653 research outputs found
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
Predictive processing, perceptual presence, and sensorimotor theory
International audienc
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
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
- …