2,192 research outputs found
Broad Band X-ray Spectra of M31 Sources with BeppoSAX
We present the first spectral study of the X-ray emitting stellar sources in
M31 in the energy band from 0.1 to 10 keV. We find that the globular cluster
sources have spectral characteristics consistent with those of the Milky Way
object, namely that the spectrum can be described by a thermal model with 6-20
keV from 2 to 10 keV. Evidence of high absorption in some of these sources is
most likely an indication that they lie in or behind the HI ring in the disk of
the galaxy.
We also find one peculiar globular cluster source, with spectral
characteristics more typically associated with either High Mass X-ray Binaries
or black hole candidates. We therefore suggest that either the source has been
wrongly identified with a globular cluster or that the system contains a Black
Hole. We confirm earlier report that the spectrum of the bulge of M31 is
consistent with the superposition of many LMXB spectra. It is likely that a
large fraction of the 15-30 keV detection obtained from the PDS instrument is
associated with the bulge, thus extending the spectral data for this complex of
sources up to 30 keV. The high energy part of the spectrum can be parameterized
with typical LMXB spectra, while at low energies an additional component is
required.Comment: Accepted for publication in A&A, main journa
The viscous disk of GRS 1915+105
GRS 1915+105, one of the two known galactic microquasars, shows an extremely
complex variability in the X-ray band, comparable to no other X-ray source in
the sky. Making use of RXTE/PCA data, we have analyzed the X-ray spectral
distribution throughout the variability. We find that all variations can be
attributed to the rapid appearance and disappearance of the inner region of an
optically-thick accretion disk. Since the time scale for each event is related
to the maximum radius of the disappearing region, the difference in time
structure is due to the time distribution of such radii. The observed relation
between the extent of the missing inner region of the disk and the duration of
an event is in remarkable agreement with the expected radius dependence of the
viscous time scale of the radiation-dominated region of an accretion disk.Comment: 6 pages, 3 figures. To appear in The Active X-ray Sky: Results from
BeppoSAX and Rossi-XTE, Nuclear Physics B Proceedings Supplements, L. Scarsi,
H. Bradt, P. Giommi and F. Fiore (eds.), Elsevier Science B.
Sub-Subgiants in the Old Open Cluster M67?
We report the discovery of two spectroscopic binaries in the field of the old
open cluster M67 -- S1063 and S1113 -- whose positions in the color-magnitude
diagram place them approximately 1 mag below the subgiant branch. A ROSAT study
of M67 independently discovered these stars to be X-ray sources. Both have
proper-motion membership probabilities greater than 97%; precise center-of-mass
velocities are consistent with the cluster mean radial velocity. S1063 is also
projected within one core radius of the cluster center. S1063 is a single-lined
binary with a period of 18.396 days and an orbital eccentricity of 0.206. S1113
is a double-lined system with a circular orbit having a period of 2.823094
days. The primary stars of both binaries are subgiants. The secondary of S1113
is likely a 0.9 Mo main-sequence star, which implies a 1.3 Mo primary star. We
have been unable to explain securely the low apparent luminosities of the
primary stars; neither binary contain stars presently limited in radius by
their Roche lobes. We speculate that S1063 and S1113 may be the products of
close stellar encounters involving binaries in the cluster environment, and may
define alternative stellar evolutionary tracks associated with mass-transfer
episodes, mergers, and/or dynamical stellar exchanges
Revisiting the Thermal Stability of Radiation-dominated Thin Disks
The standard thin disk model predicts that when the accretion rate is over a
small fraction of the Eddington rate, which corresponds to L \ga 0.06
L_{Edd}, the inner region of the disk is radiation-pressure-dominated and
thermally unstable. However, observations of the high/soft state of black hole
X-ray binaries with luminosity well within this regime (0.01L_{Edd} \la L \la
0.5L_{Edd}) indicate that the disk has very little variability, i.e., quite
stable. Recent radiation magnetohydrodynamic simulations of a vertically
stratified shearing box have confirmed the absence of the thermal instability.
In this paper, we revisit the thermal stability by linear analysis, taking into
account the role of magnetic field in the accretion flow. By assuming that the
field responses negatively to a positive temperature perturbation, we find that
the threshold of accretion rate above which the disk becomes thermally unstable
increases significantly compared with the case of not considering the role of
magnetic field. This accounts for the stability of the observed sources with
high luminosities. Our model also presents a possible explanation as to why
only GRS 1915+105 seems to show thermally unstable behavior. This peculiar
source holds the highest accretion rate (or luminosity) among the known high
state sources, which is well above the accretion rate threshold of the
instability.Comment: 13 pages, 2 figures, accepted by Ap
The Nature and Cause of Spectral Variability in LMC X-1
We present the results of a long-term observation campaign of the
extragalactic wind-accreting black-hole X-ray binary LMC X-1, using the
Proportional Counter Array on the Rossi X-Ray Timing Explorer (RXTE). The
observations show that LMC X-1's accretion disk exhibits an anomalous
temperature-luminosity relation. We use deep archival RXTE observations to show
that large movements across the temperature-luminosity space occupied by the
system can take place on time scales as short as half an hour. These changes
cannot be adequately explained by perturbations that propagate from the outer
disk on a viscous timescale. We propose instead that the apparent disk
variations reflect rapid fluctuations within the Compton up-scattering coronal
material, which occults the inner parts of the disk. The expected relationship
between the observed disk luminosity and apparent disk temperature derived from
the variable occultation model is quantitatively shown to be in good agreement
with the observations. Two other observations support this picture: an inverse
correlation between the flux in the power-law spectral component and the fitted
inner disk temperature, and a near-constant total photon flux, suggesting that
the inner disk is not ejected when a lower temperature is observed.Comment: 35 pages, 10 figures, to be published in Ap
The faint "heartbeats" of IGR J17091-3624: an exceptional black-hole candidate
We report on the first 180 days of RXTE observations of the outburst of the
black hole candidate IGR J17091-3624. This source exhibits a broad variety of
complex light curve patterns including periods of strong flares alternating
with quiet intervals. Similar patterns in the X-ray light curves have been seen
in the (up to now) unique black hole system GRS 1915+105. In the context of the
variability classes defined by Belloni et al. (2000) for GRS 1915+105, we find
that IGR J17091-3624 shows the \nu, \rho, \alpha, \lambda, \beta and \mu
classes as well as quiet periods which resemble the \chi class, all occurring
at 2-60 keV count rate levels which can be 10-50 times lower than observed in
GRS 1915+105. The so-called \rho class "heartbeats" occur as fast as every few
seconds and as slow as ~100 seconds, tracing a loop in the hardness-intensity
diagram which resembles that previously seen in GRS 1915+105. However, while
GRS 1915+105 traverses this loop clockwise, IGR J17091-3624 does so in the
opposite sense. We briefly discuss our findings in the context of the models
proposed for GRS 1915+105 and find that either all models requiring near
Eddington luminosities for GRS 1915+105-like variability fail, or IGR
J17091-3624 lies at a distance well in excess of 20 kpc or, it harbors one of
the least massive black holes known (< 3 M_sun).Comment: 7 pages, 5 figures, published on ApJ Letter
Multiwavelength spectral evolution during the 2011 outburst of the very faint X-ray transient Swift J1357.2-0933
We report our multiwavelength study of the 2011 outburst evolution of the
newly discovered black hole candidate X-ray binary Swift J1357.2-0933. We
analysed the Swift X-ray telescope and Ultraviolet/Optical telescope (UVOT)
data taken during the ~7 months duration of the outburst. It displayed a 2-10
keV X-ray peak luminosity of ~1E35(D/1.5 kpc)^2 erg s-1 which classifies the
source as a very faint X-ray transient. We found that the X-ray spectrum at the
peak was consistent with the source being in the hard state, but it softened
with decreasing luminosity, a common behaviour of black holes at low
luminosities or returning to quiescence from the hard state. The correlations
between the simultaneous X-ray and ultraviolet/optical data suggest a system
with a black hole accreting from a viscous disc that is not irradiated. The
UVOT filters provide the opportunity to study these correlations up to
ultraviolet wavelengths a regime so far unexplored. If the black hole nature is
confirmed, Swift J1357.2-0933 would be one of the very few established black
hole very-faint X-ray transients.Comment: 6 pages, 2 tables, 5 figures. Accepted by MNRA
Accretion Disc Wind Variability in the States of the Microquasar GRS 1915+105
Continuing our study of the role and evolution of accretion disc winds in the
microquasar GRS 1915+105, we present high-resolution spectral variability
analysis of the beta and gamma states with the Chandra High Energy Transmission
Grating Spectrometer. By tracking changes in the absorption lines from the
accretion disc wind, we find new evidence that radiation links the inner and
outer accretion discs on a range of time-scales. As the central X-ray flux
rises during the high-luminosity gamma state, we observe the progressive
over-ionization of the wind. In the beta state, we argue that changes in the
inner disc leading to the ejection of a transient 'baby jet' also quench the
highly-ionized wind from the outer disc. Our analysis reveals how the state,
structure, and X-ray luminosity of the inner accretion disc all conspire to
drive the formation and variability of highly-ionized accretion disc winds.Comment: Accepted for publication in MNRAS. 11 pages, 7 figures, uses
mn2e.cls. Comments welcom
Swift Observations of MAXI J1659-152: A Compact Binary with a Black Hole Accretor
We report on the detection and follow-up high cadence monitoring observations
of MAXI J1659-152, a bright Galactic X-ray binary transient with a likely
black-hole accretor, by Swift over a 27 day period after its initial outburst
detection. MAXI J1659-152 was discovered almost simultaneously by Swift and
MAXI on 2010 Sept 25, and was monitored intensively from the early stages of
the outburst through the rise to a brightness of ~0.5 Crab by the Swift XRT,
UVOT, and BAT. We present temporal and spectral analysis of the Swift
observations. The broadband light-curves show variability characteristic of
black-hole candidate transients. We present the evolution of thermal and
non-thermal components of the 0.5-150 keV combined X-ray spectra during the
outburst. MAXI J1659-152 displays accretion state changes typically associated
with black-hole binaries, transitioning from its initial detection in the Hard
State, to the Steep Power-Law State, followed by a slow evolution towards the
Thermal State, signified by an increasingly dominant thermal component
associated with the accretion disk, although this state change did not complete
before Swift observations ended. We observe an anti-correlation between the
increasing temperature and decreasing radius of the inner edge of the accretion
disk, suggesting that the inner edge of the accretion disk in-falls towards the
black-hole as the disk temperature increases. We observed significant evolution
in the absorption column during the initial rise of the outburst, with the
absorption almost doubling, suggestive of the presence of an evolving wind from
the accretion disk. We detect quasi- periodic oscillations that evolve with the
outburst, as well as irregular shaped dips that recur with a period of
2.42\pm0.09 hours, strongly suggesting an orbital period that would make MAXI
J1659-152 the shortest period black-hole binary yet known.Comment: Accepted for publication in ApJ. 9 pages, 7 figure
Rms-flux relation of Cyg X-1 with RXTE: dipping and nondipping cases
The rms (root mean square) variability is the parameter for understanding the
emission temporal properties of X-ray binaries (XRBs) and active galactic
nuclei (AGN).
The rms-flux relation with Rossi X-ray Timing Explorer (RXTE) data for the
dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our
results show that there exist the linear rms-flux relations in the frequency
range 0.1-10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of
the dipping case in the low energy band. For the high energy band, the slopes
of the dipping and nondipping cases are hardly constant within errors. The
explanations of the results have been made by means of the ``Propagating
Perturbation'' model of Lyubarskii (1997).Comment: 15 pages, 12 figures, Accepted for publication in Astrophysics &
Space Scienc
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