480 research outputs found
Faint X-ray Sources in the Globular Cluster Terzan 5
We report our analysis of a Chandra X-ray observation of the rich globular
cluster Terzan 5, in which we detect 50 sources to a limiting 1.0-6 keV X-ray
luminosity of 3*10^{31} ergs/s within the half-mass radius of the cluster.
Thirty-three of these have L_X>10^{32} ergs/s, the largest number yet seen in
any globular cluster. In addition to the quiescent low-mass X-ray binary (LMXB,
identified by Wijnands et al.), another 12 relatively soft sources may be
quiescent LMXBs. We compare the X-ray colors of the harder sources in Terzan 5
to the Galactic Center sources studied by Muno and collaborators, and find the
Galactic Center sources to have harder X-ray colors, indicating a possible
difference in the populations. We cannot clearly identify a metallicity
dependence in the production of low-luminosity X-ray binaries in Galactic
globular clusters, but a metallicity dependence of the form suggested by Jordan
et al. for extragalactic LMXBs is consistent with our data.Comment: 15 pages, 10 figures (3 color). Resubmitted to ApJ after
incorporating referee comments. v2: Added references to introductio
XMM-Newton Spectroscopy of the Accretion-Driven Millisecond X-ray Pulsar XTE J1751-305 in Outburst
We present an analysis of the first high-resolution spectra measured from an
accretion-driven millisecond X-ray pulsar in outburst. We observed XTE
J1751-305 with XMM-Newton on 2002 April 7 for approximately 35 ksec. Using a
simple absorbed blackbody plus power-law model, we measure an unabsorbed flux
of (6.6 +/- 0.1) * 10^(-10) erg/cm^2/s (0.5--10.0 keV). A hard power-law
component (Gamma = 1.44 +/- 0.01) contributes 83% of the unabsorbed flux in the
0.5-10.0 keV band, but a blackbody component (kT = 1.05 +/- 0.01 keV) is
required. We find no clear evidence for narrow or broad emission or absorption
lines in the time-averaged spectra, and the sensitivity of this observation has
allowed us to set constraining upper-limits on the strength of important
features. The lack of line features is at odds with spectra measured from some
other X-ray binaries which share some similarities with XTE J1751-305. We
discuss the implications of these findings on the accretion flow geometry in
XTE J1751-305.Comment: 5 pages, 3 figures (2 color). ApJ Letters, accepted. Uses
emulateapj.st
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
Chandra/HETGS Spectroscopy of the Galactic Black Hole GX 339-4: A Relativistic Iron Line and Evidence for a Seyfert-like Warm Absorber
We observed the Galactic black hole GX 339-4 with the Chandra High Energy
Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its
2002-2003 outburst. The sensitivity of this observation provides an
unprecedented glimpse of a Galactic black hole at about a tenth of the
luminosity of the outburst peak. The continuum spectrum is well described by a
model consisting of multicolor disk blackbody (kT = 0.6 keV) and power-law
(Gamma = 2.5) components. X-ray reflection models yield improved fits. A
strong, relativistic Fe K-alpha emission line is revealed, indicating that the
inner disk extends to the innermost stable circular orbit. The breadth of the
line is sufficient to suggest that GX 339-4 may harbor a black hole with
significant angular momentum. Absorption lines from H-like and He-like O, and
He-like Ne and Mg are detected, as well as lines which are likely due to Ne II
and Ne III. The measured line properties make it difficult to associate the
absorption with the coronal phase of the interstellar medium. A scenario
wherein the absorption lines are due to an intrinsic AGN-like warm-absorber
geometry -- perhaps produced by a disk wind in an extended disk-dominated state
-- may be more viable. We compare our results to Chandra observations of the
Galactic black hole candidate XTE J1650-500, and discuss our findings in terms
of prominent models for Galactic black hole accretion flows and connections to
supermassive black holes.Comment: 20 pages, 11 postscript figure files (many in color), uses
emulateapj.sty and apjfonts.sty, slightly expanded, accepted for publication
in Ap
Chandra and Swift observations of the quasi-persistent neutron star transient EXO 0748-676 back to quiescence
The quasi-persistent neutron star X-ray transient and eclipsing binary EXO
0748-676 recently started the transition to quiescence following an accretion
outburst that lasted more than 24 years. We report on two Chandra and twelve
Swift observations performed within five months after the end of the outburst.
The Chandra spectrum is composed of a soft, thermal component that fits to a
neutron star atmosphere model with kT^inf~0.12 keV, joined by a hard powerlaw
tail that contributes ~20% of the total 0.5-10 keV unabsorbed flux. The
combined Chandra/Swift data set reveals a relatively hot and luminous quiescent
system with a temperature of kT^inf~0.11-0.13 keV and a bolometric thermal
luminosity of ~8.1E33-1.6E34 (d/7.4 kpc)^2 erg/s. We discuss our results in the
context of cooling neutron star models.Comment: Accepted for publication in MNRAS Letters, moderate revision
according to referee report, added one plot to figure 2 and included new
Swift observations, 5 pages, 2 figure
Simultaneous Chandra and RXTE Spectroscopy of the Microquasar H~1743-322: Clues to Disk Wind and Jet Formation from a Variable Ionized Outflow
We observed the bright phase of the 2003 outburst of the Galactic black hole
candidate H 1743-322 in X-rays simultaneously with Chandra and RXTE on four
occasions. The Chandra/HETGS spectra reveal narrow, variable (He-like) Fe XXV
and (H-like) Fe XXVI resonance absorption lines. In the first observation, the
Fe XXVI line has a FWHM of 1800 +/- 400 km/s and a blue-shift of 700 +/- 200
km/s, suggesting that the highly ionized medium is an outflow. Moreover, the Fe
XXV line is observed to vary significantly on a timescale of a few hundred
seconds in the first observation, which corresponds to the Keplerian orbital
period at approximately 1 E+4 gravitational radii. Our models for the
absorption geometry suggest that a combination of geometric effects and
changing ionizing flux are required to account for the large changes in line
flux observed between observations, and that the absorption likely occurs at a
radius less than 1 E+4 radii for a 10 Msun black hole. Viable models for the
absorption geometry include cyclic absorption due to an accretion disk
structure, absorption in a clumpy outflowing disk wind, or possibly a
combination of these two. If the wind in H 1743-322 has unity filling factor,
the highest implied mass outflow rate is 20 percent of the Eddington mass
accretion rate. This wind may be a hot precursor to the Seyfert-like,
outflowing "warm absorber" geometries recently found in the Galactic black
holes GX 339-4 and XTE J1650-500. We discuss these findings in the context of
ionized Fe absorption lines found in the spectra of other Galactic sources, and
connections to warm absorbers, winds, and jets in other accreting systems.Comment: 18 pages, 7 figures, 5 in color, subm. to ApJ. Uses emulateapj.sty
and apjfonts.st
New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non
core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in
the cluster is now 43, the largest sample in any globular cluster so far. For
the identifications we used near-ultraviolet (NUV) and optical images from the
Hubble Space Telescope, in combination with X-ray results from the Chandra
X-ray Observatory. This allowed us to build the deepest NUV CV luminosity
function of the cluster to date. We found that the CVs in 47 Tuc are more
concentrated towards the cluster center than the main sequence turnoff stars.
We compared our results to the CV populations of the core-collapsed globular
clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per
unit mass than those two other clusters. That suggests that dynamical
interactions in core-collapsed clusters play a major role creating new CVs. In
47 Tuc, the CV population is probably dominated by primordial and old
dynamically formed systems. We estimated that the CVs in 47 Tuc have total
masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function
of the CVs in the three clusters is bimodal. Additionally, we discuss a
possible double degenerate system and an intriguing/unclassified object.
Finally, we present four systems that could be millisecond pulsar companions
given their X-ray and NUV/optical colors. For one of them we present very
strong evidence for being an ablated companion. The other three could be CO- or
He-WDs.Comment: Published on MNRAS. 31 Pages, 23 Figures, 5 Tables. Minor changes
with respect to previous arXiv versio
High frequency quasi-periodic oscillations in the black hole X-ray transient XTE J1650-500
We report the detection of high frequency variability in the black hole X-ray
transient XTE J1650-500. A quasi-periodic oscillation (QPO) was found at 250 Hz
during a transition from the hard to the soft state. We also detected less
coherent variability around 50 Hz, that disappeared when the 250 Hz QPO showed
up. There are indications that when the energy spectrum hardened the QPO
frequency increased from ~110 Hz to ~270 Hz, although the observed frequencies
are also consistent with being 1:2:3 harmonics of each other. Interpreting the
250 Hz as the orbital frequency at the innermost stable orbit around a
Schwarzschild black hole leads to a mass estimate of 8.2 Msun. The spectral
results by Miller et al.(2002, ApJ, 570, L69), which suggest considerable black
hole spin, would imply a higher mass.Comment: Submitted to ApJ, 12 pages including 2 figure
Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust
In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing
binary EXO 0748-676 started a transition from outburst to quiescence, after it
had been actively accreting for more than 24 years. In a previous work, we
discussed Chandra and Swift observations obtained during the first five months
after this transition. Here, we report on further X-ray observations of EXO
0748-676, extending the quiescent monitoring to 1.6 years. Chandra and
XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal
component that is well-fitted by a neutron star atmosphere model. An additional
hard powerlaw tail is detected that changes non-monotonically over time,
contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux.
The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal
bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the
inferred neutron star effective temperature decreases from ~124 to 109 eV. We
interpret the observed decay as cooling of the neutron star crust and show that
the fractional quiescent temperature change of EXO 0748-676 is markedly smaller
than observed for three other neutron star X-ray binaries that underwent
prolonged accretion outbursts.Comment: Moderate textual revisions according to referee report, accepted for
publication in MNRA
RXTE observations of the neutron star low-mass X-ray binary GX 17+2: correlated X-ray spectral and timing behavior
We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutron
star low-mass X-ray binary and Z source GX 17+2. A study was performed of the
properties of the noise components and quasi-periodic oscillations (QPOs) as a
function of the spectral properties, with the main goal to study the relation
between the frequencies of the horizontal branch and upper kHz QPOs. It was
found that when the upper kHz QPO frequency is below 1030 Hz these frequencies
correlate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the first
source in which this is observed. We also found that the frequency difference
of the high frequency QPOs was not constant and that the quality factors (Q
values) of the HBO, its second harmonic, and the kHz QPOs are similar, and vary
almost hand in hand by a factor of more than three. Observations of the normal
branch oscillations during two type I X-ray bursts showed that their absolute
amplitude decreased as the flux from the neutron star became stronger. We
discuss these and other findings in terms of models that have been proposed for
these phenomena. We also compare the behavior of GX 17+2 and other Z sources
with that of black hole sources and consider the possibility that the mass
accretion rate might not be driving force behind all spectral and variability
changes.Comment: 35 pages, including 14 figures. Accepted for publication in ApJ.
Revised discussion, one new figure, and some minor figure changes with
respect to old versio
- âŠ