J. Finke

Modeling the Extragalactic Background Light from Stars and Dust

The extragalactic background light (EBL) from the far-infrared through the visible and extending into the ultraviolet is thought to be dominated by starlight, either through direct emission or through absorption and reradiation by dust. This is the most important energy range for absorbing γ-rays from distant sources such as blazars and gamma-ray bursts and producing electron-positron pairs. In previous work, we presented EBL models in the optical through ultraviolet by consistently taking into account the star formation rate (SFR), initial mass function (IMF), and dust extinction, and treating stars on the main sequence as blackbodies. This technique is extended to include post-main-sequence stars and reprocessing of starlight by dust. In our simple model, the total energy absorbed by dust is assumed to be re-emitted as three blackbodies in the infrared, one at 40 K representing warm, large dust grains, one at 70 K representing hot, small dust grains, and one at 450 K representing polycyclic aromatic hydrocarbons. We find that our best-fit model combining the Hopkins and Beacom SFR using the Cole et al. parameterization with the Baldry and Glazebrook IMF agrees with available luminosity density data at a variety of redshifts. Our resulting EBL energy density is quite close to the lower limits from galaxy counts, though in two cases below the lower limits, and agrees fairly well with other recent EBL models shortward of about 5 μm. Deabsorbing TeV γ-ray spectra of various blazars with our EBL model gives results consistent with simple shock acceleration theory. We also find that the universe should be optically thin to γ-rays with energies less than 20 GeV.

SYNCHROTRON SELF-COMPTON ANALYSIS OF TeV X-RAY-SELECTED BL LACERTAE OBJECTS

We introduce a methodology for analysis of multiwavelength data from X-ray-selected BL Lac (XBL) objects detected in the TeVregime. By assuming that the radioYthroughYX-ray flux from XBLs is nonthermal synchrotron radiation emitted by isotropically distributed electrons in the randomly oriented magnetic field of a relativistic blazar jet, we obtain the electron spectrum. This spectrum is then used to deduce the synchrotron self-Compton (SSC) spectrum as a function of the Doppler factor, magnetic field, and variability timescale. The variability timescale is used to infer the comoving blob radius from light-travel time arguments, leaving only two parameters. With this approach, we accurately simulate the synchrotron and SSC spectra of flaring XBLs in the Thomson through Klein-Nishina regimes. Photoabsorption by interactions with internal jet radiation and the intergalactic background light (IBL) is included.Dopplerfactors,magneticfields,andabsolutejetpowersareobtainedbyfittingtheH.E.S.S.andSwiftdataof the recent giant TeV flare observed from PKS 2155� 304. For the H.E.S.S. and Swift data from 2006 July 28 and 30, respectively, Doppler factors k60 and absolute jet powers k10 46 ergs s � 1 are required for a synchrotron/SSC model to give a good fit to the data, for a low intensity of the IBL and a ratio of 10 times more energy in hadrons than nonthermal electrons. Fits are also madetoa TeVflare observedin 2001fromMrk 421 which require Doppler factorsk30 and jet powers k10 45 ergs s � 1 . Subject headingg BLLacertaeobjects:general — BLLacertaeobjects:individual(PKS2155� 304,Mrk421) — galaxies: active — radiation mechanisms: nonthermal

SBS 0846+513: a new γ-ray-emitting narrow-line Seyfert 1 galaxy

We report Fermi Large Area Telescope (LAT) observations of the radio-loud active galactic nucleus SBS 0846+513 (z = 0.5835), optically classified as a narrow-line Seyfert 1 galaxy, together with new and archival radio-to-X-ray data. The source was not active at γ-ray energies during the first two years of Fermi operation. A significant increase in activity was observed during 2010 October–2011 August. In particular, a strong γ-ray flare was observed in 2011 June reaching an isotropic γ-ray luminosity (0.1–300 GeV) of 1.0 × 10^(48) erg s^(−1), comparable to that of the brightest flat spectrum radio quasars, and showing spectral evolution in γ rays. An apparent superluminal velocity of (8.2 ± 1.5)c in the jet was inferred from 2011 to 2012 Very Long Baseline Array (VLBA) images, suggesting the presence of a highly relativistic jet. Both the power released by this object during the flaring activity and the apparent superluminal velocity are strong indications of the presence of a relativistic jet as powerful as those of blazars. In addition, variability and spectral properties in radio and γ-ray bands indicate blazar-like behaviour, suggesting that, except for some distinct optical characteristics, SBS 0846+513 could be considered as a young blazar at the low end of the blazars black hole mass distribution.

COMPTON DOMINANCE AND THE BLAZAR SEQUENCE

Does the “blazar sequence” exist, or is it a result of a selection effect, due to the difficulty in measuring the redshifts of blazars with both high synchrotron peak frequencies (10 15 Hz) and luminosities (10 46 erg s −1 )? We explore this question with a sample of blazars from the second catalog of active galactic nuclei from the Fermi Large Area Telescope. The Compton dominance, the ratio of the peak of the Compton to the synchrotron peak luminosities, is essentially a redshift-independent quantity and thus crucial to answering this question. We find that a correlation exists between Compton dominance and the peak frequency of the synchrotron component for all blazars in the sample, including ones with unknown redshift. We then construct a simple model to explain the blazar properties in our sample, where the difference between sources is due to only the magnetic field of the blazar jet emitting region, the external radiation field energy density, and the jet angle to the line of sight, with the magnetic field strength and external energy density being correlated. This model can reproduce the trends of the blazars in the sample and predicts that blazars may be discovered in the future with high synchrotron peak frequencies and luminosities. At the same time the simple model reproduces the lack of high-synchrotron-peaked blazars with high Compton dominances (1).

Synchrotron Radiation from Ultra-High Energy Protons and the Fermi Observations of GRB 080916C

Fermi gamma-ray telescope data of GRB 080916C with ~1e55 erg in apparent isotropic gamma-ray energy, show a several second delay between the rise of 100 MeV - GeV radiation compared with keV - MeV radiation. Here we show that synchrotron radiation from cosmic ray protons accelerated in GRBs, delayed by the proton synchrotron cooling timescale in a jet of magnetically-dominated shocked plasma moving at highly relativistic speeds with bulk Lorentz factor Gamma ~ 500, could explain this result. A second generation electron synchrotron component from attenuated proton synchrotron radiation makes enhanced soft X-ray to MeV gamma-ray emission. Long GRBs with narrow, energetic jets accelerating particles to ultra-high energies could explain the Auger observations of UHE cosmic rays from sources within 100 Mpc for nano-Gauss intergalactic magnetic fields. The total energy requirements in a proton synchrotron model are proportional to Gamma^(16/3). This model for GRB 080916C is only plausible if Gamma ~< 500 and the jet opening angle is ~ 1 degree.

ON THE BREAK IN THE FERMI-LARGE AREA TELESCOPE SPECTRUM OF 3C 454.3

Fermi Gamma-ray Space Telescope observations of the flat spectrum radio quasar 3C 454.3 show a spectral-index change ΔΓ 1.2 ± 0.3 at break energy E br 2.4 ± 0.3 GeV. Such a sharp break is inconsistent with a cooling electron distribution and is poorly fit with a synchrotron self-Compton model. We show that a combination of two components, namely, the Compton-scattered disk and broad-line region (BLR) radiations, explains this spectral break and gives a good fit to the quasi-simultaneous radio, optical/UV, X-ray, and γ-ray spectral energy distribution observed in 2008 August. A sharp break can be produced independent of the emitting regions distance from the central black hole if the BLR has a gradient in density R –2, consistent with a wind model for the BLR.Fermi Gamma ray Space Telescope observations of the flat spectrum radio quasar 3C 454.3 show a spectral-index change ∆Γ ∼= 1.2 ± 0.3 at break energy Ebr ≈ 2.4 ± 0.3 GeV. Such a sharp break is inconsistent with a cooling electron distribution and is poorly fit with a synchrotron self-Compton model. We show that a combination of two components, namely the Compton-scattered disk and broad-line region (BLR) radiations, explains this spectral break and gives a good fit to the quasi-simultaneous radio, optical/UV, X-ray, and γ-ray spectral energy distribution observed in 2008 August. A sharp break can be produced independent of the emitting region’s distance from the central black hole if the BLR has a gradient in density ∝ R, consistent with a wind model for the BLR. Subject headings: gamma rays: galaxies — Quasar: 3C 454.3 — radiation mechanisms: non-thermal

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.

The Hard VHE γ-Ray Emission in High-Redshift TeV Blazars: Comptonization of Cosmic Microwave Background Radiation in an Extended Jet?

Observations of very-high-energy (VHE; E > 250 GeV) γ-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting the observed VHE spectrum for γγ absorption, even by the lowest plausible level of the IIBR, provides evidence for a very hard (photon spectral index Γph < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE γ-ray spectrum poses a serious challenge to the conventional synchrotron self-Compton (SSC) interpretation of the VHE emission of TeV blazars and suggests the emergence of a separate emission component beyond a few hundred GeV. Here we propose that such a very hard, slowly variable VHE emission component in TeV blazars may be produced via Compton upscattering of cosmic microwave background (CMB) photons by shock-accelerated electrons in an extended jet. For the case of 1ES 1101–232, this component could dominate the bolometric luminosity of the extended jet if the magnetic fields are of the order of typical intergalactic magnetic fields (B ~ 10 μG) and if electrons are still being accelerated out to TeV energies (γ 4 × 106) on kiloparsec scales along the jet.

THE STELLAR CONTRIBUTION TO THE EXTRAGALACTIC BACKGROUND LIGHT AND ABSORPTION OF HIGH-ENERGY GAMMA RAYS

TeV {gamma}-rays from distant astrophysical sources are attenuated due to electron-positron pair creation by interacting with ultraviolet/optical to infrared photons which fill the universe and are collectively known as the extragalactic background light (EBL). We model the {approx}0.1-10 eV starlight component of the EBL derived from expressions for the stellar initial mass function (IMF), star formation history of the universe, and wavelength-dependent absorption of a large sample of galaxies in the local universe. These models are simultaneously fitted to the EBL data as well as to the data on the stellar luminosity density in our local universe. We find that the models with modified Salpeter A IMF together with Cole et al. or Hopkins and Beacom star formation history best represent available data. Since no dust emission is included, our calculated EBL models can be interpreted as the lower limits in the {approx}0.1-1 eV range. We present simple analytic fits to the best-fit EBL model evolving with redshift. We then proceed to calculate {gamma}-ray opacities and absorption of {approx}10-300 GeV {gamma}-rays coming from different redshifts. We discuss implications of our results for the Fermi Gamma Ray Space Telescope and ground-based Air Cherenkov Telescopes.

Constraints on the Intergalactic Magnetic Field with Gamma-Ray Observations of Blazars

Gamma rays from distant blazars interact with the extragalactic background light, creating electron-positron pairs, and reducing the gamma-ray flux measured by ground-based atmospheric Cherenkov gamma-ray telescopes. These pairs can Compton-scatter the cosmic microwave background, creating a gamma-ray signature observable by the Fermi Large Area Telesope (LAT). The signature is also dependent on the intergalactic magnetic field (IGMF), since it can deflect the pairs from our line of sight, reducing the gamma-ray emission. We present preliminary constraints on the IGMF using Fermi-LAT and Cherenkov telescope observations, ruling out both very large and very small values of the IGMF strength.