Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session U4: New High Energy Views of the GalaxyInvited Session
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Sponsoring Units: DAP Chair: Julie McEnery, NASA Room: Ballroom C |
Monday, April 18, 2016 3:30PM - 4:06PM |
U4.00001: The HAWC Galactic Plane Survey Invited Speaker: Michelle Hui The High Altitude Water Cherenkov (HAWC) Observatory is an all-sky surveying instrument that covers 2/3 of the sky in 24 hours. It is designed with an emphasis on continuous sky coverage for transient events, and on the measurement of extended and large-scale structures. The array is located in Sierra Negra, Mexico at an elevation of 4,100 m and was inaugurated in March 2015. The HAWC array consists of 300 water Cherenkov detectors and is sensitive to extensive air showers triggered by cosmic rays and gamma rays from 100 GeV to $>$100 TeV. Thanks to its modular design, data taking began in Summer 2013 with 1/3 of the array. Analysis of the first year of data with the partial array shows detections that are coincident with known TeV supernova remnants and pulsar wind nebulae along the Galactic plane. Spectral and morphological analyses are ongoing to study the particle population and acceleration mechanism of these objects. With a growing data set taken with the completed array, source searches are underway for both point-like and extended emission along the Galactic plane, which contain many objects such as pulsar wind nebulae, young star clusters, and binaries. [Preview Abstract] |
Monday, April 18, 2016 4:06PM - 4:42PM |
U4.00002: The Origin of the GeV Galactic Center Excess Invited Speaker: Tracy Slatyer Studies of data from the Fermi Gamma-Ray Space Telescope have revealed bright gamma-ray emission from the central regions of our galaxy, with a spatial and spectral profile consistent with annihilating dark matter. However, recent model-independent analyses suggest that rather than originating from dark matter, the GeV excess may arise from a surprising new population of as-yet-unresolved gamma-ray point sources in the heart of the Milky Way. I will discuss the status of possible explanations for the excess, and prospects for the future. [Preview Abstract] |
Monday, April 18, 2016 4:42PM - 5:18PM |
U4.00003: NuSTAR results from the Galactic Center -- diffuse emission Invited Speaker: Charles Hailey The Nuclear Spectroscopic Telescope Array (NuSTAR) was launched in June 2012. It carried the first true, hard X-ray (\textgreater \textasciitilde 10 keV-79 keV) focusing telescopes into orbit. Its twin telescopes provide 10 times better angular resolution and 100 times better sensitivity than previously obtainable in the hard X-ray band. Consequently NuSTAR is able to resolve faint diffuse structures whose hard X-rays offer insight into some of the most energetic processes in the Galactic Center. One of the surprising discoveries that NuSTAR made in the Galactic Center is the central hard X-ray emission (CHXE). The CHXE is a diffuse emission detected from \textasciitilde 10 keV to beyond 50 keV in X-ray energy, and extending spatially over a region \textasciitilde 8 parsecs x \textasciitilde 4 parsecs in and out of the plane of the galaxy respectively, and centered on the supermassive black hole Sgr A*. The CHXE was speculated to be due to a large population of unresolved black hole X-ray binaries, millisecond pulsars (MSP), a class of highly magnetized white dwarf binaries called intermediate polars, or to particle outflows from Sgr A*. The presence of an unexpectedly large population of MSP in the Galactic Center would be particularly interesting, since MSP emitting at higher energies and over a much larger region have been posited to be the origin of the gamma-ray emission that is also ascribed to dark matter annihilation in the galaxy. In addition, the connection of the CHXE to the \textasciitilde 9000 unidentified X-ray sources in the central \textasciitilde 100 pc detected by the Chandra Observatory, to the soft X-ray emission detected by the Chandra and XMM/Newton observatories in the Galactic Center, and to the hard X-ray emission detected by both the RXTE and INTEGRAL observatories in the Galactic Ridge, is unclear. I review these results and present recent NuSTAR observations that potentially resolve the origin of the CHXE and point to a unified origin for all these X-ray emissions. Two other noteworthy classes of diffuse structures in the Galactic Center will be discussed. The first class are the giant molecular clouds, which are strong hard X-ray emitters. These hard X-rays are believed to be produced when one or more giant outbursts from the supermassive black hole Sgr A*, more than a century ago, resulted in hard X-rays being reflected from the clouds, and detected only today. I discuss how these hard X-rays are used to elucidate the past history of the supermassive black hole, and to compare and contrast these past giant outbursts with those observed from the supermassive black hole more recently. The second class are non-thermal filaments, magnetized structures with both radio and soft X-ray emission that have now been shown by NuSTAR to be hard X-ray emitters. The electrons generating the hard X-rays observed in one of these filaments are the most energetic that have been observed in the galaxy. The filaments are a heterogeneous class of hard X-ray emitters, and the various mechanisms by which they produce hard X-ray emission will be discussed. Future NuSTAR observations of the Galactic Center with NuSTAR will also be discussed. [Preview Abstract] |
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