RXTE observations, we extract a minimal timescale over the complete range of transitions for 4U 1608-52 during the 2002 and 2007 outbursts and the 1999 and 2000 outbursts for Aql X-1. We present evidence for a strong positive correlation between this minimal timescale and a similar timescale extracted from the corresponding power spectra of these sources.
With our neutron star crust cooling code {\tt NSCool} we track the thermal evolution of the neutron star in Aql X-1 over the full accretion outburst history from 1996 until 2015. For the first time, we model many outbursts (23 outbursts were detected) collectively and in great detail. This allows us to investigate the influence of previous outbursts on the internal temperature evolution and to test different neutron star crust cooling scenarios. Aql X-1 is an ideal test source for this purpose, because it shows frequent, short outbursts and thermally dominated quiescence spectra. The source go…
We present the luminosity dwell-time distributions during the hard states of low-mass X-ray binaries containing a neutron star, 4U 1608-52 and AqlX-1, observed with MAXI/GSC. The luminosity distributions show a steep cut-off in the low-luminosity side at $\sim1.0 \times 10^{36}$ erg s$^{-1}$ in both the two sources. The cut-off implies a rapid luminosity decrease in their outburst decay phases, and the feature can be interpreted as due the propeller effect. We estimated the surface magnetic field of the neutron star to be (0.5--1.6) $\times 10^8$ G in 4U 1608-52 and (0.6--1.9) $\times 10^8$ G …
We report on the quiescent X-ray properties of the recently discovered transiently accreting 11 Hz X-ray pulsar in the globular cluster Terzan 5. Using two archival Chandra observations, we demonstrate that the quiescent spectrum of this neutron star low-mass X-ray binary is soft and can be fit to a neutron star atmosphere model with a temperature of kT^inf~73 eV. A powerlaw spectral component is not required by the data and contributes at most ~20% to the total unabsorbed 0.5-10 keV flux of ~9E-14 erg/cm2/s. Such a soft quiescent spectrum is unusual for neutron stars with relatively high infe…
We report the result of a new Chandra observation of the black hole X-ray transient H 1705-250 in quiescence. H 1705-250 was barely detected in the new 50 ks Chandra observation. With 5 detected counts, we estimate the source quiescent luminosity to be Lx~9.1e30 erg/s in the 0.5-10 keV band (adopting a distance of 8.6 kpc). This value is in line with the quiescent luminosities found among other black hole X-ray binaries with similar orbital periods. By using images taken with the Faulkes Telescope North, we derive a refined position of H 1705-250. We also present the long-term lightcurve of th…
Aql X--1 is one of the most prolific low mass X-ray binary transients (LMXBTs) showing outbursts almost annually. We present the results of our spectral analyses of RXTE/PCA observations of the 2000 and the 2011 outbursts. We investigate the spectral changes related to the changing disk-magnetosphere interaction modes of Aql X--1. The X-ray light curves of the outbursts of LMXBTs typically show phases of fast rise and exponential decay. The decay phase shows a "knee" where the flux goes from the slow decay to the rapid decay stage. We assume that the rapid decay corresponds to a weak propeller…
The structure and composition of the crust of neutron stars plays an important role in their thermal and magnetic evolution, hence in setting their observational properties. One way to study the crust properties is to measure how it cools after it has been heated during an accretion outburst in a low-mass X-ray binary (LMXB). Such studies have shown that there is a tantalizing source of heat, of currently unknown origin, that is located in the outer layers of the crust and has a strength that varies between different sources and different outbursts. With the aim of understanding the mechanism …
Accretion via disks can make neutron stars in low-mass X-ray binaries (LMXBs) fast spinning, and some of these stars are detected as millisecond pulsars. Here we report a practical way to find out if a neutron star in a transient LMXB has reached the spin equilibrium by disk--magnetosphere interaction alone, and if not, to estimate this spin equilibrium frequency. These can be done using specific measurable source luminosities, such as the luminosity corresponding to the transition between the accretion and propeller phases, and the known stellar spin rate. Such a finding can be useful to test…
We studied the transient neutron-star low-mass X-ray binary GRS 1747-312, located in the globular cluster Terzan 6, in its quiescent state after its outburst in August 2004, using an archival XMM-Newton observation. A source was detected in this cluster and its X-ray spectrum can be fitted with the combination of a soft, neutron-star atmosphere model and a hard, power-law model. Both contributed roughly equally to the observed 0.5-10 keV luminosity (~4.8X10^33 erg/s). This type of X-ray spectrum is typically observed for quiescent neutron-star X-ray transients that are perhaps accreting in qui…
In 2018, we reported our discovery of a dozen quiescent X-ray binaries in the central parsec (pc) of the Galaxy (Hailey et al. 2018). In a recent follow-up paper (Mori et al. 2021), we published an extended analysis of these sources and other X-ray binaries (XRBs) in the central pc and beyond, showing that most if not all of the 12 non-thermal sources are likely black hole low-mass X-ray binary (BH-LMXB) candidates. In response, Maccarone et al. 2022 (TM22 hereafter) argued, primarily on the claim that neutron star low-mass X-ray binaries (NS-LMXBs) often do not have short outburst recurrence …
We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the first quiescent UV detections of these objects. All are detected at a much higher level than expected from their companion stars alone and are significant detections of the accretion flow. Three of the four UV excesses can be characterized by a black body of temperature 5000-13,000K, hotter than expected for the quiescent outer disk. A good fit could not be found …
A search for optical counterparts in quiescent black hole X-ray transients
Instituto de Astrofísica de Canarias（加那利天体物理研究所）；Universidad de La Laguna（拉古纳大学）；University of Sheffield（谢菲尔德大学）；University of Cambridge（剑桥大学）；Radboud University（拉德堡德大学）
The Be/X-ray transient GRO J1750-27 exhibited a type-II (giant) outburst in 2015. After the source transited to quiescence, we triggered our multi-year Chandra monitoring programme to study its quiescent behaviour. The programme was designed to follow the cooling of a potentially heated neutron-star crust due to accretion of matter during the preceding outburst, similar to what we potentially have observed before in two other Be/X-ray transients, namely 4U 0115+63 and V 0332+53. However, unlike for these other two systems, we do not find any strong evidence that the neutron-star crust in GRO J…
We present high time-resolution ULTRACAM optical and NOTCam infrared observations of the edge-on black hole X-ray transient Swift J1357.2-0933. Our data taken in 2012 and 2013 show the system to be at its pre-outburst magnitude and so the system is in quiescence. In contrast to other X-ray transients, the quiescent light curves of Swift J1357.2-0933 do not show the secondary star's ellipsoidal modulation. The optical light curve is dominated by variability with an optical fractional rms of ~35 per cent, a factor of >3 larger than what is observed in other systems at similar time-resolution. Op…
We report on a series of Swift/XRT observations, performed between February and 22 March 2012, during the quiescent state of the neutron-star X-ray binary SAX J1750.8-2900. In these observations, the source was either just detected or undetected, depending on the exposure length (which ranged from ~0.3 to ~3.8 ks). The upper limits for the non-detections were consistent with the detected luminosities (when fitting a thermal model to the spectrum) of ~1E34 erg/s (0.5-10 keV). This level is consistent with what has been measured previously for this source in quiescence. However, on March 17 the …
The presence of some X-ray sources in the Galactic Centre region which show variability, but do not show outbursts in over a decade of monitoring has been used to argue for the presence of a large population of stellar mass black holes in this region. A core element of the arguments that these objects are accreting black holes is the claim that neutron stars (NSs) in low mass X-ray binaries (LMXBs) do not have long transient recurrence times. We demonstrate in this paper that about half of the known transient LMXBs with clear signatures for NS primaries have recurrence times in excess of a dec…
The observed evolution of the broad-band spectral energy distribution (SED) in NS X-ray Nova Aql X-1 during the rise phase of a bright FRED-type outburst in 2013 can be understood in the framework of thermal emission from unstationary accretion disc with temperature radial distribution transforming from a single-temperature blackbody emitting ring into the multi-colour irradiated accretion disc. SED evolution during the hard to soft X-ray state transition looks curious, as it can not be repro- duced by the standard disc irradiation model with a single irradiation parameter for NUV, Optical and…
Transitional millisecond pulsars (tMSPs) switch between an accretion-powered state without radio pulsations and a rotation-powered state with radio pulsations. In the former state, they are X-ray bright, in the latter X-ray dim. Soft X-ray transients (SXTs) undergo similar switches in X-ray, between "high" states with bright X-ray outbursts and a "low" state of quiescence. The upper limit on the quiescent X-ray luminosity of SXT 1H 1905+000 suggests that its luminosity might be similar to that of the known tMSPs. A detection of radio pulsations would link SXTs more strongly with tMSPs; and thu…
We present the results of continued monitoring of the quiescent neutron star low-mass X-ray binary XTE J1701-462 with Chandra and Swift. A new Chandra observation from 2010 October extends our tracking of the neutron star surface temperature from ~800 days to ~1160 days since the end of an exceptionally luminous 19 month outburst. This observation indicates that the neutron star crust may still be slowly cooling toward thermal equilibrium with the core; another observation further into quiescence is needed to verify this. The shape of the overall cooling curve is consistent with that of a brok…
In quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars, the origin of the thermal X-ray component may be either release of heat from the core of the neutron star, or continuing low-level accretion. In general, heat from the core should be stable on timescales $<10^4$ years, while continuing accretion may produce variations on a range of timescales. While some quiescent neutron stars (e.g. Cen X-4, Aql X-1) have shown variations in their thermal components on a range of timescales, several others, particularly those in globular clusters with no detectable nonthermal hard X-rays …
Tidal Disruption Events (TDEs) have been found to show a preference for post-starburst (PS) and quiescent Balmer-strong (QBS) galaxies. This preference can be used to help find TDEs in transient surveys. But what other transients might "contaminate" such a search, and by how much? We examine all reported transients coincident with the centers of galaxies in the French & Zabludoff (2018) catalog of spectroscopically confirmed PS and QBS galaxies and photometrically identified PS and QBS galaxy candidates. We find that TDEs and Type Ia supernovae (SNe) are the only types of transients classified…
EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides in the globular cluster Terzan 5. We studied the transient during its quiescent state using 18 Chandra observations of the cluster acquired between 2003 and 2016. We found an extremely variable source, with a luminosity variation in the 0.5-10 keV energy range of $\sim3$ orders of magnitude (between $3\times10^{31}$ erg s$^{-1}$ and $2\times10^{34}$ erg s$^{-1}$) on timescales from years down to only a few days. Using an absorbed power-law model to fit its quiescent spectra, we obtained a typical photon index of $\sim1…
We present Rossi X-Ray Timing Explorer and Swift observations made during the final three weeks of the 2006-2007 outburst of the super-Eddington neutron star (NS) transient XTE J1701-462, as well as Chandra and XMM-Newton observations covering the first ~800 days of the subsequent quiescent phase. The source transitioned quickly from active accretion to quiescence, with the luminosity dropping by over 3 orders of magnitude in ~13 days. The spectra obtained during quiescence exhibit both a thermal component, presumed to originate in emission from the NS surface, and a non-thermal component of u…
Magnetars are neutron stars (NSs) with extreme magnetic fields of strength $5 \times 10^{13}$ - $10^{15}$ G. They exhibit transient, highly energetic events, such as short X-ray flashes, bursts and giant flares, all of which are powered by their enormous magnetic energy. Quiescent magnetars have X-ray luminosities between $10^{29}$ and $10^{35}$ erg/s, and are further classified as either persistent or transient magnetars. Their X-ray emission is modulated with the rotational period of the NS, with a typical relative amplitude (so-called pulsed fraction) between 10-58 per cent, implying that t…
X-ray spectral analysis of quiescent low-mass X-ray binaries (LMXBs) has been one of the most common tools to measure the radius of neutron stars (NSs) for over a decade. So far, this method has been mainly applied to NSs in globular clusters, primarily because of their well-constrained distances. Here, we study Chandra data of seven transient LMXBs in the Galactic plane in quiescence to investigate the potential of constraining the radius (and mass) of the NSs inhabiting these systems. We find that only two of these objects had X-ray spectra of sufficient quality to obtain reasonable constrai…
We study long-term thermal evolution of neutron stars in soft X-ray transients (SXTs), taking the deep crustal heating into account consistently with the changes of the composition of the crust. We collect observational estimates of average accretion rates and thermal luminosities of such neutron stars and compare the theory with observations. We perform simulations of thermal evolution of accreting neutron stars, considering the gradual replacement of the original nonaccreted crust by the reprocessed accreted matter, the neutrino and photon energy losses, and the deep crustal heating due to n…
Context. Supergiant fast X-ray transients are a subclass of high mass X-ray binaries that host a neutron star accreting mass from the wind of its OB supergiant companion. They are characterized by an extremely pronounced and rapid variability in X-rays, which still lacks an unambiguous interpretation. A number of deep pointed observations with XMM-Newton have been carried out to study the quiescent emission of these sources and gain insight into the mechanism that causes their X-ray variability. Aims. We continued this study by using three XMM-Newton observations of the two supergiant fast X-r…
We report on two short XMM-Newton observations performed in August 2006 and February 2007 during the quiescence state of the enigmatic black hole candidate system IGR J17091-3624. During both observations the source was clearly detected. Although the errors on the estimated fluxes are large, the source appears to be brighter by several tens of percents during the February 2007 observation compared to the August 2006 observation. During both observations the 2-10 keV luminosity of the source was close to ~10^{33} erg/s for an assumed distance of 10 kpc. However, we note that the distance to thi…
We study the quiescent luminosities of accreting neutron stars by a new mechanism as neutrino heating for additional deep crustal heating, where the neutrino heating is produced by charged pions decay from the nuclear collisions on the surface of neutron star during its active accretion. For low mass neutron star($\lesssim1.4~M_{\odot}$), as the neutrino heating is little($\lesssim1$ MeV per accreted nucleon) or there would be no neutrino heating, the quiescent luminsoity will be not affected or slightly affected. While for massive neutron star ($\gtrsim2~M_{\odot}$), the quiescent luminosity …
We present near-infrared polarimetric observations of the black hole X-ray binaries Swift J1357.2-0933 and A0620-00. In both sources, recent studies have demonstrated the presence of variable infrared synchrotron emission in quiescence, most likely from weak compact jets. For Swift J1357.2-0933 we find that the synchrotron emission is polarized at a level of 8.0 +- 2.5 per cent (a 3.2 sigma detection of intrinsic polarization). The mean magnitude and rms variability of the flux (fractional rms of 19-24 per cent in K_s-band) agree with previous observations. These properties imply a continuousl…
It is assumed that accreting neutron stars (NSs) in LMXBs are heated due to the compression of the existing crust by the accreted matter which gives rise to nuclear reactions in the crust. It has been shown that most of the energy is released deep in the crust by pycnonuclear reactions involving low-Z elements. We discuss if NSs in very-faint X-ray transients (VFXTs; those which have peak X-ray luminosities < 1E36 erg/s) can be used to test this model. Unfortunately we cannot conclusively answer this because of the large uncertainties in our estimates of the accretion rate history of those VFX…
We monitored the neutron star low-mass X-ray binary SAX J1750.8-2900 after the end of its 2015/2016 outburst using the X-ray Telescope (XRT) aboard Swift to detect possible post-outburst 'rebrightenings', similar to those seen after its 2008 outburst. We did not detect any rebrightening behaviour, suggesting that the physical mechanism behind the rebrightening events is not always active after each outburst of the source. Any model attempting to explain these rebrightenings should thus be able to reproduce the different outburst profiles of the source at different times. Surprisingly, our Swif…
We report on a Chandra Director's Discretionary Time observation of the globular cluster Terzan 5, carried out ~7 weeks after the cessation of the 2010 outburst of the newly discovered transiently accreting 11 Hz X-ray pulsar. We detect a thermal spectrum that can be fitted with a neutron star atmosphere model with a temperature for an observer at infinity of kT~100 eV, and a quiescent thermal bolometric luminosity of Lq~2E33 erg/s for an assumed distance of 5.5 kpc. The thermal emission is elevated above the quiescent base level measured in 2003 and 2009, i.e., prior to the recent accretion o…
The radiative mechanism of the black hole X-ray transients (BHXTs) in their quiescent states (defined as the 2-10 keV X-ray luminosity $\le 10^{34}{\rm \,erg\,s^{-1}}$) remains unclear. In this work, we investigate the quasi-simultaneous quiescent state spectrum (including radio, infrared, optical, ultraviolet and X-ray) of two BHXTs, A0620--00 and XTE J1118+480. We find that, these two sources can be well described by a coupled accretion -- jet model. More specifically, most of the emission (radio up-to infrared, and the X-ray waveband) comes from the collimated relativistic jet. Emission fro…
MAXI J0556-332 is the hottest transient accreting neutron star at the beginning of its quiescence. A theoretical model with crustal heating indicates that an additional shallow heat source of Qshallow > 6 MeV per accreted nucleon is required in the shallow outer crust with respect to the deeper star crust by considering the observed decline in accretion rate at the end of outburst. However, the physical source of this shallow heating is still unclear. In the present investigation, we performed stellar evolutionary calculations, adopting the effects of outburst behavior of the accretion rate. A…
We present a model of a viscously evolving accretion disc around a magnetized neutron star. The model features the varying outer radius of the hot ionized part of the disc due to cooling and the varying inner radius of the disc due to interaction with the magnetosphere. It also includes hindering of accretion on the neutron star because of the centrifugal barrier and irradiation of the outer disc and companion star by X-rays from the neutron star and disc. When setting inner boundary conditions, we take into account that processes at the inner disc occur on a time scale much less than the visc…
The measured spin-down rates in quiescence of the transient accreting millisecond pulsars IGR J00291+5934, XTE J1751-305, SAX J1808.4-3658, and Swift J1756.9-2508 have been used to estimate the magnetic moments of these objects assuming standard magnetic dipole braking. It is shown that this approach leads to an overestimate, if the amount of residual accretion is enough to distort the magnetosphere away from a force-free configuration, through magnetospheric mass loading or crushing, so that the lever arm of the braking torque migrates inside the light cylinder. We derive an alternative spin-…
We present new Chandra X-ray observations of the transient black hole X-ray binary MAXI J1659-152 in quiescence. These observations were made more than one year after the end of the source's 2010-2011 outburst. We detect the source at a 0.5-10 keV flux of 2.8(8)e-15 erg/cm^2/s, which corresponds to a luminosity of ~1.2e31 (d/ 6 kpc)^2 erg/s. This level, while being the lowest at which the source has been detected, is within factors of ~2 of the levels seen at the end of the initial decay of the outburst and soon after a major reflare of the source. The quiescent luminosity of MAXI J1659-152, w…
We show that the X-ray outburst light curves of four transient anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), namely XTE J1810-197, SGR 0501+4516, SGR 1627-41 and CXOU J164710.2-455216, can be produced by the fallback disk model that was also applied to the outburst light curves of persistent AXPs and SGRs in our earlier work. The model solves the diffusion equation for the relaxation of a disk which has been pushed back by a soft gamma-ray burst. The sets of main disk parameters used for these transient sources are very similar to each other and to those employed in our e…
An accretion outburst onto a neutron star transient heats the neutron star's crust out of thermal equilibrium with the core. After the outburst the crust thermally relaxes toward equilibrium with the neutron star core and the surface thermal emission powers the quiescent X-ray light curve. Crust cooling models predict that thermal equilibrium of the crust will be established $\approx 1000 \, \mathrm{d}$ into quiescence. Recent observations of the cooling neutron star transient MXB 1659-29, however, suggest that the crust did not reach thermal equilibrium with the core on the predicted timescal…
The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ~10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT obser…
The neutron star low-mass X-ray binary and intermittent millisecond X-ray pulsar HETE J1900.1-2455 returned to quiescence in late 2015, after a prolonged accretion outburst of ~10 yr. Using a Chandra observation taken ~180 d into quiescence we detect the source at a luminosity of ~4.5E31 (D/4.7 kpc)^2 erg/s (0.5-10 keV). The X-ray spectrum can be described by a neutron star atmosphere model with a temperature of ~54 eV for an observer at infinity. We perform thermal evolution calculations based on the 2016 quiescent data and a <98 eV temperature upper limit inferred from a Swift observation ta…
Burst-only sources are X-ray sources showing up only during short bursts but with no persistent emission (at least with the monitoring instrument which led to their discovery). These bursts have spectral characteristics consistent with thermonuclear (type I) burst from the neutron star surface, linking burst-only sources to neutron star X-ray binary transients. We have carried out a series of snapshot observations of the entire sample of burst-only sources with the Swift satellite. We found a few sources in outburst and detect faint candidates likely representing their quiescent counterparts. …
Stars may be tidally disrupted if, in a single orbit, they are scattered too close to a supermassive black hole (SMBH). Tidal disruption events are thought to power luminous but short-lived accretion episodes that can light up otherwise quiescent SMBHs in transient flares. Here we explore a more gradual process of tidal stripping where stars approach the tidal disruption radius by stellar evolution while in an eccentric orbit. After the onset of mass transfer, these stars episodically transfer mass to the SMBH every pericenter passage giving rise to low-level flares that repeat on the orbital …
We report on the optical detection of the black hole X-ray transient MAXI J1659-152 during its quiescent state. By using the Canada France Hawaii Telescope (CFHT), we observed MAXI J1659-152 about 7 months after the end of an X-ray outburst. The optical counterpart of MAXI J1659-152 is clearly detected with a r'-band magnitude of 23.6-23.8. The detection confirms that the optical emission of MAXI J1659-152 during quiescence is relatively bright comparing to other black hole X-ray transients. This implies that the distance to MAXI J1659-152 is 4.6-7.5 kpc for a M2 dwarf companion star, or 2.3-3…
The transient neutron star low-mass X-ray binary EXO 1745-248, located in the globular cluster Terzan 5, was detected during its quiescent state with Chandra in 2003. The source displayed a 0.5-10 keV X-ray luminosity of Lq ~ 1E33 (D/5.5kpc)^2 erg/s, which was completely dominated by hard non-thermal emission. This is at odds with other non-pulsating neutron stars that typically show detectable soft thermal emission at such quiescent luminosities. Here, we use three additional Chandra observations, performed in 2009 and 2011, to further study the quiescent properties of EXO 1745-248. We find t…
