Some questions raised by Fermi-LAT data about blazars are summarized, along with attempts at solutions within the context of leptonic models. These include both spectral and statistical questions, including the origin of the GeV breaks in low-synchrotron peaked blazars, the location of the gamma-ray emission sites, the correlations in the spectral energy distributions with luminosity, and the difficulty of synchrotron/SSC models to fit the spectra of some TeV blazars.
The 11 June 2008 launch of the Gamma-ray Large Area Space Telescope, subsequently renamed the Fermi Gamma ray Space Telescope, opened a new era into studies of radio galaxies and blazars. The Large Area Telescope (LAT) on Fermi, with a peak on-axis effective of ≈ 8000 cm 2 and a single photon angular resolution for 68% containment at 1 GeV of ≈ 0.8 • [1], represents an improvement by 1 -2 orders of magnitude in sensitivity over its predecessors EGRET on the Compton Gamma-ray Observatory and AGILE [2]. In the Third EGRET Catalog of Gamma Ray Sources [3], 66 high-confidence ( > ∼ 5σ) detections of blazars and one probable radio galaxy, Centaurus A were reported. Ten high-latitude sources at high galactic latitude (|b| > 10 • ) were detected with significance > 10σ. Now, just over three years into the Fermi mission, three important lists of Fermi-LAT AGNs have been or are about to be published:
(i) The LAT Bright AGN Sample (LBAS) [4]. Based on the first three months of scientific operations of the Fermi satellite, the LBAS consists of 106 high-latitude sources with T S > ∼ 100, or significance > ∼ 10σ. It consists of 58 flat spectrum radio quasars (FSRQs), 42 BL Lac objects, 2 radio galaxies, namely, Centaurus A and NGC 1275 (3C 84), and 4 blazars with unknown classification. (ii) The First LAT AGN catalog (1LAC) [5]. This catalog contains 671 sources that are associated at high confidence with 709 AGNs (some γ-ray sources have multiple associations). The associations are based on counterpart catalogs, such as the flat-spectrum 8.4 GHz CRATES (Combined Radio All-Sky Targeted Eight GHz Survey) Catalog [6] and the Roma BZCAT blazar catalog [7]. The 1LAC clean sample is a subset of the highconfidence associations, and consists of 599 AGNs with no multiple associations or other analysis flags. These sources subdivide into 275 BL Lac objects, 248 FSRQs, 50 blazars of unknown type (either because a source lacks an optical spectrum, or the optical spectrum is inadequate to determine if it is a BL Lac or FSRQ), and 26 other AGNs. These “other” AGNs include 7 radio galaxies/misaligned AGNs, 4 radio-loud narrow line Seyfert 1 (RL-NLSy1) galaxies, 4 narrow line radio galaxies, 3 starburst galaxies (which of course are not actually AGNs), and 9 other sources. (iii) The Second LAT AGN catalog (2LAC) [8]. The 2LAC contains 1016 sources that are associated at high confidence with AGNs. The 2LAC clean sample consists of 885 sources (46% larger than the 1LAC clean sample), consisting of 395 BL Lacs, 310 FSRQs, 156 blazars of unknown type, 8 misaligned AGNs, 4 RL-NLSy1 galaxies, 10 AGNs of other types, and 2 starburst galaxies (NGC 4945 has fallen out of the list).
Important questions that are impacted by or arise from the Fermi-LAT observations of blazars and radio galaxies, and interpretation that relates to these questions within the context of leptonic models, will occupy the remainder of this short review.
After summarizing the standard leptonic model for blazars, we consider some important questions raised by the Fermi-LAT data, including (i) Limitations of one-zone synchrotron/SSC model for TeV blazars (ii) Relation between aligned and misaligned jet sources (iii) Validity and meaning of the blazar sequence (iv) GeV spectral cutoffs of LSP and ISP blazars (v) Super-Eddington luminosities of blazars
The general outlines of blazar leptonic models are now well established for the standard one-zone model within the context of blazar unification [9]. In this paradigm, the basic ingredients of a blazar are an accretion disk, regions of broad and narrow emission-line gas, and a dusty IR torus. Radio-loudness is a consequence of collimated relativistic outflows of plasma directed along the polar axis of the accretion disk. When observing at small angles to the plane of the accretion disk, the torus gas conceals the broad-line region formed close to the black hole, so that only narrow emission lines are detected [10]. These are the Type II Seyfert galaxies or, if radio-loud, narrow line radio galaxies. Observations at larger angles to the plane of the accretion disk, where the torus no longer obscures the central region, reveal the broad emission lines. These are the Type I Seyferts or, if radio-loud, the broad-line radio galaxies.
When viewed at very small angles with respect to the jet axis, the Doppler-boosted nonthermal radiation overwhelms the accretion-disk radiation and makes the familiar twohumped broadband blazar continuum [11]. The featureless continuum is made by nonthermal particles accelerated at shocks formed, for example, by colliding shells [12] or through sheared flows in structured jets [13]. In the standard blazar model for flaring activity, a compact plasma zone filled with nonthermal electrons and positrons radiate synchrotron photons that form the highly polarized radio continuum that extends to optical frequencies in flat spectrum radio quasars (FSRQs), and to X-ray frequencies in the high-synchrotron
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