We analyse four XMM-Newton observations of the neutron-star low-mass X-ray binary EXO 0748$-$676 in quiescence. We fit the spectra with an absorbed neutron-star atmosphere model, without the need for a high-energy (power-law) component; with a 95 per cent confidence the power-law contributes less than 1 per cent to the total flux of the source in $0.5-10.0$ keV. The fits show significant residuals at around 0.5 keV which can be explained by either a hot gas component around the neutron star or a moderately broad emission line from a residual accretion disc. The temperature of the neutron-star has decreased significantly compared to the previous observation, from 124 eV to 105 eV, with the cooling curve being consistent with either an exponential decay plus a constant or a (broken) power law. The best-fitting neutron-star mass and radius can be better constrained if we extend the fits down to the lowest possible energy available. For an assumed distance of 7.1 kpc, the best-fitting neutron-star mass and radius are $2.00_{-0.24}^{+0.07}~M_\odot$ and $11.3_{-1.0}^{+1.3}$ km if we fit the spectrum over the $0.3-10$ keV range, but $1.50_{-1.0}^{+0.4}~M_\odot$ and $12.2_{-3.6}^{+0.8}$ km if we restrict the fits to the $0.5-10$ keV range. We finally discuss the effect of the assumed distance to the source upon the best-fitting neutron-star mass and radius. As systematic uncertainties in the deduced mass and radius depending on the distance are much larger than the statistical errors, it would be disingenuous to take these results at face value.
In this work, we perform the detailed analysis of absorption features in spectra of magnetar candidates observed by XMM-Newton satellite. No significant line-like feature has been found. This negative result may indicate the possible presence of smoothing out the absorption features mechanisms.
Accretion onto magnetized neutron stars is expected to be centrifugally inhibited at low accretion rates. Several sources, however, are known to pulsate in quiescence at luminosities below the theoretical limit predicted for the onset of the centrifugal barrier. The source 1A 0535+262 is one of them. Here we present the results of an analysis of a ~50 ks long XMM-Newton observation of 1A 0535+262 in quiescence. At the time of the observation, the neutron star was close to apastron, and the source had remained quiet for two orbital cycles. In spite of this, we detected a pulsed X-ray flux of ~3…
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 the deep observations of the "bursting pulsar" GRO J1744-28, which were performed with XMM-Newton and aimed to clarify the origin of its X-ray emission in quiescence. We detect the source at a luminosity level of $\sim10^{34}$ erg s$^{-1}$ with an X-ray spectrum that is consistent with the power law, blackbody, or accretion-heated neutron star atmosphere models. The improved X-ray localization of the source allowed us to confirm the previously identified candidate optical counterpart as a relatively massive G/K~III star at 8 kpc close to the Galactic center, implying an almost fac…
The neutron star low-mass X-ray binary EXO 0748-676 started a transition from outburst to quiescence in August 2008, after more than 24 years of continuous accretion. The return of the source to quiescence has been monitored extensively by several X-ray observatories. Here, we report on four XMM-Newton observations elapsing a period of more than 19 months and starting in November 2008. The X-ray spectra show a soft thermal component which we fit with a neutron star atmosphere model. Only in the first observation do we find a significant second component above ~ 3 keV accounting for ~ 7 % of th…
IGR J17091-3624 is a low mass X-ray binary (LMXB) which received wide attention from the community thanks to its similarities with the bright black hole system GRS 1915+105. Both systems exhibit a wide range of highly structured X-ray variability during outburst, with time scales from few seconds to tens of minutes, which make them unique in the study of mass accretion in LMXBs. In this work we present a general overview into the long-term evolution of IGR J17091-3624, using Swift/XRT observations from the onset of the 2011-2013 outburst in February 2011 to the end of the last bright outburst …
We report the results from archival XMM-Newton and INTEGRAL observations of the Supergiant Fast X-ray Transient (SFXT) IGR J18483-0311 in quiescence. The 18-60 keV hard X-ray behaviour of the source is presented here for the first time, it is characterized by a spectral shape ($Γ$ about 2.5) similar to that during outburst activity and the lowest measured luminosity level is about 10^34 erg s^-1. The 0.5-10 keV luminosity state, measured by XMM-Newton during the apastron passage, is about one order of magnitude lower and it is reasonably fitted by an absorbed black body model yielding paramete…
Recently, the neutron star X-ray binary EXO 0748-676 underwent a transition to quiescence. We analyzed an XMM-Newton observation of this source in quiescence, where we fitted the spectrum with two different neutron-star atmosphere models. From the fits we constrained the allowed parameter space in the mass-radius diagram for this source for an assumed range of distances to the system. Comparing the results with different neutron-star equations of state, we constrained the distance to EXO 0748-676. We found that the EOS model 'SQM1' is rejected by the atmosphere model fits for the known distanc…
The low mass X-ray binary 4U 2129+47 was discovered during a previous X-ray outburst phase and was classified as an accretion disk corona source. A 1% delay between two mid-eclipse epochs measured ~22 days apart was reported from two XMM-Newton observations taken in 2005, providing support to the previous suggestion that 4U 2129+47 might be in a hierarchical triple system. In this work we present timing and spectral analysis of three recent XMM-Newton observations of 4U 2129+47, carried out between November 2007 and January 2008. We found that absent the two 2005 XMM-Newton observations, all o…
IGR J18483-0311 was discovered with INTEGRAL in 2003 and later classified as a supergiant fast X-ray transient. It was observed in outburst many times, but its quiescent state is still poorly known. Here we present the results of XMM-Newton, Swift, and Chandra observations of IGRJ18483-0311. These data improved the X-ray position of the source, and provided new information on the timing and spectral properties of IGR J18483-0311 in quiescence. We report the detection of pulsations in the quiescent X-ray emission of this source, and give for the first time a measurement of the spin-period deriv…
In this paper we report on a long multi-wavelength observational campaign of the supergiant fast X-ray transient prototype IGR J17544-2619. A 150 ks-long observation was carried out simultaneously with XMM-Newton and NuSTAR, catching the source in an initial faint X-ray state and then undergoing a bright X-ray outburst lasting about 7 ks. We studied the spectral variability during outburst and quiescence by using a thermal and bulk Comptonization model that is typically adopted to describe the X-ray spectral energy distribution of young pulsars in high mass X-ray binaries. Although the statist…
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it had been actively accreting for more than 24 years. In a previous work, we discussed Chandra and Swift observations obtained during the first five months after this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 years. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well-fitted by a neutron star atmosphere model. A…
We have performed the first sensitive X-ray observation of the low-mass X-ray binary SAX J1750.8-2900 in quiescence with XMM-Newton. The spectrum was fit to both a classical black body model, and a non-magnetized, pure hydrogen neutron star atmosphere model. A power law component was added to these models, but we found that it was not required by the fits. The distance to SAX J1750.8-2900 is known to be D = 6.79 kpc from a previous analysis of photospheric radius expansion bursts. This distance implies a bolometric luminosity (as given by the NS atmosphere model) of (1.05 +/- 0.12) x 10^34 (D/…
We report on INTEGRAL, Swift and XMM-Newton observations of IGR J17511-3057 performed during the outburst that occurred between March 23 and April 25, 2015. The source reached a peak flux of 0.7(2)E-9 erg/cm$^2$/s and decayed to quiescence in approximately a month. The X-ray spectrum was dominated by a power-law with photon index between 1.6 and 1.8, which we interpreted as thermal Comptonization in an electron cloud with temperature > 20 keV . A broad (σ ~ 1 keV) emission line was detected at an energy (E = 6.9$^{+0.2}_{-0.3}$ keV) compatible with the Kα transition of ionized Fe, suggesting a…
We present optical and infrared photometry of the low-mass X-ray binary EXO 0748-676 in quiescence for the first time in 24 years since it became X-ray active in 1985. We find the counterpart at average magnitudes of R=22.4 and J=21.3. We monitored the source approximately nightly through 2008 November to 2009 January. During this time there was considerable night-to-night optical variability but no long term trends were apparent. The night-to-night variability reveals a periodicity of P=0.159331+/-0.000012d, consistent with the X-ray orbital period to within 0.01%. This indicates that the qui…
Hen 3-1341 is a symbiotic binary system consisting of a white dwarf and a red giant star that is one of about ten symbiotics that show hints of jets. The bipolar jets have been detected through displaced components of emission lines during its outburst from 1998 to 2004. These components disappeared when Hen 3-1341 reached quiescence. On February 23, 2012, Hen 3-1341 started a new outburst with the emergence of new bipolar jets on March 3, 2012. We observed Hen 3-1341 during quiescence with XMM-Newton in March 2010 with an effective exposure time of 46.8 ks and with Swift on March 8-11, 2012 a…
Magnetars are neutron stars having extreme magnetic field strengths. Study of their emission properties in quiescent state can help understand effects of a strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar that was discovered in 2008 during an outburst, which has recently returned to quiescence. We report its spectral and timing properties measured with new and archival observations from the Chandra X-Ray Observatory, XMM-Newton, and Suzaku. We found that the quiescent spectrum is best fit by a power-law plus two blackbody model, with temperatures of kT_low~0.26 keV and kT_hi…
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. …
Crustal cooling of accretion-heated neutron stars provides insight into the stellar interior of neutron stars. The neutron star X-ray transient, KS~1731$-$260, was in outburst for 12.5 years before returning to quiescence in 2001. We have monitored the cooling of this source since then through {\it Chandra} and {\it XMM-Newton} observations. Here, we present a 150 ks {\it Chandra} observation of KS~1731$-$260 taken in August 2015, about 14.5 years into quiescence, and 6 years after the previous observation. We find that the neutron star surface temperature is consistent with the previous obser…
We have analysed a series of Suzaku data and one dataset of XMM-Newton of the SU UMa type dwarf nova VW Hyi in optical quiescence. The observed spectra in the 0.2-10 keV band are moderately well represented by multi-temperature thermal plasma emission models with a maximum temperature of 5-9 keV and bolometric luminosity of 2.4-5.2 $\times$ 10$^{30}$ erg s$^{-1}$. The mass accretion rate derived from the hard X-ray spectra does not show any clear trend as a function of time since the last superoutburst, in contradiction to theoretical predictions of the disc behaviour of a SU UMa type dwarf no…
We present a multi-wavelength timing study of the eclipsing low mass X-ray binary EXO 0748-676 (UY Vol) using XMM-Newton when the source was in a hard spectral state. The orbital optical and X-ray light curves show a fairly large amount of intensity modulation in the 7 observations taken during September-November, 2003, covering 36 complete binary orbits of EXO 0748-676. While assessing the non-burst variability, simultaneously in the optical and X-ray light curves, we find that they are not correlated at reprocessing or orbital time-scales, but are weakly correlated at a few 1000s of seconds …
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 …
The neutron star low-mass X-ray binary SWIFT J1749.4-2807 is the only known eclipsing accreting millisecond X-ray pulsar. In this manuscript we perform a spectral characterization of the system throughout its 2021, two-week-long outburst, analyzing 11 NICER observations and quasi-simultaneous XMM-Newton and NuSTAR single observations at the outburst peak. The broadband spectrum is well-modeled with a black body component with a temperature of $\sim$0.6 keV, most likely consistent with a hot spot on the neutron star surface, and a Comptonisation spectrum with power-law index $Γ\sim 1.9$, arisin…
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…
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…
CONTEXT - Transient low-mass X-ray binaries (LMXBs) often show outbursts lasting typically a few-weeks and characterized by a high X-ray luminosity ($L_{x} \approx 10^{36}-10^{38}$ erg/sec), while for most of the time they are found in X-ray quiescence ($L_X\approx10^{31} -10^{33}$ erg/sec). EXO 1745-248 is one of them. AIMS - The broad-band coverage, and the sensitivity of instrument on board of {\xmm} and {\igr}, offers the opportunity to characterize the hard X-ray spectrum during {\exo} outburst. METHODS - In this paper we report on quasi-simultaneous {\xmm} and {\igr} observations of the …
Observations of accreting neutron stars (NS) with strong magnetic fields can be used not only for studying the accretion flow interaction with NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating neutron star (magnetosphere) with the in-falling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray …
We present phase resolved optical spectroscopy and X-ray timing of the neutron star X-ray binary EXO 0748-676 after the source returned to quiescence in the fall of 2008. The X-ray light curve displays eclipses consistent in orbital period, orbital phase and duration with the predictions and measurements before the return to quiescence. Halpha and He I emission lines are present in the optical spectra and show the signature of the orbit of the binary companion, placing a lower limit on the radial velocity semi-amplitude of K2>405 km/s. Both the flux in the continuum and the emission lines show…
The propeller effect should cut off accretion in fast-spinning neutron star high mass X-ray binaries (HMXBs) at low mass transfer rates. However, accretion continues in some HMXBs at $L_{x} < 10^{34}$ erg s$^{-1}$, as evidenced by continuing pulsations. Indications of spectral softening in systems in the propeller regime suggest that some HMXBs are undergoing fundamental changes in their accretion regime. A 39 ks \textit{XMM-Newton} observation of the transient HMXB V0332+53 found it at a very low X-ray luminosity ($L_{x} \sim 4\times 10^{32}$ erg s${^{-1}}$). A power-law spectral fit requires…
WZ Sagittae is the prototype object of a subclass of dwarf novae, with rare and long (super)outbursts, in which a white dwarf primary accretes matter from a low mass companion. High-energy observations offer the possibility of a better understanding of the disk-accretion mechanism in WZ Sge-like binaries. We used archival XMM-Newton and Swift data to characterize the X-ray spectral and temporal properties of WZ Sge in quiescence. We performed a detailed timing analysis of the simultaneous X-ray and UV light curves obtained with the EPIC and OM instruments on board XMM-Newton in 2003. We employ…
We present results from multi-wavelength simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cyg in quiescence. Our coverage with NuSTAR provides the very first opportunity to study the X-ray spectrum of V404 Cyg at energies above 10 keV. The unabsorbed broad-band (0.3--30 keV) quiescent luminosity of the source is 8.9$\times$10$^{32}$ erg s$^{-1}$ for a distance of 2.4 kpc. The source shows clear variability on short time scales (an hour to a couple of hours) in radio, soft X-ray and hard X-ray bands in the form of multiple flares. The broad-band X-ray spectra obtain…
The transient X-ray source CXOM31 004205.77+411330.43 exhibited several outbursts during our long-term monitoring campaign of ~monthly observations of the M31 center with Chandra. However, the decay profile appears to be unlike those observed from Galactic transients. We followed up the 2011 outburst with two ~B band HST/ACS observations, one in outburst and the other in quiescence, and used difference imaging to search for a counterpart; this would be dominated by re-processed X-ray emission from the disk. We found a counterpart with B = 28.21+/-0.16. An XMM-Newton observation from a previous…
XMM-Newton has produced an extensive X-ray source catalogue in which the standard pipeline determines the variability of sufficiently bright sources through chi-square and fractional variability tests. Faint sources, however, are not automatically checked for variability, thus overlooking faint, short timescale transients. Our goal is to find new faint, fast transients in XMM-Newton EPIC-pn observations. To that end we have created the EPIC-pn XMM-Newton Outburst Detector (EXOD) algorithm, which we run on the EPIC-pn data available in the 3XMM-DR8 catalogue. In EXOD, we compute the whole-field…
The accretion behaviour in low-mass X-ray binaries (LMXBs) at low luminosities, especially at <E34 erg/s, is not well known. This is an important regime to study to obtain a complete understanding of the accretion process in LMXBs, and to determine if systems that host neutron stars with accretion-heated crusts can be used probe the physics of dense matter (which requires their quiescent thermal emission to be uncontaminated by residual accretion). Here we examine ultraviolet (UV) and X-ray data obtained when EXO 0748-676, a crust-cooling source, was in quiescence. Our Hubble Space Telescope s…
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…
We show for five DN systems, SS Cyg, VW Hyi, RU Peg, WW Cet and T Leo that the UV and X-ray power spectra of their time variable light curves are similar in quiescence. All of them show a break in their power spectra, which in the framework of the model of propagating fluctuations indicates inner disk truncation. We derive the inner disk radii for these systems in a range (10-3)$\times10^{9}$ cm. We analyze the RXTE data of SS Cyg in outburst and compare it with the power spectra, obtained during the period of quiescence. We show that during the outburst the disk moves towards the white dwarf …
The recent discovery of a milli-second radio pulsar experiencing an accretion outburst similar to those seen in low mass X-ray binaries, has opened up a new opportunity to investigate the evolutionary link between these two different neutron star manifestations. The remarkable X-ray variability and hard X-ray spectrum of this object can potentially serve as a template to search for other X-ray binary/radio pulsar transitional objects. Here we demonstrate that the transient X-ray source XMM J174457-2850.3 near the Galactic center displays similar X-ray properties. We report on the detection of …
Determining the maximum possible neutron star (NS) mass places limits on the equation of state (EoS) of ultra-dense matter. The mass of NSs in low mass X-ray binaries can be determined from the binary mass function, providing independent constraints are placed on both the binary inclination and mass ratio. In eclipsing systems, they relate via the totality duration. EXO 0748-676 is an eclipsing NS low mass X-ray binary with a binary mass function estimated using stellar emission lines from the irradiated face of the companion. The NS mass is thus known as a function of mass ratio. Here we mode…
We have analyzed 3 observations of the High Mass X-ray Binary A0535+26 performed by the Rossi X-ray Timing Explorer (RXTE) 3, 5, and 6 months after the last outburst in 2011 February. We detect pulsations only in the second observation. The 3-20 keV spectra can be fit equally well with either an absorbed power law or absorbed thermal bremsstrahlung model. Re-analysis of 2 earlier RXTE observations made 4 years after the 1994 outburst, original BeppoSAX observations 2 years later, re-analysis of 4 EXOSAT observations made 2 years after the last 1984 outburst, and a recent XMM-Newton observation…
We report on the study of 14 XMM-Newton observations of the magnetar SGR 1806-20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of $\lvert\dotν\rvert\approx1.35\times10^{-11}$ Hz s$^{-1}$, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained consta…
We present the X-ray analysis of dwarf nova EY Cyg using the 45 ks \textit{XMM-Newton} observatory archival data obtained in quiescence. We find orbital modulations in X-rays. We simultaneously fitted EPIC pn, MOS1 and MOS2 data using a model for interstellar medium absorption (\textit{tbabs}) and a multi-temperature plasma emission model with a power-law distribution of temperatures (CEVMKL) as expected from low accretion rate quiescent dwarf novae. The \textit{XMM-Newton} EPIC spectra of the source yields a maximum temperature $kT_{\rm max}$ $\sim$ 14.9$^{+3.3}_{-2.2}$ keV with an unabsorbed…
We took advantage of the large collecting area and good timing capabilities of the EPIC cameras on-board XMM-Newton to investigate the accretion geometry onto the magnetized neutron star hosted in the high mass X-ray binary EXO 2030+375 during the rise of a source Type-I outburst in 2014. We carried out a timing and spectral analysis of the XMM-Newton observation as function of the neutron star spin phase. We used a phenomenological spectral continuum model comprising the required fluorescence emission lines. Two neutral absorption components are present: one covering fully the source and one …
We investigate the early phase of the first state change during the 2008 September-November outburst of H1743-322, first detected by the INTEGRAL satellite. We analyse INTEGRAL, RXTE, Swift, and XMM/Newton observations, which provide coverage of the quiescence to outburst evolution in the 3-200 keV range every few days. The energy spectra are well fitted by a phenomenological model consisting of an exponentially cut-off power law plus a disc component. A more physical model of thermal Comptonisation (and a disc) represents the spectra equally well. In a first phase (up to MJD 54760), the photo…
We have monitored the transient neutron star low-mass X-ray binary 1RXS J180408.9-342058 in quiescence after its ~4.5 month outburst in 2015. The source has been observed using Swift and XMM-Newton. Its X-ray spectra were dominated by a thermal component. The thermal evolution showed a gradual X-ray luminosity decay from ~18 x 10^32 to ~4 x 10^32 (D/5.8 kpc)^2 erg s^{-1} between ~8 and ~379 d in quiescence, and the inferred neutron star surface temperature (for an observer at infinity; using a neutron star atmosphere model) decreased from ~100 to ~71 eV. This can be interpreted as cooling of a…
A Multi-Wavelength Picture of the Surprisingly Short 2024/25 Outburst from EXO 0748-676
Durham University（杜伦大学）；McGill University（麦吉尔大学）；University of Warwick（华威大学）；New York University Abu Dhabi（阿布扎比纽约大学）；INAF, Osservatorio Astronomico di Brera（意大利国家天体物理研究所布里拉天文台）；Anton Pannekoek Institute for Astronomy, Universiteit van Amsterdam（阿姆斯特丹大学安东·潘内克天文研究所）；Yale University（耶鲁大学）
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…
A revolution in galaxy cluster science is only a few years away. The survey machines eROSITA and Euclid will provide cluster samples of never-before-seen statistical quality. XMM-Newton will be the key instrument to exploit these rich datasets in terms of detailed follow-up of the cluster hot gas content, systematically characterizing sub-samples as well as exotic new objects.
We observed the slowly revolving pulsar 1E 161348-5055 (1E 1613, spin period of 6.67 h) in the supernova remnant RCW 103 twice with XMM-Newton and once with the Very Large Telescope (VLT). The VLT observation was performed on 2016 June 30, about a week after the detection of a large outburst from 1E 1613. At the position of 1E 1613, we found a near-infrared source with K_S = 20.68 +/- 0.12 mag that was not detected (K_S > 21.2 mag) in data collected with the same instruments in 2006, during X-ray quiescence. Its position and behavior are consistent with a counterpart in the literature that was…
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…
The mass and radius of the neutron star (NS) in low-mass X-ray binaries can be obtained by fitting the X-ray spectrum of the NS in quiescence, and the mass and radius constrains the properties of dense matter in NS cores. A critical ingredient for spectral fits is the composition of the NS atmosphere: hydrogen atmospheres are assumed in most prior work, but helium atmospheres are possible if the donor star is a helium white dwarf. Here we perform spectral fits to XMM, Chandra, and ROSAT data of a quiescent NS in the globular cluster M13. This NS has the smallest inferred radius from previous s…
We present phase-resolved optical spectroscopy of the counterpart to the neutron star low mass X-ray binary EXO 0748-676, almost one year after it turned into quiescence. The spectra display prominent Hbeta and Hgamma and weak Fe II lines in emission. An average of all the spectra (corrected for the orbital motion) also exhibits a very weak line from Mg I. Tomographic reconstructions show that the accretion disc is not contributing to the optical line emission, which is instead dominated by the irradiated hemisphere of the companion star facing the neutron star. We could not detect absorption …
SAX J2224.9+5421 is an extraordinary neutron star low-mass X-ray binary. Albeit discovered when it exhibited a ~10-s long thermonuclear X-ray burst, it had faded to a 0.5-10 keV luminosity of Lx<8E32 (D/7.1 kpc)^2 erg/s only ~8 hr later. It is generally assumed that neutron stars are quiescent (i.e., not accreting) at such an intensity, raising questions about the trigger conditions of the X-ray burst and the origin of the faint persistent emission. We report on a ~51 ks XMM-Newton observation aimed to find clues explaining the unusual behavior of SAX J2224.9+5421. We identify a likely counter…
