1,158 research outputs found
A closer look at the X-ray transient XTE J1908+094: identification of two new near-infrared candidate counterparts
We had reported in Chaty, Mignani, Israel (2002) on the near-infrared (NIR)
identification of a possible counterpart to the black hole candidate XTE
J1908+094 obtained with the ESO/NTT. Here, we present new, follow-up, CFHT
adaptive optics observations of the XTE J1908+094 field, which resolved the
previously proposed counterpart in two objects separated by about 0.8".
Assuming that both objects are potential candidate counterparts, we derive that
the binary system is a low-mass system with a companion star which could be
either an intermediate/late type (A-K) main sequence star at a distance of 3-10
kpc, or a late-type (K) main sequence star at a distance of 1-3 kpc.
However, we show that the brighter of the two objects (J ~ 20.1, H ~ 18.7, K' ~
17.8) is more likely to be the real counterpart of the X-ray source. Its
position is more compatible with our astrometric solution, and colours and
magnitudes of the other object are not consistent with the lower limit of 3 kpc
derived independently from the peak bolometric flux of XTE J1908+094. Further
multi-wavelength observations of both candidate counterparts are crucial in
order to solve the pending identification.Comment: accepted for publication in MNRAS, 5 pages, 3 figure
Constraining the neutron star equation of state using XMM-Newton
We have identified three possible ways in which future XMM-Newton
observations can provide significant constraints on the equation of state of
neutron stars. First, using a long observation of the neutron star X-ray
transient CenX-4 in quiescence one can use the RGS spectrum to constrain the
interstellar extinction to the source. This removes this parameter from the
X-ray spectral fitting of the pn and MOS spectra and allows us to investigate
whether the variability observed in the quiescent X-ray spectrum of this source
is due to variations in the soft thermal spectral component or variations in
the power law spectral component coupled with variations in N_H. This will test
whether the soft thermal spectral component can indeed be due to the hot
thermal glow of the neutron star. Potentially such an observation could also
reveal redshifted spectral lines from the neutron star surface. Second,
XMM-Newton observations of radius expansion type I X-ray bursts might reveal
redshifted absorption lines from the surface of the neutron star. Third,
XMM-Newton observations of eclipsing quiescent low-mass X-ray binaries provide
the eclipse duration. With this the system inclination can be determined
accurately. The inclination determined from the X-ray eclipse duration in
quiescence, the rotational velocity of the companion star and the
semi-amplitude of the radial velocity curve determined through optical
spectroscopy, yield the neutron star mass.Comment: 4 pages, 1 figure, proceedings of the XMM-Newton workshop, June 2007,
accepted for publication in A
A new bursting X-ray transient: SAX J1750.8-2900
We have analysed in detail the discovery measurements of the X-ray burster
SAX J1750.8-2900 by the Wide Field Cameras on board BeppoSAX in spring 1997, at
a position ~1.2 degrees off the Galactic Centre. The source was in outburst on
March 13th when the first observation started and showed X-ray emission for ~ 2
weeks. A total of 9 bursts were detected, with peak intensities varying from ~
0.4 to 1.0 Crab in the 2-10 keV range. Most bursts showed a fast rise time (~
1s), an exponential decay profile with e-folding time of ~ 5s, spectral
softening during decay, and a spectrum which is consistent with few keV
blackbody radiation. These features identify them as type-I X-ray bursts of
thermonuclear origin. The presence of type-I bursts and the source position
close to the Galactic Centre favours the classification of this object as a
neutron star low mass X-ray binary. X-ray emission from SAX J1750.8-2900 was
not detected in the previous and subsequent Galactic bulge monitoring, and the
source was never seen bursting again.Comment: 13 pages, 3 Postscript figures, aaspp4 styl
Detection of a 1258 Hz high-amplitude kilohertz quasi-periodic oscillation in the ultra-compact X-ray binary 1A 1246-588
We have observed the ultra-compact low-mass X-ray binary (LMXB) 1A 1246-588
with the Rossi X-ray Timing Explorer (RXTE). In this manuscript we report the
discovery of a kilohertz quasi-periodic oscillation (QPO) in 1A 1246-588. The
kilohertz QPO was only detected when the source was in a soft high-flux state
reminiscent of the lower banana branch in atoll sources. Only one kilohertz QPO
peak is detected at a relatively high frequency of 1258+-2 Hz and at a single
trial significance of more than 7 sigma. Kilohertz QPOs with a higher frequency
have only been found on two occasions in 4U 0614+09. Furthermore, the frequency
is higher than that found for the lower kilohertz QPO in any source, strongly
suggesting that the QPO is the upper of the kilohertz QPO pair often found in
LMXBs. The full-width at half maximum is 25+-4 Hz, making the coherence the
highest found for an upper kilohertz QPO. From a distance estimate of ~6 kpc
from a radius expansion burst we derive that 1A 1246-588 is at a persistent
flux of ~0.2-0.3 per cent of the Eddington flux, hence 1A 1246-588 is one of
the weakest LMXBs for which a kilohertz QPO has been detected. The
root-mean-square (rms) amplitude in the 5-60 keV band is 27+-3 per cent, this
is the highest for any kilohertz QPO source so far, in line with the general
anti-correlation between source luminosity and rms amplitude of the kilohertz
QPO peak identified before. Using the X-ray spectral information we produce a
colour-colour diagram. The source behaviour in this diagram provides further
evidence for the atoll nature of the source.Comment: 4 pages, 3 figures, accepted for publication in MNRA
The BeppoSAX WFC X-ray source catalogue
We present the catalogue of X-ray sources detected by the two Wide Field
Cameras (WFCs) in complete observations on board BeppoSAX during its 6 years of
operational lifetime, between April 1996 and April 2002. The BeppoSAX WFCs were
coded mask instruments sensitive in the 2-28 keV energy band with a 40x40
square degree fields of view, pointing in opposite directions and
perpendicularly to the BeppoSAX Narrow Field Instruments (NFI). The WFCs were
usually operated simultaneously to NFI observations, each lasting up to several
days. WFCs observed thus the entire sky several times with a typical
sensitivity of 2 to 10 mCrab. A systematic analysis of all WFC observations in
the BeppoSAX archive has been carried out using the latest post-mission release
of the WFC analysis software and calibrations. The catalogue includes 253
distinct sources, obtained from a total sample of 8253 WFC detections. We
describe the basic statistical properties of the sample and present a six-year
history of two celestial calibration X-ray sources.Comment: 15 pages, 11 figures, Catalogue, Accepted for publication on A&
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