37,077 research outputs found
Initial low/hard state, multiple jet ejections and X-ray/radio correlations during the outburst of XTE J1859+226
We have studied the 1999 soft X-ray transient outburst of XTE J1859+226 at
radio and X-ray wavelengths. The event was characterised by strong variability
in the disc, corona and jet - in particular, a number of radio flares
(ejections) took place and seemed well-correlated with hard X-ray events.
Apparently unusual for the `canonical soft' X-ray transient, there was an
initial period of low/hard state behaviour during the rise from quiescence but
prior to the peak of the main outburst - we show that not only could this
initial low/hard state be an ubiquitous feature of soft X-ray transient
outbursts but that it could also be extremely important in our study of
outburst mechanisms.Comment: 12 pages, Accepted for publication in MNRA
A Swift survey of accretion onto stellar-mass black holes
We present a systemic analysis of all of the stellar mass black hole binaries
(confirmed & candidate) observed by the Swift observatory up to June 2010. The
broad Swift bandpass enables a trace of disk evolution over an unprecedented
range in flux and temperature. The final data sample consists of 476 X-ray
spectra containing greater than 100 counts, in the 0.6 -- 10 keV band. This is
the largest sample of high quality CCD spectra of accreting black holes
published to date. In addition, strictly simultaneous data at optical/UV
wavelengths are available for 255 (54%) of these observations.
The data are modelled with a combination of an accretion disk and a hard
spectral component. For the hard component we consider both a simple power-law
and a thermal Comptonization model. An accretion disk is detected at greater
than the 5sigma confidence level in 61% of the observations. Lightcurves and
color-color diagrams are constructed for each system. Hardness luminosity and
disk fraction luminosity diagrams are constructed and are observed to be
consistent with those typically observed by RXTE, noting the sensitivity below
2 keV provided by Swift. The observed spectra have an average luminosity of ~
1% Eddington, though we are sensitive to accretion disks down to a luminosity
of 10^{-3} L_Edd. Thus this is also the largest sample of such cool accretion
disks studied to date. (abridged)Comment: 28 pages, 5 tables, 21 figures, Accepted for publication in ApJ. This
is the final revised versio
Do the Spectra of Soft X-ray Transients Reveal Bulk Motion Inflow Phenomenon?
We present our analysis of the high-energy radiation from black hole (BH)
transients, using archival data obtained primarily with RXTE observatory, and a
comprehensive test of the bulk motion Comptonization (BMC) model for the
high-soft state continuum. The emergent spectra of over 30 separate
measurements of GRO J1655-40, GRS 1915+105, GRS 1739-278, 4U 1630-47 XTE
J1755-32, and EXO~1846-031 X-ray sources are successfully fitted by the BMC
model, which has been derived from basic physical principles in previous work.
This in turn provides direct physical insight into the innermost observable
regions where matter impinging upon the event horizon can effectively be
directly viewed. The BMC model is characterized by three parameters: the disk
color temperature, a geometric factor related to the illumination of the black
hole (BH) site by the disk and a spectral index related to the efficiency of
the bulk motion upscattering. For the case of GRO J1655-40, where there are
distance and mass determinations, a self consistency check of the BMC model has
been made. Using model parameters we present new, independent constraints on
the black hole mass, mass accretion rate and the distance for the
aforementioned sources. Notable is the relationship between the color
temperature and flux, which for GRO J1655-40 is entirely distinct from a simple
T^4 dependence, and consistent with the disk model we have invoked - standard
Shakura-Sunyaev's disk. This allows us to impose an important estimation of the
hardness parameter, the ratio of the color temperature to the effective
temperature - we find T_h~2.6, higher than previous estimates used in the
literature.Comment: 50 pages, 8 figures, accepted for publication in the Astrophysical
Journal (scheduled for the May 20, 1999 issue
X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectral
properties of a large sample of sources detected serendipitously with the
XMM-Newton observatory in 25 selected fields. The survey covers a total solid
angle of ~3.5 deg2 and contains 1137 sources with ~10E-15 < S0.5-10 < 10E-12
erg cm-2 s-1. We find evidence for hardening of the average X-ray spectra of
the sources towards fainter fluxes. We interpret this as indicating a higher
degree of photoelectric absorption amongst the fainter population. Absorption
is detected at 95% confidence in 20% of the sources, but it could certainly be
present in many other sources below our detection capabilities. For Broad Line
AGNs (BLAGNs), we detect absorption in ~10% of the sources with column
densities in the range 10E21 - 10E22 cm-2. The fraction of absorbed Narrow
Emission Line galaxies (NELGs, most with intrinsic X-ray luminosities >10E43
erg s-1, and therefore classified as type 2 AGNs) is significantly higher
(40%), with a hint of moderately higher columns. We do not find evidence for a
redshift evolution of the underlying power law index of BLAGNs, which stays
roughly constant at Gamma ~1.9, with intrinsic dispersion of 0.4. A small
fraction (~7%) of BLAGNs and NELGs require the presence of a soft excess, that
we model as a black body with temperature ranging from 0.1 to 0.3 keV.
Comparing our results on absorption to popular X-ray background synthesis
models, we find absorption in only ~40% of the sources expected. This is due to
a deficiency of heavily absorbed sources (with NH ~10E22 - 10E24 cm-2) in our
sample in comparison with the models. We therefore conclude that the synthesis
models require some revision in their specific parameters.Comment: 20 pages, 30 Postscript figures, A&A in pres
Multi-Temperature Blackbody Spectra of Thin Accretion Disks With and Without a Zero-Torque Inner Boundary Condition
The standard spectral model for analyzing the soft component of thermal
emission from a thin accretion disk around a black hole is the
multi-temperature blackbody (MTB) model. The widely used implementation of this
model, which is known as ``diskbb,'' assumes nonzero torque at the inner edge
of the accretion disk. This assumption is contrary to the classic and current
literature on thin-disk accretion, which advocates the use of a zero-torque
boundary condition. Consequently, we have written code for a zero-torque model,
``ezdiskbb,'' which we compare to the nonzero-torque model diskbb by fitting
RXTE spectra of three well-known black hole binaries: 4U 1543-47, XTE
J1550-564, and GRO J1655-40. The chief difference we find is that the
zero-torque model gives a value for the inner disk radius that is about 2.2
times smaller than the value given by diskbb. This result has important
implications, especially for the determination of black-hole angular momentum
and mass accretion rate.Comment: 27 pages, 9 figures, submitted to Ap
Short term aperiodic variability of X-ray binaries: its origin and implications
In this review I briefly describe the latest advances in studies of aperiodic
variability of accreting X-ray binaries and outline the model which currently
describe the majority of observational appearances of variability of accreting
sources in the best way. Then I concentrate on the case of luminous accreting
neutron star binaries (in the soft/high spectral state), where study of
variability of X-ray emission of sources allowed us to resolve long standing
problem of disentangling the contribution of accretion disk and
boundary/spreading layer components to the time average spectrum of sources.
The obtained knowledge of the shape of the spectrum of the boundary layer
allowed us to make estimates of the mass and radii of accreting neutron stars.Comment: 11 pages, 5 figures. Proceedings article of the conference "Cool
Discs, Hot Flows: The Varying Faces of Accreting Compact Objects", Ed. M.
Axelsson, AIP Conference Proceedings 105
Disk Dominated States of 4U 1957+11: Chandra, XMM, and RXTE Observations of Ostensibly the Most Rapidly Spinning Galactic Black Hole
We present simultaneous Chandra-HETG and RXTE observations of a moderate flux
`soft state' of the black hole candidate 4U1957+11. These spectra, having a
minimally discernible hard X-ray excess, are an excellent test of modern disk
atmosphere models that include the effects of black hole spin. The HETG data
show that the soft disk spectrum is only very mildly absorbed with N_H =1-2 X
10^{21} cm^-2. These data additionally reveal 13.449 A NeIX absorption
consistent with the warm/hot phase of the interstellar medium. The fitted disk
model implies a highly inclined disk around a low mass black hole rapidly
rotating with normalized spin a*~1. We show, however, that pure Schwarzschild
black hole models describe the data extremely well, albeit with large disk
atmosphere ``color-correction'' factors. Standard color-correction factors can
be attained if one additionally incorporates mild Comptonization. We find that
the Chandra observations do not uniquely determine spin. Similarly, XMM/RXTE
observations, taken only six weeks later, are equally unconstraining. This lack
of constraint is partly driven by the unknown mass and unknown distance of
4U1957+11; however, it is also driven by the limited bandpass of Chandra and
XMM. We therefore present a series of 48 RXTE observations taken over the span
of several years and at different brightness/hardness levels. These data prefer
a spin of a*~1, even when including a mild Comptonization component; however,
they also show evolution of the disk atmosphere color-correction factors. If
the rapid spin models with standard atmosphere color-correction factors of
h_d=1.7 are to be believed, then the RXTE observations predict that 4U1957+11
can range from a 3 M_sun black hole at 10 kpc with a*~0.83 to a 16 M_sun black
hole at 22 kpc with a* ~ 1, with the latter being statistically preferred.Comment: 16 pages (emulateapj style). Accepted for Publication in the
Astrophysical Journa
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