Fermi Gamma-ray Space Telescope

Papers Relevant to Fermi Science - Week of January 24, 2011

Title: High-energy emission from star-forming galaxies
Authors: Massimo Persic (INAF and INFN, Trieste), Yoel Rephaeli (Tel-Aviv University and University of California, San Diego)
Comments: Invited talk at SciNeGHE2010 (8th Wotkshop on Science with the New Generation of High Energy Gamma-ray Experiments): Gamma-ray Astrophysics in the Multimessenger Context (Trieste, Sept.8-10, 2010)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, the steady-state energy distributions of these particles in the starburst galaxies M82 and NGC253 can be determined with a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region, and a radial profile is assumed for the magnetic field. The resulting radiative yields of electrons and protons are calculated: the predicted >100MeV and >100GeV fluxes are in agreement with the corresponding quantities measured with the orbiting Fermi telescope and the ground-based VERITAS and HESS Cherenkov telescopes. The cosmic-ray energy densities in central regions of starburst galaxies, as inferred from the radio and γ-ray measurements of (respectively) non-thermal synchrotron and π0-decay emission, are Up=O(100) eV cm-3, i.e. at least an order of magnitude larger than near the Galactic center and in other non-very-actively star-forming galaxies. These very different energy density levels reflect a similar disparity in the respective supernova rates in the two environments. A Lγ ∝ SFR1.4 relationship is then predicted, in agreement with preliminary observational evidence.

arXiv: 1101.4404

Title: Radio variability of 1st 3-months Fermi blazars at 5 GHz: affected by interstellar scintillation?
Authors: X. Liu (1), Z. Ding (1,2), J. Liu (1), N. Marchili (3), T.P. Krichbaum (3), ((1) Urumqi Observatory, NAOCAS, (2) Graduate University of CAS, (3) MPIfR)
Comments: 4 pages, 4 figures, in proceedings of 'Multiwavelength Variability of Blazars' in Guangzhou Uni. of China, 22-24, Sep. 2010, to appear in JAA
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Blazars from the first-three-months Fermi-AGN list were observed with the Urumqi 25m radio telescope at 5GHz in IDV (Intra-Day Variability) mode and inter-month observation mode. A significant correlation between the flux density at 5GHz and the γ-ray intensity for the Fermi-LAT detected blazars is seen. There is a higher IDV detection rate in Fermi detected blazars than those reported for other samples. Stronger variability appears at lower Galactic latitudes; IDV appears to be stronger in weaker sources, indicating that the variability is affected by interstellar scintillation.

arXiv: 1101.4482

Title: MAGIC discovery of VHE Emission from the FSRQ PKS 1222+21
Authors: J. Aleksić, L. Antonelli, P. Antoranz, M. Backes, J. Barrio, D. Bastieri, J. Becerra González, W. Bednarek, A. Berdyugin, K. Berger, E. Bernardini, A. Biland, O. Blanch, R. Bock, A. Boller, G. Bonnoli, D. Borla Tridon, I. Braun, Dr. Thomas Bretz, A. Cañellas, E. Carmona, A. Carosi, P. Colin, E. Colombo, J. Contreras, J. Cortina, L. Cossio, S. Covino, F. Dazzi, A. De Angelis, E. De Cea del Pozo, Dr. Barbara De Lotto, Dr. Carlos Delgado Mendez, A. Diago Ortega, M. Doert, A. Domìnguez, D. Dominis Prester, D. Dorner, M. Doro, D. Elsaesser, D. Ferenc, M. Fonseca, L. Font, C. Fruck, R. Garcìa Lòpez, M. Garczarczyk, D. Garrido, G. Giavitto, N. Godinović, D. Hadasch, D. Häfner, A. Herrero, D. Hildebrand, D. Höhne-Mönch, J. Hose, D. Hrupec, B. Huber, et al. (92 additional authors not shown)
Comments: 6 pages, 5 figure; Submitted to ApJ Letters
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Very High Energy (VHE) γ-ray emission from the flat spectrum radio quasar PKS 1222+21 (4C 21.35, z=0.432) was detected with the MAGIC Cherenkov telescopes during a short observation (~0.5 h) performed on 2010 June 17. The MAGIC detection coincides with high energy MeV/GeV γ-ray activity measured by the Large Area Telescope (LAT) on board the Fermi satellite. The VHE spectrum measured by MAGIC extends from about 70 GeV up to at least 400 GeV and can be well described by a power law dN/dE ∝ E^-Γ with a photon index Γ = 3.75±0.27stat±0.2syst. The averaged integral flux above 100 GeV is (4.56±0.46) x 10-10 cm-2 s-1. The VHE flux measured by MAGIC varies significantly within the 30 min exposure implying a flux doubling time of about 10 min. The absence of a spectral cutoff constrains the γ-ray emission region outside the Broad Line Region, which would otherwise absorb the VHE γ-rays. Together with the detected fast variability, this challenges present emission models from jets in flat spectrum radio quasars. Moreover, the combined Fermi/LAT and MAGIC spectral data yield constraints on the density of the Extragalactic Background Light in the UV-optical to near-infrared range that are compatible with recent models.

arXiv: 1101.4645
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Title: Bounding the Time Delay between High-energy Neutrinos and Gravitational-wave Transients from Gamma-ray Bursts
Authors: Bruny Baret, Imre Bartos, Boutayeb Bouhou, Alessandra Corsi, Irene Di Palma, Corinne Donzaud, Véronique Van Elewyck, Chad Finley, Gareth Jones, Antoine Kouchner, Szabolcs Màrka, Zsuzsa Màrka, Luciano Moscoso, Eric Chassande-Mottin, Maria Alessandra Papa, Thierry Pradier, Peter Raffai, Jameson Rollins, Patrick Sutton
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Extragalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc)

We derive a conservative coincidence time window for joint searches of gravita-tional-wave (GW) transients and high-energy neutrinos (HENs, with energies above 100GeV), emitted by gamma-ray bursts (GRBs). The last are among the most interesting astrophysical sources for coincident detections with current and near-future detectors. We take into account a broad range of emission mechanisms. We take the upper limit of GRB durations as the 95% quantile of the T90's of GRBs observed by BATSE, obtaining a GRB duration upper limit of ~150s. Using published results on high-energy (>100MeV) photon light curves for 8 GRBs detected by Fermi LAT, we verify that most high-energy photons are expected to be observed within the first ~150s of the GRB. Taking into account the breakout-time of the relativistic jet produced by the central engine, we allow GW and HEN emission to begin up to 100s before the onset of observable gamma photon production. Using published precursor time differences, we calculate a time upper bound for precursor activity, obtaining that 95% of precursors occur within ~250s prior to the onset of the GRB. Taking the above different processes into account, we arrive at a time window of tHEN - tGW ∈ [-500s,+500s]. Considering the above processes, an upper bound can also be determined for the expected time window of GW and/or HEN signals coincident with a detected GRB, tGW - tGRB ≈ tHEN - tGRB ~ [-350s,+150s].

arXiv: 1101.4669
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Title: The High Time Resolution Universe Survey II: Discovery of 5 Millisecond Pulsars
Authors: S. D. Bates, M. Bailes, N. D. R. Bhat, M. Burgay, S. Burke-Spolaor, N. D'Amico, A. Jameson, S. Johnston, M. J. Keith, M. Kramer, L. Levin, A. Lyne, S. Milia, A. Possenti, B. Stappers, W. van Straten
Comments: 11 pages, 5 figures, 4 tables, for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); High Energy Astrophysical Phenomena (astro-ph.HE)

We present the discovery of 5 millisecond pulsars found in the mid-Galactic latitude portion of the High Time Resolution Universe (HTRU) Survey. The pulsars have rotational periods from ~2.3 to ~7.5 ms, and all are in binary systems with orbital periods ranging from ~0.3 to ~150 d. In four of these systems, the most likely companion is a white dwarf, with minimum masses of ~0.2 Solar Masses. The other pulsar, J1731-1847, has a very low mass companion and exhibits eclipses, and is thus a member of the "black widow" class of pulsar binaries. These eclipses have been observed in bands centred near frequencies of 700, 1400 and 3000 MHz, from which measurements have been made of the electron density in the eclipse region. These measurements have been used to examine some possible eclipse mechanisms. The eclipse and other properties of this source are used to perform a comparison with the other known eclipsing and "black widow" pulsars. These new discoveries occupy a short-period and high-dispersion measure (DM) region of parameter space, which we demonstrate is a direct consequence of the high time and frequency resolution of the HTRU survey. The large implied distances to our new discoveries makes observation of their companions unlikely with both current optical telescopes and the Fermi Gamma-ray Space Telescope. The extremely circular orbits make any advance of periastron measurements highly unlikely. No relativistic Shapiro delays are obvious in any of the systems, although the low flux densities would make their detection difficult unless the orbits were fortuitously edge-on.

arXiv: 1101.4778
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Title: TeV blazars and their distance
Authors: Elisa Prandini, Giacomo Bonnoli, Laura Maraschi, Mose' Mariotti, Fabrizio Tavecchio
Comments: Contribution to "Cosmic Radiation Fields: Sources in the early Universe", Desy, Germany, November 9-12, 2010; 6 pages, 3 figures
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

Recently, a new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed, with the underlying assumption that the Very High Energy (VHE) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are related to the real redshifts by a simple linear relation, that has been used to infer the unknown distance of blazars. The sample will be revised with the up-to-date spectra in both TeV and GeV bands, the method tested with the more recent EBL models and finally applied to the unknown distance blazars detected at VHE.

arXiv: 1101.5005

Title: Cosmological effects on the observed flux and fluence distributions of gamma-ray bursts: Are the most distant bursts in general the faintest ones?
Authors: Attila Meszaros, Jakub Ripa, Felix Ryde
Comments: Accepted for publication in Astronomy and Astrophysics, 5 figures, 7 tables
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Several claims have been put forward that an essential fraction of long-duration BATSE gamma-ray bursts should lie at redshifts larger than 5. This point-of-view follows from the natural assumption that fainter objects should, on average, lie at larger redshifts. However, redshifts larger than 5 are rare for bursts observed by Swift, seemingly contradicting the BATSE estimates. The purpose of this article is to clarify this contradiction. We derive the cosmological relationships between the observed and emitted quantities, and we arrive at a prediction that can be tested on the ensembles of bursts with determined redshifts. This analysis is independent on the assumed cosmology, on the observational biases, as well as on any gamma-ray burst model. Four different samples are studied: 8 BATSE bursts with redshifts, 13 bursts with derived pseudo-redshifts, 134 Swift bursts with redshifts, and 6 Fermi bursts with redshifts. The controversy can be explained by the fact that apparently fainter bursts need not, in general, lie at large redshifts. Such a behaviour is possible, when the luminosities (or emitted energies) in a sample of bursts increase more than the dimming of the observed values with redshift. In such a case dP(z)/dz > 0 can hold, where P(z) is either the peak-flux or the fluence. All four different samples of the long bursts suggest that this is really the case. This also means that the hundreds of faint, long-duration BATSE bursts need not lie at high redshifts, and that the observed redshift distribution of long Swift bursts might actually represent the actual distribution.

arXiv: 1101.5040
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Title: On the Galactic Center being the main source of Galactic Cosmic Rays evidenced by recent cosmic rays and gamma rays observations
Authors: Yiqing Guo, Zhaoyang Feng, Qiang Yuan, Cheng Liu, Hongbo Hu
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

There had been many studies on the possibility that the Galactic Center (GC) is the main source of the galactic cosmic rays (GCRs). The recent observations on the sharp knee of the cosmic rays spectrum at PeV energy and the excesses of the electron and positron at sub-TeV suggest that the GCRs are likely coming from a single source. The observation of Fermi bubbles around the GC and the 511 keV line at GC now seem to provide more evidences that the explosion at the GC 107 years ago should contribute to the production of the majority of the GCRs.

arXiv: 1101.5192
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Title: Fermi Large Area Telescope Detection of Bright Gamma-ray Outbursts from a Peculiar Quasar 4C +21.35
Authors: Y. T. Tanaka, L. Stawarz, D. J. Thompson, F. D'Ammando, S. J. Fegan, B. Lott, D. L. Wood, C. C. Cheung, J. Finke, S. Buson, L. Escande, S. Saito, M. Ohno, T. Takahashi, D. Donato, J. Chiang, M. Giroletti, F. K. Schinzel, G. Iafrate, F. Longo
Comments: 24 pages, 4 figures; Submitted to ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

In this paper we report on the two-year-long Fermi-LAT observation of the peculiar blazar 4C +21.35 (PKS 1222+216). This source was in a quiescent state from the start of science operations of the Fermi Gamma-ray Space Telescope in 2008 August until 2009 September, and then became more active, with gradually increasing flux and some moderately-bright flares. In 2010 April and June, 4C +21.35 underwent a very strong GeV outburst composed of several major flares characterized by rise and decay timescales of the order of a day. During the outburst, the GeV spectra of 4C +21.35 displayed a broken power-law form with spectral breaks observed near 1-3 GeV photon energies. We demonstrate that, at least during the major flares, the jet in 4C +21.35 carried a total kinetic luminosity comparable to the total accretion power available to feed the outflow. We also discuss the origin of the break observed in the flaring spectra of 4C +21.35. We show that, in principle, a model involving annihilation of the GeV photons on the He II Lyman recombination continuum and line emission of "broad line region" clouds may account for such. However, we also discuss the additional constraint provided by the detection of 4C +21.35 at 0.07-0.4 TeV energies by the MAGIC telescope, which coincided with one of the GeV flares of the source. We argue that there are reasons to believe that the ≤TeV emission of 4C +21.35 (as well as the GeV emission of the source, if co-spatial), is not likely to be produced inside the broad line region zone of highest ionization (~ 1017 cm from the nucleus), but instead originates further away from the active center, namely around the characteristic scale of the hot dusty torus surrounding the 4C +21.35 nucleus (~ 1019 cm).

arXiv: 1101.5339
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