187 research outputs found
On the Lack of Type I X-ray Bursts in Black Hole X-ray Binaries: Evidence for the Event Horizon?
Type I X-ray bursts are very common in neutron star X-ray binaries, but no
Type I burst has been seen in the dozen or so binaries in which the accreting
compact star is too massive to be a neutron star and therefore is identified as
a black hole candidate. We have carried out a global linear stability analysis
of the accumulating fuel on the surface of a compact star to identify the
conditions under which thermonuclear bursts are triggered. Our analysis, which
improves on previous calculations, reproduces the gross observational trends of
bursts in neutron star systems. It further shows that, if black hole candidates
have surfaces, they would very likely exhibit instabilities similar to those
that lead to Type I bursts on neutron stars. The lack of bursts in black hole
candidates is thus significant, and indicates that these objects have event
horizons. We discuss possible caveats to this conclusion.Comment: 11 pages, 1 figure, to appear in 1 August 2002 edition of
Astrophysical Journal Letters, significant changes to the methods, results
unchange
X-ray Dust Scattering at Small Angles: The Complete Halo around GX13+1
The exquisite angular resolution available with Chandra should allow
precision measurements of faint diffuse emission surrounding bright sources,
such as the X-ray scattering halos created by interstellar dust. However, the
ACIS CCDs suffer from pileup when observing bright sources, and this creates
difficulties when trying to extract the scattered halo near the source. The
initial study of the X-ray halo around GX13+1 using only the ACIS-I detector
done by Smith, Edgar & Shafer (2002) suffered from a lack of sensitivity within
50'' of the source, limiting what conclusions could be drawn.
To address this problem, observations of GX13+1 were obtained with the
Chandra HRC-I and simultaneously with the RXTE PCA. Combined with the existing
ACIS-I data, this allowed measurements of the X-ray halo between 2-1000''.
After considering a range of dust models, each assumed to be smoothly
distributed with or without a dense cloud along the line of sight, the results
show that there is no evidence in this data for a dense cloud near the source,
as suggested by Xiang et al. (2005). Finally, although no model leads to
formally acceptable results, the Weingartner & Draine (2001) and nearly all of
the composite grain models from Zubko, Dwek & Arendt (2004) give poor fits.Comment: 8 pages, 6 figures, accepted for publication in Ap
Anti-phase Modulation of Electron- and Hole-like States in Vortex Core of Bi2Sr2CaCu2Ox Probed by Scanning Tunneling Spectroscopy
In the vortex core of slightly overdoped Bi2Sr2CaCu2Ox, the electron-like and
hole-like states have been found to exhibit spatial modulations in anti-phase
with each other along the Cu-O bonding direction. Some kind of
one-dimensionality has been observed in the vortex core, and it is more clearly
seen in differential conductance maps at lower biases below +-9 mV
Lithium production on a low-mass secondary in a black hole soft X-ray transient
We examine production of Li on the surface of a low-mass secondary in a black
hole soft X-ray transient (BHSXT) through the spallation of CNO nuclei by
neutrons which are ejected from a hot (> 10 MeV) advection-dominated accretion
flow (ADAF) around the black hole. Using updated binary parameters, cross
sections of neutron-induced spallation reactions, and mass accretion rates in
ADAF derived from the spectrum fitting of multi-wavelength observations of
quiescent BHSXTs, we obtain the equilibrium abundances of Li by equating the
production rate of Li and the mass transfer rate through accretion to the black
hole. The resulting abundances are found to be in good agreement with the
observed values in seven BHSXTs. We note that the abundances vary in a
timescale longer than a few months in our model. Moreover, the isotopic ratio
Li6/Li7 is calculated to be about 0.7--0.8 on the secondaries, which is much
higher than the ratio measured in meteorites. Detection of such a high value is
favorable to the production of Li via spallation and the existence of a hot
accretion flow, rather than an accretion disk corona system in quiescent BHSXT.Comment: 4 pages, 3 figures, and 2 tables, submitted to Astrophyscal Jounal
Letter
Geometrical Distance Determination using Type I X-ray Bursts
With the excellent angular resolution of the Chandra X-ray Observatory, it is
possible to geometrically determine the distance to variable Galactic sources,
based on the phenomenon that scattered radiation appearing in the X-ray halo
has to travel along a slightly longer path than the direct, unscattered
radiation. By measuring the delayed variability, constraints on the source
distance can be obtained if the halo brightness is large enough to dominate the
point spread function (PSF) and to provide sufficient statistics. The distance
to Cyg X-3, which has a quasi-sinusoidal light curve, has been obtained with
this approach by Predehl et al. Here we examine the feasibility of using the
delayed signature of type I X-ray bursts as distance indicators. We use
simulations of delayed X-ray burst light curves in the halo to find that the
optimal annular region and energy band for a distance measurement with a
grating observation is roughly 10-50" and 1-5 keV respectively, assuming
Chandra's effective area and PSF, uniformly distributed dust, the input
spectrum and optical depth to GX 13+1, and the Weingartner & Draine
interstellar grain model. We find that the statistics are dominated by Poisson
noise rather than systematic uncertainties, e.g., the PSF contribution to the
halo. Using Chandra, a distance measurement to such a source at 4 (8) kpc could
be made to about 23% (30%) accuracy with a single burst with 68% confidence. By
stacking many bursts, a reasonable estimate of systematic errors limit the
distance measurement to about 10% accuracy.Comment: 7 pages, 4 figures; Accepted for publication in Ap
On the enigmatic X-ray Source V1408 Aql (=4U 1957+11)
Models for the characteristically soft X-ray spectrum of the compact X-ray
source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization
to multicolor accretion disk models. We critically examine the X-ray spectrum
of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics
(ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray
Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray
spectral models to these data, we favor an interpretation of the X-ray spectrum
as being due to an accretion disk viewed at large inclination angles. Evidence
for this hypothesis includes long term (117 day, 235 day, 352 day)
periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as
being due to a warped precessing disk, and a 1 keV feature in the ASCA data,
which we interpret as being the blend of L fluorescence features from a disk
atmosphere or wind. We also present timing analysis of the RXTE data and find
upper limits of 4% for the root mean square (rms) variability between
f=0.001-16 Hz. The situation of whether the compact object is a black hole or
neutron star is still ambiguous; however, it now seems more likely that an
X-ray emitting, warped accretion disk is an important component of this system.Comment: High Frequency Power Spectrum corrected for unflagged `data dropouts'
(described in Appendix) and correct upper limits for variability presented.
All energy spectra and long term variability sections unchanged. Additional
references and acknowledgements added. 13 pages in emulateapj.st
P-Process Nucleosynthesis inside Supernova-Driven Supercritical Accretion Disks
We investigate p-process nucleosynthesis in a supercritical accretion disk
around a compact object of 1.4 M_solar, using the self-similar solution of an
optically thick advection dominated flow. Supercritical accretion is expected
to occur in a supernova with fallback material accreting onto a new-born
compact object. It is found that appreciable amounts of p-nuclei are
synthesized via the p-process in supernova-driven supercritical accretion disks
(SSADs) when the accretion rate m_dot = M_dot c^2/(16 L_Edd) >10^5, where L_Edd
is the Eddington luminosity. Abundance profiles of p-nuclei ejected from SSADs
have similar feature to those of the oxygen/neon layers in Type II supernovae
when the abundance of the fallback gas far from the compact object is that of
the oxygen/neon layers in the progenitor. The overall abundance profile is in
agreement with that of the solar system. Some p-nuclei, such as Mo, Ru, Sn, and
La, are underproduced in the SSADs as in Type II supernovae. If the fallback
gas is mixed with a small fraction of proton through Rayleigh-Taylor
instability during the explosion, significant amounts of Mo92 are produced
inside the SSADs. Ru96 and La138 are also produced when the fallback gas
contains abundant proton though the overall abundance profile of p-nuclei is
rather different from that of the solar system. The p-process nucleosynthesis
in SSADs contributes to chemical evolution of p-nuclei, in particular Mo92, if
several percents of fallback matter are ejected via jets and/or winds.Comment: 15 pages, 7 figures included, 3 tables, LaTeX emulateapj5.sty,
accepted for publication by the Astronomical Journal (March, 2003
Thermonuclear Stability of Material Accreting onto a Neutron Star
We present a global linear stability analysis of nuclear fuel accumulating on
the surface of an accreting neutron star and we identify the conditions under
which thermonuclear bursts are triggered. The analysis reproduces all the
recognized regimes of hydrogen and helium bursts, and in addition shows that at
high accretion rates, near the limit of stable burning, there is a regime of
``delayed mixed bursts'' which is distinct from the more usual ``prompt mixed
bursts.'' In delayed mixed bursts, a large fraction of the fuel is burned
stably before the burst is triggered. Bursts thus have longer recurrence times,
but at the same time have somewhat smaller fluences. Therefore, the parameter
alpha, which measures the ratio of the energy released via accretion to that
generated through nuclear reactions in the burst, is up to an order of
magnitude larger than for prompt bursts. This increase in alpha near the
threshold of stable burning has been seen in observations. We explore a wide
range of mass accretion rates, neutron star radii and core temperatures, and
calculate a variety of burst properties. From a preliminary comparison with
data, we suggest that bursting neutron stars may have hot cores, with T_{core}
>~ 10^{7.5} K, consistent with interior cooling via the modified URCA or
similar low-efficiency process, rather than T_{core} ~ 10^7 K, as expected for
the direct URCA process. There is also an indication that neutron star radii
are somewhat small <~ 10 km. Both of these conclusions need to be confirmed by
comparing more careful calculations with better data.Comment: 67 pages, 19 figures, final version to appear in The Astrophysical
Journal, vol. 599, no. 1, Dec. 10, 200
Peculiar spectral and power spectral behaviour of the LMXB GX 13+1
We present results of an analysis of all 480 ks of Rossi X-ray Timing
Explorer Proportional Counter Array data obtained from 17 May 1998 to 11
October 1998 on the luminous low mass X-ray binary GX 13+1. We analysed the
spectral properties in colour-colour diagrams (CDs) and hardness-intensity
diagrams (HIDs) and fitted the power spectra with a multi-Lorentzian model. GX
13+1 traces out a curved track in the CDs on a time scale of hours, which is
very reminiscent of a standard atoll track containing an island, and lower and
upper banana branch. However, both count rate and power spectral properties
vary along this track in a very unusual way, not seen in any other atoll or Z
source. The count rate, which varied by a factor of ~1.6, along a given track
first decreases and then increases, causing the motion through the HIDs to be
in the opposite sense to that in the CD, contrary to all other Z and atoll
sources. Along a CD track, the very low frequency noise uniquely decreases in
amplitude from ~5 to ~2% (rms). The high frequency noise amplitude decreases
from ~4% to less than 1% and its characteristic frequency decreases from ~10 to
\~5 Hz. The 57-69 Hz quasi-periodic oscillation (QPO) found earlier is also
detected, and no kHz QPOs are found. In addition the entire track shows secular
motion on a time scale of about a week. The average count rate as well as the
amplitude of the very low frequency noise correlate with this secular motion.
We discuss a possible explanation for the peculiar properties of GX 13+1 in
terms of an unusual orientation or strength of a relativistic jet.Comment: 15 pages, 13 figures. Accepted for publication in A&
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