302 research outputs found
A study of spectra of Cyg X-3 observed by BeppoSAX
We model the ~1-200 keV spectra of Cygnus X-3 observed by BeppoSAX. The
continuum, modeled by Comptonization in a hybrid plasma, is modified by the
strongly ionized plasma of the stellar wind of the Wolf-Rayet companion star.
Discrete absorption and emission spectral features are modeled with XSTAR. The
model has been applied to phase-resolved spectra in the hard and soft spectral
states.Comment: 3 pages, to appear in the Proceedings of the Conference "X-ray
Diagnostics of Astrophysical Plasmas", Cambridge, November 15-17, 2004.
Replacement: minor changes in text and acknowledgement
Jet radio emission in Cygnus X-1 and its orbital modulation
We present results of our detailed theoretical study of the observed orbital
modulation of the radio emission in Cyg X-1. The modulation occurs due to
free-free absorption in the wind from the companion star varying with the
orbital phase, and our results put strong constraints on the spatial
distribution of the jet radio emission at the frequencies of 2-15 GHz. A
crucial role in enhancing the asymmetry of the wind absorption suffered by the
jet emission is played by the irradiation by X-rays emitted in the vicinity of
the black hole. This increases the wind temperature by more than order of
magnitude with respect to that of the wind of an isolated supergiant. The
observed phase lags of the minima of the radio emission with respect to the
spectroscopic zero phase strongly imply the bulk of the mass of the jet is
nonrelativistic (~5 x 10^8 cm/s) within the jet core. The jet can, however,
become relativistic outside the core. Also, the jet can have a two-component
structure, being slow on the outside and fast inside, in which case its
synchrotron-emitting part may be relativistic already in the core. We also
consider the observed superorbital modulation of the radio emission (with the
period of ~150 d) and find it can be explained by a jet precession both causing
variable wind absorption and changing the jet Doppler factor. Finally, we
consider the case of Cyg X-3, and show that its lack of observable orbital
radio modulation (in spite of strong modulation of X-rays) is explained by that
system being both much more compact and much more luminous than Cyg X-1.Comment: Accepted for publication in MNRAS; new revised version with small
changes; 13 pages, 16 figure
What can Simbol-X do for gamma-ray binaries?
Gamma-ray binaries have been uncovered as a new class of Galactic objects in
the very high energy sky (> 100 GeV). The three systems known today have hard
X-ray spectra (photon index ~ 1.5), extended radio emission and a high
luminosity in gamma-rays. Recent monitoring campaigns of LSI +61 303 in X-rays
have confirmed variability in these systems and revealed a spectral hardening
with increasing flux. In a generic one-zone leptonic model, the cooling of
relativistic electrons accounts for the main spectral and temporal features
observed at high energy. Persistent hard X-ray emission is expected to extend
well beyond 10 keV. We explain how Simbol-X will constrain the existing models
in connection with Fermi Space Telescope measurements. Because of its
unprecedented sensitivity in hard X-rays, Simbol-X will also play a role in the
discovery of new gamma-ray binaries, giving new insights into the evolution of
compact binaries.Comment: 4 pages, 1 figure, Proceedings of the 2nd International Simbol-X
symposium held in Paris, 2-5 December 200
Spectral variability in Cygnus X-3
We model the broad-band X-ray spectrum of Cyg X-3 in all states displayed by
this source as observed by the Rossi X-ray Timing Explorer. From our models, we
derive for the first time unabsorbed spectral shapes and luminosities for the
full range of spectral states. We interpret the unabsorbed spectra in terms of
Comptonization by a hybrid electron distribution and strong Compton reflection.
We study the spectral evolution and compare with other black hole as well as
neutron star sources. We show that a neutron star accretor is not consistent
with the spectral evolution as a function of Ledd and especially not with the
transition to a hard state. Our results point to the compact object in Cyg X-3
being a massive, ~30 Msun black hole.Comment: 14 pages, 9 figures, accepted for publication in MNRA
On the Merging Cluster Abell 578 and Its Central Radio Galaxy 4C +67.13
Here we analyze radio, optical, and X-ray data for a peculiar cluster Abell
578. This cluster is not fully relaxed and consists of two merging sub-systems.
The brightest cluster galaxy, CGPG 0719.8+6704, is a pair of interacting
ellipticals with projected separation 10 kpc, the brighter of which hosts
the radio source 4C +67.13. The Fanaroff-Riley type-II radio morphology of 4C
+67.13 is unusual for central radio galaxies in local Abell clusters. Our new
optical spectroscopy revealed that both nuclei of the CGPG 0719.8+6704 pair are
active, albeit at low accretion rates corresponding to the Eddington ratio
(for the estimated black hole masses of and ). The gathered X-ray ({\it Chandra})
data allowed us to confirm and to quantify robustly the previously noted
elongation of the gaseous atmosphere in the dominant sub-cluster, as well as a
large spatial offset (\,kpc projected) between the position of the
brightest cluster galaxy and the cluster center inferred from the modeling of
the X-ray surface brightness distribution. Detailed analysis of the brightness
profiles and temperature revealed also that the cluster gas in the vicinity of
4C\,+67.13 is compressed (by a factor of about ) and heated (from
\,keV up to 2.7\,keV), consistent with the presence of a weak shock
(Mach number ) driven by the expanding jet cocoon. This would then
require the jet kinetic power of the order of \,erg\,s,
implying either a very high efficiency of the jet production for the current
accretion rate, or a highly modulated jet/accretion activity in the system.Comment: 12 pages, 11 figures, accepted for publication in Ap
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A classification of the X-ray and radio states of Cyg X-3 and their long-term correlations
We present a detailed classification of the X-ray states of Cyg X-3 based on
the spectral shape and a new classification of the radio states based on the
long-term correlated behaviour of the radio and soft X-ray light curves. We
find a sequence of correlations, starting with a positive correlation between
the radio and soft X-ray fluxes in the hard spectral state, changing to a
negative one at the transition to soft spectral states. The temporal evolution
can be in either direction on that sequence, unless the source goes into a very
weak radio state, from which it can return only following a major radio flare.
The flare decline is via relatively bright radio states, which results in a
hysteresis loop on the flux-flux diagram. We also study the hard X-ray light
curve, and find its overall anticorrelation with the soft X-rays. During major
radio flares, the radio flux responds exponentially to the level of a hard
X-ray high-energy tail. We also specify the detailed correspondence between the
radio states and the X-ray spectral states. We compare our results to those of
black-hole and neutron-star binaries. Except for the effect of strong
absorption and the energy of the high-energy break in the hard state, the X-ray
spectral states of Cyg X-3 closely correspond to the canonical X-ray states of
black-hole binaries. Also, the radio/X-ray correlation closely corresponds to
that found in black-hole binaries, but it significantly differs from that in
neutron-star binaries. Overall, our results strongly support the presence of a
black hole in Cyg X-3.Comment: MNRAS, in pres
Absorption of high-energy gamma rays in Cygnus X-3
The microquasar Cygnus X-3 was detected at high energies by the gamma-ray
space telescopes AGILE and Fermi. The gamma-ray emission is transient,
modulated with the orbital period and seems related to major radio flares, i.e.
to the relativistic jet. The GeV gamma-ray flux can be substantially attenuated
by internal absorption with the ambient X-rays. In this study, we examine
quantitatively the effect of pair production in Cygnus X-3 and put constraints
on the location of the gamma-ray source. Cygnus X-3 exhibits complex temporal
and spectral patterns in X-rays. During gamma-ray flares, the X-ray emission
can be approximated by a bright disk black body component and a non-thermal
tail extending in hard X-rays, possibly related to a corona above the disk. We
calculate numerically the exact optical depth for gamma rays above a standard
accretion disk. Emission and absorption in the corona are also investigated.
GeV gamma rays are significantly absorbed by soft X-rays emitted from the inner
parts of the accretion disk. The absorption pattern is complex and anisotropic.
Isotropization of X-rays due to Thomson scattering in the companion star wind
tends to increase the gamma-ray opacity. Gamma rays from the corona suffer from
strong absorption by photons from the disk and cannot explain the observed
high-energy emission, unless the corona is unrealistically extended. The lack
of absorption feature in the GeV emission indicates that high-energy gamma rays
should be located at a minimum distance ~10^8-10^10 cm from the compact object.
The gamma-ray emission is unlikely to have a coronal origin.Comment: 11 pages, 9 figures, accepted for publication in Astronomy and
Astrophysic
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