Session X14: Active Galactic Nuclei and General Astrophysics

Detection of Extended Emission from Fornax A and Measurement of the Extragalactic Background Light

Prior to the launch of Fermi in 2008, the radio galaxy Fornax A was identified as one of the few extragalactic objects that might be detected as spatially extended above 100 MeV. However, even though it was detected with high confidence in the first 2 years of the mission, it was not determined to be an extended source. Recently, the Fermi-LAT collaboration developed a new event-level analysis called Pass 8 which yields a larger acceptance, a better angular and energy resolution, as well as smaller systematic uncertainties. The~improvements provided with Pass 8 combined with a longer exposure means that the spatial extension of Fornax A is significantly detected, making it only the second extragalactic gamma-ray source so far to show extent. Details of this measurement will be presented along with~modeling of the emission above 100 MeV.   

A Bright TeV Flare from the Blazar B2 1215+303 Detected by VERITAS

The extragalactic TeV sky is dominated by blazars, a class of active galactic nuclei (AGN) believed to be powered by supermassive black holes, with ultra-relativistic particle jets pointed close to our line of sight. B2 1215+303 is one such blazar that was first detected at TeV energies by the MAGIC atmospheric Cherenkov telescope, and subsequently by VERITAS in observations carried out between 2009 and 2012. In February 2014, during routine observations of the blazar 1ES 1218+304, which lies in the same field of view as B2 1215+303, VERITAS detected a massive flare from B2 1215+303, which lasted for less than a day. The peak TeV gamma-ray emission was found to exceed 3 times the Crab Nebula flux in the same energy range, making B2 1215+303 one of the most luminous TeV blazars detected to date. We will present results from the VERITAS observations of this source. We examine the variability detected in B2 1215+303 and use the gamma-ray data to estimate the Doppler factor of the jet of B2 1215+303.   

On the Origin of High Energy Emission from Blazars

Continuous monitoring of the gamma-ray sky by the Fermi Large Area Telescope has opened the door to quasi-simultaneous broadband observations of blazars that are addressing open questions about the nature and origin of high energy blazar emission. We present an overview of results from modeling broadband observations of gamma-ray loud blazars under the TANAMI program which monitors gamma-ray loud AGN in the southern hemisphere at multiple wavelengths. While more observations are needed and the modeling is evolving, some tantalizing patterns are already emerging.   

Monitoring TeV Gamma-ray Sources for Flaring States with HAWC

The flux of many TeV gamma-ray emitters exhibits time variability. Detection of these flaring states across multiple wavelengths will lead to a better understanding of the acceleration processes occurring in the source. The High-Altitude Water Cherenkov (HAWC) Observatory is an extensive air-shower detector located near Pico de Orizaba in Mexico which is sensitive to TeV gamma rays. Designed as a survey instrument, the HAWC detector has a large field of view and nearly 100\% uptime. This makes HAWC an ideal instrument to monitor sources for transient flaring states. We will present a method of monitoring sources using a Bayesian blocks algorithm to detect changes in the flux and report on the sensitivity of the method. We also discuss results from several bright AGN flares which occurred during the construction phase of HAWC.   

The Systematics of Relativistically Beamed Jets from Active Galaxies and the Blazar Divide

We investigate the systematics of the properties of highly relativistic jets at multiple frequencies, including gamma-ray data from the Fermi telescope and MOJAVE radio imaging on parsec scales. We test the hypothesis that the blazar divide constitutes a dichotomy. We also explore possible measures of the Doppler factor for these highly Doppler-beamed active galactic nuclei.   

Exceptional X-ray Weak Quasars: Implications for Accretion Flows

Actively accreting supermassive black holes (SMBHs) are found, nearly universally, to create luminous X-ray emission, and this point underlies the utility of X-ray surveys for finding growing SMBHs throughout the Universe. However, there are exceptions to this rule that provide novel insights, including PHL 1811 analogs and some weak-line quasars. We have been systematically studying such X-ray weak quasars with the Chandra X-ray Observatory, aiming (1) to define their optical-to-X-ray spectral energy distributions, (2) to measure their basic X-ray spectral properties, and (3) to establish the optical/UV emission-line and continuum properties that most directly trace X-ray weakness. Many of these type 1 quasars show unusually hard X-ray spectra, suggesting that small-scale absorption/reflection has a primary role in causing their X-ray weakness and distinctive emission-line properties. Physical considerations indicate that this small-scale absorber/reflector may be the geometrically thick inner accretion disk expected to form if PHL 1811 analogs and weak-line quasars have unusually high SMBH accretion rates.   

The Distribution and Annihilation of Dark Matter Around Black Holes

We consider a simple model for dark matter self-annihilation where the cross section depends on energy, analogous to many standard model particle interactions. Since the only known way to accelerate dark matter is through gravitational forces, we focus on the collisions of particles orbiting around black holes. To do so, we use a Monte Carlo code to integrate the geodesic orbits of test particles around Kerr black holes, generating a distribution function of both bound and unbound populations of dark matter particles. From this distribution function, we calculate annihilation rates and observable gamma-ray spectra. We find that for rapidly spinning black holes, the collisional Penrose process can reach efficiencies of $>$600\%, leading to a high-energy tail in the annihilation spectrum. Furthermore, the high particle densities and large proper volume of the region immediately surrounding the horizon ensures that the observed flux from these extreme events is non-negligable.   

Search for Astrophysical Tau Neutrinos with Three Years of IceCube Data

High-energy cosmic neutrinos are expected to be produced in extremely energetic astrophysical sources such as active galactic nuclei (AGNs) and gamma ray bursts (GRBs). The IceCube neutrino observatory has recently detected a diffuse astrophysical neutrino flux at 5.7$\sigma $. One of the outstanding questions regarding astrophysical neutrinos is their flavor composition. Most standard oscillation scenarios predict tau neutrinos in the astrophysical flux, which have a negligible background from cosmic ray induced atmospheric neutrinos. I will present the analysis method and results from a recent search for astrophysical tau neutrinos with three years of IceCube data.