716 research outputs found
Is Radiation of Quantized Black Holes Observable?
If primordial black holes (PBH) saturate the present upper limit on the dark
matter density in our Solar system and if their radiation spectrum is discrete,
the sensitivity of modern detectors is close to that necessary for detecting
this radiation. This conclusion is not in conflict with the upper limits on the
PBH evaporation rate.Comment: 6 pages, 1 figure (reproduced properly in pdf file
INTEGRAL probes the morphology of the Crab nebula in hard X-rays/soft gamma-rays
Aims. We use the IBIS/ISGRI telescope on-board INTEGRAL to measure the
position of the centroid of the 20-200 keV emission from the Crab region.
Methods. We find that the astrometry of the IBIS telescope is affected by the
temperature of the IBIS mask during the observation. After correcting for this
effect, we show that the systematic errors in the astrometry of the telescope
are of the order of 0.5 arcsec. In the case of the Crab nebula and several
other bright sources, the very large number of photons renders the level of
statistical uncertainty in the centroid smaller or comparable to this value.
Results. We find that the centroid of the Crab nebula in hard X-rays (20-40
keV) is shifted by 8.0 arcsec with respect to the Crab pulsar in the direction
of the X-ray centroid of the nebula. A similar shift is also found at higher
energies (40-100 and 100-200 keV). We observe a trend of decreasing shift with
energy, which can be explained by an increase in the pulsed fraction. To
differentiate between the contribution of the pulsar and the nebula, we divide
our data into an on-pulse and off-pulse sample. Surprisingly, the nebular
emission (i.e., off-pulse) is located significantly away from the X-ray
centroid of the nebula. Conclusions. In all 3 energy bands (20-40, 40-100, and
100-200 keV), we find that the centroid of the nebula is significantly offset
from the predicted position. We interpret this shift in terms of a cut-off in
the electron spectrum in the outer regions of the nebula, which is probably the
origin of the observed spectral break around 100 keV. From a simple
spherically-symmetric model for the nebula, we estimate that the electrons in
the external regions of the torus (d ~ 0.35 pc from the pulsar) reach a maximal
energy slightly below 10^14 eV.Comment: 7 pages, 8 figures, accepted for publication in A&
INTEGRAL observations of V0332+53 in outburst
We present the analysis of a 100ksec Integral(3-100kev) observation of the
transient X-ray pulsar V0332+53 inoutburst. The source is pulsating at
P=4.3751+/-0.0002s with a clear double pulse from 6 kev to 60 kev. The average
flux was ~550mCrab between 20 kev and 60 kev. We modeled the broad band
continuum from 5 kev to 100 kev with a power-law modified by an exponential cut
off. We observe three cyclotron lines: the fundamental line at 24.9+/-0.1 kev,
the first harmonic at 50.5+/-0.1 kev as well as the second harmonic
at71.7+/-0.8 kev, thus confirming the discovery of the harmonic lines by Coburn
et al. (2005) in RXTE data.Comment: 4 pages, 3 figures. Accepted for publication in A&A Letter
South-West extension of the hard X-ray emission from the Coma cluster
We explore the morphology of hard (18-30 keV) X-ray emission from the Coma
cluster of galaxies. We analyze a deep (1.1 Ms) observation of the Coma cluster
with the ISGRI imager on board the \emph{INTEGRAL} satellite. We show that the
source extension in the North-East to South-West (SW) direction ()
significantly exceeds the size of the point spread function of ISGRI, and that
the centroid of the image of the source in the 18-30 keV band is displaced in
the SW direction compared to the centroid in the 1-10 keV band. To test the
nature of the SW extension we fit the data assuming different models of source
morphology. The best fit is achieved with a diffuse source of elliptical shape,
although an acceptable fit can be achieved assuming an additional point source
SW of the cluster core. In the case of an elliptical source, the direction of
extension of the source coincides with the direction toward the subcluster
falling onto the Coma cluster. If the SW excess is due to the presence of a
point source with a hard spectrum, we show that there is no obvious X-ray
counterpart for this additional source, and that the closest X-ray source is
the quasar EXO 1256+281, which is located from the centroid of the
excess. The observed morphology of the hard X-ray emission clarifies the nature
of the hard X-ray "excess" emission from the Coma cluster, which is due to the
presence of an extended hard X-ray source SW of the cluster core.Comment: 7pages, 10 figure
Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds
Context : A 1000-s flare from a new hard X-ray transient, IGR J08408-4503,
was observed by INTEGRAL on May 15, 2006 during the real-time routine
monitoring of IBIS/ISGRI images performed at the INTEGRAL Science Data Centre.
The flare, detected during a single one-hour long pointing, peaked at 250 mCrab
in the 20-40 keV energy range.
Aims : Multi-wavelength observations, combining high-energy and optical data,
were used to unveil the nature of IGR J08408-4503.
Methods : A search in all INTEGRAL public data for other bursts from IGR
J08408-4503 was performed, and the detailed analysis of another major flare is
presented. The results of two Swift Target of Opportunity observations are also
described. Finally, a study of the likely optical counterpart, HD 74194, is
provided.
Results : IGR J08408-4503 is very likely a supergiant fast X-ray transient
(SFXT) system. The system parameters indicate that the X-ray flares are
probably related to the accretion of wind clumps on a compact object orbiting
about 1E13 cm from the supergiant HD 74194. The clump mass loss rate is of the
order of 1E-6 solar mass/yr.
Conclusions : Hard X-ray flares from SFXTs allow to probe the stellar winds
of massive stars, and could possibly be associated with wind perturbations due
to line-driven instabilities.Comment: 5 pages with 5 figures. Published as a Letter in Astronomy &
Astrophysic
- …