Session H11: X-Ray and Gamma-Ray Astrophysics

Energy Spectrum of Cygnus Diffuse Excess with MILAGRO

An analysis of the diffuse spectrum can help further the understanding of high-energy cosmic rays in our Galaxy. Models of the propagation of cosmic rays in the galaxy result in a predicted diffuse gamma-ray flux for the sky. EGRET has observed an excess of diffuse gamma rays (relative to this model) in the GeV range. Milagro has also measured an excess of diffuse gamma rays at 15TeV, which was also well above the model's prediction. The TeV excess in the Cygnus Region of the galaxy shows the largest diffuse excess in the Milagro data. In order to understand this excess better, the data was reanalyzed. Using a new analysis technique allows us to reconstruct the spectrum of the diffuse emission. In my presentation, I will present results based on the new analysis technique that include the complete Milagro data set.   

The Mass Distribution of Stellar-Mass Black Holes

Until the first gravitational wave detection involving a stellar-mass black hole, X-ray binary systems provide the only observational window on the properties of these objects. We have used a sample of fifteen low-mass X-ray binary systems and five high-mass X-ray binary systems for which dynamical measurements of the black hole mass exist to place constraints on the mass distribution of stellar-mass black holes. We fit the low-mass X-ray binary black hole masses alone and the combined sample of twenty black hole masses to five parametric and five non-parametric models for the underlying mass distribution. We can extract useful information from this plethora of models because of new techniques we developed that allow for the efficient calculation of the relative Bayesian posterior probabilities of the models. Surprisingly from a theoretical perspective, we find strong evidence in both the low-mass and combined samples for a ``mass gap'' between the maximum neutron star mass of $\sim 3 M_\odot$ and the minimum black hole mass, in agreement with studies from other groups; we find the minimum black hole mass to be $4.3 M_\odot$ for the best-fitting model of the low-mass X-ray binaries and $4.5 M_\odot$ for the best-fitting model of the combined sample (both at 90\% confidence).   

Fermi-LAT observations of transient and flaring systems in the Galaxy

The Fermi Large Area Telescope (LAT) views the entire gamma-ray sky ($\sim$20 MeV to $>$300 GeV) every three hours. The all-sky coverage provides an excellent opportunity for the discovery of new types of gamma-ray transients. Ongoing survey observations over more than two years have been successful in revealing gamma-ray flares from several notable Galactic objects. The LAT has detected the nova V407 Cygni, a first in gamma rays. The LAT has also found two short flares from the Crab Nebula in the 100 MeV to 1 GeV range. I will discuss what this activity reveals about particle acceleration in astrophysical sources and consider the prospects for future transient discoveries with the LAT.   

The Chandra Observations of BL Lac Markarian 421 in the High-Emission State

Markarian 421 is a well-established BL Lac-type active galactic nucleus (AGN). It is one of the brightest and extremely high variable TeV blazar. The TeV gamma-ray emission observed from this source varies in flux dramatically. In November 2009 we observed Markarian 421 for three 5-ks exposures with the Chandra X-ray Observatory during the period of its high TeV gamma-ray emission state. These observations have contributed to the broadband multi-wavelength observational campaign that involved a number of space-born and ground-based experiments. The analysis results showing the evolution of the X-ray energy spectrum across the flare will be presented at the conference.   

Recent Highlights from the VERITAS Blazar Science and Observing Program

Blazars comprise the most abundant class of extragalactic sources detected at very high energies. The imaging atmospheric Cherenkov telescope VERITAS, an array of four 12m telescopes located near Tucson, Arizona, has detected 17 blazars. This number is steadily growing, in part thanks to improved source candidate identification through the utilization of the Fermi Gamma-Ray Space Telescope. As it enters its fourth year, the VERITAS blazar observing program remains focused on the discovery of new sources, but with specific long term blazar-related science goals also at the forefront. This talk will highlight recent results from VERITAS blazar observations, including new source discoveries, results from multi-wavelength campaigns, and the scientific prospects for the growing population of intermediate-frequency~peaked BL Lac objects, as well as large redshift high-frequency peaked BL Lacs, within the VERITAS blazar catalog.   

Study of HAWC Sensitivity to AGN

The High Altitude Water Cherenkov (HAWC) experiment is the next generation extended air shower array. Construction of the project has already begun in Sierra Negra, Mexico at an altitude of 4100 m. With improved energy resolution, lower energy threshold, and better angular resolution, the completed HAWC array is predicted to be 10 --15 times more sensitive than MILAGRO, the highly successful predecessor to HAWC. Here, we examine Monte Carlo simulations to calculate the sensitivity of the HAWC detector to the existing population of active galactic nuclei (AGN). Moreover, we simulate heightened gamma-ray emission from the AGNs with varying flaring time-scales and obtain the corresponding HAWC sensitivity to these AGNs. Combining HAWC's extended energy coverage over 50 GeV to 100 TeV and a very high duty-cycle, HAWC will trigger multi-wavelength campaigns for transient AGNs. Finally, I investigate the implications of the observations of AGNs at the highest energies ($>$ 30 TeV) on our present understanding of the infrared background radiation.   

TeV gamma rays from BL-Lacs and photon - hidden sector paraphoton kinetic mixing

Extra U(1) gauge bosons can arise in some embeddings of the Standard Model in string theory; they are referred to as hidden sector paraphotons here. We propose that the weak coupling of hidden sector paraphotons to Standard Model photons results in a mechanism whereby gamma rays from distant astrophysical objects can propagate vast distances through the intergalactic medium without absorption or pair conversion. This mechanism is used to show that TeV gamma rays from distant BL-Lacertae objects may kinetically intermix with weakly interacting paraphotons in a way that helps to explain their observed energy spectra. Laser based photon regeneration experiments may test this model in the very near future.   

The Unresolved Blazar Component of the Extragalactic Gamma Ray Background

We present a new theoretical estimate of the contribution of unresolved blazars to the extragalactic $\gamma$-ray background (EGB). We find that the Fermi source count data do not rule out a scenario in which the EGB is dominated by emission from unresolved blazars. The spectrum of unresolved FSRQs, when accounting for the energy-dependent effects of source confusion, could be consistent with the combined spectrum of both the low-energy EGRET EGB measurements between 50 and 200 MeV and the Fermi-LAT EGB measurements above 200 MeV.