Fermi Science Support Center

Papers Relevant to Fermi Science - Week of April 25, 2011

Title: Exploring the dark accelerator HESS J1745-303 with Fermi Large Area Telescope
Authors: C. Y. Hui (1), E. M. H. Wu (2), J. H. K. Wu (2), R. H. H. Huang (3), K. S. Cheng (2), P. H. T. Tam (3), A. K. H. Kong (3) ((1) Chungnam National University, (2) University of Hong Kong, (3) National Tsing Hua University)
Comments: 8 pages, 3 figures, 1 table, accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present a detailed analysis of the γ-ray emission from HESS J1745-303 with the data obtained by the Fermi Gamma-ray Space Telescope in the first ~29 months observation.The source can be clearly detected at the level of ~18σ and ~6σ in 1-20 GeV and 10-20 GeV respectively. Different from the results obtained by the Compton Gamma-ray Observatory, we do not find any evidence of variability. Most of emission in 10-20 GeV is found to coincide with the region C of HESS J1745-303. A simple power-law is sufficient to describe the GeV spectrum with a photon index of Γ ~ 2.6. The power-law spectrum inferred in the GeV regime can be connected to that of a particular spatial component of HESS J1745-303 in 1-10 TeV without any spectral break. These properties impose independent constraints for understanding the nature of this "dark particle accelerator".

arXiv: 1104.4836

Title: Radio-Optical-Gamma-Ray properties of MOJAVE AGN detected by Fermi/LAT
Authors: T.G. Arshakian, J. León-Tavares, M. Boettcher, J. Torrealba, V.H. Chavushyan, M.L. Lister, E. Ros, J.A. Zensus
Comments: 11 pages, 10 figures, 4 tables, submitted to A&A
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Aims. We use a sample of 83 core-dominated active galactic nuclei (AGN) selected from the MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) radio-flux-limited sample and detected with the Fermi Large Area Telescope (LAT) to study the relations between non-simultaneous radio, optical, and γ-ray measurements. Methods. We perform a multi-band statistical analysis to investigate the relations between the emissions in different bands and reproduce these relations by modeling of the spectral energy distributions of blazars. Results. There is a significant correlation between the γ-ray luminosity and the optical nuclear and radio (15 GHz) luminosities of blazars. We report a well defined positive correlation between the γ-ray luminosity and the radio-loudness for quasars and BL Lacertae type objects (BL Lacs). A strong positive correlation is found between the radio luminosity and the γ-ray-optical loudness for quasars, while a negative correlation between the optical luminosity and the γ-ray-radio loudness is present for BL Lacs. Modeling of these correlations with a simple leptonic jet model for blazars indicates that variations of the accretion disk luminosity (and hence the jet power) is able to reproduce the trends observed in most of the correlations. To reproduce all observed correlations, variations of several parameters, such as the accretion power, jet viewing angle, Lorentz factor, and magnetic field of the jet, are required.

arXiv: 1104.4946

Title: Discovery of VHE gamma-ray emission from the W49 region with H.E.S.S
Authors: Francois Brun, Mathieu de Naurois, Werner Hofmann, Svenja Carrigan, Arache Djannati-Ataï, Stefan Ohm, for the H.E.S.S. Collaboration
Comments: 6 pages, 3 figures. Proc. of the 25th TEXAS Symposium on Relativistic Astrophysics. To appear in Proceedings of Science
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The W49 region hosts two bright radio sources: the star forming region W49A and the supernova remnant W49B. The 106 Msolar Giant Molecular Cloud W49A is one of the most luminous giant radio HII regions in our Galaxy and hosts several active, high-mass star formation sites. The mixed-morphology supernova remnant W49B has one of the highest surface brightness in radio of all the SNRs of this class in our Galaxy and is one of the brightest ejecta-dominated SNRs in X-rays. Infrared observations evidenced that W49B is interacting with molecular clouds and Fermi recently reported the detection of a coincident bright, high-energy gamma-ray source. Observations by the H.E.S.S. telescope array resulted in the significant detection of VHE gamma-ray emission from the W49 region, compatible with VHE emission from the SNR W49B. The results, the morphology and the origin of the VHE emission are presented in the multi-wavelength context and the implications on the origin of the signal are discussed.

arXiv: 1104.5003

Title: GeV Emission from Collisional Magnetized Gamma Ray Bursts
Authors: P. Mészáros, M.J. Rees
Comments: ApJ(Lett.), in press
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Magnetic fields may play a dominant role in gamma-ray bursts, and recent observations by the Fermi satellite indicate that GeV radiation, when detected, arrives delayed by seconds from the onset of the MeV component. Motivated by this, we discuss a magnetically dominated jet model where both magnetic dissipation and nuclear collisions are important. We show that, for parameters typical of the observed bursts, such a model involving a realistic jet structure can reproduce the general features of the MeV and a separate GeV radiation component, including the time delay between the two. The model also predicts a multi-GeV neutrino component.

arXiv: 1104.5025

Title: The 2010 May Flaring Episode of Cygnus X-3 in Radio, X-Rays, and γ-Rays
Authors: Peter K. G. Williams, John A. Tomsick, Arash Bodaghee, Geoffrey C. Bower, Guy G. Pooley, Katja Pottschmidt, Jérôme Rodriguez, Jörn Wilms, Simone Migliari, Sergei A. Trushkin
Comments: 6 pages, 3 figures, accepted ApJL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Galaxy Astrophysics (astro-ph.GA)

In 2009, Cygnus X-3 (Cyg X-3) became the first microquasar to be detected in the GeV γ-ray regime, via the satellites Fermi and AGILE. The addition of this new band to the observational toolbox holds promise for building a more detailed understanding of the relativistic jets of this and other systems. We present a rich dataset of radio, hard and soft X-ray, and γ-ray observations of Cyg X-3 made during a flaring episode in 2010 May. We detect a ~3-d softening and recovery of the X-ray emission, followed almost immediately by a ~1-Jy radio flare at 15 GHz, followed by a 4.3σ γ-ray flare (E > 100 MeV) ~1.5 d later. The radio sampling is sparse, but we use archival data to argue that it is unlikely the γ-ray flare was followed by any significant unobserved radio flares. In this case, the sequencing of the observed events is difficult to explain in a model in which the γ-ray emission is due to inverse Compton scattering of the companion star's radiation field. Our observations suggest that other mechanisms may also be responsible for γ-ray emission from Cyg X-3.

arXiv: 1104.5248

Title: The Milky Way's Fermi Bubbles: Echoes of the Last Quasar Outburst?
Authors: K. Zubovas, A. R. King, S. Nayakshin
Comments: 6 pages, 0 figures; accepted for publication in MNRAS
Subjects: Galaxy Astrophysics (astro-ph.GA)

Fermi-LAT has recently detected two gamma ray bubbles disposed symmetrically with respect to the Galactic plane. The bubbles have been suggested to be in a quasi-steady state, inflated by ongoing star formation over the age of the Galaxy. Here we propose an alternative picture where the bubbles are the remnants of a large-scale wide-angle outflow from Sgr A*, the SMBH of our Galaxy. Such an outflow would be a natural consequence of a short but bright accretion event on to Sgr A* if it happened concurrently with the well known star formation event in the inner 0.5 pc of the Milky Way ~ 6 Myr ago. We find that the hypothesised near-spherical outflow is focussed into a pair of symmetrical lobes by the greater gas pressure along the Galactic plane. The outflow shocks against the interstellar gas in the Galaxy bulge. Gamma--ray emission could be powered by cosmic rays created by either \sgra\ directly or accelerated in the shocks with the external medium. The Galaxy disc remains unaffected, agreeing with recent observational evidence that supermassive black holes do not correlate with galaxy disc properties. We estimate that an accreted mass ~ 2 x 103 Msun is needed for the accretion event to power the observed Fermi-LAT lobes. Within a factor of a few this agrees with the mass of the young stars born during the star formation event. This estimate suggests that roughly 50% of the gas was turned into stars, while the rest accreted onto Sgr A*. One interpretation of this is a reduced star formation efficiency inside the Sgr A* accretion disc due to stellar feedback, and the other a peculiar mass deposition geometry that resulted in a significant amount of gas falling directly inside the inner ~ 0.03 pc of the Galaxy.

arXiv: 1104.5443

Title: GRB observations by Fermi LAT revisited: new candidates found
Authors: G. I. Rubtsov, M. S. Pshirkov, P. G. Tinyakov
Comments: 5 pages, 2 tables
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We search the Fermi-LAT photon database for an extended gamma-ray emission which could be associated with any of the 581 previously detected gamma-ray bursts (GRBs) visible to the Fermi-LAT. For this purpose we compare the number of photons with energies E > 100 MeV and E > 1 GeV which arrived in the first 1500 seconds after the burst from the same region, to the expected background. We require that the expected number of false detections does not exceed 0.05 for the entire search and find the high-energy emission in 19 bursts, four of which (GRB 081009, GRB 090720B, GRB 100911 and GRB 100728A) were previously unreported. The first three are detected at energies above 100 MeV, while the last one shows a statistically significant signal only above 1 GeV.

arXiv: 1104.5476

Title: Gamma-ray Constraints on Effective Interactions
Authors: Kingman Cheung, Po-Yan Tseng, Tzu-Chiang Yuan
Comments: 22 pages, 4 figures
Subjects: High Energy Physics - Phenomenology (hep-ph); Cosmology and Extragalactic Astrophysics (astro-ph.CO)

Using an effective interaction approach to describe the interactions between the dark matter particle and the light degrees of freedom of the standard model, we calculate the gamma-ray flux due to the annihilation of the dark matter into quarks, followed by fragmentation into neutral pions which subsequently decay into photons. By comparison to the mid-latitude data released from the Fermi-LAT experiment, we obtain useful constraints on the size of the effective interactions and they are found to be comparable to those deduced from collider, gamma-ray line and anti-matter search experiments. However, the two operators induced by scalar and vector exchange among fermionic dark matter and light quarks that contribute to spin-independent cross sections are constrained more stringently by the recent XENON100 data.

arXiv: 1104.5329

Title: Rest-frame properties of 32 gamma-ray bursts observed by the Fermi Gamma-Ray Burst Monitor
Authors: D.Gruber, J. Greiner, A. von Kienlin, A. Rau, M. S. Briggs, V. Connaughton, A. Goldstein, A. J. van der Horst, M. Nardini, P. N. Bhat, E. Bissaldi, J. M. Burgess, V. L. Chaplin, R. Diehl, G. J. Fishman, G. Fitzpatrick, S.Foley, M. H. Gibby, M. M. Giles, S. Guiriec, R. M. Kippen, C. Kouveliotou, L. Lin, S. McBreen, C. A. Meegan, F. Olivares E., W. S. Paciesas, R. D. Preece, D. Tierney, C. Wilson-Hodge
Comments: accepted by A&A
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Aims: In this paper we study the main spectral and temporal properties of gamma-ray bursts (GRBs) observed by Fermi/GBM. We investigate these key properties of GRBs in the rest-frame of the progenitor and test for possible intra-parameter correlations to better understand the intrinsic nature of these events. Methods: Our sample comprises 32 GRBs with measured redshift that were observed by GBM until August 2010. 28 of them belong to the long-duration population and 4 events were classified as short/hard bursts. For all of these events we derive, where possible, the intrinsic peak energy in the νFν spectrum (Ep,rest), the duration in the rest-frame, defined as the time in which 90% of the burst fluence was observed (T90,rest) and the isotropic equivalent bolometric energy (Eiso). Results: The distribution of Ep,rest has mean and median values of 1.1 MeV and 750 keV, respectively. A log-normal fit to the sample of long bursts peaks at ~800 keV. No high-Ep population is found but the distribution is biased against low Ep values. We find the lowest possible Ep that GBM can recover to be ~ 15 keV. The T90,rest distribution of long GRBs peaks at ~10 s. The distribution of Eiso has mean and median values of 8.9 x 1052 erg and 8.2 x 1052 erg, respectively. We confirm the tight correlation between Ep,rest and Eiso (Amati relation) and the one between Ep,rest and the 1-s peak luminosity (Lp) (Yonetoku relation). Additionally, we observe a parameter reconstruction effect, i.e. the low-energy power law index α gets softer when Ep is located at the lower end of the detector energy range. Moreover, we do not find any significant cosmic evolution of neither Ep,rest nor T90,rest.

arXiv: 1104.5495

Title: Detection of High-Energy Gamma-Ray Emission during the X-ray flaring activity in GRB100728A
Authors: The Fermi Collaboration, L. Piro
Comments: accepted for publication in ApJ Letters. Contact authors: E. Troja, L. Piro, V. Vasileiou, S. Cutini, J. E. McEnery
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present the simultaneous Swift and Fermi observations of the bright GRB100728A and its afterglow. The early X-ray emission is dominated by a vigorous flaring activity continuing until 1 ks after the burst. In the same time interval high energy emission is significantly detected by the Fermi/LAT. Marginal evidence of GeV emission is observed up to later times. We discuss the broadband properties of this burst within both the internal and external shock scenarios, with a particular emphasis on the relation between X-ray flares, the GeV emission and a continued long-duration central engine activity as their power source.

arXiv: 1104.5496