384 research outputs found
The behavior of subluminous X-ray transients near the Galactic center as observed using the X-ray telescope aboard Swift
In this paper we report on the spectral analysis of seven X-ray transients,
which were found to be active during a monitoring campaign of the Galactic
center carried out in 2006 and 2007 using the X-ray telescope aboard the Swift
satellite. This campaign detected new outbursts of five known X-ray transients
and discovered two new systems. Their 2-10 keV peak luminosities range from
1E34 to 6E36 erg/s. Two of the sources discussed in this paper are confirmed
neutron star systems (AX J1745.6-2901 and GRS 1741-2853), while the five others
have an unknown nature. We discuss the characteristics of the observed
outbursts and the duty cycles of the various systems. Several of the detected
transients seem to undergo enhanced X-ray activity with levels intermediate
between quiescence and full outburst. We discuss the possibility that the
subluminous appearance of the eclipsing X-ray burster AX J1745.6-2901 is due to
line-of-sight effects. We detected two type-I X-ray bursts with a duration of
50-60 seconds from AX J1745.6-2901, which we discuss in view of the bursting
behavior of low-luminosity X-ray transients. Assuming that we are dealing with
accreting neutron stars and black holes, we estimate the time-average accretion
rate, Mdot, of the transients, which is an important input parameter for binary
evolution models that attempt to explain the nature of subluminous X-ray
transients. Our estimates lie in the range of 3E-13 Msun/yr < Mdot < 1E-10
Msun/yr, if the systems are neutron star X-ray binaries and between 4E-14
Msun/yr < Mdot < 2E-11 Msun/yr for a scenario where the accreting object is a
black hole. Some of the systems have such low estimated mass-accretion rates
that they possibly pose a challenge for binary evolution models.Comment: Accepted for publication in A&A, some textual changes according to
referee's report, 13 pages, 4 figure
A four-year baseline Swift study of enigmatic X-ray transients located near the Galactic center
We report on continued monitoring observations of the Galactic center carried
out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This
campaign revealed activity of the five known X-ray transients AX J1745.6-2901,
CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC
J174538.0-290022. All these sources are known to undergo very faint X-ray
outbursts with 2-10 keV peak luminosities of Lx,peak~1E34-1E36 erg/s, although
the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS
1741-2853 can also become brighter (Lx,peak~1E36-1E37 erg/s). We discuss the
observed long-term lightcurves and X-ray spectra of these five enigmatic
transients. In 2008, AX J1745.6-2901 returned to quiescence following an
unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was
active in 2009 and displayed the brightest outburst ever recorded for this
source, reaching up to a 2-10 keV luminosity of Lx~1E37 (D/7.2 kpc)^2 erg/s.
This system appears to undergo recurrent accretion outbursts approximately
every 2 years. Furthermore, we find that the unclassified transient XMM
J174457-2850.3 becomes bright only during short episodes (days) and is often
found active in between quiescence (Lx~1E32 erg/s) and its maximum outburst
luminosity of Lx~1E36 erg/s. CXOGC J174535.5-290124 and CXOGC J174538.0-290022,
as well as three other very-faint X-ray transients that were detected by Swift
monitoring observations in 2006, have very low time-averaged mass-accretion
rates of ~< 2E-12 Msun/yr. Despite having obtained two years of new data in
2008 and 2009, no new X-ray transients were detected.Comment: Minor textual revisions according to referee report, accepted for
publication in A&
Near-Infrared Counterparts to Chandra X-ray Sources toward the Galactic Center. I. Statistics and a Catalog of Candidates
We present a catalog of 5184 candidate infrared counterparts to X-ray sources
detected towards the Galactic center. The X-ray sample contains 9017 point
sources detected in this region by the Chandra X-ray Observatory, including
data from a recent deep survey of the central 2 x 0.8 deg of the Galactic
plane. A total of 6760 of these sources have hard X-ray colors, and the
majority of them lie near the Galactic center, while most of the remaining 2257
soft X-ray sources lie in the foreground. We cross-correlated the X-ray source
positions with the 2MASS and SIRIUS near-infrared catalogs, which collectively
contain stars with a 10-sigma limiting flux of K_s<=15.6 mag. In order to
distinguish absorbed infrared sources near the Galactic center from those in
the foreground, we defined red and blue sources as those which have H-K_s>=0.9
and <=0.9 mag, respectively. We find that 5.8(1.5)% of the hard X-ray sources
have real infrared counterparts, of which 228(99) are red and 166(27) are blue.
The red counterparts are probably comprised of WR/O stars, HMXBs, and
symbiotics near the Galactic center. We also find that 39.4(1.0)% of the soft
X-ray sources have blue infrared counterparts; most of these are probably
coronally active dwarfs in the foreground. There is a noteworthy collection of
~20 red counterparts to hard X-ray sources near the Sagittarius-B H II region,
which are probably massive binaries that have formed within the last several
Myr. For each of the infrared matches to X-ray sources in our catalog we
derived the probability that the association is real, based on the results of
the cross-correlation analysis. The catalog will serve spectroscopic surveys to
identify infrared counterparts to X-ray sources near the Galactic center.Comment: Submitted to ApJ January 16, 2009; accepted July 21, 2009; 30 pages,
6 figure
On the interpretation of the multicolour disc model for black hole candidates
We present a critical analysis of the usual interpretation of the multicolour
disc model parameters for black hole candidates in terms of the inner radius
and temperature of the accretion disc. Using a self-consistent model for the
radiative transfer and the vertical temperature structure in a Shakura-Sunyaev
disc, we simulate the observed disc spectra, taking into account doppler
blurring and gravitational redshift, and fit them with multicolour models. We
show not only that such a model systematically underestimates the value of the
inner disc radius, but that when the accretion rate and/or the energy
dissipated in the corona are allowed to change the inner edge of the disc, as
inferred from the multicolour model, appears to move even when it is in fact
fixed at the innermost stable orbit.Comment: 4 pages including 2 figures, accepted for publication in MNRA
Near-infrared counterparts to Chandra X-ray sources toward the Galactic Center. II. Discovery of Wolf-Rayet stars and O supergiants
We present new identifications of infrared counterparts to the population of
hard X-ray sources near the Galactic center detected by the Chandra X-ray
Observatory. We have confirmed 16 new massive stellar counterparts to the X-ray
population, including nitrogen-type (WN) and carbon-type (WC) Wolf-Rayet stars,
and O supergiants. For the majority of these sources, the X-ray photometry is
consistent with thermal emission from plasma having temperatures in the range
of kT=1-8 keV or non-thermal emission having power-law indices in the range of
-1<gamma<3, and X-ray luminosities in the range of Lx~1e32-1e34 erg/s. Several
sources have exhibited X-ray variability of several factors between separate
observations. The X-ray properties are not a ubiquitous feature of single
massive stars but are typical of massive binaries, in which the high-energy
emission is generated by the collision of supersonic winds, or by accretion
onto a compact companion. However, the possibility of intrinsic hard X-ray
generation from single stars cannot be completely ruled out. The spectral
energy distributions of these sources exhibit significant infrared excess,
attributable to free-free emission from ionized stellar winds, supplemented by
hot dust emission in the case of the WC stars. With the exception of one object
located near the outer regions of the Quintuplet cluster, most of the new stars
appear isolated or in loose associations. Seven hydrogen-rich WN and O stars
are concentrated near the Sagittarius B HII region, while other similar stars
and more highly evolved hydrogen-poor WN and WC stars lie scattered within ~50
pc, in projection, of Sagitarrius A West. We discuss various mechanisms capable
of generating the observed X-rays and the implications these stars have for
massive star formation in the Galaxy's Central Molecular Zone.Comment: Accepted to ApJ on December 5, 2009. 61 pages, including 17 figure
Faint Infrared Flares from the Microquasar GRS 1915+105
We present simultaneous infrared and X-ray observations of the Galactic
microquasar GRS 1915+105 using the Palomar 5-m telescope and Rossi X-ray Timing
Explorer on July 10, 1998 UT. Over the course of 5 hours, we observed 6 faint
infrared (IR) flares with peak amplitudes of mJy and durations
of seconds. These flares are associated with X-ray
soft-dip/soft-flare cycles, as opposed to the brighter IR flares associated
with X-ray hard-dip/soft-flare cycles seen in August 1997 by Eikenberry et al.
(1998). Interestingly, the IR flares begin {\it before} the X-ray oscillations,
implying an ``outside-in'' origin of the IR/X-ray cycle. We also show that the
quasi-steady IR excess in August 1997 is due to the pile-up of similar faint
flares. We discuss the implications of this flaring behavior for understanding
jet formation in microquasars.Comment: 10 pages, 4 figures Accepted for publication in ApJ Letter
Discovery of X-ray eclipses from the transient source CXOGC J174540.0-290031 with XMM-Newton
We present the XMM-Newton observations obtained during four revolutions in
Spring and Summer 2004 of CXOGC J174540.0-290031, a moderately bright transient
X-ray source, located at only 2.9" from SgrA*. We report the discovery of sharp
and deep X-ray eclipses, with a period of 27,961+/-5 s and a duration of about
1,100+/-100 s, observed during the two consecutive XMM revolutions from August
31 to September 2. No deep eclipses were present during the two consecutive XMM
revolutions from March 28 to April 1, 2004. The spectra during all four
observations are well described with an absorbed power law continuum. While our
fits on the power law index over the four observations yield values that are
consistent with Gamma=1.6-2.0, there appears to be a significant increase in
the column density during the Summer 2004 observations, i.e. the period during
which the eclipses are detected. The intrinsic luminosity in the 2-10 keV
energy range is almost constant with 1.8-2.3 x 10^34 (d_8kpc)^2 erg/s over the
four observations. In the framework of eclipsing semidetached binary systems,
we show that the eclipse period constrains the mass of the assumed
main-sequence secondary star to less than 1.0 M_odot. Therefore, we deduce that
CXOGC J174540.0-290031 is a low-mass X-ray binary (LMXB). Moreover the eclipse
duration constrains the mass of the compact object to less than about 60
M_odot, which is consistent with a stellar mass black hole or a neutron star.
The absence of deep X-ray eclipses during the Spring 2004 observations could be
explained if the centroid of the X-ray emitting region moves from a position on
the orbital plane to a point above the compact object, possibly coincident with
the base of the jet which was detected in radio at this epoch. [Abstract
truncated].Comment: A&A, accepted for publication (10 pages, 8 figures, 2 Tables
Possible 38 day X-ray period of KS1731-260
We report the detection of a 38 day period in the X-ray flux of the transient
burster KS1731-260. The narrow peak of periodicity was detected during ~TJD
10150--11050 when the source had a high and relatively stable X-ray flux. After
\~TJD 11100 the source became strongly variable on a time scale of months that
contaminates the search for the 38 day periodicity. The detected period can not
be a binary period. The binary with Roche lobe overflow has in this case large
radii of the secondary and of the accretion disk. Disk and secondary star
illumination by X-ray flux from luminous neutron star would lead to high
infrared brightness of the binary. That clearly contradicts the infrared data
even for the brightest infrared sources within CHANDRA error box of KS1731-260.
Remaining possibility is that observed periodicity is connected with the
accretion disk precession, similar to that was observed for SS 433, Her X-1,
Cyg X-1 etc.Comment: 4 pages, 5 figures. Submitted to A&A Letter
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