Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session R5: The Galactic Center ExcessFocus Undergraduate
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Sponsoring Units: DAP Chair: Timothy Brandt, Institute for Advanced Study Room: Virginia B |
Monday, January 30, 2017 10:45AM - 11:21AM |
R5.00001: The GeV Excess in the Galactic Center Invited Speaker: Tim Linden Over the last seven years, Fermi-LAT observations have convincingly found an excess in gamma-ray emission emanating from the center of the Milky Way galaxy. The excess has three definitive properties: (1) it has a hard spectrum that peaks at an energy of ~2 GeV, (2) it extends from within 0.1 degrees to more than 10 degrees from Sgr A* with a three-dimensional intensity that falls roughly as $r^{-2},$ (3) it is approximately spherically symmetric. Several models for this excess have been formulated, including the collective emission from a population of individually dim gamma-ray pulsars, outbursts of cosmic-ray electrons from the central molecular zone, or potentially even dark matter annihilation. In this overview, I will discuss the observational data, and the arguments for and against each theoretical model. Additionally, I will discuss significant improvements in gamma-ray diffuse emission modeling that enhance our understanding of high energy astrophysics near the Galactic center, and will describe the impact of these models on our understanding of the gamma-ray excess. [Preview Abstract] |
Monday, January 30, 2017 11:21AM - 11:33AM |
R5.00002: Evidence for Spatial Variation in the High-Energy Spectrum of the Galactic Center Excess Anna Kwa, Shunsaku Horiuchi, Manoj Kaplinghat Fermi Large Area Telescope observations towards the Milky Way center have revealed a spatially extended source of gamma rays in excess of the modeled astrophysical backgrounds. Possible explanations for this `galactic center excess' include weakly-interacting massive particle dark matter annihilations, unresolved milllisecond pulsars, and cosmic-ray outbursts from the galactic center. I will discuss an analysis comparing the the spatial morphology and spectrum of the excess signal in the innermost few degrees of the galactic center versus the outlying sky regions. We find that the excess spectrum above $\sim$10 GeV is spatially varying: the spectrum extends above these energies outside of $\sim5^\circ$ in galactocentric radius, but cuts off sharply by $\sim$10 GeV in the innermost few degrees. If interpreted as a real feature of the excess, this radial variation in the spectrum has important implications for both astrophysical and dark matter interpretations of the galactic center excess. Single-component dark matter annihilation models face challenges in reproducing this variation; on the other hand, a population of unresolved millisecond pulsars contributing both prompt and secondary inverse Compton emission may be able to explain the spectrum as well as its spatial dependency. [Preview Abstract] |
Monday, January 30, 2017 11:33AM - 11:45AM |
R5.00003: The GeV Galactic Center Excess – Uncertainties in the Measured Spectrum and Morphology and Dark Matter Implications Andrea Albert, Dmitry Malyshev, Anna Franckowiak, Luigi Tilbaldo, Mattia Di Mauro Gamma-ray emission from dark matter (DM) annihilation is expected to peak toward the Galactic center (GC), which shines in gamma rays due to interactions of cosmic rays with interstellar gas and radiation in the Galaxy, and many energetic objects along the line of sight. Many groups have found an excess in the Fermi Large Area Telescope (LAT) data toward the GC peaking around a few GeV. Morphological and spectral arguments have been made to support both DM and more standard astrophysical interpretations. We assess the uncertainties on the morphology and spectrum of the excess related to modeling the various components of gamma-ray emission in that region, using 6.5 years of LAT data. We consider uncertainties in the distribution of interstellar gas along the line of sight, in the low-latitude emission from the Fermi bubbles, and in the abundance of cosmic-ray sources in the innermost Galaxy. The excess persists in all the models considered, though the spectrum varies significantly. To test the robustness of a DM interpretation, we search for DM-like excesses along the Galactic plane. We find no significant detection of DM and set upper limits that assume the DM is centrally peaking in the inner Galaxy. Our limits are competitive with limits from dwarf galaxies and other targets. [Preview Abstract] |
Monday, January 30, 2017 11:45AM - 11:57AM |
R5.00004: Implications of tension between observations of the bright Galactic Center Excess and dim dwarf galaxies Ryan Keeley, Kevork Abazajian We incorporate Milky Way dark matter halo profile uncertainties, as well as an accounting of diffuse gamma-ray emission uncertainties in dark matter annihilation models for the Galactic Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma Ray Space Telescope. The range of particle annihilation rate and masses expand when including these unknowns. However, two of the most precise empirical determinations of the Milky Way halo's local density and density profile leave the signal region to be in considerable tension with dark matter annihilation searches from combined dwarf galaxy analyses for single-channel dark matter annihilation models. Accordingly, we accurately quantify this tension in a joint likelihood analysis. We determine which particle dark matter models are favored in the case of a high local dark matter density scenario and in the case where dark matter differentiates between the GC and dwarfs, specifying what part of the parameter space for such models is most favored. Intended for the focus session on the GeV excess [Preview Abstract] |
Monday, January 30, 2017 11:57AM - 12:09PM |
R5.00005: Probing dim point sources in the inner Milky Way using PCAT Tansu Daylan, Stephen K. N. Portillo, Douglas P. Finkbeiner Poisson regression of the Fermi-LAT data in the inner Milky Way reveals an extended gamma-ray excess. An important question is whether the signal is coming from a collection of unresolved point sources, possibly old recycled pulsars, or constitutes a truly diffuse emission component. Previous analyses have relied on non-Poissonian template fits or wavelet decomposition of the Fermi-LAT data, which find evidence for a population of dim point sources just below the 3FGL flux limit. In order to be able to draw conclusions about the flux distribution of point sources at the dim end, we employ a Bayesian trans-dimensional MCMC framework by taking samples from the space of catalogs consistent with the observed gamma-ray emission in the inner Milky Way. The software implementation, PCAT (Probabilistic Cataloger), is designed to efficiently explore that catalog space in the crowded field limit such as in the galactic plane, where the model PSF, point source positions and fluxes are highly degenerate. We thus generate fair realizations of the underlying MSP population in the inner galaxy and constrain the population characteristics such as the radial and flux distribution of such sources. [Preview Abstract] |
Monday, January 30, 2017 12:09PM - 12:21PM |
R5.00006: Searches for point sources in the Galactic Center region Mattia Di Mauro Several groups have demonstrated the existence of an excess in the gamma-ray emission around the Galactic Center (GC) with respect to the predictions from a variety of Galactic Interstellar Emission Models (GIEMs) and point source catalogs. The origin of this excess, peaked at a few GeV, is still under debate. A possible interpretation is that it comes from a population of unresolved Millisecond Pulsars (MSPs) in the Galactic bulge. We investigate the detection of point sources in the GC region using new tools which the Fermi-LAT Collaboration is developing in the context of searches for Dark Matter (DM) signals. These new tools perform very fast scans iteratively testing for additional point sources at each of the pixels of the region of interest. We show also how to discriminate between point sources and structural residuals from the GIEM. We apply these methods to the GC region considering different GIEMs and testing the DM and MSPs intepretations for the GC excess. Additionally, we create a list of promising MSP candidates that could represent the brightest sources of a MSP bulge population. [Preview Abstract] |
Monday, January 30, 2017 12:21PM - 12:33PM |
R5.00007: The Fermi Galactic Center excess as a signal from Bursts of Cosmic-Rays Ilias Cholis, Carmelo Evoli, Francesca Calore, Tim Linden, Christoph Weniger, Dan Hooper The possible gamma-ray excess in the inner Galaxy and the Galactic center suggested by Fermi-LAT observations has triggered great interest in the astro-particle physics community. Among its various interpretations have been WIMP dark matter annihilations, gamma-ray emission from a population of millisecond pulsars, or emission from cosmic rays injected in a sequence of burst-like events or continuously at the Galactic Center. Accounting for the model systematics coming from the Galactic diffuse emission in the inner part of our Galaxy, we will present a comprehensive study of the latter possibility. A small series of outbursts, which took place approximately a million years ago, and 100s of thousands years ago and more recently can account for the observed gamma-ray signal. Furthermore strong re-acceleration and rapid synchrotron cooling conditions within the inner tens of pc of the Galaxy, could explain the hard spectrum of the observed gamma-ray excess. In fact, a connection to the Fermi Bubbles can be made. [Preview Abstract] |
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