348 research outputs found
Recurrent ~24 h Periods in RXTE ASM Data
Analysis of data from the Rossi X-ray Timing Explorer satellite's All Sky
Monitor instrument for several X-ray binary sources has identified a recurrent
\~24 h period. This period is sometimes highly significant, giving rise to the
possibility of it being identified as an orbital or super-orbital period.
Further analysis has revealed the same period in a number of other X-ray
sources. As a result this period has been discounted as spurious, described
variously as arising from daily variations in background levels and beating
between the sampling period and long-term secular trends in the light curves.
We present here an analysis of the spurious periods and show that the dominant
mechanism is in fact spectral leakage of low-frequency power present in the
light curves.Comment: 9 Pages, 10 figures, 1 table, submitted to PASA 20th December 2004.
Added 1 page of text and 3 figures to clarify results and discussion.
Resubmitted 16th May 2005. Accepted 25th June 200
Detection of Low-Hard State Spectral and Timing Signatures from the Black Hole X-Ray Transient XTE J1650-500 at Low X-Ray Luminosities
Using the Chandra X-ray Observatory and the Rossi X-ray Timing Explorer, we
have studied the black hole candidate (BHC) X-ray transient XTE J1650-500 near
the end of its 2001-2002 outburst after its transition to the low-hard state at
X-ray luminosities down to L = 1.5E34 erg/s (1-9 keV, assuming a source
distance of 4 kpc). Our results include a characterization of the spectral and
timing properties. At the lowest sampled luminosity, we used an 18 ks Chandra
observation to measure the power spectrum at low frequencies. For the 3 epochs
at which we obtained Chandra/RXTE observations, the 0.5-20 keV energy spectrum
is consistent with a spectral model consisting of a power-law with interstellar
absorption. We detect evolution in the power-law photon index from 1.66 +/-
0.05 to 1.93 +/- 0.13 (90% confidence errors), indicating that the source
softens at low luminosities. The power spectra are characterized by strong
(20-35% fractional rms) band-limited noise, which we model as a zero-centered
Lorentzian. Including results from an RXTE study of XTE J1650-500 near the
transition to the low-hard state by Kalemci et al. (2003), the half-width of
the zero-centered Lorentzian (roughly where the band-limited noise cuts off)
drops from 4 Hz at L = 7E36 erg/s (1-9 keV, absorbed) to 0.067 +/- 0.007 Hz at
L = 9E34 erg/s to 0.0035 +/- 0.0010 Hz at the lowest luminosity. While the
spectral and timing parameters evolve with luminosity, it is notable that the
general shapes of the energy and power spectra remain the same, indicating that
the source stays in the low-hard state. This implies that the X-ray emitting
region of the system likely keeps the same overall structure, while the
luminosity changes by a factor of 470. We discuss how these results may
constrain theoretical black hole accretion models.Comment: 11 pages, accepted by ApJ after minor revision
Low State of the X-ray Burster SLX 1732-304 in the Globular Cluster Terzan 1 According to RXTE Data
Observations of the X-ray burster SLX 1732-304 in the globular cluster Terzan
1 with the PCA/RXTE instrument in April 1997 are presented. The source was in a
low state; its flux in the standard X-ray band was half the flux recorded by
the ART-P/Granat telescope also during its low state. At the same time, its
spectrum was softer than the ART-P spectrum; it was well described by a power
law with a photon index of 2.3 without any evidence of a high-energy cutoff.Comment: 8 pages, 3 figures. Accepted to publication in Astronomy Letter
Evaluating Spectral Models and the X-ray States of Neutron-Star X-ray Transients
We propose a hybrid model to fit the X-ray spectra of atoll-type X-ray
transients in the soft and hard states. This model uniquely produces luminosity
tracks that are proportional to T^4 for both the accretion disk and boundary
layer. The model also indicates low Comptonization levels for the soft state,
gaining a similarity to black holes in the relationship between Comptonization
level and the strength of integrated rms variability in the power density
spectrum. The boundary layer appears small, with a surface area that is roughly
constant across soft and hard states. This result may suggestion that the NS
radius is smaller than its inner-most stable circular orbit.Comment: 15 pages, 15 figures, accepted for publication in the Ap
X-Ray Light Curves of Gamma-ray Bursts Detected with the All-Sky Monitor on RXTE
We present X-ray light curves (1.5-12 keV) for fifteen gamma-ray bursts
(GRBs) detected by the All-Sky Monitor on the Rossi X-ray Timing Explorer. We
compare these soft X-ray light curves with count rate histories obtained by the
high-energy (>12 keV) experiments BATSE, Konus-Wind, the BeppoSAX Gamma-Ray
Burst Monitor, and the burst monitor on Ulysses. We discuss these light curves
within the context of a simple relativistic fireball and synchrotron shock
paradigm, and we address the possibility of having observed the transition
between a GRB and its afterglow. The light curves show diverse morphologies,
with striking differences between energy bands. In several bursts, intervals of
significant emission are evident in the ASM energy range with little or no
corresponding emission apparent in the high-energy light curves. For example,
the final peak of GRB 970815 as recorded by the ASM is only detected in the
softest BATSE energy bands. We also study the duration of bursts as a function
of energy. Simple, singly-peaked bursts seem consistent with the E^{-0.5} power
law expected from an origin in synchrotron radiation, but durations of bursts
that exhibit complex temporal structure are not consistent with this
prediction. Bursts such as GRB 970828 that show many short spikes of emission
at high energies last significantly longer at low energies than the synchrotron
cooling law would predict.Comment: 15 pages with 20 figures and 2 tables. In emulateapj format. Accepted
by ApJ
A Giant Glitch in the Energetic 69 ms X-ray Pulsar AXS J161730-505505
We present new results on the recently discovered 69 ms X-ray pulsar AXS
J161730-505505, the sixth youngest sample of all known pulsars. We have
undertaken a comprehensive X-ray observing campaign of AXS J161730-505505 with
the ASCA, SAX, and XTE observatories and follow its long term spin-down history
between 1989 and 1999, using these, archival GINGA and ASCA data sets, and the
radio ephemeris. The spin-down is not simply described by a linear function as
originally thought, but instead we find evidence of a giant glitch (|Delta P/P|
> 10E-6) between 1993 August and 1997 September, perhaps the largest yet
observed from a young pulsar. The glitch is well described by steps in the
period and its first derivative accompanied by a persistent second derivative
similar to those in the Vela pulsar. The pulse profile of AXS J161730-505505
presents a single asymmetric peak which is maintained over all observation
epochs. The energy spectrum is also steady over time, characterized by a highly
absorbed power-law with a photon index 1.4 +/- 0.2, consistent with that found
for other young rotation powered pulsars.Comment: 6 pages with 2 figures, LaTex, emulateapj.sty. To appear in the
Astrophysical Journal Letter
On the Correlated X-ray and Optical Evolution of SS Cygni
We have analyzed the variability and spectral evolution of the prototype
dwarf nova system SS Cygni using RXTE data and AAVSO observations. A series of
pointed RXTE/PCA observations allow us to trace the evolution of the X-ray
spectrum of SS Cygni in unprecedented detail, while 6 years of optical AAVSO
and RXTE/ASM light curves show long-term patterns. Employing a technique in
which we stack the X-ray flux over multiple outbursts, phased according to the
optical light curve, we investigate the outburst morphology. We find that the
3-12 keV X-ray flux is suppressed during optical outbursts, a behavior seen
previously, but only in a handful of cycles. The several outbursts of SS Cygni
observed with the more sensitive RXTE/PCA also show a depression of the X-rays
during optical outburst. We quantify the time lags between the optical and
X-ray outbursts, and the timescales of the X-ray recovery from outburst. The
optical light curve of SS Cygni exhibits brief anomalous outbursts. During
these events the hard X-rays and optical flux increase together. The long-term
data suggest that the X-rays decline between outburst. Our results are in
general agreement with modified disk instability models (DIM), which invoke a
two-component accretion flow consisting of a cool optically thick accretion
disk truncated at an inner radius, and a quasi-spherical hot corona-like flow
extending to the surface of the white dwarf. We discuss our results in the
framework of one such model, involving the evaporation of the inner part of the
optically thick accretion disk, proposed by Meyer & Meyer-Hofmeister (1994).Comment: 24 pages, 8 figures, 2 tables, accepted for publication in Ap
The Galactic black hole transient H1743-322 during outburst decay: connections between timing noise, state transitions and radio emission
Multi-wavelength observations of Galactic black hole transients during
outburst decay are instrumental for our understanding of the accretion geometry
and the formation of outflows around black hole systems. H1743-322, a black
hole transient observed intensely in X-rays and also covered in the radio band
during its 2003 decay, provides clues about the changes in accretion geometry
during state transitions and also the general properties of X-ray emission
during the intermediate and the low-hard states. In this work, we report on the
evolution of spectral and temporal properties in X-rays and the flux in the
radio band with the goal of understanding the nature of state transitions
observed in this source. We concentrate on the transition from the thermal
dominant state to the intermediate state that occurs on a timescale of one day.
We show that the state transition is associated with a sudden increase in
power-law flux. We determine that the ratio of the power-law flux to the
overall flux in the 3--25 keV band must exceed 0.6 to observe strong timing
noise. Even after the state transition, once this ratio was below 0.6, the
system transited back to the thermal dominant state for a day. We show that the
emission from the compact radio core does not turn on during the transition
from the thermal dominant state to the intermediate state but does turn on when
the source reaches the low-hard state, as seen in 4U 1543-47 and GX 339-4. We
find that the photon index correlates strongly with the QPO frequency and
anti-correlates with the rms amplitude of variability. We also show that the
variability is more likely to be associated with the power-law emission than
the disk emission.Comment: 23 pages, 5 Figures, 1 Table, accepted for publication in Ap
Relativistic Iron Emission and Disk Reflection in Galactic Microquasar XTE J1748-288
We report evidence for an Fe K-alpha fluorescence line feature and disk
reflection in the Very High, High, and Low State X-ray spectra of the galactic
microquasar XTE J1748-288 during its June~1998 outburst. Spectral analyses are
made on data gathered throughout the outburst by the Rossi X-ray Timing
Explorer Proportional Counter Array. Gaussian line, relativistic disk emission
line, and ionized disk reflection models are fit to the data. In the Very High
State the line profile appears strongly redshifted, consistent with disk
emission from the innermost stable orbits around a maximally rotating Kerr
black hole. In the High State the line profile is less redshifted and
increasingly prominent. The Low State line profile is very strong (~0.5 keV
equivalent width) and centered at 6.7 +/- 0.10 keV; disk line emission model
fits indicate that the inner edge of the disk fluctuates between ~20 and ~100
gravitational radii in this state. The disk reflection fraction is traced
through the outburst; reflection from an ionized disk is preferred in the VHS
and HS, and reflection from a relatively neutral disk is preferred in the LS.
We discuss the implications of our findings for the binary system dynamics and
accretion flow geometry in XTE J1748-288.Comment: 32 pages, 8 figures, 5 tables. ApJ accepted 8/16/0
X-Ray Observations of V4641 SGR (= SAX J1819.3-2525) During the Brief and Violent Outburst of 2003
We present the results of detailed analysis of pointed X-ray observations by
RXTE PCA/HEXTE of the black hole X-ray binary (BHXRB) system V4641 Sgr (= SAX
J1819.3-2525) during its outburst of August 2003. Soft X-ray (3-20 keV) flux
variations by factors of 10 or more on timescales of minutes or shorter were
seen. The rapid and strong variability of this source sets it apart from
typical XRBs. In spite of large luminosity fluctuations, the spectral state of
the source did not change significantly during the dwells which suggests that
the physical emission processes did not change much during the observations.
The energy spectra during the dwells were dominated by a hard Comptonized
powerlaw component, indicative of the canonical low/hard state observed in
other BHXRBs. No soft thermal component was found in three out of the four RXTE
pointings. However spectral deconvolution of the observation with largest
average luminosity suggests an obscured, hot accretion disk. During one of the
observations we detected a short term (about 100s) soft X-ray dropout which is
apparently due to variability in the observed column density. Strong Fe
K fluorescent emisssion line near 6.5 keV was detected with large
equivalent widths in the range of 700 - 1000eV. In the temporal domain, the
Fourier power spectra were dominated by red noise below a few Hz. Poisson noise
dominated at higher frequencies and no high frequency features were detected.
The strong Comptonized spectra, broad iron emission line, absence of disk
component in the spectra, absence of any timing variability above few Hz and
occasional large changes in the column density along the line-of-sight, all
support an enshrouded black hole with occasional outflow and a very dynamic
environment.Comment: 27 pages, 10 figures (1 color figure), accepted for publication in
the Astrophysical Journal. It is tentatively scheduled for the ApJ 01
February 2006, v637, 2 issu
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