1,017 research outputs found
The faint 2005 hard state outburst of Aquila X-1 seen by INTEGRAL and RXTE
We report on the spectral analysis of RXTE and INTEGRAL data of the 2005
April outburst of the transient Atoll source Aql X-1. Although this outburst is
one of the faintest ever detected for this source in the soft X-rays
(RXTE/ASM), one of our INTEGRAL observations, taken close to the soft X-ray
peak, shows that the source flux was quite high, with a 20-200 keV flux of 2.05
x 10^-9 erg cm^-2 s^-1. On this occasion we detect the source up to 150 keV for
the first time. We compare and discuss the similarity of the source behavior
with that of black hole transients especially XTE J1550-564.Comment: 4 pages 2 figures, Accepted for publication in A&A as a RESEARCH NOT
Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton
We have identified four new black hole candidates in M31 globular clusters
using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The
X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the
highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts
from the neutron star transients Aql X-1 or 4U 1608-452; the outburst
apparently started ~45 days earlier than the observed peak, hence the
luminosity could have been considerably higher. We identified XB082, XB153 and
XB185 as BHCs by observing "low state" emission spectra at luminosities that
exceed the threshold for neutron star binaries. The probability that these are
neutron star systems with anisotropic emission beamed toward us is <4E-4, and
their variability suggests emission from a single source. We therefore conclude
that these systems likely contain black holes rather than neutron stars. We
have now identified 4 persistently bright BHCs in the region; the probability
that these are all background AGN is <1E-20. According to theory, the donors
could be tidally captured main sequence stars, or white dwarves in
ultra-compact binaries. We find that GCs that are particularly massive (XB082)
or metal rich (XB144) can host bright X-ray sources in addition to those that
are both (XB163). Our method may reveal BHCs in other bright X-ray sources.Comment: Accepted for publication in ApJ. 17 pages, 5 figure
State transitions in the 2001/2002 outburst of XTE J1650-500
We present a study of the X-ray transient and black hole candidate XTE
J1650-500 during its 2001/2002 outburst. The source made two state transitions
between the hard and soft states, at luminosity levels that differed by a
factor of ~5-10. The first transition, between hard and soft, lasted for ~30
days and showed two parts; one part in which the spectral properties evolve
smoothly away from the hard state and another that we identify as the 'steep
power law state'. The two parts showed different behavior of the Fe K emission
line and QPO frequencies. The second transition, from soft to hard, lasted only
\~15 days and showed no evidence of the presence of the 'steep power law
state'. Comparing observations from the early rise and the decay of the
outburst, we conclude that the source can be in the hard state in a range of
more 10^4 in luminosity. We briefly discuss the state transitions in the
framework of a two-flow model.Comment: 4 pages, 5 figures. To appear in Proc. of the II BeppoSAX Meeting:
"The Restless High-Energy Universe" (Amsterdam, May 5-8, 2003), E.P.J. van
den Heuvel, J.J.M. in 't Zand, and R.A.M.J. Wijers Ed
Asymmetry Effects on Nuclear Fragmentation
We show the possibility of extracting important information on the symmetry
term of the Equation of State () directly from multifragmentation
reactions using stable isotopes with different charge asymmetries. We study
n-rich and n-poor collisions at using a new stochastic
transport approach with all isospin effects suitably accounted for. For central
collisions a chemical component in the spinodal instabilities is clearly seen.
This effect is reduced in the neck fragmentation observed for semiperipheral
collisions, pointing to a different nature of the instability. In spite of the
low asymmetry tested with stable isotopes the results are showing an
interesting and promising dependence on the stiffness of the symmetry term,
with an indication towards an increase of the repulsion above normal density.Comment: 8 pages (Latex), 7 Postscript figures, CRIS2000 Conference,
Acicastello, Italy, May 22-26, (2000), Nucl. Phys. A (in press
Higher Order Variability Properties of Accreting Black Holes
To better constrain the emission mechanism underlying the hard state of
galactic black hole candidates, we use high-time resolution RXTE lightcurves
for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for
these objects, the skewness and the Fourier bispectrum. Similar analyses, in
particular using the skewness measure, have been attempted before, but the
photon collection area of RXTE allows us to present results of much greater
statistical significance. The results for the two objects are qualitatively
similar, reinforcing the idea that the same basic mechanisms are at work in
both. We find a significantly positive skewness for variability timescales less
than second, and a {\it negative} skewness for timescale sec.
Such a skewness pattern cannot be reproduced by the simplest shot variability
models where individual shots have a fixed profile and intensity and are
uncorrelated in time. Further evidence against simple shot models comes from
the significant detection of a non-zero bicoherence for Fourier periods sec, implying that significant coupling does exist between variations
on these timescales. We discuss how current popular models for variability in
black hole systems can be modified to match these observations. Using simulated
light curves, we suggest that the most likely way to reproduce this observed
behavior is to have the variability come in groups of many shots, with the
number of shots per unit time fitting an envelope function which has a rapid
rise and slow decay, while the individual shots have a slow rise and a rapid
decay. Invoking a finite energy reservoir that is depleted by each shot is a
natural way of producing the required shot correlations.Comment: Accepted to MNRAS, 10 pages, 7 figures, substantially improved
discussion of errors, minor changes to interpretation sectio
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