1,056 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
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
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
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
SAXJ1712.6-3739: a persistent hard X-ray source as monitored with INTEGRAL
The X-ray source SAXJ1712.6-3739 is a very weak Low Mass X-ray Binary
discovered in 1999 with BeppoSAX and located in the Galactic Center. This
region has been deeply investigated by the INTEGRAL satellite with an
unprecedented exposure time, giving us an unique opportunity to study the hard
X-ray behavior also for weak objects. The spectral results are based on the
systematic analysis of all INTEGRAL observations covering the source position
performed between February 2003 and October 2006. SAXJ1712.6-3739 did not shows
any flux variation along this period as well as compared to previous BeppoSAX
observation. Hence, to better constrain the physical parameters we combined
both instrument data. Long INTEGRAL monitoring reveals, for the first time,
that this X-ray burster is a weak persistent source, displaying a X-ray
spectrum extended to high energy and spending most of the time in a low
luminosity hard state. The broad-band spectrum is well modeled with a simple
Comptonized model with a seed photons temperature of ~0.5keV and an electron
temperature of ~24keV. The low mass accretion rate (~2x10^{-10} Msun/yr), the
long bursts recurrence time, the small sizes of the region emitting the seed
photons consisting with the inner disk radius and the high luminosity ratio in
the 40-100keV and 20-40keV band, are all features common to the Ultra Compact
source class.Comment: accepted A&
X-ray variability during the quiescent state of the neutron-star X-ray transient in the globular cluster NGC 6440
The globular cluster NGC 6440 is known to harbor a bright neutron-star X-ray
transient. We observed the globular cluster with Chandra on two occasions when
the bright transient was in its quiescent state in July 2000 and June 2003
(both observations were made nearly 2 years after the end of their preceding
outbursts). The quiescent spectrum during the first observation is well
represented by a two component model (a neutron-star atmosphere model plus a
power-law component which dominates at energies above 2 keV). During the second
observation (which was roughly of equal duration to the first observation) we
found that the power-law component could no longer be detected. Our spectral
fits indicate that the effective temperature of the neutron-star surface was
consistent between the two observations. We conclude that the effect of the
change in power-law component caused the 0.5-10 keV flux to be a factor of ~2
lower during the second observation compared to the first observation. We
discuss plausible explanations for the variations, including variable residual
accretion onto the neutron star magnetosphere or some variation in the
interaction of the pulsar wind with the matter still outflowing from the
companion star.Comment: 18 pages, 3 color figs, 1 b&w figures, 3 tables; discussion expanded;
accepted for publication in Ap
Six new candidate ultracompact X-ray binaries
Ultracompact X-ray binaries (UCXBs) appear able to sustain accretion onto the
compact accretor at rates lower than in wider X-ray binaries. This may be
understood by the smaller accretion disks in UCXBs: a lower X-ray luminosity
suffices to keep a disk completely ionized through irradiation and, thus, keep
the viscosity at a sufficiently high level to allow effective transport of
matter to the compact object. We employ this distinguishing factor on data from
RXTE and BeppoSAX to identify six new candidate UCXBs, thus increasing the
population by one quarter. The candidates are drawn from the population of
persistently accreting and type-I X-ray bursting low-mass X-ray binaries. The
X-ray bursts establish the low-mass X-ray binary nature and provide a handle on
the accretion rate. We find that the low accretion rates are supported by the
long burst recurrence times and the hard X-ray spectra of the persistent
emission as derived from the 2nd INTEGRAL catalog of soft gamma-ray sources. We
discuss the peculiar light curves of some new UCXB candidates.Comment: Section 2 corrected and improved thanks to comments by J.-P. Lasota.
Accepted for publication in Astronomy and Astrophysic
Sedimentation and Type I X-ray Bursts at Low Accretion Rates
Neutron stars, with their strong surface gravity, have interestingly short
timescales for the sedimentation of heavy elements. Motivated by observations
of Type I X-ray bursts from sources with extremely low persistent accretion
luminosities, L_X < 10^{36}\usp\ergspersecond (\simeq
0.01\ensuremath{L_{\mathrm{Edd}}}), we study how sedimentation affects the
distribution of isotopes and the ignition of H and He in the envelope of an
accreting neutron star. For local mass accretion rates \mdot \lesssim
10^{-2}\medd (for which the ignition of H is unstable), where \medd =
8.8\times 10^{4}\nsp\gpscps, the helium and CNO elements sediment out of the
accreted fuel before reaching a temperature where H would ignite. Using
one-zone calculations of the thermonuclear burning, we find a range of
accretion rates for which the unstable H ignition does not trigger unstable He
burning. This range depends on the emergent flux from reactions in the deep
neutron star crust; for F = 0.1\nsp\MeV(\dot{m}/\mb), the range is 3\times
10^{-3}\medd\lesssim\mdot\lesssim 10^{-2}\medd. We speculate that sources
accreting in this range will build up a massive He layer that later produces an
energetic and long X-ray burst. At mass accretion rates lower than this range,
we find that the H flash leads to a strong mixed H/He flash. Surprisingly, even
at accretion rates \mdot \gtrsim 0.1\medd, although the H and He do not
completely segregate, the H abundance at the base of the accumulated layer is
still reduced. While following the evolution of the X-ray burst is beyond the
scope of this introductory paper, we note that the reduced proton-to-seed ratio
favors the production of \iso{12}{C}--an important ingredient for subsequent
superbursts.Comment: 15 pages, 14 figures, submitted to ApJ, revised versio
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
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