2 research outputs found
The Brightest Black Holes
I suggest that there are two classes of ultraluminous X-ray sources (ULXs),
corresponding to super-Eddington mass inflow in two situations: (a)
thermal-timescale mass transfer in high-mass X-ray binaries, and (b)
long-lasting transient outbursts in low-mass X-ray binaries. These two classes
are exemplified by SS433 and microquasars like GRS 1915+105 respectively. The
observed ULX population is a varying mixture of the two, depending on the star
formation history of the host galaxy. ULXs in galaxies with vigorous star
formation (such as the Antennae) are generally SS433--like, while ULXs in
elliptical galaxies must be of the microquasar type. The latter probably have
significantly anisotropic radiation patterns. They should also be variable, but
demonstrating this may require observations over decades. The close analogy
between models of X-ray binaries and active galactic nuclei (AGN) suggests that
there should exist an apparently super-Eddington class of the latter, which may
be the ultrasoft AGN, and a set of X-ray binaries with Doppler--boosted X-ray
emission. These are presumably a subset of the ULXs, but remain as yet
unidentified.Comment: 4 pages, no figures; accepted for MNRAS Letter
Constraints on particle acceleration in SS433/W50 from MAGIC and H.E.S.S. observations
Context. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system.Aim. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses (Porb~ 13.1 days) and precession of the circumstellar disk (Ppre ~ 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations.Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to ~40â80 h, depending on the region, was employed.Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be âČ 10â12â10â13 TeVâ1 cmâ2 sâ1 in an energy interval ranging from ~few Ă 100 GeV to ~few TeV. Integral flux limits down to ~ 10â12â10â13 ph cmâ2 sâ1 and ~ 10â13â10â14 ph cmâ2 sâ1 are obtainedat 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jetregions and on the physics of the jet/medium interactions.Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, qp †2.5 Ă 10â5, to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields âł10 ÎŒG is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with Eeâ up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.Key words: gamma rays: general / stars: black holes / X-rays: binaries / ISM: jets and outflow