The quiescent counterpart of the peculiar X-ray burster SAX J2224.9+5421

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 counterpart that is detected at Lx~5E31 (D/7.1 kpc)^2 erg/s (0.5-10 keV) and has a soft X-ray spectrum that can be described by a neutron star atmosphere model with a temperature of ~50 eV. This would suggest that SAX J2224.9+5421 is a transient source that was in quiescence during our XMM-Newton observation and experienced a very faint (ceasing) accretion outburst at the time of the X-ray burst detection. We consider one other potential counterpart that is detected at Lx~5E32 (D/7.1 kpc)^2 erg/s and displays an X-ray spectrum that is best described by power law with a photon index of ~1.7. Similarly hard X-ray spectra are seen for a few quiescent neutron stars and may be indicative of a relatively strong magnetic field or the occurrence of low-level accretion.Comment: 7 pages, 2 figures, 2 tables. Accepted to Ap

A direct measurement of the heat release in the outer crust of the transiently accreting neutron star XTE J1709-267

The heating and cooling of transiently accreting neutron stars provides a powerful probe of the structure and composition of their crust. Observations of superbursts and crust cooling of accretion-heated neutron stars require more heat release than is accounted for in current models. Obtaining firm constraints on the depth and magnitude of this extra heat is challenging and therefore its origin remains uncertain. We report on Swift and XMM-Newton observations of the transient neutron star low-mass X-ray binary XTE J1709-267, which were made in 2012 September-October when it transitioned to quiescence after a ~10-week long accretion outburst. The source is detected with XMM-Newton at a 0.5-10 keV luminosity of Lx~2E34 (D/8.5 kpc)^2 erg/s. The X-ray spectrum consists of a thermal component that fits to a neutron star atmosphere model and a non-thermal emission tail, which each contribute ~50% to the total emission. The neutron star temperature decreases from ~158 to ~152 eV during the ~8-hour long observation. This can be interpreted as cooling of a crustal layer located at a column density of y~5E12 g/cm^2 (~50 m inside the neutron star), which is just below the ignition depth of superbursts. The required heat generation in the layers on top would be ~0.06-0.13 MeV per accreted nucleon. The magnitude and depth rule out electron captures and nuclear fusion reactions as the heat source, but it may be accounted for by chemical separation of light and heavy nuclei. Low-level accretion offers an alternative explanation for the observed variability.Comment: 6 pages, 4 figures, 1 table, accepted to ApJ Letters. Minor changes according to referee report, revised version includes a discussion on the alternative interpretation of residual accretio

Searching for the most powerful thermonuclear X-ray bursts with the Neil Gehrels Swift Observatory

We searched for thermonuclear X-ray bursts from Galactic neutron stars in all event mode data of the Neil Gehrels Swift Observatory collected until March 31, 2018. In particular, we are interested in the intermediate-duration bursts (shell flashes fueled by thick helium piles) with the ill-understood phenomenon of strong flux fluctuations. Nine such bursts have been discussed in the literature to date. Swift is particularly suitable for finding additional examples. We find and list a total of 134 X-ray bursts; 44 are detected with BAT only, 41 with XRT only, and 49 with both. Twenty-eight bursts involve automatic slews. We find 12 intermediate-duration bursts, all detected in observations involving automatic slews. Five show remarkably long Eddington-limited phases in excess of 200 s. Five show fluctuations during the decay phase; four of which are first discussed in the present study. We discuss the general properties of the fluctuations, considering also 7 literature cases. In general two types of fluctuations are observed: fast ones, with a typical timescale of 1 s and up and downward fluctuations of up to 70%, and slow ones, with a typical timescale of 1 min and only downward fluctuations of up to 90%. The latter look like partial eclipses because the burst decay remains visible in the residual emission. We revisit the interpretation of this phenomenon in the context of the new data set and find that it has not changed fundamentally despite the expanded data set. It is thought to be due to a disturbance of the accretion disk by outflowing matter and photons, causing obscuration and reflection due to Thompson scattering in an orbiting highly ionized cloud or structure above or below the disk. We discuss in detail the most pronounced burster SAX J1712.6-3739. One of the bursts from this source is unusual in that it lasts longer than 5600 s, but does not appear to be a superburst.Comment: Accepted for publication in Astronomy & Astrophysics, 29 pages, 12 figures. Version 2 has 3 bursts from IGR J17480-2446 re-identified to 2 from Swift J174805.3-244637 and 1 from EXO 1745-24

The Swift X-ray monitoring campaign of the center of the Milky Way

In 2006 February, shortly after its launch, Swift began monitoring the center of the Milky Way with the onboard X-Ray Telescope using short 1-ks exposures performed every 1-4 days. Between 2006 and 2014, over 1200 observations have been obtained, amounting to ~1.2 Ms of exposure time. This has yielded a wealth of information about the long-term X-ray behavior of the supermassive black hole Sgr A*, and numerous transient X-ray binaries that are located within the 25'x25' region covered by the campaign. In this review we highlight the discoveries made during these first nine years, which includes 1) the detection of seven bright X-ray flares from Sgr A*, 2) the discovery of the magnetar SGR J1745-29, 3) the first systematic analysis of the outburst light curves and energetics of the peculiar class of very-faint X-ray binaries, 4) the discovery of three new transient X-ray sources, 5) exposing low-level accretion in otherwise bright X-ray binaries, and 6) the identification of a candidate X-ray binary/millisecond radio pulsar transitional object. We also reflect on future science to be done by continuing this Swift's legacy campaign of the Galactic center, which includes high-cadence monitoring of how the interaction between the gaseous object `G2' and Sgr A* plays out in the future.Comment: 13 pages, 6 figures, 4 tables. Invited review to appear in Elsevier's Journal of High Energy Astrophysics dedicated issue "Swift: 10 years of discovery

The X-ray flaring properties of Sgr A* during six years of monitoring with Swift

Starting in 2006, Swift has been targeting a region of ~21'X21' around Sagittarius A* (Sgr A*) with the onboard X-ray telescope. The short, quasi-daily observations offer an unique view of the long-term X-ray behavior of the supermassive black hole. We report on the data obtained between 2006 February and 2011 October, which encompasses 715 observations with a total accumulated exposure time of ~0.8 Ms. A total of six X-ray flares were detected with Swift, which all had an average 2-10 keV luminosity of Lx (1-4)E35 erg/s (assuming a distance of 8 kpc). This more than doubles the number of such bright X-ray flares observed from Sgr A*. One of the Swift-detected flares may have been softer than the other five, which would indicate that flares of similar intensity can have different spectral properties. The Swift campaign allows us to constrain the occurrence rate of bright (Lx > 1E35 erg/s) X-ray flares to be ~0.1-0.2 per day, which is in line with previous estimates. This analysis of the occurrence rate and properties of the X-ray flares seen with Swift offers an important calibration point to asses whether the flaring behavior of Sgr A* changes as a result of its interaction with the gas cloud that is projected to make a close passage in 2013.Comment: 8 pages, 5 figures, 3 tables. Shortened, accepted to Ap

Predictive processing, perceptual presence, and sensorimotor theory

International audienc

The Galactic center X-ray transients AX J1745.6-2901 and GRS 1741-2853

AX J1745.6-2901 and GRS 1741-2853 are two transient neutron star low-mass X-ray binaries that are located within ~10' from the Galactic center. Multi-year monitoring observations with the Swift/XRT has exposed several accretion outbursts from these objects. We report on their updated X-ray light curves and renewed activity that occurred in 2010-2013.Comment: 2 pages, 1 figure, 1 table. To appear in conference proceedings of IAU symposium 303 "The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus

X-ray softening in the new X-ray transient XTE J1719-291 during its 2008 outburst decay

The X-ray transient XTE J1719-291 was discovered with RXTE/PCA during its outburst in 2008 March, which lasted at least 46 days. Its 2-10 keV peak luminosity is 7E35 erg/s assuming a distance of 8 kpc, which classifies the system as a very faint X-ray transient. The outburst was monitored with Swift, RXTE, Chandra and XMM-Newton. We analysed the X-ray spectral evolution during the outburst. We fitted the overall data with a simple power-law model corrected for absorption and found that the spectrum softened with decreasing luminosity. However, the XMM-Newton spectrum can not be fitted with a simple one-component model, but it can be fitted with a thermal component (black body or disc black body) plus power-law model affected by absorption. Therefore, the softening of the X-ray spectrum with decreasing X-ray luminosity might be due to a change in photon index or alternatively it might be due to a change in the properties of the soft component. Assuming that the system is an X-ray binary, we estimated a long-term time-averaged mass accretion rate of ~ 7.7E-13 M_sun/yr for a neutron star as compact object and ~ 3.7E10-13 M_sun/yr in the case of a black hole. Although no conclusive evidence is available about the nature of the accretor, based on the X-ray/optical luminosity ratio we tentatively suggest that a neutron star is present in this system.Comment: Accepted for publication in MNRAS. 8 pages, 4 figures, 2 table