Session B8: Galactic Gamma Rays

SNR observations with VERITAS

The acceleration of particles in supernova shocks is widely accepted as the source of cosmic rays below the ``knee'' in the spectrum at 3 $\times$ $10^{15}$ eV. The subsequent production of TeV gamma-rays, either via leptonic processes or hadron-hadron interaction in the surrounding media, gives us a window into these processes. Using VERITAS, an array of 4 Imaging Atmospheric Cherenkov Telescopes located at Mt. Hopkins near Tucson, Arizona, the VERITAS collaboration is searching for evidence of cosmic ray production in nearby supernova remnants. We present a summary of observations and analysis of several supernova remnants observed during the 2007-2008 and 2008-2009 observing seasons.   

VERITAS Observations of Galactic Compact Objects

After 3 years of observations, VERITAS has accrued a significant amount of data on galactic compact objects which are believed to be possible sources of TeV gamma rays such as high and low mass X-ray binaries and magnetars. We present the results of these observations, focusing specifically on LS I +61 303 which is one of only 3 binaries to be reliably detected in the TeV gamma-ray regime. Although LS I +61 303 has been extensively studied, the nature of high energy emission from the source is still poorly understood. We present the results of multi-wavelength observations conducted between 2006 and 2009 on LS I +61 303 utilizing data from VERITAS in the TeV gamma-ray regime, RXTE and Swift in the hard X-ray regime, and Fermi in the GeV gamma-ray regime. While the source is variable in all three of these energy regimes, a coherent relation (or lack thereof) between emission in these bands has not yet emerged.   

VERITAS observations of Geminga

The Geminga x-ray source was first detected by SAS-2 and COS-B, and has been identified as a radio quiet pulsar associated with a 300,000 year old supernova remnant. During 2007 VERITAS performed observations to search for potential TeV gamma ray emission from the Geminga pulsar and the region near Geminga. In this presentation, we describe these measurements and the analysis of these observations.   

Search for emission at 100 GeV from the Crab pulsar in correlation with giant pulses in radio

Pulsars are sources of non-thermal emission from radio energies to about 100 GeV gamma rays. In radio, some pulsars emit strong pulses, which last between a few microseconds and a few nanoseconds. The origin of these giant pulses is not understood but some argue that the emission of gamma-rays and giant pulses is tightly linked. Studies in gamma rays with the EGRET satellite did not find correlated emission with giant pulses. We present results of simultaneous radio-GeV observations with GBT and VERITAS that aim to correlate for the first time radio giant pulses with gamma rays above 100 GeV.   

Search with VERITAS and SGARFACE for bursts of gamma-rays with energy greater than 100 MeV and with duration shorter than 15 $\mu$s

Bursts of gamma rays above 100 MeV and with duration less than 15 $\mu$s might be produced by pulsars (Lyutikov, 2007), by explosions of primordial black holes (Hawking, 1974), or via some as-yet undiscovered mechanism, for example during gamma-ray bursts. Bursts of nearly simultaneous gamma-rays above 100 MeV can be detected by their Cherenkov radiation using ground-based telescopes. SGARFACE piggy-backs on the Whipple 10 m imaging air-Cherenkov telescope to search for bursts of gamma-rays with duration less than 15 $\mu$s. To eliminate cosmic- ray background events that mimic the expected image parameters of bursts, we search for correlations with VERITAS. VERITAS is the most sensitive array of air-Cherenkov telescopes in the northern hemisphere and is located about 7 km distant from the SGARFACE experiment. The fluence sensitivity of this search is 0.2 (0.01) photons/m$^2$ for 1 (10) GeV gamma-rays during a 100 ns burst. About 80 hours of observations have been taken with both instruments pointed at the same source in the sky. We present results of the search for bursts produced by the Crab pulsar, by GRBs, and by the explosion of primordial black holes.   

Gamma-Rays from Pion Production in the Milky Way Halo

Low-energy cosmic rays (CRs) below the ``knee'' are believed to be produced by galactic sources (supernova remnant shocks, pulsars) and are trapped in the galactic magnetic fields whereas higher-energy CRs, up to $\sim10^{18}$~eV, are leaking from the Galaxy. In a recent work, a model of the ``Galactosphere'' --- the up-scaled analog of the Heliosphere --- has been suggested to exist around the Milky Way and produce lower-energy CRs at the galactic termination and bow shocks. We proposed the observational test of the model via pion-produced gamma-rays generated by these TeV ``anomalous extra-galactic CRs'' propagating through the Galactic halo and interacting with hydrogen gas in high-velocity clouds (HVCs) and inside the Galaxy. We estimate the peak of the gamma-ray spectrum to be in the 10~GeV range, hence the signal from HVCs and the north-south galactic asymmetry of the gamma-ray background are potentially detectable by LAT on board of FermiGST.   

A Cosmic-Ray and Thermally Driven Kiloparsec-scale Outflow from the Milky Way

We review the importance of cosmic-ray pressure in helping to drive kpc-scale galactic outflows. In particular, we examine the case of the Milky Way, and outline a theory that the ``Galactic X-ray Bulge'' discovered by Snowden et al. (1997) is the signature of a large-scale outflow driven by combined thermal and cosmic-ray pressure. We confront this model with observations of the synchrotron halo from Haslam et al. (1981), and discuss the constraints that these observations place on the wind model and perhaps any model of the ``Galactic X-ray Bulge''. We also outline further advances to the model including a more detailed cosmic-ray diffusion model, and the possible role of clumping and mass loading in the outflow.   

Understanding the Galactic Diffuse Emission with the HAWC Gamma-Ray Telescope

Knowledge of the Galactic diffuse emission can further our understanding of the origin and propagation of the cosmic radiation throughout the Galaxy. At energies above 1 GeV the EGRET observed an excess over predictions based upon our knowledge of the matter and radiation fields throughout the Galaxy. At higher energies (near 10 TeV), the Milagro detector measured an even larger excess from the direction of the Cygnus Region and an excess consistent with that measured by EGRET from other directions. The Fermi telescope has failed to confirm the GeV excess measured by EGRET and this has deepened the mystery of the TeV excess. The HAWC telescope will be able to resolve the diffuse emission into regions as small as 5 degrees across and measure the energy spectrum of the diffuse emission within these regions. HAWC measurements should enable one to understand the different contributions (nuclei and electrons) to the diffuse emission and thereby map out the cosmic-ray electron distribution throughout the Galaxy at energies near 10 TeV. In this talk I will discuss the capabilities of the HAWC Observatory and its promise for understanding the origin and propagation of cosmic rays within the Galaxy.