86 research outputs found
Which Stars Form Black Holes and Neutron Stars?
I describe the current state of our knowledge of the mapping between the
initial masses of stars and the compact objects -- particularly neutron stars
and black holes -- that they produce. Most of that knowledge is theoretical in
nature, and relies on uncertain assumptions about mass loss through winds,
binary mass transfer, and the amount of mass ejected during a supernovae.
Observational constraints on the initial masses of stars that produce neutron
stars and black holes is scarce. They fall into three general categories: (1)
models of the stars that produced the supernova remnants associated with known
compact objects, (2) scenarios through with high mass X-ray binaries were
produced, and (3) associations between compact objects and coeval clusters of
stars for which the minimum masses of stars that have undergone supernovae are
known. I focus on the last category as the most promising in the near term. I
describe three highly-magnetized neutron stars that have been associated with
progenitors that had initial masses of 30\msun, and evaluate the prospects
of finding further associations between star clusters and compact objects.Comment: 8 pages, 3 figures. Uses aipproc.cls. To appear in the proceedings of
the conference``The Multicoloured Landscape of Compact Objects and their
Explosive Origins'', 2006 June 11--24, Cefalu, Sicily, to be published by AI
Millisecond Oscillations During Thermonuclear X-ray Bursts
I review the basic phenomenology and theory of the millisecond brightness
oscillations observed during thermonuclear X-ray bursts from 13 of
approximately 70 accreting neutron stars in low-mass X-ray binaries. Compelling
observations indicate that the oscillations are produced by surface brightness
patterns on the rapidly rotating neutron stars. However, it remains to be
understood (1) why the brightness patterns producing them persist for up to 15
s during an X-ray burst, whereas the burning should cover the entire surface in
less than 1 s, and (2) why the frequencies drift upward by about 5 Hz during
the course of the burst. These peculiarities can probably be explained by
taking into account the expansion of the surface layers caused by the burning,
zonal flows that form due to pressure gradients between the equator and poles,
and Rossby-Alfven modes that are excited in the surface ocean. Further progress
toward understanding how burning progresses on the surface of the neutron star
can be made with a next-generation X-ray timing mission, which would provide a
larger sample of sources with oscillations, detect sideband signals produced by
the spectrum of modes that should be excited in the neutron star ocean, and
measure harmonic structure in the profiles of the oscillations. These
observations would be crucial for measuring the distribution of the rotation
rates of neutron stars, the progression of unstable nuclear burning in the
accreted ocean, and the curvature of the space-time around the neutron star.Comment: Review article for "X-Ray Timing 2003: Rossi and Beyond", ed. P.
Kaaret, F. K. Lamb, & J. H. Swank (Melville, NY: American Institute of
Physics). 6 pages, including 5 figure
Mid-infrared emission from dust around quiescent low-mass X-ray binaries
We report the discovery of excess 4.5 and 8 μm emission from three quiescent black hole low-mass X-ray binaries, A0620−00, GS 2023+338, and XTE J1118+480, and the lack of similar excess emission from Cen X-4. The mid-infrared emission from GS 2023+338 probably originates in the accretion disk. However, the excess emission from A0620−00 and XTE J1118+480 is brighter and peaks at longer wavelengths, and thus most likely originates from circumbinary dust that is heated by the light of the secondary star. For these two sources, we find that the inner edges of the dust distributions lie near 1.7 times the binary separations, which are the minimum radii at which circumbinary disks would be stable against tidal disruption. The excesses are weak at 24 μm, which implies that the dust does not extend beyond about 3 times the binary separations. The total masses of circumbinary material are between 10^22 and 10^24 g. The material could be the remains of fallback disks produced in supernovae, or material from the companions injected into circumbinary orbits during mass transfer
Discovery of hot supergiant stars near the Galactic center
We report new results of a campaign to find Wolf-Rayet and O (WR/O) stars and
high-mass X-ray binaries (HMXBs) in the Galactic center. We searched for
candidates by cross-correlating the 2MASS catalog with a deep Chandra catalog
of X-ray point sources in the Radio Arches region. Following up with K-band
spectroscopy, we found two massive stellar counterparts to CXOGC
J174555.3-285126 and CXOGC J174617.0-285131, which we classify as a broad-lined
WR star of sub-type WN6b and an O Ia supergiant, respectively. Their X-ray
properties are most consistent with those of known colliding-wind binaries in
the Galaxy and the Large Magellanic Cloud, although a scenario involving
low-rate accretion onto a compact object is also possible. The O Ia star lies
4.4 pc in projection from the Quintuplet cluster, and has a radial velocity
consistent with that of the Quintuplet, suggesting that this star might have
escaped from the cluster. We also present the discovery of a B2 Ia supergiant,
which we identified as a candidate massive star using 8 micron Spitzer maps of
the Galactic center in a region near the known massive X-ray-emitting star
CXOGC J174516.1-290315. We discuss the origin of these stars in the context of
evolving stellar clusters in the Galactic center.Comment: 21 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Discovery of a 270 Hz X-Ray Burst Oscillation in the X-Ray Dipper 4U 1916-053
We report the discovery of a highly coherent oscillation in a type-I X-ray
burst observed from 4U 1916-053 by the Rossi X-ray Timing Explorer (RXTE). The
oscillation was most strongly detected approx. 1 s after the burst onset at a
frequency of 269.3 Hz, and it increased in frequency over the following 4
seconds of the burst decay to a maximum of around 272 Hz. The total measured
drift of 3.58 +/- 0.41 Hz (1 sigma) represents the largest fractional change in
frequency (1.32 +/- 0.15 %) yet observed in any burst oscillation. If the
asymptotic frequency of the oscillation is interpreted in terms of a decoupled
surface burning layer, the implied neutron star spin period is around 3.7 ms.
However, the expansion of the burning layer required to explain frequency drift
during the burst is around 80 m, substantially larger than expected
theoretically (assuming rigid rotation). The oscillation was not present in the
persistent emission before the burst, nor in the initial rise. When detected
its amplitude was 6-12% (RMS) with a roughly sinusoidal profile. The burst
containing the oscillation showed no evidence for photospheric radius
expansion, while at least 5 of the other 9 bursts observed from the source by
RXTE during 1996 and 1998 did. No comparable oscillations were detected in the
other bursts. A pair of kilohertz quasi-periodic oscillations (QPOs) has been
previously reported from this source with a mean separation of 348 +/- 12 Hz.
4U 1916-053 is the first example of a source where the burst oscillation
frequency is significantly smaller than the frequency separation of the kHz
QPOs.Comment: 8 pages, 2 figures, 2 tables; accepted for ApJ Letter
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
Massive Stellar X-ray Sources in the Galactic Center
We present results of a spectroscopic survey of bright near-infrared
counterparts to X-ray point sources from a deep Chandra survey of the Galactic nuclear bulge. K-band spectroscopy has revealed 13 new Wolf-Rayet and
O-supergiant counterparts to Chandra sources in the Galactic center (GC). Although they are systematically softer in X-rays than the general GC source population of accretion powered cataclysmic variables (CVs), their X-ray colors indicate a hard component consistent with emission from plasmas with E > 2 keV. Such hard X-ray emission is not ubiquitous among single Wolf-Rayet and O stars,
but is common among Wolf-Rayet+OB binaries with colliding supersonic winds.
Although we regard colliding-wind binary hypothesis as the most likely scenario,
it remains possible that several of these objects are wind-accreting neutron stars
or black holes in supergiant high-mass X-ray binaries, or extraordinary single
stars emitting hard X-rays
- …