468 research outputs found
XMMU J174716.1-281048: a "quasi-persistent" very faint X-ray transient?
The X-ray transient XMMU J174716.1-281048 was serendipitously discovered with
XMM-Newton in 2003. It lies about 0.9 degrees off the Galactic Centre and its
spectrum shows a high absorption (~8 x 10E22 cm^(-2)). Previous X-ray
observations of the source field performed in 2000 and 2001 did not detect the
source, indicative of a quiescent emission at least two orders of magnitude
fainter. The low luminosity during the outburst (~5 x 10E34 erg/s at 8 kpc)
indicates that the source is a member of the ``very faint X-ray transients''
class. On 2005 March 22nd the INTEGRAL satellite caught a possible type-I X-ray
burst from the new INTEGRAL source IGR J17464-2811, classified as fast X-ray
transient. This source was soon found to be positionally coincident, within the
uncertainties, with XMMU J174716.1-281048. Here we report data analysis of the
X-ray burst observed with the IBIS and JEM-X telescopes and confirm the type-I
burst nature. We also re-analysed XMM-Newton and Chandra archival observations
of the source field. We discuss the implications of these new findings,
particularly related to the source distance as well as the source
classification.Comment: 4 pages, 8 figures, accepted for publication in A&A Letter
Linking burst-only X-ray binary sources to faint X-ray transients
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. In addition, we observed
three quasi-persistent faint X-ray binary transients. Finally we discuss
burst-only sources and quasi-persistent sources in the framework of neutron
star transients.Comment: 9 pages, 8 figures. Accepted for publication on Ap
Spectral properties of the soft excess pulsar RX J0059.2-7138 during its 2013 outburst
We report on an X-ray observation of the Be X-ray Binary Pulsar RX
J0059.2-7138, performed by XMM-Newton in March 2014. The 19 ks long observation
was carried out about three months after the discovery of the latest outburst
from this Small Magellanic Cloud transient, when the source luminosity was Lx ~
10 erg/s. A spin period of P=2.762383(5) s was derived, corresponding to
an average spin-up of
s from the only previous period measurement, obtained more than 20
years earlier. The time-averaged continuum spectrum (0.2-12 keV) consisted of a
hard power-law (photon index ~0.44) with an exponential cut-off at a
phase-dependent energy (20-50 keV) plus a significant soft excess below about
0.5 keV. In addition, several features were observed in the spectrum: an
emission line at 6.6 keV from highly ionized iron, a broad feature at 0.9-1 keV
likely due to a blend of Fe L-shell lines, and narrow emission and absorption
lines consistent with transitions in highly ionized oxygen, nitrogen and iron
visible in the high resolution RGS data (0.4-2.1 keV). Given the different
ionization stages of the narrow line components, indicative of photoionization
from the luminous X-ray pulsar, we argue that the soft excess in RX
J0059.2-7138 is produced by reprocessing of the pulsar emission in the inner
regions of the accretion disc.Comment: Accepted for publication in Mon. Not. R. Astron. Soc. 9 pages, 5
figure
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
XMM-Newton and NuSTAR simultaneous X-ray observations of IGR J11215-5952
We report the results of an XMM-Newton and NuSTAR coordinated observation of
the Supergiant Fast X-ray Transient (SFXT) IGRJ11215-5952, performed on
February 14, 2016, during the expected peak of its brief outburst, which
repeats every about 165 days. Timing and spectral analysis were performed
simultaneously in the energy band 0.4-78 keV. A spin period of 187.0 +/- 0.4 s
was measured, consistent with previous observations performed in 2007. The
X-ray intensity shows a large variability (more than one order of magnitude) on
timescales longer than the spin period, with several luminous X-ray flares
which repeat every 2-2.5 ks, some of which simultaneously observed by both
satellites. The broad-band (0.4-78 keV) time-averaged spectrum was well
deconvolved with a double-component model (a blackbody plus a power-law with a
high energy cutoff) together with a weak iron line in emission at 6.4 keV
(equivalent width, EW, of 40+/-10 eV). Alternatively, a partial covering model
also resulted in an adequate description of the data. The source time-averaged
X-ray luminosity was 1E36 erg/s (0.1-100 keV; assuming 7 kpc). We discuss the
results of these observations in the framework of the different models proposed
to explain SFXTs, supporting a quasi-spherical settling accretion regime,
although alternative possibilities (e.g. centrifugal barrier) cannot be ruled
out.Comment: 13 pages, 11 figures, accepted for publication on The Astrophysical
Journa
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