71 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
On the two types of steady hard X-ray states of GRS 1915+105
Using the data of 5 years of RXTE observations we investigate the X-ray
spectral and timing properties of GRS 1915+105 during the hard steady states.
According to the results of our simultaneous X-ray spectral and timing analysis
the behavior the source during the hard steady states can be reduced to a
couple of major distinct types. i) Type I states: The dominant hard component
of the energy spectrum has characteristic quasi- exponential cut-off at 50-120
keV. The broad-band power density spectrum of the source shows significant high
frequency noise component with a cut-off at 60-80 Hz. ii) Type II states: The
hard spectral component has a break in its slope at ~12-20 keV. The high
frequency part of the power density spectrum fades quickly lacking significant
variability at frequencies higher than ~30 Hz. These two types of the X-ray
hard states are also clearly distinguished by their properties in the radio
band: while during the type I observations the source tends to be
'radio-quiet', the type II observations are characterized by high level of
radio flux ('plateau' radio states). In this work we demonstrate aforementioned
differences using the data of 12 representative hard steady state observations.
We conclude that the difference between these two types can be probably
explained in terms of different structure of the accretion flow in the
immediate vicinity of the compact object due to presence of relativistic
outflow of matter.Comment: 16 pages, including 3 figures, submitted to Astrophysical Journal
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
Detection of a Series of X-ray Dips Associated with a Radio Flare in GRS 1915+105
We report the detection of a series of X-ray dips in the Galactic black hole
candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with
the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on
board the Indian satellite IRS-P3. The observations were made after the source
made a transition from a steady low-hard state to a chaotic state which
occuered within a few hours. Dips of about 20-160 seconds duration are observed
on most of the days. The X-ray emission outside the dips shows a QPO at ~ 4 Hz
which has characteristics similar to the ubiquitous 0.5 - 10 Hz QPO seen during
the low-hard state of the source. During the onset of dips this QPO is absent
and also the energy spectrum is soft and the variability is low compared to the
non-dip periods. These features gradually re-appear as the dip recovers. The
onset of the occurrence of a large number of such dips followed the start of a
huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as
the cause for mass ejection due to the evacuation of matter from an accretion
disk around the black hole. We propose that a super-position of a large number
of such dip events produces a huge radio jet in GRS 1915+105.Comment: 18 pages, 7 figures, Accepted for publication in Ap
X-ray Spectroscopy of the Cluster of Galaxies Abell 1795 with XMM-Newton
The initial results from XMM-Newton observations of the rich cluster of
galaxies Abell 1795 are presented. The spatially-resolved X-ray spectra taken
by the European Photon Imaging Cameras (EPIC) show a temperature drop at a
radius of kpc from the cluster center, indicating that the ICM is
cooling. Both the EPIC and the Reflection Grating Spectrometers (RGS) spectra
extracted from the cluster center can be described by an isothermal model with
a temperature of keV. The volume emission measure of any cool
component ( keV) is less than a few % of the hot component at the cluster
center. A strong OVIII Lyman-alpha line was detected with the RGS from the
cluster core. The O abundance and its ratio to Fe at the cluster center is
0.2--0.5 and 0.5--1.5 times the solar value, respectively.Comment: Accepted: A&A Letters, 2001, 6 page
On the artificial nature of aperiodic variability in XMM-Newton observations of M 31 X-ray sources and the ultraluminous X-ray source NGC 4559 ULX-7
Context: Power density spectra (PDS) that are characteristic of low mass X-ray binaries (LMXBs) have been previously reported for M 31 X-ray sources, observed by XMM-Newton. However, we have recently discovered that these PDS result from the improper addition/subtraction of non-simultaneous lightcurves.
Aims: To understand the properties and origins of the artefact.
Methods: We re-analysed our XMM-Newton observations of M 31 with non-simultaneous and simultaneous lightcurves, then combined simulated lightcurves at various intensities with various offsets and found that the artefact is more dependent on the offset than the intensity.
Results: The lightcurves produced by the XMM-Newton Science Analysis Software (SAS) are non-synchronised by default. This affects not only the combination of lightcurves from the three EPIC detectors (MOS1, MOS2 and pn), but also background subtraction in the same CCD. It is therefore imperative that all SAS-generated lightcurves are synchronised by time filtering, even if the whole observation is to be used. We also find that the reported timing behaviour for NGC 4559 ULX-7 was also contaminated by the artefact; there is no significant variability in the correctly-combined lightcurves of NGC 4559 ULX-7. Hence, the classification of this source as an intermediate-mass black hole is no longer justified.
Conclusions: While previous timing results from M 31 have been proven wrong, and also the broken power law PDS in NGC 4559 ULX-7, XMM-Newton was able to detect aperiodic variability in just 3 ks of observations of NGC 5408 ULX1. Hence XMM-Newton remains a viable tool for analysing variability in extra-galactic X-ray sources
XMM-Newton discovery of transient X-ray pulsar in NGC 1313
We report on the discovery and analysis of the transient X-ray pulsar XMMU
J031747.5-663010 detected in the 2004 November 23 XMM-Newton observation of the
spiral galaxy NGC 1313. The X-ray source exhibits pulsations with a period
P~765.6 s and a nearly sinusoidal pulse shape and pulsed fraction ~38% in the
0.3-7 keV energy range. The X-ray spectrum of XMMU J031747.5-663010 is hard and
is well fitted with an absorbed simple power law of photon index ~1.5 in the
0.3-7 keV energy band. The X-ray properties of the source and the absence of an
optical/UV counterpart brighter than 20 mag allow us to identify XMMU
J031747.5-663010 as an accreting X-ray pulsar located in NGC 1313. The
estimated absorbed 0.3-7 keV luminosity of the source L~1.6\times 10^{39}
ergs/s, makes it one of the brightest X-ray pulsars known. Based on the
relatively long pulse period and transient behaviour of the source, we classify
it as a Be binary X-ray pulsar candidate. XMMU J031747.5-663010 is the second
X-ray pulsar detected outside the Local Group, after transient 18 s pulsating
source CXOU J073709.1+653544 discovered in the nearby spiral galaxy NGC 2403.Comment: 6 pages, 4 figures. Accepted for publication in MNRAS. Updated to
match the accepted versio
Supersoft X-ray sources in M31: II. ROSAT-detected supersoft sources in the ROSAT, Chandra and XMM eras
We have performed Chandra observations during the past 3 years of 5 of the
M31 supersoft X-ray sources (SSS) discovered with ROSAT. Surprisingly, only one
of these sources has been detected, despite a predicted detection of about
20-80 counts. This has motivated a thorough check of the ROSAT M31 survey I
data, including a relaxation of the hardness ratio requirement used to select
SSS. This increases the number of SSS identified in survey I by 7. We then
carried out a comparison with the ROSAT M31 survey II dataset which had
hitherto not been explicitly investigated for SSS. We find that most of the
ROSAT survey I sources are not detected, and only two new SSS are identified.
The low detection rate in the ROSAT survey II and our Chandra observations
implies that the variability time scale of SSS is a few months. If the majority
of these sources are close-binary SSS with shell hydrogen burning, it further
implies that half of these sources predominantly experience large mass transfer
rates.Comment: accepted for publ. in ApJ; 2 ps-figures; high-quality figures
available at http://www.mpe.mpg.de/~jcg/publis.htm
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