58 research outputs found
Detection of a 5-Hz QPO from X-ray Nova GRS 1739-278
The X-ray nova GRS1739-278 flared up near the Galactic center in the spring
of 1996. Here we report on the discovery of a ~5-Hz quasi-periodic oscillations
(QPO) in RXTE/PCA observations of GRS1739-278. The QPO were only present when
the source was in its very high state, and disappeared later, when it made a
transition down into the high state. We present the energy spectra of this
black hole candidate measured in both high and very high states, and discuss
the similarities between this system and other X-ray transients.Comment: 12 pages, 4 figures, accepted for publication in ApJ Letter
RXTE observations of Galactic microquasar XTE J1748--288 during its 1998 outburst
We present an analysis of the RXTE observations of the recently discovered
Galactic microquasar XTE J1748--288 during its 1998 outburst. The spectral
evolution of the source during the outburst can be considered a sequence of
qualitatively distinct states. During the first observations, corresponding to
the maximum of X-ray flux, the spectrum of the source consisted of a dominating
hard power law component and a soft thermal component, which can be described
by the model of multicolor disk emission. The hard component contributed >80%
to the X-ray luminosity in the 3-25 keV energy band. Overall two-component
spectral shape is an attribute of Very High state (VHS) observed previously in
BHC, but the domination of hard component is unusual. Later on, as the X-ray
source faded, its energy spectrum qualitatively changed, showing High (HS) and
then Low (LS) states, both typical for black hole binaries. As the energy
spectrum changed, the fast variability also evolved dramatically. Initially the
power density spectrum was formed by a dominating band-limited noise component,
QPO features at 20-30 Hz and at ~0.5 Hz, and a very low frequency noise
component. After a significant decrease of the contribution of the hard
spectral component the amplitude of the fractional variability decreased by an
order of magnitude and the PDS spectrum adopted a power-law shape with a broad
QPO peak around 0.03 Hz. When the system switched to the LS, the PDS shape
changed again and the QPOs have not been detected since. When the source was
observed in VHS, a clear correlation between QPO parameters and X-ray flux was
seen.Comment: 9 pages, 7 figures, accepted for publication in MNRA
X-ray Spectrum of the Black Hole Candidate X1755-338
We report the first detection of a hard power-law tail in the X-ray spectrum
of the black hole candidate (BHC) binary X1755-338, which was observed in 1989
March-September during the TTM Galactic Centre survey. In addition, an
ultrasoft thermal component with a temperature of ~1.1-1.4 keV was also
detected. We demonstrate that the soft and hard X-ray components of X1755-338
vary independently, as in the spectra of the well known BHCs LMC X-1, LMC X-3
and GX339-4 in their high (intensity) state. If the hard tail observed from
X1755-338 is generated near the black hole by energetic electrons up-scattering
low energy photons, the un-correlated variations imply that the soft X-rays
from accretion disc may not be the main photon seeds needed for inverse
Comptonization. The TTM observations strongly suggest that X1755-338 does
indeed belong to the family of BHCs.Comment: 4 pages, compressed and uuencoded postscript file, to be published in
MNRAS (Letters
The discovery of 2.78 hour periodic modulation of the X-ray flux from globular cluster source Bo 158 in M31
We report the discovery of periodic intensity dips in the X-ray source XMMU
J004314.1+410724, in the globular cluster Bo158 in M31. The X-ray flux was
modulated by ~83% at a period of 2.78 hr (10017 s) in an XMM-Newton observation
taken 2002 Jan 6-7. The X-ray intensity dips show no energy dependence. We
detected weaker dips with the same period in observations taken 2000 June 25
(XMM-Newton) and 1991 June 26 (ROSAT/PSPC). The amplitude of the modulation has
been found to be anticorrelated with source X-ray flux: it becomes lower when
the source intensity rises. The energy spectrum of Bo158 was stable from
observation to observation, with a characteristic cutoff at ~4-6 keV. The
photo-electric absorption was consistent with the Galactic foreground value. No
significant spectral changes were seen in the course of the dips. If the 2.78
hr cycle is the binary period of Bo158 the system is highly compact, with a
binary separation of ~10e11 cm. The association of the source with a globular
cluster, together with spectral parameters consistent with Galactic neutron
star sources, suggests that X-rays are emitted by an accreting neutron star.
The properties of Bo 158 are somewhat reminiscent of the Galactic X-ray sources
exhibiting a dip-like modulations. We discuss two possible mechanisms
explaining the energy-independent modulation observed in Bo 158: i) the
obscuration of the central source by highly ionized material that scatters
X-rays out of the line of sight; ii) partial covering of an extended source by
an opaque absorber which occults varying fractions of the source.Comment: 10 pages, 4 figures, ApJ, submitted, uses emulateapj styl
RXTE Observations of an Outburst of Recurrent X-ray Nova GS 1354-644
We present the results of Rossi X-ray Timing Explorer observations of GS
1354-644 during a modest outburst in 1997-1998. The source is one of a handful
of black hole X-ray transients that are confirmed to be recurrent in X-rays. A
1987 outburst of the same source observed by Ginga was much brighter, and
showed a high/soft spectral state. In contrast the 1997-1998 outburst showed a
low/hard spectral state. Both states are typical for black hole binaries. The
RXTE All Sky Monitor observed an outburst duration of 150 to 200 days. PCA and
HEXTE observations covered ~70 days near the maximum of the light curve and
during the flux decline. Throughout the observations, the spectrum can be
approximated by Compton upscattering of soft photons by energetic electrons.
The hot electron cloud has a temperature kT ~30 keV and optical depth tau~4--5.
To fit the data well an additional iron fluorescent line and reflection
component are required, which indicates the presence of optically thick cool
material, most probably in the outer part of the accretion disk. Dramatic fast
variability was observed, and has been analyzed in the context of a shot noise
model. The spectrum appeared to be softest at the peaks of the shot-noise
variability. The shape of the power spectrum was typical for black hole systems
in a low/hard state. We note a qualitative difference in the shape of the
dependence of fractional variability on energy, when we compare systems with
black holes and with neutron stars. Since it is difficult to discriminate these
systems on spectral grounds, at least in their low/hard states, this new
difference might be important.Comment: 12 pages, 9 figures, accepted for publication in ApJ (Feb. 2000,
v.530), uses emulateapj.st
Observations of the Optical Counterpart to XTE J1118+480 During Outburst by the ROTSE-I Telescope
The X-ray nova XTE J1118+480 exhibited two outbursts in the early part of
2000. As detected by the Rossi X-ray Timing Explorer (RXTE), the first outburst
began in early January and the second began in early March. Routine imaging of
the northern sky by the Robotic Optical Transient Search Experiment (ROTSE)
shows the optical counterpart to XTE J1118+480 during both outbursts. These
data include over 60 epochs from January to June 2000. A search of the ROTSE
data archives reveal no previous optical outbursts of this source in selected
data between April 1998 and January 2000. While the X-ray to optical flux ratio
of XTE J1118+480 was low during both outbursts, we suggest that they were full
X-ray novae and not mini-outbursts based on comparison with similar sources.
The ROTSE measurements taken during the March 2000 outburst also indicate a
rapid rise in the optical flux that preceded the X-ray emission measured by the
RXTE by approximately 10 days. Using these results, we estimate a pre-outburst
accretion disk inner truncation radius of 1.2 x 10^4 Schwarzschild radii.Comment: 9 pages, 1 table, 2 figure
Fast X-ray Transients and Their Connection to Gamma-Ray Bursts
Fast X-ray transients (FXTs) with timescales from seconds to hours have been
seen by numerous space instruments. We have assembled archival data from
Ariel-5, HEAO-1 (A-1 and A-2), WATCH, ROSAT, and Einstein to produce a global
fluence-frequency relationship for these events. Fitting the log N-log S
distribution over several orders of magnitude to simple power law we find a
slope of -1.0. The sources of FXTs are undoubtedly heterogeneous, the -1 power
law is an approximate result of the summation of these multiple sources. Two
major contributions come from gamma-ray bursts and stellar flares.
Extrapolating from the BATSE catalog of GRBs, we find that the fraction of
X-ray flashes that can be the X-ray counterparts of gamma-ray bursts is a
function of fluence. Certainly most FXTs are not counterparts of standard
gamma-ray bursts. The fraction of FXTs from non-GRB sources, such as magnetic
stars, is greatest for the faintest FXTs. Our understanding of the FXT
phenomenon remains limited and would greatly benefit from a large, homogeneous
data set, which requires a wide-field, sensitive instrument.Comment: 36 pages, 8 figure
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