Gamma-ray emission from the globular clusters Liller 1, M80, NGC 6139, NGC 6541, NGC 6624, and NGC 6752

Globular clusters (GCs) are emerging as a new class of gamma-ray emitters, thanks to the data obtained from the Fermi Gamma-ray Space Telescope. By now, eight GCs are known to emit gamma-rays at energies >100~MeV. Based on the stellar encounter rate of the GCs, we identify potential gamma-ray emitting GCs out of all known GCs that have not been studied in details before. In this paper, we report the discovery of a number of new gamma-ray GCs: Liller 1, NGC 6624, and NGC 6752, and evidence for gamma-ray emission from M80, NGC 6139, and NGC 6541, in which gamma-rays were found within the GC tidal radius. With one of the highest metallicity among all GCs in the Milky Way, the gamma-ray luminosity of Liller 1 is found to be the highest of all known gamma-ray GCs. In addition, we confirm a previous report of significant gamma-ray emitting region next to NGC 6441. We briefly discuss the observed offset of gamma-rays from some GC cores. The increasing number of known gamma-ray GCs at distances out to ~10 kpc is important for us to understand the gamma-ray emitting mechanism and provides an alternative probe to the underlying millisecond pulsar populations of the GCs.Comment: 22 pages, 7 figures, 2 tables; ApJ, in pres

BL Lacertae Objects and the Extragalactic Gamma-Ray Background

A tight correlation between gamma-ray and radio emission is found for a sample of BL Lacertae (BL Lac) objects detected by Fermi Gamma-ray Space Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The gamma-ray emission of BL Lac objects exhibits strong variability, and the detection rate of gamma-ray BL Lac objects is low, which may be related to the gamma-ray duty cycle of BL Lac objects. We estimate the gamma-ray duty cycle ~ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of gamma-ray emission with radio emission and the estimated gamma-ray duty cycle, we derive the gamma-ray luminosity function (LF) of BL Lac objects from their radio LF. Our derived gamma-ray LF of BL Lac objects can almost reproduce that calculated with the recently released Fermi bright active galactic nuclei (AGN) sample. We find that about 45% of the extragalactic diffuse gamma-ray background (EGRB) is contributed by BL Lac objects. Combining the estimate of the quasar contribution to the EGRB in the previous work, we find that about 77% of the EGRB is contributed by BL Lac objects and radio quasars.Comment: 12 pages, 4 figures, submitte

Gamma-Ray Light Curves and Spectra of Models for Type-Ia Supernovae

Based on detailed Monte Carlo calculations, we present gamma-ray energy deposition functions, gamma-ray light curves, and gamma-ray spectra for a large set of theoretical models of Type Ia supernovae including ''classical'' detonation and deflagration, delayed detonation, explosions of low mass white dwarfs, and tamped detonation scenarios. Our computations show that models for Type Ia supernovae can be discriminated and the absolute amount of Ni-56 synthesized in the event can be determined on the basis of the gamma-ray light curves and spectra if gamma-ray measurements are combined with observations at other wavelengths, e.g., in the optical band. We discuss at which times gamma-ray observations are most suitable and needed from the theoretical point of view. The implication of the upper limit in the gamma-ray flux by CGRO experiment for our understanding of SN 1991 T is discussed. We find that this limit is consistent with both the optical light curve and the implied distance (12.5 Mpc), i.e., several models can be ruled out by the gamma-ray observations.Astronom

Fermi LAT measurements of diffuse gamma-ray emission: results at the first-year milestone

For more than one year the Fermi Large Area Telescope has been surveying the gamma-ray sky from 20 MeV to more than 300 GeV with unprecedented statistics and angular resolution. One of the key science targets of the Fermi mission is diffuse gamma-ray emission. Galactic interstellar gamma-ray emission is produced by interactions of high-energy cosmic rays with the interstellar gas and radiation field. We review the most important results on the subject obtained so far: the non-confirmation of the excess of diffuse GeV emission seen by EGRET, the measurement of the gamma-ray emissivity spectrum of local interstellar gas, the study of the gradient of cosmic-ray densities and of the X(CO)=N(H2)/W(CO) ratio in the outer Galaxy. We also catch a glimpse at diffuse gamma-ray emission in the Large Magellanic Cloud. These results allow the improvement of large-scale models of Galactic diffuse gamma-ray emission and new measurements of the extragalactic gamma-ray background.Comment: Contribution to the Workshop SciNeGHe 2009/Gamma-ray Physics in the LHC era (Assisi - Italy, Oct. 7-9 2009); 10 pages, 6 figure

GeV Analysis of Mixed Morphology Supernova Remnants Interacting with Molecular Clouds

The first remnants detected by the Fermi Gamma-ray Space Telescope were of the type of mixed-morphology supernova remnants interacting with molecular clouds. In this paper we are presenting preliminary results of the gamma-ray analysis of 4 selected mixed morphology remnants, G359.1-0.5, G337.8-0.1, G001.0-0.1, and G346.6-0.2, as well as G349.7+0.2, in the 0.2 - 300 GeV energy range from the data collected by Fermi Gamma-ray Space Telescope for 3 years. G359.1-0.5, G337.8-0.1, and G349.7+0.2 were all detected with significances above 5 sigma. The excess distribution of G359.1-0.5 shows extended gamma-ray emission, which coincides with the TeV gamma-ray source HESS J1745-303. G337.8-0.1 also shows an extended nature.Comment: High Energy Gamma-Ray Astronomy: 5th International Meeting on High Energy Gamma-Ray Astronom

Gamma Rays from Star Formation in Clusters of Galaxies

Star formation in galaxies is observed to be associated with gamma-ray emission. The detection of gamma rays from star-forming galaxies by the Fermi Large Area Telescope (LAT) has allowed the determination of a functional relationship between star formation rate and gamma-ray luminosity (Ackermann et. al. 2012). Since star formation is known to scale with total infrared (8-1000 micrometers) and radio (1.4 GHz) luminosity, the observed infrared and radio emission from a star-forming galaxy can be used to quantitatively infer the galaxy's gamma-ray luminosity. Similarly, star forming galaxies within galaxy clusters allow us to derive lower limits on the gamma-ray emission from clusters, which have not yet been conclusively detected in gamma rays. In this study we apply the relationships between gamma-ray luminosity and radio and IR luminosities derived in Ackermann et. al. 2012 to a sample of galaxy clusters from Ackermann et. al. 2010 in order to place lower limits on the gamma-ray emission associated with star formation in galaxy clusters. We find that several clusters have predicted lower limits on gamma-ray emission that are within an order of magnitude of the upper limits derived in Ackermann et. al. 2010 based on non-detection by Fermi-LAT. Given the current gamma-ray limits, star formation likely plays a significant role in the gamma-ray emission in some clusters, especially those with cool cores. We predict that both Fermi-LAT over the course of its lifetime and the future Cherenkov Telescope Array will be able to detect gamma-ray emission from star-forming galaxies in clusters.Comment: 17 pages, 2 figures, 2 tables. Minor revisions made to match version accepted to Ap

Short Gamma Ray Bursts as possible electromagnetic counterpart of coalescing binary systems

Coalescing binary systems, consisting of two collapsed objects, are among the most promising sources of high frequency gravitational waves signals detectable, in principle, by ground-based interferometers. Binary systems of Neutron Star or Black Hole/Neutron Star mergers should also give rise to short Gamma Ray Bursts, a subclass of Gamma Ray Bursts. Short-hard-Gamma Ray Bursts might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most short Gamma Ray Bursts originate from binaries of Neutron Star or Black Hole/Neutron Star mergers, we outline here the possibility to associate short Gamma Ray Bursts as electromagnetic counterpart of coalescing binary systems.Comment: 4 pages, 1 figur

Swift Pointing and Gravitational-Wave Bursts from Gamma-Ray Burst Events

The currently accepted model for gamma-ray burst phenomena involves the violent formation of a rapidly rotating solar-mass black hole. Gravitational waves should be associated with the black-hole formation, and their detection would permit this model to be tested. Even upper limits on the gravitational-wave strength associated with gamma-ray bursts could constrain the gamma-ray burst model. This requires joint observations of gamma-ray burst events with gravitational and gamma-ray detectors. Here we examine how the quality of an upper limit on the gravitational-wave strength associated with gamma-ray bursts depends on the relative orientation of the gamma-ray-burst and gravitational-wave detectors, and apply our results to the particular case of the Swift Burst-Alert Telescope (BAT) and the LIGO gravitational-wave detectors. A result of this investigation is a science-based ``figure of merit'' that can be used, together with other mission constraints, to optimize the pointing of the Swift telescope for the detection of gravitational waves associated with gamma-ray bursts.Comment: iop style, 1 figure, 6 pages, presented at GWDAW 200