Session M4: Invited Session: Hot Topics in Astrophysics

Hot News from the Milky Way's Central Black Hole

The recent discovery of a dense, cold cloud (dubbed ``G2'') approaching the supermassive black hole at our Galactic Center (Sgr A*) offers an unprecedented opportunity to test models of black hole accretion and its associated feedback. G2's orbit is eccentric and the cloud already shows signs of tidal disruption by the black hole. High-energy emission from the Sgr A*/G2 encounter will likely rise toward pericenter (Spring 2014) and continue over the next several years as the material circularizes. This encounter may also enhance Sgr A*'s flare activity across the electromagnetic spectrum. I will present intensive multiwavelength campaigns (X-ray through radio) aimed at studying the radiation properties of Sgr A* as G2 breaks up and feeds the accretion flow, to constrain the rates and emission mechanisms of faint X-ray flares, and to detect G2 itself as it is shocked and heated.   

Hot News from NuSTAR about black hole spin

Measurement of black hole spin has the potential to enhance our understanding in a wide variety of key astrophysical topics, including galaxy formation and the growth of supermassive black holes, supernova/GRB explosions, and relativistic jets. The best methods for measuring black hole spin currently available are anchored in X-ray spectroscopy, and ultimately rely on constraining the radius of the innermost stable circular orbit (ISCO), which relates directly to spin. Although such measurements are in their relative infancy, substantial progress has been made over the last few years. NuSTAR has undertaken a major program, coordinated with XMM, Swift and Suzaku, to obtain the highest-quality broad band X-ray spectra from AGN and BH binaries to date, with the aim of obtaining spin constraints. The quality of the data not only allows us to make robust constraints, but also challenge the physical assumptions inherent in the relativistic reflection models primarily utilized for these measurements. We review the current status of this program, highlighting in particular some of the early observational results obtained.   

Hot News from the HAWC Gamma-Ray Observatory

The High-Altitude Water Cherenkov (HAWC) TeV Gamma-Ray Observatory is currently under construction at a site about two hours' drive east of Puebla, Mexico on the Sierra Negra plateau (4100 m a.s.l.). HAWC is unique among TeV gamma-ray instruments in that it can observe large portions of the sky simultaneously, and covers half the sky every 24 hours. Therefore, the detector is particularly well-suited to measure extended and large-scale structures in the sky such as diffuse galactic gamma-ray emission and large- and small-scale anisotropies. Discoveries of other extended unidentified objects at TeV energies, for example collocated with the ``Fermi Bubbles,'' and the observation of transient phenomena such as GRBs, are also possible. The construction of HAWC funded through NSF, DoE, and CONACyT, is expected to be complete by Fall 2014. Data are already being collected during construction with an increasingly sensitive detector allowing for synchronous observations with instruments at other wavebands like the Fermi Space Telescope. Analysis of the data set reveals significant anisotropies in the arrival directions of cosmic rays, both on small (below 10s of degrees) and large angular scales. A number of gamma-ray hot spots are also observed along the Galactic plane and the data have been searched for high-energy emission from GRBs detected at lower energies. I will present first results and scientific potential of the experiment.\\[4pt] We acknowledge the support from: US National Science Foundation (NSF); US Department of Energy Office of High-Energy Physics; The Laboratory Directed Research and Development (LDRD) program of Los Alamos National Laboratory; CONACyT, Mexico; Red de F\'isica de Altas Energ\'as, Mexico; DGAPA-UNAM, Mexico; and the University of Wisconsin Alumni Research Foundation.