S. W. Digel

Broad-line Radio Galaxies Observed with Fermi-LAT: The Origin of the GeV γ-Ray Emission

We report on a detailed investigation of the γ-ray emission from 18 broad line radio galaxies (BLRGs) base d on two years ofFermi Large Area Telescope (LAT) data. We confirm the previously re po ted detections of 3C 120 and 3C 111 in the GeV photon energy range; a detailed loo k at the temporal characteristics of the observedγ-ray emission reveals in addition possible flux variability in both sources. No statistically significant γ-ray detection of the other BLRGs was however found in the con sidered dataset. Though the sample size studied is small, what appears to differentiate 3C 111 and 3C 120 from the BLRGs not yet detected in γrays is the particularly strong nuclear radio flux. This findi g, together with the indications of the γ-ray flux variability and a number of other arguments presented, indi cate that the GeV emission of BLRGs is most likely dominated by the beamed radiation of relativistic jets obse rved at intermediate viewing angles. In this paper we also analyzed a comparison sample of high accretion-rate Se yfert 1 galaxies, which can be considered radioquiet counterparts of BLRGs, and found none were detected in γ-rays. A simple phenomenological hybrid model applied for the broad-band emission of the discussed r adio-loud and radio-quiet type 1 active galaxies suggests that the relative contribution of the nuclear jets to he accreting matter is ≥ 1% on average for BLRGs, whilst ≤ 0.1% for Seyfert 1 galaxies. Subject headings: radiation mechanisms: non-thermal — galaxies: active — gal axies: individual (3C 111, 3C 120) — galaxies: jets — gamma rays: galaxies — X-rays: gala xiesWe report on a detailed investigation of the γ-ray emission from 18 broad-line radio galaxies (BLRGs) based on two years of Fermi Large Area Telescope data. We confirm the previously reported detections of 3C 120 and 3C 111 in the GeV photon energy range; a detailed look at the temporal characteristics of the observed γ-ray emission reveals in addition possible flux variability in both sources. No statistically significant γ-ray detection of the other BLRGs was found, however, in the considered data set. Though the sample size studied is small, what appears to differentiate 3C 111 and 3C 120 from the BLRGs not yet detected in γ-rays is the particularly strong nuclear radio flux. This finding, together with the indications of the γ-ray flux variability and a number of other arguments presented, indicates that the GeV emission of BLRGs is most likely dominated by the beamed radiation of relativistic jets observed at intermediate viewing angles. In this paper we also analyzed a comparison sample of high-accretion-rate Seyfert 1 galaxies, which can be considered radio-quiet counterparts of BLRGs, and found that none were detected in γ-rays. A simple phenomenological hybrid model applied for the broadband emission of the discussed radio-loud and radio-quiet type 1 active galaxies suggests that the relative contribution of the nuclear jets to the accreting matter is ≥1% on average for BLRGs, whereas it is ≤0.1% for Seyfert 1 galaxies.

Refining the Associations of the Fermi Large Area Telescope Source Catalogs

The Fermi-Large Area Telescope (LAT) First Source Catalog (1FGL) was released in 2010 February and the Fermi-LAT 2-Year Source Catalog (2FGL) appeared in 2012 April, based on data from 24 months of operation. Since they were released, many follow up observations of unidentified γ-ray sources have been performed and new procedures for associating γ-ray sources with potential counterparts at other wavelengths have been developed. Here we review and characterize all of the associations as published in the 1FGL and 2FGL catalogs on the basis of multifrequency archival observations. In particular, we located 177 spectra for the low-energy counterparts that were not listed in the previous Fermi catalogs, and in addition we present new spectroscopic observations of eight γ-ray blazar candidates. Based on our investigations, we introduce a new counterpart category of “candidate associations” and propose a refined classification for the candidate low-energy counterparts of the Fermi sources. We compare the 1FGL-assigned counterparts with those listed in 2FGL to determine which unassociated sources became associated in later releases of the Fermi catalogs. We also search for potential counterparts to all of the remaining unassociated Fermi sources. Finally, we prepare a refined and merged list of all of the associations of 1FGL plus 2FGL that includes 2219 unique Fermi objects. This is the most comprehensive and systematic study of all the associations collected for the γ-ray sources available to date. We conclude that 80% of the Fermi sources have at least one known plausible γ-ray emitter within their positional uncertainty regions.

Fermi-LAT Observations of High-energy Behind-the-limb Solar Flares

We report on the Fermi-LAT detection of high-energy emission from the behind-the-limb (BTL) solar flares that occurred on 2013 October 11, and 2014 January 6 and September 1. The Fermi-LAT observations are associated with flares from active regions originating behind both the eastern and western limbs, as determined by STEREO. All three flares are associated with very fast coronal mass ejections (CMEs) and strong solar energetic particle events. We present updated localizations of the >100 MeV photon emission, hard X-ray (HXR) and EUV images, and broadband spectra from 10 keV to 10 GeV, as well as microwave spectra. We also provide a comparison of the BTL flares detected by Fermi-LAT with three on-disk flares and present a study of some of the significant quantities of these flares as an attempt to better understand the acceleration mechanisms at work during these occulted flares. We interpret the HXR emission to be due to electron bremsstrahlung from a coronal thin-target loop top with the accelerated electron spectra steepening at semirelativistic energies. The >100 MeV gamma-rays are best described by a pion-decay model resulting from the interaction of protons (and other ions) in a thick-target photospheric source. The protons are believed to have been accelerated (to energies >10 GeV) in the CME environment and precipitate down to the photosphere from the downstream side of the CME shock and landed on the front side of the Sun, away from the original flare site and the HXR emission.

Fermi-LAT observations of high- and intermediate-velocity clouds: tracing cosmic rays in the halo of the Milky Way

It is widely accepted that cosmic rays (CRs) up to at least PeV energies are Galactic in origin. Accelerated particles are injected into the interstellar medium where they propagate to the farthest reaches of the Milky Way, including a surrounding halo. The composition of CRs coming to the solar system can be measured directly and has been used to infer the details of CR propagation that are extrapolated to the whole Galaxy. In contrast, indirect methods, such as observations of gamma-ray emission from CR interactions with interstellar gas, have been employed to directly probe the CR densities in distant locations throughout the Galactic plane. In this article we use 73 months of data from the Fermi Large Area Telescope in the energy range between 300 MeV and 10 GeV to search for gamma-ray emission produced by CR interactions in several high- and intermediate-velocity clouds located at up to ~ 7 kpc above the Galactic plane. We achieve the first detection of intermediate-velocity clouds in gamma rays and set upper limits on the emission from the remaining targets, thereby tracing the distribution of CR nuclei in the halo for the first time. We find that the gamma-ray emissivity per H atom decreases with increasing distance from the plane at 97.5% confidence level. This corroborates the notion that CRs at the relevant energies originate in the Galactic disk. The emissivity of the upper intermediate-velocity Arch hints at a 50% decline of CR densities within 2 kpc from the plane. We compare our results to predictions of CR propagation models.

ON THE NATURE OF THE GAMMA-RAY SOURCE 2FGL J1823.8+4312: THE DISCOVERY OF A NEW CLASS OF EXTRAGALACTIC X-RAY SOURCES

One of the unsolved mysteries of gamma-ray astronomy concerns the nature of the unidentified gamma-ray sources. Recently, using the Second Fermi LAT source catalog (2FGL) and the Wide-field Infrared Survey Explorer (WISE) archive, we discovered that the WISE counterparts of gamma-ray blazars, a class of active galactic nuclei, delineate a region (the WISE Gamma-ray Strip) in the 3-dimensional infrared color space well separated from the locus of the other astronomical objects. Based on this result, we built an association procedure to recognize if there areWISE blazar candidates within the positional uncertainty region of the unidentified gamma-ray sources. Here we report on our analysis of 2FGL J1823.8+4312, a gamma-ray active galactic nucleus of uncertain type associated with the X-ray source 1RXS J182418.7+430954 according to the 2FGL, to verify whether it is a blazar. Applying our association method we found two sources with IR colors typical of gamma-ray blazars, located within the 99.9% confidence region of 2FGL J1823.8+4312: WISE J182352.33+431452.5 and WISE J182409.25+431404.7. Then we searched in the Chandra, NVSS and SDSS archival observations for their counterparts. We discovered that WISE J182352.33+431452.5, our preferred gamma-ray blazar candidate according to our WISE association procedure, is detected in the optical and in the X-raysmorexa0» but not in the radio, making it extremely unusual if it is a blazar. Given its enigmatic spectral energy distribution, we considered the possibility that it is a radio faint blazar or the prototype of a new class of extragalactic sources, our conclusion is independent of whether WISE J182352.33+431452.5 is the actual counterpart of 2FGL J1823.8+4312.«xa0less

The Origins of the Gamma-Ray Flux Variations of NGC 1275 Based on Eight Years of Fermi-LAT Observations

We present an analysis of 8 years of Fermi-LAT ( > 0.1 GeV) gamma-ray data obtained for the radio galaxy NGC 1275. The gamma-ray flux from NGC 1275 is highly variable on short (~ days to weeks) timescales, and has steadily increased over this 8-year timespan. By examining the changes in its flux and spectral shape in the LAT energy band over the entire dataset, we found that its spectral behavior changed around 2011 February (~ MJD 55600). The gamma-ray spectra at the early times evolve largely at high energies, while the photon indices were unchanged in the latter times despite rather large flux variations. To explain these observations, we suggest that the flux changes in the early times were caused by injection of high-energy electrons into the jet, while later, the gamma-ray flares were caused by a changing Doppler factor owing to variations in the jet Lorentz factor and/or changes in the angle to our line of sight. To demonstrate the viability of these scenarios, we fit the broad-band spectral energy distribution data with a one-zone synchrotron self-Compton (SSC) model for flaring and quiescent intervals before and after 2011 February. To explain the gamma-ray spectral behavior in the context of the SSC model, the maximum electron Lorentz factor would have changed in the early times, while a modest change in the Doppler factor adequately fits the quiescent and flaring state gamma-ray spectra in the later times.