Session Q13: Very High Energy Gamma Ray Detection and Cosmic Ray Propagation

The TeV Gamma-Ray Sky Observed by Milagro

Milagro has performed the deepest survey of the Northern Hemisphere TeV gamma-ray sky. The observatory is sensitive to gamma rays from below 1 TeV to greater than 100 TeV. The analysis technique weights events by the ratio of the probability that the detected event is due to a gamma ray or a background hadronic cosmic ray. The hadronic cosmic ray background produces penetrating particles in the extensive air showers that are less prevalent in the gamma ray initiated showers. Two different sets of weights were selected to search for gamma-ray sources with spectra extending to the highest energies and to search for gamma-ray sources in which spectra cut off at a few TeV. In this talk we will present the location and flux of the sources detected for each of these two spectral assumptions. Most of the strongly detected Milagro sources are coincident with high luminosity pulsars detected at lower energies by the Fermi satellite.   

Energy spectrum of Galactic gamma-ray sources detected by Milagro

The Milagro Gamma-Ray Observatory has detected numerous sources of VHE gamma-rays with a median energy above 10 TeV. The large collection area, high duty cycle and large aperture give Milagro unprecedented sensitivity particularly at E = 30 -100 TeV. In this energy regime, the contribution from inverse-Compton scattering of accelerated electrons is expected to attenuate, so the highest energy events are likely due to hadronic interactions. Gamma-ray sources with spectra that extend to 100 TeV without cutting off are therefore strong candidates for acceleration sites for Galactic cosmic rays. I will present the spectra from 5-100 TeV for all the high significance gamma-ray sources detected by Milagro.   

Physics with the HAWC Gamma-Ray Observatory

The High Altitude Water Cherenkov (HAWC) gamma-ray observatory will be a wide field of view, continuously operated, TeV gamma-ray observatory. HAWC is a natural extension of Milagro, which has demonstrated the ability to detect, at TeV energies many of the galactic sources which have been observed by the Fermi LAT in the GeV energy range. Since Milagro was a first generation detector constructed in a preexisting reservoir at a relatively low elevation (2640m), what Fermi was able to see in several months took Milagro near seven years to see. HAWC will be constructed as a joint Mexican-US collaboration on the Sierra Negra Mountain in Mexico at an elevation of 4100m. The design of HAWC was optimized using the lessons learned from Milagro and will be $\sim $15 times more sensitive than Milagro when completed. This improvement in sensitivity will allow HAWC to measure or constrain the TeV spectra of most of the Fermi discovered GeV sources. In addition, above 100 GeV HAWC will be more sensitive than the Fermi and be the only ground-based instrument capable of detecting prompt emission from gamma-ray bursts in this energy regime. This talk will present the physics capabilities of HAWC as well as its status.   

Constraining the Propagation of Galactic Cosmic Rays Using Measurements of the Composition and Energy Spectra of Dominantly Secondary Isotopes

Using measurements from the Cosmic Ray Isotope Spectrometer (CRIS) on-board the Advanced Composition Explorer (ACE), we report isotopic abundances and energy spectra for dominantly secondary isotopes, produced by interstellar fragmentation of heavier species, in the energy range of 50-500 MeV/nucleon. We also consider secondary-to-primary ratios that are fit using a simple leaky box model of cosmic ray transport in the Galaxy combined with a spherically symmetric solar modulation model. CRIS measurements reported here include data from two consecutive solar minima, between 1997-1998 and 2008-2009, when the solar magnetic field was of opposite polarity. These results are used to constrain the propagation of cosmic rays in the Galaxy, as well as better understand how the effects of gradient and curvature drifts in the interplanetary magnetic field change over the solar cycle.   

On the positron fraction and models of cosmic ray propagation

The recent observations of the positron fraction in cosmic rays by PAMELA has created much excitement because of its possible connection with annihilation or decay of dark matter in the Galaxy, or with a variety of astrophysical processes taking place in nearby pulsars or supernova explosions. The PAMELA instrument measured a positron fraction of $\sim $ 0.0673 at $\sim $ 1.64 GeV, which decreases to $\sim $0.0483 at $\sim $6.83 GeV, and thereupon increases monotonically, reaching a value of $\sim $0.137 at mean energy of 82.55 GeV. It is this monotonic increase that has been called anomalous, as it does not conform to the predictions of the leaky box model with the residence time of cosmic rays decreasing with increasing energy. Accordingly the possibility of the reacceleration of positrons in the cosmic ray sources, and the effects of inhomogeneous distribution of cosmic ray sources about the solar system, have been discussed. Detailed comments on these suggestions may be found in Ahlers et al. and in Mertsch et al.. We wish to point out here that very general arguments based on cosmic-ray propagation models indicate that the positron fraction should increase at high energies, and asymptotically reach a value of $\sim $0.6 at the highest energies. Furthermore, we show that these observations support the nested leaky-box models for cosmic ray propagation.   

A Method for Constraining Galactic Magnetic Field Models Using Data from the Pierre Auger Observatory

Ultra-high energy cosmic rays are deflected by magnetic fields during propagation from their extra-galactic sources. However, these fields are poorly understood. We present a method for constraining the parameter space of galactic magnetic field models by comparing test statistics between backtracked data and isotropic expectations for assumed source and cosmic ray composition hypotheses. Different galactic magnetic field models are scanned using data from the Pierre Auger Observatory under various assumed source and composition hypotheses.   

Cross-Correlation of UHECRs with the Local Matter Distribution

The cross-correlation of UHECR arrival directions with the local matter distribution is a powerful tool for analyzing the anisotropy of cosmic ray events. Because the nearby matter distribution is known to high resolution from the 2 MASS Redshift Survey, the fractional fluctuations in the cross-correlation are reduced by the factor $\sim (n N)^{-\frac{1}{2}}$, where $n$ is the number of UHECR events and $N$ the number of galaxies, as compared to $\sim n^{-1}$ for UHECR auto-correlations. The observed cross-correlation between UHECRs detected with the Pierre Auger Observatory and 2MRS galaxies is incompatible with the UHECRs coming from an isotropic distribution, and compatible with their sources being associated with galaxies. The dependence of the significance of the correlation on the energy threshold of the UHECRs and on the depth of the galaxy sample carries important information, which can be interpreted by simulations with mock UHECR catalogs.   

Search for nearby extragalactic sources of the highest energy cosmic rays

We update the analysis of the correlation between the arrival directions of the highest energy cosmic rays ($E>57$~EeV) observed with the Pierre Auger Southern Observatory and the positions of nearby extragalactic objects.