Fermi Gamma-ray Space Telescope

Papers Relevant to Fermi Science - Week of June 27, 2011

Title: Diffuse Galactic Gamma Rays at intermediate and high latitudes. I. Constraints on the ISM properties
Authors: I. Cholis, M. Tavakoli, C. Evoli, L. Maccione, P. Ullio
Comments: 20 pages, 13 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We study the high latitude (|b|>10°) diffuse γ-ray emission in the Galaxy in light of the recently published data from the Fermi collaboration at energies between 100 MeV and 100 GeV. The unprecedented accuracy in these measurements allows to probe and constrain the properties of sources and propagation of cosmic rays (CRs) in the Galaxy, as well as confirming conventional assumptions made on the interstellar medium (ISM). Using the publicly available DRAGON code, that has been shown to reproduce local measurements of CRs, we study assumptions made in the literature on HI and H2 gas distributions in the ISM, and non spatially uniform models of diffusion in the Galaxy. By performing a combined analysis of CR and γ-ray spectra, we derive constraints on the properties of the ISM gas distribution and the vertical scale height of galactic CR diffusion, which may have implications also on indirect Dark Matter detection. We also discuss some of the possible interpretations of the break at ~230 GV in CR protons and helium spectra, recently observed by PAMELA and their impact on γ-rays.

arXiv: 1106.5073
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Title: Blazar Compton efficiencies: Fermi, external photons and the Sequence
Authors: J. A. Gupta, I. W. A. Browne, M. W. Peel
Comments: 6 pages, 5 figures. Submitted to MNRAS
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

The Fermi-LAT survey provides a large sample of blazars selected on the strength of their inverse Compton emission. We cross-correlate the first Fermi-LAT catalogue with the CRATES radio catalogue and use this sample to investigate whether blazar gamma-ray luminosities are influenced by the availability of external photons to be up-scattered. Using the 8.4 GHz flux densities of their compact radio cores as a proxy for their jet power, we calculate their Compton Efficiency parameters, which measure the ability of jets to convert power in the form of ultra-relativistic electrons into Compton gamma-rays. We find no clear differences in Compton efficiencies between BL Lac objects and FSRQs and no anti-correlation between Compton efficiency and synchrotron peak frequency. This suggests that the scattering of external photons is energetically unimportant compared to the synchrotron self-Compton process. These results contradict the predictions of the blazar sequence.

arXiv: 1106.5172
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Title: Dark Matter and Synchrotron Emission from Galactic Center Radio Filaments
Authors: Tim Linden, Dan Hooper, Farhad Yusef-Zadeh
Comments: 10 pages, 5 Figures, Submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Galaxy Astrophysics (astro-ph.GA); High Energy Physics - Phenomenology (hep-ph)

The inner degrees of the Galactic center contain a large population of filamentary structures observed at radio frequencies. These so-called non-thermal radio filaments (NRFs) trace magnetic field lines and have attracted significant interest due to their hard (Sv ∝ ν-0.1±0.4) synchrotron emission spectra. The origin of these filaments remains poorly understood. We show that the electrons and positrons created through the annihilations of a relatively light (~5-10 GeV) dark matter particle with the cross section predicted for a simple thermal relic can provide a compelling match to the intensity, spectral shape, and flux variation of the NRFs. Furthermore, the characteristics of the dark matter particle necessary to explain the synchrotron emission from the NRFs is consistent with those required to explain the excess γ-ray emission observed from the Galactic center by the Fermi-LAT, as well as the direct detection signals observed by CoGeNT and DAMA/LIBRA.

arXiv: 1106.5493
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Title: The Cosmological Impact of Luminous TeV Blazars I: Implications of Plasma Instabilities for the Intergalactic Magnetic Field and Extragalactic Gamma-Ray Background
Authors: Avery E. Broderick (1,2,3), Philip Chang (1,4), Christoph Pfrommer (5,1) ((1) Canadian Institute for Theoretical Astrophysics, (2) Perimeter Institute for Theoretical Physics, (3) University of Waterloo, (4) University of Wisconsin-Milwaukee, (5) Heidelberg Institute for Theoretical Studies)
Comments: 21 pages, 10 figures, submitted to the ApJ
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

Inverse-Compton cascades initiated by energetic gamma rays (E>100 GeV) enhance the GeV emission from bright, extragalactic TeV sources. The absence of this emission from bright TeV blazars has been used to constrain the intergalactic magnetic field (IGMF), and the stringent limits placed upon the unresolved extragalactic gamma-ray background (EGRB) by Fermi has been used to argue against a large number of such objects at high redshifts. However, these are predicated upon the assumption that inverse-Compton scattering is the primary energy-loss mechanism for the ultra-relativistic pairs produced by the annihilation of the energetic gamma rays on extragalactic background light photons. Here we show that for sufficiently bright TeV sources (isotropic-equivalent luminosities >1042 erg s-1) plasma beam instabilities, specifically the "oblique" instability, present a plausible mechanism by which the energy of these pairs can be dissipated locally, heating the intergalactic medium. Since these instabilities typically grow on timescales short in comparison to the inverse-Compton cooling rate, they necessarily suppress the inverse-Compton cascades. As a consequence, this places a severe constraint upon efforts to limit the IGMF from the lack of a discernible GeV bump in TeV sources. Similarly, it considerably weakens the Fermi limits upon the evolution of blazar populations. Specifically, we construct a TeV-blazar luminosity function from those objects presently observed and find that it is very well described by the quasar luminosity function at z~0.1, shifted to lower luminosities and number densities, suggesting that both classes of sources are regulated by similar processes. Extending this relationship to higher redshifts, we show that the magnitude and shape of the EGRB above ~10 GeV is naturally reproduced with this particular example of a rapidly evolving TeV-blazar luminosity function.

arXiv: 1106.5494
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Title: The Cosmological Impact of Luminous TeV Blazars II: Rewriting the Thermal History of the Intergalactic Medium
Authors: Philip Chang (1,2), Avery E. Broderick (1,3,4), Christoph Pfrommer (5,1) ((1) Canadian Institute for Theoretical Astrophysics, (2) University of Wisconsin-Milwaukee, (3) Perimeter Institute for Theoretical Physics, (4) University of Waterloo, (5) Heidelberg Institute for Theoretical Studies)
Comments: 23 pages, 14 figures, submitted to the ApJ
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

The Universe is opaque to extragalactic very high-energy gamma rays (VHEGRs, E>100 GeV) because they annihilate and pair produce on the extragalactic background light. The resulting ultra-relativistic pairs are assumed to lose energy through inverse Compton scattering of CMB photons. In Broderick et al. (2011, Paper I of this three paper series), we argued that instead powerful plasma instabilities in the ultra-relativistic pair beam dissipate the kinetic energy of the TeV-generated pairs locally, heating the intergalactic medium (IGM). Here, we explore the effect of this heating upon the thermal history of the IGM. We collate the observed extragalactic VHEGR sources to determine a local VHEGR heating rate and correct for the pointed nature of VHEGR observations using Fermi observations of high and intermediate peaked BL Lacs. Because the local extragalactic VHEGR flux is dominated by TeV blazars, we tie the TeV blazar luminosity density to the quasar luminosity density, and produce a VHEGR heating rate as a function of redshift. This heating is relatively homogeneous for z<4 with increasing spatial variation at higher redshift (order unity at z~6). This new heating process dominates photoheating at low redshift and the inclusion of TeV blazar heating qualitatively and quantitatively changes the structure and history of the IGM. TeV blazars produce a uniform volumetric heating rate that is sufficient to increase the temperature of the mean density IGM by nearly an order of magnitude, and at low densities by substantially more, naturally producing an inverted equation of state inferred by observations of the Lyα forest, a feature that is difficult to reconcile with standard reionization models. Finally, we close with a discussion on the possibility of detecting this hot low-density IGM, but find that such measurements are currently not feasible. (abridged)

arXiv: 1106.5504
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Title: Sub-luminous γ-Ray pulsars
Authors: R. W. Romani, M. Kerr, H. A. Craig, S. Johnston, I. Cognard, D. A. Smith
Comments: 9 pages, 4 figures; accepted to the Astrophysical Journal
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Most pulsars observed by the Fermi LAT have γ-ray luminosities scaling with spindown power Ė as Lγ ≈ (Ė x 1033 erg s-1)1/2. However, there exist one detection and several upper limits an order of magnitude or more fainter than this trend. We describe these 'sub-luminous' γ-ray pulsars, and discuss the case for this being an orientation effect. Of the 12 known young radio pulsars with Ė>1034 erg s-1 and d<2kpc several are substantially sub-luminous. The limited available geometrical constraints favor aligned geometries for these pulsars, although no one case for alignment is compelling. In this scenario GeV emission detected from such sub-luminous pulsars can be due to a lower altitude, lower-power accelerator gap.

arXiv: 1106.5762
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Title: Breit-Wigner Enhancement Considering the Dark Matter Kinetic Decoupling
Authors: Xiao-Jun Bi, Peng-Fei Yin, Qiang Yuan
Subjects: High Energy Physics - Phenomenology (hep-ph); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

In the paper we study the Breit-Wigner enhancement of dark matter (DM) annihilation considering the kinetic decoupling in the evolution of DM freeze-out at the early universe. Since the DM temperature decreases much faster (as 1/R2) after kinetic decoupling than that in kinetic equilibrium (as 1/R) we find the Breit-Wigner enhancement of DM annihilation rate after the kinetic decoupling will affect the DM relic density significantly. Focusing on the model parameters that trying to explain the anomalous cosmic positron/electron excesses observed by PAMELA/Fermi/ATIC we find the elastic scattering Xf -> Xf is not efficient to keep dark matter in kinetic equilibrium, and the kinetic decoupling temperature Tkd is comparable to the chemical decoupling temperature Tf ~ O(10) GeV. The reduction of the relic density after Tkd is significant and leads to a limited enhancement factor ~ O(102). Therefore it is difficult to explain the anomalous positron/electron excesses in cosmic rays by DM annihilation and give the correct DM relic density simultaneously in the minimal Breit-Wigner enhancement model.

arXiv: 1106.6027
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