458 research outputs found
ASCA observations of the galactic bulge hard x-ray source GRS 1758--258
GRS 1758-258 is one of the few persistent hard X-ray emitters (E>100 keV) in
the Galaxy. Using the ASCA satellite, we have obtained the first detailed data
on GRS 1758-258 in the 1-10 keV range, where previous observations were
affected by confusion problems caused by the nearby strong source GX5-1. The
spectrum is well described by a power law with photon index 1.7 without strong
Fe emission lines. A prominent soft excess, as observed with ROSAT when the
hard X-ray flux was in a lower intensity state, was not detected. However, the
presence of a soft spectral component, accounting for at most 5% of the 0.1-300
keV flux, cannot be excluded. The accurate measurement of interstellar
absorption (N_H=(1.5+-0.1) x 10^22 cm -2) corresponds to an optical extinction
which definitely excludes the presence of a massive companion.Comment: 7 pages, AAS latex [11pt,aaspptwo,flushrt,tighten], + 1.ps figure
Accepted for pubblication in ApJ, 09 02 96 Also available at
http://fy.chalmers.se/~haardt/personal/curr.html Figures 1 and 2 available
upon request at [email protected]
Imaging extended sources with coded mask telescopes: Application to the INTEGRAL IBIS/ISGRI instrument
Context. In coded mask techniques, reconstructed sky images are
pseudo-images: they are maps of the correlation between the image recorded on a
detector and an array derived from the coded mask pattern. Aims. The
INTEGRAL/IBIS telescope provides images where the flux of each detected source
is given by the height of the local peak in the correlation map. As such, it
cannot provide an estimate of the flux of an extended source. What is needed is
intensity sky images giving the flux per solide angle as typically done at
other wavelengths. Methods. In this paper, we present the response of the
INTEGRAL IBIS/ISGRI coded mask instrument to extended sources. We develop a
general method based on analytical calculations in order to measure the
intensity and the associated error of any celestial source and validated with
Monte-Carlo simulations. Results. We find that the sensitivity degrades almost
linearly with the source extent. Analytical formulae are given as well as an
easy-to-use recipe for the INTEGRAL user. We check this method on IBIS/ISGRI
data but these results are general and applicable to any coded mask telescope.Comment: 9 pages, 6 figures, Accepted for publication in A&
First observations of the X-ray transient EXO 2030+375 with IBIS/ISGRI
We present a first INTEGRAL observation of the 42s transient X-ray pulsar EXO
2030+375 with IBIS/ISGRI. The source was detected during Cyg X-1 observations
in December 2002. We analyzed observations during the outburst period from 9 to
21 December 2002 with a total exposure time of ~770 kiloseconds. EXO 2030+375
was almost always detected during single ~30 minute exposures in the 18-45
energy bands. The source light curve shows the characteristic outburst shape
observed in this source.Comment: 4 pages, 3 figures (1 in CMYK color), accepted by Astronomy and
Astrophysics, INTEGRAL special issue, 200
Echoes of multiple outbursts of Sagittarius A* revealed by Chandra
The relatively rapid spatial and temporal variability of the X-ray radiation
from some molecular clouds near the Galactic center shows that this emission
component is due to the reflection of X-rays generated by a source that was
luminous in the past, most likely the central supermassive black hole,
Sagittarius A*. Studying the evolution of the molecular cloud reflection
features is therefore a key element to reconstruct Sgr A*'s past activity. The
aim of the present work is to study this emission on small angular scales in
order to characterize the source outburst on short time scales. We use Chandra
high-resolution data collected from 1999 to 2011 to study the most rapid
variations detected so far, those of clouds between 5' and 20' from Sgr A*
towards positive longitudes. Our systematic spectral-imaging analysis of the
reflection emission, notably of the Fe Kalpha line at 6.4 keV and its
associated 4-8 keV continuum, allows us to characterize the variations down to
15" angular scale and 1-year time scale. We reveal for the first time abrupt
variations of few years only and in particular a short peaked emission, with a
factor of 10 increase followed by a comparable decrease, that propagates along
the dense filaments of the Bridge cloud. This 2-year peaked feature contrasts
with the slower 10-year linear variations we reveal in all the other molecular
structures of the region. Based on column density constraints, we argue that
these two different behaviors are unlikely to be due to the same illuminating
event. The variations are likely due to a highly variable active phase of Sgr
A* sometime within the past few hundred years, characterized by at least two
luminous outbursts of a few-year time scale and during which the Sgr A*
luminosity went up to at least 10^39 erg/s.Comment: 17 pages, 16 figures, Accepted for publication in Astronomy &
Astrophysic
XMM-Newton Observation of the Black Hole Microquasar GRS 1758-258
The XMM-Newton X-ray observatory pointed the galactic black hole candidate
and microquasar GRS 1758-258 in September 2000 for about 10 ks during a program
devoted to the scan of the Galactic Center regions. Preliminary results from
EPIC MOS camera data are presented here. The data indicate that the source
underwent a state transition from its standard low-hard state to an
intermediate state. For the first time in this source the ultra-soft component
of the accretion disk, which black hole binaries display in intermediate or
high-soft states, was clearly detected and measured thanks to the high spectral
capabilities of XMM-Newton.Comment: To appear in the Proc. of the Gamma-Ray Astrophysics 2001 Symposium,
4-6 April 2001, Baltimore, Maryland, U.S.A.. American Institute of Physics
(AIP) series: 5 pages, 6 PS figures, latex, uses aipproc.cls aipproc.st
The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
We present for the first time a study of the 3-200 keV broad band spectra of
the bursting atoll source 4U 1728-34 (GX 354-0) along its hardness intensity
diagram. The analysis was done using the INTEGRAL public and Galactic Center
deep exposure data ranging from February 2003 to October 2004. The spectra are
well described by a thermal Comptonization model with an electron temperature
from 35 keV to 3 keV and Thomson optical depth, tau_T, from 0.5 to 5 in a slab
geometry. The source undergoes a transition from an intermediate/hard to a soft
state where the source luminosity increases from 2 to 12% of Eddington. We have
also detected 36 type I X-ray bursts two of which show photospheric radius
expansion. The energetic bursts with photospheric radius expansion occurred at
an inferred low mass accretion rate per unit area of \dot m ~ 1.7x10E3 g/cm2/s,
while the others at a higher one between 2.4x10E3 - 9.4x10E3 g/cm2/s. For
4U1728-34 the bursts' total fluence, and the bursts' peak flux are
anti-correlated with the mass accretion rate. The type I X-ray bursts involve
pure helium burning either during the hard state, or during the soft state of
the source.Comment: 11 pages, 7 figures, and 2 tables. Accepted for publication in A&
Variation of the X-ray non-thermal emission in the Arches cloud
The origin of the iron fluorescent line at 6.4 keV from an extended region
surrounding the Arches cluster is debated and the non-variability of this
emission up to 2009 has favored the low-energy cosmic-ray origin over a
possible irradiation by hard X-rays. By probing the variability of the Arches
cloud non-thermal emission in the most recent years, including a deep
observation in 2012, we intend to discriminate between the two competing
scenarios. We perform a spectral fit of XMM-Newton observations collected from
2000 to 2013 in order to build the Arches cloud lightcurve corresponding to
both the neutral Fe Kalpha line and the X-ray continuum emissions. We reveal a
30% flux drop in 2012, detected with more than 4 sigma significance for both
components. This implies that a large fraction of the studied non-thermal
emission is due to the reflection of an X-ray transient source.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
XMM-Newton observation of the brightest X-ray flare detected so far from SgrA*
We report the high S/N observation on October 3, 2002 with XMM-Newton of the
brightest X-ray flare detected so far from SgrA* with a duration shorter than
one hour (~ 2.7 ks). The light curve is almost symmetrical with respect to the
peak flare, and no significant difference between the soft and hard X-ray range
is detected. The overall flare spectrum is well represented by an absorbed
power-law with a soft photon spectral index of Gamma=2.5+/-0.3, and a peak 2-10
keV luminosity of 3.6 (+0.3-0.4) x 10^35 erg/s, i.e. a factor 160 higher than
the Sgr A* quiescent value. No significant spectral change during the flare is
observed. This X-ray flare is very different from other bright flares reported
so far: it is much brighter and softer. The present accurate determination of
the flare characteristics challenge the current interpretation of the physical
processes occuring inside the very close environment of SgrA* by bringing very
strong constraints for the theoretical flare models.Comment: Accepted for publication in A&A Letters. 4 pages, 2 figures, 1 table
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