198 research outputs found
Unveiling the environment surrounding LMXB SAX J1808.4-3658
Low-mass X-ray binaries (LMXBs) are a natural workbench to study accretion
disk phenomena and optimal background sources to measure elemental abundances
in the Interstellar medium (ISM). In high-resolution XMM-Newton spectra, the
LMXB SAX J1808.4-3658 showed in the past a neon column density significantly
higher than expected given its small distance, presumably due to additional
absorption from a neon-rich circumstellar medium (CSM). It is possible to
detect intrinsic absorption from the CSM by evidence of Keplerian motions or
outflows. For this purpose, we use a recent, deep (100 ks long),
high-resolution Chandra/LETGS spectrum of SAX J1808.4-3658 in combination with
archival data. We estimated the column densities of the different absorbers
through the study of their absorption lines. We used both empirical and
physical models involving photo- and collisional-ionization in order to
determine the nature of the absorbers. The abundances of the cold interstellar
gas match the solar values as expected given the proximity of the X-ray source.
For the first time in this source, we detected neon and oxygen blueshifted
absorption lines that can be well modeled with outflowing photoionized gas. The
wind is neon rich (Ne/O>3) and may originate from processed, ionized gas near
the accretion disk or its corona. The kinematics (v=500-1000 km/s) are indeed
similar to those seen in other accretion disks. We also discovered a system of
emission lines with very high Doppler velocities (v~24000 km/s) originating
presumably closer to the compact object. Additional observations and UV
coverage are needed to accurately determine the wind abundances and its
ionization structure.Comment: 12 pages, 10 figures, accepted for publication on A&
Discovery of an eccentric 30 days period in the supergiant X-ray binary SAX J1818.6-1703 with INTEGRAL
SAX J1818.6-1703 is a flaring transient X-ray source serendipitously
discovered by BeppoSAX in 1998 during an observation of the Galactic centre.
The source was identified as a High-Mass X-ray Binary with an OB SuperGiant
companion. Displaying short and bright flares and an unusually very-low
quiescent level implying intensity dynamical range as large as 1e3-4, the
source was classified as a Supergiant Fast X-ray Transient. The mechanism
triggering the different temporal behaviour observed between the classical
SGXBs and the recently discovered class of SFXTs is still debated. The
discovery of long orbits (>15 d) should help to discriminate between emission
models and bring constraints.
We analysed archival INTEGRAL data on SAX J1818.6-1703. We built short- and
long-term light curves and performed timing analysis in order to study the
temporal behaviour of SAX J1818.6-1703 on different time scales. INTEGRAL
revealed an unusually long orbital period of 30.0+/-0.2 d and an elapsed
accretion phase of ~6 d in the transient SGXB SAX J1818.6-1703. This implies an
elliptical orbit and constraints the possible supergiant spectral type between
B0.5-1I with eccentricities e~0.3-0.4 (for average fundamental parameters of
supergiant stars). During the accretion phase, the source behaved like
classical SGXBs. The huge variations of the observed X-ray flux can be
explained through accretion of macro-clumps formed within the stellar wind. Our
analysis strengthens the model which predicts that SFXTs behave as SGXBs but
with different orbital parameters, thus different temporal behaviour.Comment: 4 pages, 3 figures, A&A Letter in press (subm. 17/10/2008 - accept.
15/11/2008
ROSSI X-RAY TIMING EXPLORER AND BeppoSAX OBSERVATIONS OF THE TRANSIENT X-RAY PULSAR XTE J1859+083
We present observations of the 9.8 s X-ray pulsar XTE J1859+083 made with the All Sky Monitor (ASM) and Proportional Counter Array (PCA) on board the Rossi X-ray Timing Explorer, and the Wide Field Camera (WFC) on board BeppoSAX. The ASM data cover a 12 year time interval and show that an extended outburst occurred between approximately MJD 50,250 and 50,460 (1996 June 16 to 1997 January 12). The ASM data excluding this outburst interval suggest a possible modulation with a period of 60.65 ± 0.08 days. Eighteen sets of PCA observations were obtained over an approximately one month interval in 1999. The flux variability measured with the PCA appears consistent with the possible period found with the ASM. The PCA measurements of the pulse period showed it to decrease nonmonotonically and then to increase significantly. Doppler shifts due to orbital motion rather than accretion torques appear to be better able to explain the pulse period changes. Observations with the WFC during the extended outburst give a position that is consistent with a previously determined PCA error box, but which has a significantly smaller error. The transient nature of XTE J1859+083 and the length of its pulse period are consistent with it being a Be/neutron star binary. The possible 60.65 day orbital period would be of the expected length for a Be star system with a 9.8 s pulse period
Misalignment of the microquasar V4641 Sgr (SAX J1819.3--2525)
In the microquasar V4641 Sgr the spin of the black hole is thought to be
misaligned with the binary orbital axis. The accretion disc aligns with the
black hole spin by the Lense-Thirring effect near to the black hole and further
out becomes aligned with the binary orbital axis. The inclination of the radio
jets and the Fe line profile have both been used to determine the
inclination of the inner accretion disc but the measurements are inconsistent.
Using a steady state analytical warped disc model for V4641 Sgr we find that
the inner disc region is flat and aligned with the black hole up to about . Thus if both the radio jet and fluorescent emission originates in
the same inner region then the measurements of the inner disc inclination
should be the same.Comment: Accepted for publication in MNRA
Catalogue of high-mass X-ray binaries in the Galaxy ( edition)
We present a new edition of the catalogue of high-mass X-ray binaries in the
Galaxy. The catalogue contains source name(s), coordinates, finding chart,
X-ray luminosity, system parameters, and stellar parameters of the components
and other characteristic properties of 114 high-mass X-ray binaries, together
with a comprehensive selection of the relevant literature. The aim of this
catalogue is to provide the reader with some basic information on the X-ray
sources and their counterparts in other wavelength ranges (-rays, UV,
optical, IR, radio). About 60% of the high-mass X-ray binary candidates are
known or suspected Be/X-ray binaries, while 32% are supergiant/X-ray binaries.
Some sources, however, are only tentatively identified as high-mass X-ray
binaries on the basis of their X-ray properties similar to the known high-mass
X-ray binaries. Further identification in other wavelength bands is needed to
finally determine the nature of these sources. In cases where there is some
doubt about the high-mass nature of the X-ray binary this is mentioned.
Literature published before 1 October 2005 has, as far as possible, been taken
into account.Comment: 25 pages, table include
INTEGRAL high energy detection of the transient IGR J11321-5311
Context: The transient hard X-ray source IGR J11321-5311 was discovered by
INTEGRAL on June 2005, during observations of the Crux spiral arm. To date,
this is the only detection of the source to be reported by any X/gamma-ray
mission. Aims: To characterize the behaviour and hence the nature of the source
through temporal and spectral IBIS analysis. Methods: Detailed spectral and
temporal analysis has been performed using standard INTEGRAL software OSA
v.5.1. Results: To date, IGR J11321-5311 has been detected only once. It was
active for about 3.5 hours, a short and bright flare lasting about 1.5 hours is
evident in the IBIS light curve. It reached a peak flux of about 80 mCrab or
2.2x10E-9 erg cmE-2 sE-1 (20--300 keV),corresponding to a peak luminosity of
1.1x10E37 erg sE-1 (assuming a distance of 6.5 kpc). During the outburst, the
source was detected with a significance of 18 sigma (20--300 keV) and 8 sigma
(100--300 keV). The spectrum of the total outburst activity (17--300 keV) is
best fitted by the sum of a power law (Gamma=0.55+/-0.18) plus a black body
(kT=1.0{+0.2}_{-0.3} keV), with no evidence for a break up to 300 keV. A
spectral analysis at Science Window level revealed an evident hardening of the
spectrum through the outburst. The IBIS data were searched for pulsations with
no positive result. Conclusions: The X-ray spectral shape and the flaring
behaviour favour the hypothesis that IGR J11321-5311 is an Anomalous X-ray
Pulsar, though a different nature can not be firmly rejected at the present
stage.Comment: accepted for publication in A&A letter, 4 pages, 6 figure
IGR J17544-2619: A new supergiant fast X-ray transient revealed by optical/infrared observations
One of the most recent discoveries of the INTEGRAL observatory is the
existence of a previously unknown population of X-ray sources in the inner arms
of the Galaxy. IGR J17544-2619, IGR J16465-4507 and XTE J1739-302 are among
these sources. Although the nature of these systems is still unexplained, the
investigations of the optical/NIR counterparts of the two last sources,
combined with high energy data, have provided evidence of them being highly
absorbed high mass X-ray binaries with blue supergiant secondaries and
displaying fast X-ray transient behaviour. In this work we present our
optical/NIR observations of IGR J17544-2619, aimed at identifying and
characterizing its counterpart. We show that the source is a high mass X-ray
binary at a distance of 2-4 kpc with a strongly absorbed O9Ib secondary, and
discuss the nature of the system.Comment: 7 pages, 4 figures. Accepted for publication in A&
Unveiling the nature of six HMXBs through IR spectroscopy
The International Gamma-Ray Astrophyiscs Laboratory (INTEGRAL) is discovering
a large number of new hard X-ray sources, many of them being HMXBs. The
identification and spectral characterization of their optical/infrared
counterparts is a necessary step to undertake detailed study of these systems.
In particular, the determination of the spectral type is crucial in the case of
the new class of Supergiant Fast X-ray Transients (SFXTs), which show X-ray
properties common to other objects. We used the ESO/NTT SofI spectrograph to
observe proposed IR counterparts to HMXBs, obtaining Ks medium resolution
spectra (R = 1320) with a S/N >= 100. We classified them through comparison
with published atlases. We were able to spectrally classify the six sources.
This allowed us to ascribe one of them to the new class of SFXTs and confirm
the membership of two sources to this class. We confirmed the spectral
classification, derived from optical spectroscopy, of a known system, 4U
1907-09, showing for the first time its infrared spectrum. The spectral
classification was also used to estimate the distance of the sources. We
compared the extinction as derived from X-ray data with effective interstellar
extinction obtained from our data, discussing the absorption component due to
the circumstellar environment, which we observed in four systems; in
particular, intrinsic absorption seems to emerge as a typical feature of the
entire class of SFXTs.Comment: 8 pages, 6 figures, 3 tables, accepted for publication in Astronomy &
Astrophysic
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