UNLV Department of Physics and Astronomy

Forum Schedule:  Fridays 3:45pm - 4:45pm

DateSpeakerTitle & Abstract
Aug 22Prof. Andrzej Niedzwiecki
(University of Lodz, Poland)
(host: Dr. Daniel Proga)
X-ray emission from hot accretion flows
Optically thin, two-temperature accretion flows are widely considered to be responsible for the hard X-ray continuum of low-luminosity AGNs and X-ray binaries in their hard states. I briefly review the current observational status and address some open theoretical issues. I show that comparison between predictions of our recently developed (fully GR and with an exact treatment of radiative processes) model with X-ray data allows to set constraints on the unknown microphysics of hot flows, in particular the strength of magnetic field and the efficiency of electron heating. I also point out that a self-consistent implementation of hadronic processes (which must occur due to the two-temperature structure) provides an attractive explanation for some open problems. Namely, basic properties of hot flows determine the relative strengths of the synchrotron radiation of thermal electrons and non-thermal electrons from charged-pion decay, in a manner consistent with observations. In AGNs, the non-thermal synchrotron dominates the seed photon input down to ~10^-5 L_Edd and it allows to explain the X-ray spectral index–Eddington ratio relation as well as the cut-off energies measured in the best-studied AGNs; in contrast, the (standard ADAF) model with seed photons only from the thermal synchrotron does not agree with these observations. For stellar-mass black holes, non-thermal electrons from hadronic processes become important only above ~0.01 L_Edd; again in agreement with observations, we find that the thermal synchrotron provides a sufficient seed photon flux to explain evolution of black hole transients below ~0.01 L_Edd. Above ~0.01 L_Edd, the pion-decay electrons may be relevant for the non-thermal tails observed in luminous hard states of Cyg X-1 and GX 339-4 and, even more importantly, they allow to explain stability of luminous hot flows, with L ~ 0.1 L_Edd.
Aug 29
Sep 5
Sep 12
Sep 19
Sep 26
Oct 3
Oct 10
Oct 17Wayne Hayes
(host: Dr. George Rhee)
Oct 24
Oct 31Nevada Day Recess
Nov 7
Nov 14
Nov 21
Nov 28Thanksgiving Day Recess
Dec 5PHY-493 Presentations
Dec 12Finals Week

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