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 14Dr. Matthew H. Schneps, LVL Director
(Umass Boston, Computer Science)
(host: Dr. Bernard Zygelman)
The Physics of (mis)Reading
For those who read well, reading feels as if it is a completely natural process, something that is so automatic and natural it seems akin to breathing. However, for people with dyslexia who struggle to read, reading is anything but natural. In fact, reading is among one of the most complex tasks we require people to master, and the task itself places severe constraints on the "detectors" and "processors" making up the circuits in the human brain used for reading. Here, we will examine some of these fundamental constraints from an information processing perspective, and raise the question of whether the methods for reading we have been using for centuries is appropriate given our neurology, or whether reading should be redesigned in view of new technologies for reading. (This is a non-technical talk accessible to all.)
Nov 21
Nov 28Thanksgiving Day Recess
Dec 5PHY-493 Presentations
Dec 12Finals Week

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