Fermi Science Support Center

Papers Relevant to Fermi Science - Week of March 21, 2011

Title: Positron annihilation as a cosmic-ray probe
Authors: Yutaka Ohira, Kazunori Kohri, Norita Kawanaka
Comments: 5 pages, 4 figures, submitted to ApJL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Recently, the gamma-ray telescopes AGILE and Fermi observed several middle-aged supernova remnants (SNRs) interacting with molecular clouds. A plausible emission mechanism of the gamma rays is the decay of neutral pions produced by cosmic ray (CR) nuclei (hadronic process). However, observations do not rule out contributions from the bremsstrahlung radiation due to CR electrons. TeV gamma-ray telescopes also observed many SNRs and discovered many un-ID sources. It is still unclear whether the TeV gamma-ray emission is produced via the leptonic process or the hadronic process. In this Letter, we propose that the annihilation emission of secondary positrons produced by CR nuclei is a diagnostic tool of the hadronic processes. We investigate the MeV emissions from secondary positrons and electrons produced by CR protons in molecular clouds. The annihilation emission of the secondary positrons from SNRs can be robustly estimated from the observed gamma-ray flux. The expected flux of the annihilation line from SNRs observed by AGILE and Fermi is sufficient for the future Advanced Compton Telescope to detect. Moreover, the synchrotron radiation from secondary positrons and electrons and the bremsstrahlung radiation from CR protons can be also observed by the future x-ray telescope ASTRO-H.

arXiv: 1103.4140

Title: Knee structure in high-energy inverse Compton scattering of CMB photons
Authors: Satoshi Nozawa, Yasuharu Kohyama, Naoki Itoh
Comments: 11 pages, 2 figures, 1 table, submitted to Physical Review D
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

We study the inverse Compton scattering of the CMB photons off nonthermal high-energy electrons. In the previous study, assuming the power-law distribution for electrons, we derived the analytic expression for the spectral intensity function I(ω) in the Thomson approximation, which was applicable up to the photon energies of ω < O(GeV). In the present paper, we extend the previous work to higher photon energies of ω > O(GeV) by taking into account the terms dropped in the Thomson approximation, i.e., the Klein-Nishina formula. The analytic expression for I(ω) is derived with the Klein-Nishina formula. It is shown that I(ω) has a "knee" structure at ω = O(PeV). The knee, if exists, should be accessible with gamma-ray observatories such as Fermi-LAT. We propose simple analytical formulae for I(ω) which are applicable to wide photon energies from Thomson region to extreme Klein-Nishina region.

arXiv: 1103.4284

Title: Galactic centre star formation writ large in gamma-rays
Authors: Roland M. Crocker
Comments: 7 pages, 1 figure. Submitted to the Proceedings of the 25th Texas Symposium on Relativistic Astrophysics (Heidelberg, 2010)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Galaxy Astrophysics (astro-ph.GA)

We have modelled the high-energy astrophysics of the inner 200 pc of the Galaxy with a view to explaining the diffuse, broad-band (radio continuum to TeV γ-ray), non-thermal signal detected from this region. Our modelling pins down the ISM parameters for the environment wherein cosmic ray (CR) electrons and ions reside in the Galactic centre (GC). We find that the magnetic field in this region is 100-300 μG, the gas density < 60 cm-3, and that a powerful (> 200 km/s) 'super'-wind acts to remove > 95% of the cosmic rays accelerated in the region before they have time to lose their energy in situ. The ~ 1039 erg/s carried away by the GC cosmic ray protons is precisely enough to energise the ~GeV γ-ray emission from the Fermi 'bubbles' recently found to extend north and south of the GC out to distances of ~10 kpc, provided that the bubbles constitute thick targets to the GC protons and that the situation has reached steady state. In such a situation of 'saturation' the hard, uniform spectrum of the bubbles are explained and secondary electron synchrotron explains the non-thermal microwave emission found in WMAP data mirroring the bubbles. Given the very low density of the bubble plasma (<0.01 cm-3), the pp loss time in the Bubbles is > 5 Gyr. Our scenario thus has the startling implication that a GC source of nonthermal particles of time-averaged power 1039 erg/s has persisted since the youth of the Galaxy.

arXiv: 1103.4523

Title: BL Lacertae Objects and the Extragalactic Gamma-Ray Background
Authors: Fan Li, Xinwu Cao
Comments: 12 pages, 4 figures, submitted
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

A tight correlation between γ-ray and radio emission is found for a sample of BL Lacertae (BL Lac) objects detected by Fermi Gamma-ray Space Telescope (Fermi) and the Energetic Gamma-Ray Experiment Telescope (EGRET). The γ-ray emission of BL Lac objects exhibits strong variability, and the detection rate of γ-ray BL Lac objects is low, which may be related to the γ-ray duty cycle of BL Lac objects. We estimate the γ-ray duty cycle, δγ ~ 0.11, for BL Lac objects detected by EGRET and Fermi. Using the empirical relation of γ-ray emission with radio emission and the estimated γ-ray duty cycle δγ, we derive the γ-ray luminosity function (LF) of BL Lac objects from their radio LF. Our derived γ-ray LF of BL Lac objects can almost reproduce that calculated with the recently released Fermi bright active galactic nuclei (AGN) sample. We find that ~ 45% of the extragalactic diffuse γ-ray background (EGRB) is contributed by BL Lac objects. Combining the estimate of the quasar contribution to the EGRB in the previous work, we find that ~ 77% of the EGRB is contributed by BL Lac objects and radio quasars.

arXiv: 1103.4545