233 research outputs found
Outburst Morphology in the Soft X-ray Transient Aquila X-1
We present optical and near-IR (OIR) observations of the major outbursts of
the neutron star soft X-ray transient binary system Aquila X-1, from summer
1998 -- fall 2007. The major outbursts of the source over the observed timespan
seem to exhibit two main types of light curve morphologies, (a) the classical
Fast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-ray
transients and (b) the Low-Intensity State (LIS) where the
optical-to-soft-X-ray flux ratio is much higher than that seen during a FRED.
Thus there is no single correlation between the optical (R-band) and soft X-ray
(1.5-12 keV, as seen by the ASM onboard RXTE) fluxes even within the hard state
for Aquila X-1, suggesting that LISs and FREDs have fundamentally different
accretion flow properties. Time evolution of the OIR fluxes during the major
LIS and FRED outbursts is compatible with thermal heating of the irradiated
outer accretion disk. No signature of X-ray spectral state changes or any
compact jet are seen in the OIR, showing that the OIR color-magnitude diagram
(CMD) can be used as a diagnostic tool to separate thermal and non-thermal
radiation from X-ray binaries where orbital and physical parameters of the
system are reasonably well known. We suggest that the LIS may be caused by
truncation of the inner disk in a relatively high mass accretion state,
possibly due to matter being diverted into a weak outflow.Comment: Accepted for publication in ApJ. Uses aastex. 27 pages and 10 figure
Connecting GRBs and galaxies: the probability of chance coincidence
Studies of GRB host galaxies are crucial to understanding GRBs. However,
since they are identified by the superposition in the plane of the sky of a GRB
afterglow and a galaxy there is always a possibility that an association
represents a chance alignment, rather than a physical connection. We examine a
uniform sample of 72 GRB fields to explore the probability of chance
superpositions. There is typically a ~1% chance that an optical afterglow will
coincide with a galaxy by chance. While spurious host galaxy detections will,
therefore, be rare, the possibility must be considered when examining
individual GRB/host galaxy examples. It is also tempting to use the large and
uniform collection of X-ray afterglow positions to search for GRB-associated
galaxies. However, we find that approximately half of the 14 superpositions in
our sample are likely to occur by chance, so in the case of GRBs localized only
by an X-ray afterglow, even statistical studies are suspect.Comment: edited, accepted by Ap
IR Monitoring of the Microquasar GRS 1915+105: Detection of Orbital and Superhump Signatures
We present the results of seven years of K-band monitoring of the low-mass
X-ray binary GRS 1915+105. Positive correlations between the infrared flux and
the X-ray flux and X-ray hardness are demonstrated. Analysis of the frequency
spectrum shows that the orbital period of the system is
days. The phase and amplitude of the orbital modulation suggests that the
modulation is due to the heating of the face of the secondary star. We also
report another periodic signature between 31.2 and 31.6 days, most likely due
to a superhump resonance. From the superhump period we then obtain a range on
the mass ratio of the system, .Comment: 16 pages, 6 figures; v2: minor change
Period derivative of the M15 X-ray Binary AC211/X2127+119
We have combined Rossi X-ray Timing Explorer observations of X2127+119, the
low-mass X-ray binary in the globular cluster M15, with archival X-ray
lightcurves to study the stability of the 17.1 hr orbital period. We find that
the data cannot be fit by the Ilovaisky (1993) ephemeris, and requires either a
7sigma change to the period or a period derivative Pdot/P~9x10e-7 per year.
Given its remarkably low L_X/L_opt such a Pdot lends support to models that
require super-Eddington mass transfer in a q~1 binary.Comment: 11 pages, 3 figures, to be published in New Astronom
RXTE Spectral Observations of the 1996-97 Outburst of the Microquasar GRO J1655-40
Excellent coverage of the entire 16-month 1996-97 outburst cycle of GRO
J1655-40 was provided by RXTE. We present a full spectral analysis of these
data, which includes 52 PCA spectra from 2.5-20 keV and HEXTE spectra above 20
keV. We also include a nearly continuous ASM light curve with several intensity
measurements per day. The data are interpreted in the context of the multicolor
blackbody disk/power-law model. The source is observed in the very high,
high/soft, and low/hard outburst states. During the very high state, the source
exhibits intense hard flares on time scales of hours to days which are
correlated with changes in both the fitted temperature and radius of the inner
accretion disk. During the high/soft state, the spectrum is dominated by the
soft thermal emission from the accretion disk with spectral parameters that
suggest approximately constant inner disk radius and temperature. We find that
a tight relationship exists between the observed inner radius of the disk and
the flux in the power-law component. During intense hard flares, the inner disk
radius is observed to decrease by as much as a factor of three on a time scale
of days. The apparent decrease of the inner disk radius observed during the
flares may be due to the failure of the multicolor disk model caused by a
steepening of the radial temperature profile in the disk coupled with increased
spectral hardening and not physical changes of the inner disk radius. Assuming
that our spectral model is valid during periods of weak power-law emission, our
most likely value for the inner disk radius implies a* < 0.7. Such a low value
for the black hole angular momentum is inconsistent with the relativistic frame
dragging and the `diskoseismic' models as interpretations for the 300 Hz X-ray
QPO seen during some of these RXTE observations.Comment: 34 pages including 9 figures and 3 tables. Accepted for publication
in the Astrophysical Journal. Our interpretation of the data and the main
conclusions have been significantly revise
HST Observations of the Central-Cusp Globular Cluster NGC 6752. The Effect of Binary Stars on the Luminosity Function in the Core
We consider the effect of binary stars on the main-sequence luminosity
functions observed in the core of globular clusters, with specific reference to
NGC 6752. We find that mass segregation results in an increased binary fraction
at fainter magnitudes along the main-sequence. If this effect is not taken into
account when analyzing luminosity functions, erroneous conclusions can be drawn
regarding the distribution of single stars, and the dynamical state of the
cluster. In the core of NGC 6752, our HST data reveal a flat luminosity
function, in agreement with previous results. However, when we correct for the
increasing binary fraction at faint magnitudes, the LF begins to fall
immediately below the turn-off. This effect appears to be confined to the inner
core radius of the cluster.Comment: 10 pages, 3 figures Accepted to ApJ Lett Vol 513 Number
Complete Multiwavelength Evolution of Galactic Black Hole Transients During Outburst Decay II: Compact Jets and X-ray Variability Properties
We investigated the relation between compact jet emission and X-ray
variability properties of all black hole transients with multiwavelength
coverage during their outburst decays. We studied the evolution of all power
spectral components (including low frequency quasi-periodic oscillations), and
related this evolution to changes in jet properties tracked by radio and
infrared observations. We grouped sources according to their tracks in
radio/X-ray luminosity relation, and show that the standards show stronger
broadband X-ray variability than outliers at a given X-ray luminosity when the
compact jet turned on. This trend is consistent with the internal shock model
and can be important for the understanding of the presence of tracks in the
radio/X-ray luminosity relation. We also observed that the total and the QPO
rms amplitudes increase together during the earlier part of the outburst decay,
but after the compact jet turns either the QPO disappears or its rms amplitude
decreases significantly while the total rms amplitudes remain high. We discuss
these results with a scenario including a variable corona and a non-variable
disk with a mechanism for the QPO separate from the mechanism that create broad
components. Finally, we evaluated the timing predictions of the magnetically
dominated accretion flow model which can explain the presence of tracks in the
radio/X-ray luminosity relation.Comment: Accepted for publication by Ap
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