660 research outputs found
Detection of radio emission at mas scales from HESS J0632+057 with the e-EVN
HESS J0632+057 is a variable TeV gamma-ray source. The likely low energy counterparts of the source are XMMU J063259.3+054801, the B0pe-type star MWC 148, and a point-like probable non-thermal radio source
Orbital X-Ray Variability of the Microquasar LS 5039
The properties of the orbit and the donor star in the high mass X-ray binary
microquasar LS 5039 indicate that accretion processes should mainly occur via a
radiatively driven wind. In such a scenario, significant X-ray variability
would be expected due to the eccentricity of the orbit. The source has been
observed at X-rays by several missions, although with a poor coverage that
prevents to reach any conclusion about orbital variability. Therefore, we
conducted RossiXTE observations of the microquasar system LS 5039 covering a
full orbital period of 4 days. Individual observations are well fitted with an
absorbed power-law plus a Gaussian at 6.7 keV, to account for iron line
emission that is probably a diffuse background feature. In addition, we have
taken into account that the continuum is also affected by significant diffuse
background contamination. Our results show moderate power-law flux variations
on timescales of days, as well as the presence of miniflares on shorter
timescales. The new orbital ephemeris of the system recently obtained by
Casares et al. have allowed us to show, for the first time, that an increase of
emission is seen close to the periastron passage, as expected in an accretion
scenario. Moreover, the detected orbital variability is a factor of ~4 smaller
than the one expected by using a simple wind accretion model, and we suggest
that an accretion disk around the compact object could be responsible for this
discrepancy. On the other hand, significant changes in the photon index are
also observed clearly anti-correlated with the flux variations. We interpret
the overall X-ray spectral characteristics of LS 5039 in the context of X-ray
radiation produced by inverse Compton and/or synchrotron processes in the jet
of this microquasar.Comment: published in Astrophysical Journal, submission format (real number of
pages: 7, 4 figures
Post-Periastron Gamma Ray Flare from PSR B1259-63/LS 2883 as a Result of Comptonization of the Cold Pulsar Wind
We argue that the bright flare of the binary pulsar \object{PSR
B125963/LS2883} detected by the {\it Fermi} Large Area Telescope (LAT), is
due to the inverse Compton (IC) scattering of the unshocked electron-positron
pulsar wind with a Lorentz factor . The combination of
two effects both linked to the circumstellar disk (CD), is a key element in the
proposed model. The first effect is related to the impact of the surrounding
medium on the termination of the pulsar wind. Inside the disk, the "early"
termination of the wind results in suppression of its gamma-ray luminosity.
When the pulsar escapes the disk, the conditions for termination of the wind
undergo significant changes. This would lead to a dramatic increase of the
pulsar wind zone, and thus to the proportional increase of the gamma-ray flux.
On the other hand, if the parts of the CD disturbed by the pulsar can supply
infrared photons of density high enough for efficient Comptonization of the
wind, almost the entire kinetic energy of the pulsar wind would be converted to
radiation, thus the gamma-ray luminosity of the wind could approach to the
level of the pulsar's spin-down luminosity as reported by the {\it Fermi}
collaboration.Comment: 14 pages, 4 figure
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