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 3Mr. Nathan Rowe
(Full Spectrum Laser, LLC)
(host: Mr. Timothy Waters)
Stereolithography and 3D Printing
There are three kinds of 3D printers commonly in use today: Extruder based printers (FDM), photosensitive liquid curing printers (SLA), and powder melting or sintering printers (SLS). We will cover the basic capabilities of these kinds of 3D printers. Also, the relative advantages and disadvantages of each type of printer will be discussed including: printer costs, material costs, resolution, reliability, setup, cleanup, and printer capabilities. Particular focus will be given to SLA printers including: resin formulation, light sources, lifting vs sinking based construction, and build plate and vat properties. Short bio: Nathan Rowe is a UNLV alumni with his B.S. and M.S. in Mathematical Sciences. He has worked in the security industry designing software systems for CT scanners and writing computer-learning based threat-detection algorithms. He currently works at Full Spectrum Laser on the Pegasus Touch 3D SLA printer. His job mostly entails writing computational geometry software and analysis software which helps determine whether and where supports are necessary to print a given 3D model.
Oct 10
Oct 17Wayne Hayes
(host: Dr. George Rhee)
SpArcFiRe: Scalable Automated Detection of Spiral Galaxy Arm Segments
Near future ground-based surveys such as PAN-STARRS and LSST will provide images of millions of new galaxies, and space-based surveys may contain upwards of 10^11 galaxies at the resolution of the Hubble Space Telescope. Currently, the only large-scale, visual, structural information about galaxies comes from human-based surveys like Galaxy Zoo. Such surveys are useful but do not provide objective, quantitative structural measures. We introduce a new code called SpArcFiRe (Spiral Arc Finder and Reporter) that finds and quantitatively reports visible arm structure in an image of a spiral galaxy. For each arm segment found, we list the pixels in that segment, allowing image analysis on a per-arm-segment basis. We also perform a least-squares fit of a logarithmic spiral arc to the pixels in that segment, giving per-arc parameters such as the pitch angle, arm segment length, location, etc. The algorithm takes about 1 minute per galaxy, and can easily be scaled using parallelism. We have run it on all 644,000 Sloan objects that are larger than 40 pixels across and classified as "galaxy". We find a very good correlation between our quantitative description of spiral structure and the qualitative description provided by Galaxy Zoo humans. Our objective, quantitative measures of structure demonstrate the difficulty in defining exactly what constitutes a spiral "arm", leading us to prefer the term "arm segment". We find that pitch angle often varies significantly segment-to-segment in a single spiral galaxy, making it difficult to define "the" pitch angle for a single galaxy. We demonstrate how our new database of arm segments can be queried to find galaxies satisfying specific quantitative visual criteria. For example, even though our code does not explicitly find rings, a good surrogate is to look for galaxies having one long, low-pitch-angle arm-which is how our code views ring galaxies. SpArcFiRe is available at http://sparcfire.ics.uci.edu.
Oct 24Dr. Binbin Zhang
(University of Alabama, Huntsville)
(host: Dr. Bing Zhang)
How Long does a Burst Burst?
Several gamma-ray bursts (GRBs) last much longer (~ hours) in gamma-rays than typical long GRBs (~ minutes), and recently it was proposed that these "ultra-long GRBs" may form a distinct population, probably with a different (e.g. blue supergiant) progenitor than typical GRBs. However, Swift observations have suggested that many GRBs have extended central engine activities manifested as flares and internal plateaus in X-rays. We perform a comprehensive study on a large sample of Swift GRBs with XRT observations to investigate GRB central engine activity duration and to check whether ultra-long GRBs are special. We define burst duration t_{burst} based on both gamma-ray and X-ray light curves rather than using gamma-ray observations only. We show that the distribution of t_{burst} peaks at ~ 320s for the entire sample, with 17.6% GRBs having t_{burst} > 10^3 s and 5.4% GRBs having t_{burst} > 10^4 s. The distribution shows a tail at the long t_{burst} end. The existence of a separate population is not ruled out, but is also not strongly supported by the data. We discuss the theoretical implications of our results. In particular, the central engine activity duration of GRBs is generally much longer than the gamma-ray T_{90} duration, and it would be premature to make a direct connection between T_{90} and the size of the progenitor star.
Oct 31Nevada Day Recess
Nov 7Dr. Wen-fai Fong
(University of Arizona)
(host: Dr. Bing Zhang)
Setting the Stage for the Era of Gravitational Wave Discovery
The first advanced gravitational wave detectors are slated to begin operations in 2015, and will bring one of the most anticipated discoveries of the century: the direct detection of gravitational waves. The premier gravitational wave sources are the mergers of two compact objects, involving either two neutron stars or a neutron star and a black hole. While the gravitational wave signal will give insight to the basic properties of compact objects, a coincident detection at electromagnetic wavelengths will significantly leverage the event by providing precise sky localization and an association to a galaxy. The main challenge will be how to identify the correct electromagnetic counterpart amidst an otherwise dynamic sky. In this talk, I present ongoing efforts to characterize the electromagnetic signatures from compact object mergers. In particular, I present observational evidence linking mergers to two distinct counterparts: short-duration gamma-ray bursts (GRBs) and long-lived transients powered by the nucleosynthesis of heavy elements ("kilonovae"). Such observations are crucial in setting the stage for the upcoming revolutionary era of gravitational wave discovery.
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 1
(Monday 3:45PM)
Prof. Dr. Werner Becker
(Max Planck Institute for Extraterrestrial Physics)
(host: Dr. Daniel Proga)
eROSITA: Status and Scientific Prospects
eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is currently scheduled for launch in spring 2016. eROSITA will perform a deep survey of the entire X-ray sky. In the soft band (0.5-2 keV), it will be about 30 times more sensitive than ROSAT, while in the hard band (2-8 keV) it will provide the first ever true imaging survey of the sky. The design driving science is the detection of large samples of galaxy clusters to redshifts z > 1 in order to study the large scale structure in the Universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGN, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including neutron stars and pulsars, X-ray binaries, active stars and diffuse emission from supernova remnants. The talk reports on the status of eROSITA and its scientific prospects.
Dec 5PHY-493 Presentations
Dec 12Prof. Soebur Razzaque
(University of Johannesburg, South Africa)
(host: Dr. Bing Zhang)
Fermi Bubbles and IceCube Neutrino Events
The Fermi gamma-ray telescope has recently discovered two giant globular structures, called Fermi Bubbles, at the center of our Galaxy with up to a few hundred GeV energy. Intense star-formation activity at the Galactic center has been proposed as a mechanism to blow these bubbles through a wind. Appealing to hadronic interactions of high-energy cosmic rays that fill the bubble volume to produce observed gamma rays, I will show that a fraction of the 30 TeV - 2 PeV neutrinos recently detected by IceCube can originate from the Fermi Bubbles. I will also discuss possibilities to detect 10 TeV - 2 PeV gamma rays from the bubbles by IceCube and HAWC in order to verify the neutrino production model.

Past forums: Spring '14   Fall '13   Spring '13   Fall '12 Spring '12   Fall '11   Spring '11   Fall '10   Spring '10   Fall '09   Spring '09   Fall '08