Fermi Gamma-ray Space Telescope

Papers Relevant to Fermi Science - Week of August 23, 2010

Title: Decaying Dark Matter in Supersymmetric Model and Cosmic-Ray Observations
Authors: Koji Ishiwata, Shigeki Matsumoto, Takeo Moroi
Comments: 24 pages, 5 figures
Subjects: High Energy Physics - Phenomenology (hep-ph); Galaxy Astrophysics (astro-ph.GA)

We study cosmic-rays in decaying dark matter scenario, assuming that the dark matter is the lightest superparticle and it decays through a R-parity violating operator. We calculate the fluxes of cosmic-rays from the decay of the dark matter and those from the standard astrophysical phenomena in the same propagation model using the GALPROP package. We reevaluate the preferred parameters characterizing standard astrophysical cosmic-ray sources with taking account of the effects of dark matter decay. We show that, if energetic leptons are produced by the decay of the dark matter, the fluxes of cosmic-ray positron and electron can be in good agreements with both PAMELA and Fermi-LAT data in wide parameter region. It is also discussed that, in the case where sizable number of hadrons are also produced by the decay of the dark matter, the mass of the dark matter is constrained to be less than 200-300 GeV in order to avoid the overproduction of anti-proton. We also show that the cosmic γ-ray flux can be consistent with the results of Fermi-LAT observation if the mass of the dark matter is smaller than ∼ 4 TeV.

arXiv: 1008.3636
DOI:

Title: Can past gamma-ray bursts explain both INTEGRAL and ATIC/PAMELA/Fermi anomalies simultaneously?
Authors: Antoine Calvez, Alexander Kusenko
Comments: 4 pages; version accepted for publication
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Phenomenology (hep-ph)

Gamma-ray bursts (GRBs) have been invoked to explain both the 511 keV emission from the galactic bulge and the high-energy positron excess inferred from the ATIC, PAMELA, and Fermi data. While independent explanations can be responsible for these phenomena, we explore the possibility of their common GRB-related origin by modeling the GRB distribution and estimating the rates. For an expected Milky Way long GRB rate, neither of the two signals is generic; the local excess requires a 2% coincidence, while the signal from the galactic center requires a 20% coincidence with respect to the timing of the latest GRB. The simultaneous explanation requires a 0.4% coincidence. Considering the large number of statistical "trials" created by multiple searches for new physics, the coincidences of a few per cent cannot be dismissed as unlikely. Alternatively, both phenomena can be explained by GRBs if the galactic rate is higher than expected. We also show that a similar result is difficult to obtain assuming a simplified short GRB distribution.

arXiv: 1003.0045
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Title: The Fermi-LAT high-latitude Survey: Source Count Distributions and the Origin of the Extragalactic Diffuse Background
Authors: The Fermi-LAT Collaboration
Comments: Version replaced to match the published one. Contact authors: M. Ajello and A. Tramacere
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

This is the first of a series of papers aimed at characterizing the populations detected in the high-latitude sky of the Fermi-LAT survey. In this work we focus on the intrinsic spectral and flux properties of the source sample. We show that when selection effects are properly taken into account, Fermi sources are on average steeper than previously found (e.g. in the bright source list) with an average photon index of 2.40±0.02 over the entire 0.1-100 GeV energy band. We confirm that FSRQs have steeper spectra than BL Lac objects with an average index of 2.48±0.02 versus 2.18±0.02. Using several methods we build the deepest source count distribution at GeV energies deriving that the intrinsic source (i.e. blazar) surface density at F100≥10-9 ph cm-2 s-1 is 0.12+0.03-0.02 deg-2. The integration of the source count distribution yields that point sources contribute 16(±1.8)% (±7% systematic uncertainty) of the GeV isotropic diffuse background. At the fluxes currently reached by LAT we can rule out the hypothesis that point-like sources (i.e. blazars) produce a larger fraction of the diffuse emission.

arXiv: 1003.0895
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Title: Gamma-Ray Emission Concurrent with the Nova in the Symbiotic Binary V407 Cygni
Authors: The Fermi-LAT Collaboration
Comments: 38 pages, includes Supplementary Online Material; corresponding authors: C.C. Cheung, A.B. Hill, P. Jean, S. Razzaque, K.S. Wood
Journal-ref: Science 329: 817-821, 2010
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Novae are thermonuclear explosions on a white dwarf surface fueled by mass accreted from a companion star. Current physical models posit that shocked expanding gas from the nova shell can produce X-ray emission but emission at higher energies has not been widely expected. Here, we report the Fermi Large Area Telescope detection of variable γ-ray (0.1-10 GeV) emission from the recently-detected optical nova of the symbiotic star V407 Cygni. We propose that the material of the nova shell interacts with the dense ambient medium of the red giant primary, and that particles can be accelerated effectively to produce π0 decay γ-rays from proton-proton interactions. Emission involving inverse Compton scattering of the red giant radiation is also considered and is not ruled out.

arXiv: 1008.3912
DOI: 10.1126/science.1192537

Title: Fermi-GST: A new view of the gamma-ray sky
Authors: Sylvain Chaty (Aime, Université Denis Diderot - Paris Vii, Sap)
Comments: Invited review on Fermi-LAT results, 4 pages
Journal-ref: 22nd Rencontres de Blois - Particle Physics and Cosmology, Blois : France (2010) Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The Large Area Telescope on the Fermi Gamma-ray Space Telescope (FGST, ex-GLAST) provides unprecedented sensitivity for all-sky monitoring of γ-ray activity. It is an adequate telescope to detect transient sources, since the observatory scans the entire sky every three hours and allows a general search for flaring activity on daily timescale. This search is conducted automatically as part of the ground processing of the data and allows a fast response -less than a day- to transient events. Follow-up observations in X-rays, optical, and radio are then performed to attempt to identify the origin of the emission and probe the possible existence of new transient γ-ray sources in the Galaxy. Since its launch on 11th June 2008, Fermi-LAT has detected nearly 1500 γ-ray sources, nearly half of them being extragalactic. After a brief census of detected celestial objects, we report here on the LAT results focusing on Galactic transient binary systems. The Fermi-LAT has detected 2 γ-ray binaries, a microquasar and an unexpected new type of γ-ray source: a symbiotic nova.

arXiv: 1008.3970
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Title: Fermi LAT observations of cosmic-ray electrons from 7 GeV to 1 TeV
Authors: Fermi LAT collaboration
Comments: 20 pages, 23 figures, 2 tables, accepted for publication in Physical Review D - contact authors: C. Sgro', A. Moiseev
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Experiment (hep-ex); High Energy Physics - Phenomenology (hep-ph)

We present the results of our analysis of cosmic-ray electrons using about 8 million electron candidates detected in the first 12 months on-orbit by the Fermi Large Area Telescope. This work extends our previously-published cosmic-ray electron spectrum down to 7 GeV, giving a spectral range of approximately 2.5 decades up to 1 TeV. We describe in detail the analysis and its validation using beam-test and on-orbit data. In addition, we describe the spectrum measured via a subset of events selected for the best energy resolution as a cross-check on the measurement using the full event sample. Our electron spectrum can be described with a power law ∝ E-3.08±0.05 with no prominent spectral features within systematic uncertainties. Within the limits of our uncertainties, we can accommodate a slight spectral hardening at around 100 GeV and a slight softening above 500 GeV.

arXiv: 1008.3999
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Title: Fermi-LAT Study of Gamma-ray Emission in the Direction of Supernova Remnant W49B
Authors: The Fermi-LAT Collaboration
Comments: 9 pages, 10 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present an analysis of the gamma-ray data obtained with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the direction of SNR W49B (G43.3-0.2). A bright unresolved gamma-ray source detected at a significance of 38σ is found to coincide with SNR W49B. The energy spectrum in the 0.2-200 GeV range gradually steepens toward high energies. The luminosity is estimated to be 1.5x1036 (D/8 kpc)2 erg s-1 in this energy range. There is no indication that the gamma-ray emission comes from a pulsar. Assuming that the SNR shell is the site of gamma-ray production, the observed spectrum can be explained either by the decay of neutral π mesons produced through the proton-proton collisions or by electron bremsstrahlung. The calculated energy density of relativistic particles responsible for the LAT flux is estimated to be remarkably large, Ue,p>104 eV cm-3, for either gamma-ray production mechanism.

arXiv: 1008.4190
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Title: Searching for an EBL attenuation signature in the Fermi/LAT 1st year catalog data
Authors: Martin Raue (University of Hamburg)
Comments: 4 pages, 3 figures, accepted for publication in A&A (submitted May 3, 2010)
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Observations of distant sources of high-energy (HE) γ-rays are affected by attenuation resulting from the interaction of the γ-rays with low energy photons from the diffuse meta-galactic radiation fields at ultraviolet (UV) to infrared (IR) wavelengths (Extragalactic Background Light; EBL). Recently, a large data-set of HE observations from the 1st year survey of the Large Area Telescope (LAT) instrument on-board of the Fermi satellite became available, covering an energy range from 100 MeV up to 100 GeV. In this paper, the potential of such large HE data-sets to probe the density of the EBL - especially in the UV to optical - is explored. The data from the catalog is investigated for an attenuation signature in the energy range 10-100 GeV and the results are compared with the predictions from EBL model calculations. No clear signature is found. The statistics are still limited by (1) the sensitivity of Fermi/LAT to detect sources above 10 GeV, (2) the number of firmly identified sources with known redshift, both which will improve over the coming years.

arXiv: 1008.4223
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Title: Multiwavelength properties of a new Geminga-like pulsar: PSR J2021+4026
Authors: L. Trepl, C. Y. Hui, K. S. Cheng, J. Takata, Y. Wang, Z. Y. Liu, N. Wang
Comments: 11 pages, 14 figures
Journal-ref: Mon. Not. R. Astron. Soc. 405, 1339-1348 (2010)
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

In this paper, we report a detailed investigation of the multiwavelength properties of a newly detected γ-ray pulsar, PSR J2021+4026, in both observational and theoretical aspects. We firstly identify an X-ray source in the XMM-Newton serendipitous source catalogue, 2XMM J202131.0+402645, located within the 95% confidence circle of PSR J2021+4026. With an archival Chandra observation, this identification provides an X-ray position with arcsecond accuracy which is helpful in facilitating further investigations. Searching for the pulsed radio emission at the position of 2XMM J202131.0+402645 with a 25-m telescope at Urumqi Astronomical Observatory resulted in null detection and places an upper-limit of 0.1 mJy for any pulsed signal at 18 cm. Together with the emission properties in X-ray and γ-ray, the radio quietness suggests PSR J2021+4026 to be another member of Geminga-like pulsars. In the radio sky survey data, extended emission features have been identified in the γ-ray error circle of PSR J2021+4026. We have also re-analyzed the γ-ray data collected by Fermi's Large Area Telescope. We found that the X-ray position of 2XMM J202131.0+402645 is consistent with that of the optimal gamma-ray timing solution. We have further modeled the results in the context of outer gap model which provides us with constraints for the pulsar emission geometry such as magnetic inclination angle and the viewing angle. We have also discussed the possibility of whether PSR J2021+4026 has any physical association with the supernova remnant G78.2+2.1 (γ-Cygni).

arXiv: 1008.4313
DOI: 10.1111/j.1365-2966.2010.16555.x

Title: The Crab Nebula as a standard candle in very high-energy astrophysics
Authors: M. Meyer, D. Horns, H.-S. Zechlin
Comments: 13 pages, 7 figures, 7 tables, accepted for publication in A&A
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The continuum high-energy gamma-ray emission between 1 GeV and 100 TeV from the Crab Nebula has been measured for the first time in overlapping energy bands by the Fermi large-area telescope (Fermi/LAT) below ≈ 100 GeV and by ground-based imaging air Cherenkov telescopes (IACTs) above ≈ 60 GeV. To follow up on the phenomenological approach suggested by Hillas et al. (1998), the broad band spectral and spatial measurement (from radio to low-energy gamma-rays < 1 GeV) is used to extract the shape of the electron spectrum. While this model per construction provides an excellent description of the data at energies < 1 GeV, the predicted inverse Compton component matches the combined Fermi/LAT and IACT measurements remarkably well after including all relevant seed photon fields and fitting the average magnetic field to B = 124±6 (stat.)+15-6 (sys.) μG. The close match of the resulting broad band inverse Compton component with the combined Fermi/LAT and IACTs data is used to derive instrument specific energy-calibration factors. These factors can be used to combine data from Fermi/LAT and IACTs without suffering from systematic uncertainties on the common energy scale. As a first application of the cross calibration, we derive an upper limit to the diffuse gamma-ray emission between 250 GeV and 1 TeV based upon the combined measurements of Fermi/LAT and the H.E.S.S. ground-based Cherenkov telescopes. Finally, the predictions of the magneto-hydrodynamic flow model of Kennel & Coroniti (1984) are compared to the measured SED.

arXiv: 1008.4524
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Title: Cross Calibration of Imaging Air Cherenkov Telescopes with Fermi
Authors: M. Meyer, D. Horns, H.-S. Zechlin
Comments: 2009 Fermi Symposium, eConf Proceedings C091122, 5 pages, 5 figures, 3 tables
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); High Energy Astrophysical Phenomena (astro-ph.HE)

An updated model for the synchrotron and inverse Compton emission from a population of high energy electrons of the Crab Nebula is used to reproduce the measured spectral energy distribution from radio to high energy γ-rays. By comparing the predicted inverse Compton component with recent Fermi measurements of the nebula's emission, it is possible to determine the average magnetic field in the nebula and to derive the underlying electron energy distribution. The model calculation can then be used to cross calibrate the Fermi observations with ground based air shower measurements. The resulting energy calibration factors are derived and can be used for combining broad energy measurements taken with Fermi in conjunction with ground based measurements.

arXiv: 0912.3754
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Title: e± Excesses in the Cosmic Ray Spectrum and Possible Interpretations
Authors: Yi-Zhong Fan, Bing Zhang, Jin Chang
Comments: 48 pages, including 10 figures and 1 tabel. Invited review to be published in IJMPD
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Phenomenology (hep-ph)

The data collected by ATIC, PPB-BETS, Fermi-LAT and HESS all indicate that there is an electron/positron excess in the cosmic ray energy spectrum above ∼ 100 GeV, although different instrumental teams do not agree on the detailed spectral shape. PAMELA also reported a clear excess feature of the positron fraction above several GeV, but no excess in anti-protons. Here we review the observational status and theoretical models of this interesting observational feature. We pay special attention to various physical interpretations proposed in the literature, including modified supernova remnant models for the e± background, new astrophysical sources, and new physics (the dark matter models). We suggest that although most models can make a case to interpret the data, with the current observational constraints the dark matter interpretations, especially those invoking annihilation, require much more exotic assumptions than some astrophysical interpretations. Future observations may present some "smoking-gun" observational tests to differentiate among different models and to identify the correct interpretation to the phenomenon.

arXiv: 1008.4646
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Title: Fermi results on γ-ray binaries
Authors: Adam B. Hill, Richard Dubois, Diego F. Torres, on behalf of the Fermi-LAT collaboration
Comments: Chapter to be published in the book of proceedings of the 1st Sant Cugat Forum on Astrophysics, "ICREA Workshop on the high-energy emission from pulsars and their systems", held in April, 201
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The past decade has presented a revolution in the field of observational high energy γ-ray astrophysics with the advent of a new generation in ground-based TeV telescopes and subsequent GeV space telescopes. The Fermi Large Area Telescope (LAT) was launched in August 2008 and has offered unprecedented sensitivity and survey capabilities in the 30 MeV - 300 GeV energy range.
Presented here are the results from the first two years of LAT observations of galactic binary systems including the definitive detections of LS I+61°303, LS 5039 and Cyg X-3. These sources and others are discussed in context with their known TeV and X-ray properties. The LAT data provides new understandings and pose new questions about the nature of these objects. The identification of an exponential cutoff in the spectra of both LS I+61°303 and LS 5039 was unexpected and poses challenges for explaining the emission mechanisms and processes which are in operation within these systems.

arXiv: 1008.4762
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Title: Spectral evolution of Fermi/GBM short Gamma-Ray Bursts
Authors: G. Ghirlanda (1), G. Ghisellini (1), L. Nava (2), D. Burlon (3) ((1) INAF-Osservatorio Astronomico di Brera, (2) SISSA/ISAS - Trieste, (3) MPE - Garching)
Comments: 5 pages, 1 table, 3 figures. Submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We study the spectral evolution of 14 short duration Gamma Ray Bursts (GRBs) detected by the Gamma Burst Monitor (GBM) on board Fermi. We study spectra resolved in time at the level of 4-64 ms in the 8 keV-35 MeV energy range. We find a strong correlation between the observed peak energy Epeak and the flux P within individual short GRBs. The slope of the Epeak∝PS correlation for individual bursts ranges between ∼0.4 and ∼1. The rise and decay phase of individual bursts follow the same Epeak-P correlation. The same correlation holds also for the precursors present in two GRBs. There is no correlation between the low energy spectral index and the peak energy or the flux. Our results show that in our 14 short GRBs Epeak evolves in time tracking the flux. This behavior is similar to what found in the population of long GRBs and it is in agreement with the evidence that long GRBs and (the still few) short GRBs with measured redshifts follow the same rest frame Epeak-Liso correlation. Its origin is most likely to be found in the radiative mechanism that has to be the same in both classes of GRBs.

arXiv: 1008.4767
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Title: Universally Leptophilic Dark Matter From Non-Abelian Discrete Symmetry
Authors: Naoyuki Haba, Yuji Kajiyama, Shigeki Matsumoto, Hiroshi Okada, Koichi Yoshioka
Comments: 13 pages, 2 tables, 1 figure
Subjects: High Energy Physics - Phenomenology (hep-ph); High Energy Astrophysical Phenomena (astro-ph.HE)

The positron anomaly recently reported by the cosmic-ray measurements can be explained by the decaying dark matter scenario, where it decays mainly into leptons with the lifetime of O(1026) second. When the dark matter is a fermionic particle, the lifetime of this order is known to be obtained by a dimension 6 operator suppressed by the unification scale (∼1016 GeV), while such decay operators do not necessarily involve only leptons. In addition, the scenario would be spoiled if there exist lower-dimensional operators inducing the dark matter decay. We show in this letter that a single non-Abelian discrete symmetry such as A4 is possible to prohibit all such harmful (non-leptonically coupled and lower-dimensional) operators. Moreover, the dark matter decays into charged leptons in a flavor-blind fashion due to the non-Abelian flavor symmetry, which results in perfect agreements not only with the PAMELA data but also with the latest Fermi-LAT data reported very recently. We also discuss some relevance between the discrete symmetry and neutrino physics.

arXiv: 1008.4777
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