Blazar Flaring Rates Measured with GLAST

We derive the minimum observing time scales to detect a blazar at a given flux level with the LAT on GLAST in the scanning and pointing modes. Based upon Phase 1 observations with EGRET, we predict the GLAST detection rate of blazar flares at different flux levels. With some uncertainty given the poor statistics of bright blazars, we predict that a blazar flare with integral flux >~ 200e-8 ph(> 100 MeV) cm^{-2} s^{-1}, which are the best candidates for Target of Opportunity pointings and extensive temporal and spectral studies, should occur every few days.Comment: 7 pages, 2 figures, in 2nd VERITAS Symposium on TeV Astrophysiscs, ed. L. Fortson and S. Swordy, in press, New Astronomy Review

GRBs as ultra-high energy cosmic ray sources: clues from Fermi

If gamma-ray bursts are sources of ultra-high energy cosmic rays, then radiative signatures of hadronic acceleration are expected in GRB data. Observations with the Fermi Gamma-ray Space Telescope offer the best means to search for evidence of UHECRs in GRBs through electromagnetic channels. Various issues related to UHECR acceleration in GRBs are reviewed, with a focus on the question of energetics.Comment: 9 pages, to appear in the Proceedings of "The Shocking Universe: Gamma Ray Bursts and High Energy Shock Phenomena," Venice, Italy, 14-18 September 200

Theory of high-energy messengers

Knowledge of the distant high-energy universe comes from photons, ultra-high energy cosmic rays (UHECRs), high-energy neutrinos, and gravitational waves. The theory of high-energy messengers reviewed here focuses on the extragalactic background light at all wavelengths, cosmic rays and magnetic fields in intergalactic space, and neutrinos of extragalactic origin. Comparisons are drawn between the intensities of photons and UHECRs in intergalactic space, and the high-energy neutrinos recently detected with IceCube at about the Waxman-Bahcall flux. Source candidates for UHECRs and high-energy neutrinos are reviewed, focusing on star-forming and radio-loud active galaxies. HAWC and Advanced LIGO are just underway, with much anticipation.Comment: 12 pages, 8 figures; invited review for 14th International Congress in Topics in Astroparticle and Underground Physics, Torino, Italy, 7 - 11 September 201

Blazar jet physics in the age of Fermi

The impact of the Fermi Gamma-ray Space Telescope on blazar research is reviewed. This includes a brief description of the Fermi Large Area Telescope, a summary of the various classes of extragalactic sources found in the First Large Area Telescope AGN Catalog, and more detailed discussion of the flat spectrum radio quasar 3C454.3 and the BL Lac object PKS 2155-304. Some theoretical studies related to ongoing blazar research with Fermi are mentioned, including implications of gamma-ray observations of radio galaxies on blazar unification scenarios, variability in colliding shells, and whether blazars are sources of ultra-high energy cosmic rays.Comment: Minor corrections and improvements; 11 pages, 2 figures; in Proceedings of IAU Symposium 285, "Jets at all Scales," Buenos Aires, Argentina, 13-17 September, 2010, G.E. Romero, R.A. Sunyaev and T. Belloni, ed

Variability in Blazars

The kinetic energy of bulk relativistic plasma ejected from the central engine of blazars is converted into nonthermal particle energy in the comoving frame through a process of sweeping up material from the surrounding medium. The resulting deceleration of the bulk plasma introduces a number of effects which must be included in blazar modeling. For example, the varying Doppler factor means that model fits must employ time integrations appropriate to the observing times of the detectors. We find that the ratio of the peak synchrotron fluxes reached at two different photon energies is largest when viewing along the jet axis, and becomes smaller at large angles to the jet axis. This effect is important in studies of the statistics of jet sources. Variability due either to bulk plasma deceleration or radiative cooling must be distinguished in order to apply recently proposed tests for beaming from correlated X-ray and TeV observations. The blast-wave physics developed to analyze these problems implies that most of the energy injected in the comoving frame is originally in the form of nonthermal hadrons. Because plasmoid deceleration can produce rapid variability due to a changing Doppler factor, arguments against hadronic blazar models related to the long radiative cooling time scale of hadrons are not compelling.Comment: 13 pages, 3 figures,Invited talk at VERITAS Workshop on TeV Astrophysics of Extragalactic Sources, Cambridge, MA, Oct. 23-24, 1998. In press, Astroparticle Physics, ed. M. Catanese, J. Quinn, T. Weeke

Ultra-High Energy Cosmic Rays and Neutron-Decay Halos from Gamma Ray Bursts

Simple arguments concerning power and acceleration efficiency show that ultra-high energy cosmic rays (UHECRS) with energies >~ 10^{19} eV could originate from GRBs. Neutrons formed through photo-pion production processes in GRB blast waves leave the acceleration site and travel through intergalactic space, where they decay and inject a very energetic proton and electron component into intergalactic space. The neutron-decay protons form a component of the UHECRs, whereas the neutron-decay electrons produce optical/X-ray synchrotron and gamma radiation from Compton-scattered background radiation. A significant fraction of galaxies with GRB activity should be surrounded by neutron-decay halos of characteristic size ~ 100 kpc.Comment: 3 pages, in 2nd Rome Workshop on Gamma Ray Bursts in the Afterglow Era (17-20 October 2000

Prompt and Delayed High-Energy Emission from Cosmological Gamma-Ray Bursts

In the cosmological blast-wave model for gamma ray bursts (GRBs), high energy (> 10 GeV) gamma-rays are produced either through Compton scattering of soft photons by ultrarelativistic electrons, or as a consequence of the acceleration of protons to ultrahigh energies. We describe the spectral and temporal characteristics of high energy gamma-rays produced by both mechanisms, and discuss how these processes can be distinguished through observations with low-threshold Cherenkov telescopes or GLAST. We propose that Compton scattering of starlight photons by blast wave electrons can produce delayed flares of GeV -- TeV radiation.Comment: to appear in Proceedings of VERITAS Workshop on TeV Astrophysics of Extragalactic Sources, eds. M. Catanese, J. Quinn, T. Weeke

Impact of Fermi-LAT and AMS-02 results on cosmic-ray astrophysics

This article reviews a few topics relevant to Galactic cosmic-ray astrophysics, focusing on the recent AMS-02 data release and Fermi Large Area Telescope data on the diffuse Galactic gamma-ray emissivity. Calculations are made of the diffuse cosmic-ray induced p+p --> pi^0 --> 2 gamma spectra, normalized to the AMS-02 cosmic-ray proton spectrum at ~ 10 - 100 GV, with and without a hardening in the cosmic-ray proton spectrum at rigidities R >~ 300 GV. A single power-law momentum "shock" spectrum for the local interstellar medium cosmic-ray proton spectrum cannot be ruled out from the gamma-ray emissivity data alone without considering the additional contribution of electron bremsstrahlung. Metallicity corrections are discussed, and a maximal range of nuclear enhancement factors from 1.52 to 1.92 is estimated.Origins of the 300 GV cosmic-ray proton and alpha-particle hardening are discussed.Comment: In SuGAR2015, Searching for the sources of Galactic Cosmic Rays, University of Geneva, 21-23 January 2015; eds. E. Prandini & S. Toscano; 6 pages, 3 figure