2 research outputs found
Effects of non-thermal tails in Maxwellian electron distributions on synchrotron and Compton processes
We investigate how the presence of a non-thermal tail beyond a Maxwellian
electron distribution affects the synchrotron process as well as Comptonization
in plasmas with parameters typical for accretion flows onto black holes. We
find that the presence of the tail can significantly increase the net (after
accounting for self-absorption) cyclo-synchrotron emission of the plasma, which
emission then provides seed photons for Compton upscattering. Thus, the
luminosity in the thermally-Comptonized spectrum is enhanced as well. The
importance of these effects increases with both increasing Eddington ratio and
the black hole mass. The enhancement of the Comptonized synchrotron luminosity
can be as large as by factors of and for stellar and
supermassive black holes, respectively, when the energy content in the
non-thermal tail is 1 per cent.
The presence of the tail only weakly hardens the thermal Comptonization
spectrum but it leads to formation of a high-energy tail beyond the thermal
cut-off, which two effects are independent of the nature of the seed photons.
Since observations of high-energy tails in Comptonization spectra can constrain
the non-thermal tails in the electron distribution and thus the Comptonized
synchrotron luminosity, they provide upper limits on the strength of magnetic
fields in accretion flows. In particular, the measurement of an MeV tail in the
hard state of Cyg X-1 by McConnell et al. implies the magnetic field strength
in this source to be at most an order of magnitude below equipartition.Comment: 10 pages, 9 postscript figures. Accepted to MNRA
X-ray and gamma-ray spectra and variability of the black-hole candidate GX 339-4
We analyse five observations of the X-ray binary GX 339-4 by the soft
gamma-ray OSSE detector on board CGRO simultaneous with either Ginga or RXTE
observations. The source was bright during four of them, with the luminosity of
L ~ 10^{37} erg/s and the spectrum typical for hard states of accreting black
holes, and it was in an off state during the fifth one, with L ~ 10^{35} erg/s.
Our broad-band spectral fits show the mean electron energy of electrons in the
Comptonizing plasma decreasing with increasing luminosity within the hard
(bright) state. For the observation with the best statistics at soft
gamma-rays, approximately 1/4 of energy in the Comptonizing plasma is probably
carried by non-thermal electrons. Then, considering the efficiency of
Comptonized hybrid synchrotron emission allows us to obtain an upper limit on
the strength of the magnetic field in the X-ray source. Furthermore, this
synchrotron emission is capable of producing the optical spectrum observed in
an optically-high state of GX 339-4. In the off state, the hard X-ray spectrum
is consistent with being dominated by bremsstrahlung. The unusually strong Fe K
alpha line observed by the PCA during that state is found not to be intrinsic
to the source but to originate mostly in the Galactic diffuse emission.Comment: 12 pages, 7 figures (2 in colour). Accepted for publication in MNRA