The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Jun-hui Fan - One of the best experts on this subject based on the ideXlab platform.
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The radio and Gamma-Ray luminosities of blazars
Publications of the Astronomical Society of Japan, 2001Co-Authors: Lei Zhang, K. S. Cheng, Jun-hui FanAbstract:Based on the $\Gamma$-Ray data of blazars in the third EGRET catalog and radio data at 5 GHz, we studied the correlation between the radio and $\Gamma$-Ray luminosities using two statistical methods. The first method was the partial correlation analysis method, which indicates that there exist correlations between the radio and $\Gamma$-Ray luminosities in both high and low states as well as in the average case. The second method involved a comparison of expected $\Gamma$-Ray luminosity distribution with the observed data using the Kolmogorov-- Smirnov (KS) test. In the second method, we assumed that there is a correlation between the radio and $\Gamma$-Ray luminosities and that the $\Gamma$-Ray luminosity function is proportional to the radio luminosity function. The KS test indicates that the expected Gamma-Ray luminosity distributions are consistent with the observed data in a reasonable parameter range. Finally, we used different $\Gamma$-Ray luminosity functions to estimate the possible 'observed' $\Gamma$-Ray luminosity distributions by GLAST.
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The relation between Gamma-Ray and near-infrared radiation in Gamma-Ray-loud blazars
The Astrophysical Journal, 1997Co-Authors: Guang-zhong Xie, You-hong Zhang, Jun-hui FanAbstract:We collect 16 Gamma-Ray-loud blazars (seven BL Lac objects and nine flat-spectrum radio quasars) with both observed near-infrared and Gamma-Ray flux densities and find that the near-IR luminosity correlates better with Gamma-Ray luminosity than with X-Ray. Possible constraints on the Gamma-Ray emission mechanism are discussed. We suggest that the Gamma-Ray radiation is probably created by inverse Compton scattering of the radiation from hot circumnuclear dust by beamed, ultrarelativistic electrons.
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Gamma-Ray emission of blazars
Astrophysics and Space Science, 1997Co-Authors: Jun-hui FanAbstract:In this paper, the correlations of Gamma Ray with other bands are discussed for highly polarized blazars. The results show Gamma Rays are beamed. It is closely associated with the radio band emission, but not associated with optical or X-Ray emissions. Gamma Ray emission is likely from resynchrotron-self-Compton (SSC) process.
D J Thompson - One of the best experts on this subject based on the ideXlab platform.
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The Gamma-Ray Sky with Fermi
Nuclear Physics B - Proceedings Supplements, 2013Co-Authors: D J ThompsonAbstract:Abstract Gamma Rays reveal extreme, nonthermal conditions in the Universe. The Fermi Gamma-Ray Space Telescope has been exploring the Gamma-Ray sky for more than four years, enabling a search for powerful transients like Gamma-Ray bursts, solar flares, and flaring active galactic nuclei, as well as long-term studies including pulsars, binary systems, supernova remnants, and searches for predicted sources of Gamma Rays such as clusters of galaxies. Some results include a stringent limit on Lorentz invariance violation derived from a Gamma-Ray burst, unexpected Gamma-Ray variability from the Crab Nebula, a huge Gamma-Ray structure in the direction of the center of our Galaxy, and strong constraints on some Weakly Interacting Massive Particle (WIMP) models for dark matter.
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Gamma Ray astrophysics: the EGRET results
Reports on Progress in Physics, 2008Co-Authors: D J ThompsonAbstract:Cosmic Gamma Rays provide insight into some of the most dynamic processes in the Universe. At the dawn of a new generation of Gamma-Ray telescopes, this review summarizes results from the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory, the principal predecessor mission studying high-energy photons in the 100 MeV energy range. EGRET viewed a Gamma-Ray sky dominated by prominent emission from the Milky Way, but featuring an arRay of other sources, including quasars, pulsars, Gamma-Ray bursts and many sources that remain unidentified. A central feature of the EGRET results was the high degree of variability seen in many Gamma-Ray sources, indicative of the powerful forces at work in objects visible to Gamma-Ray telescopes.
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Gamma Ray Pulsars
Cosmic Gamma-Ray Sources, 2004Co-Authors: D J ThompsonAbstract:High-energy Gamma Rays are a valuable tool for studying particle acceleration and radiation in the magnetospheres of energetic pulsars. The seven or more pulsars seen by instruments on the Compton Gamma Ray Observatory (CGRO) show that: the light curves usually have double-peak structures (suggesting a broad cone of emission); Gamma Rays are frequently the dominant component of the radiated power; and all the spectra show evidence of a high-energy turnover. For all the known Gamma-Ray pulsars, multiwavelength observations and theoretical models based on such observations offer the prospect of gaining a broad understanding of these rotating neutron stars. The Gamma-Ray Large Area Space Telescope (GLAST), now in planning for a launch in 2006, will provide a major advance in sensitivity, energy range, and sky coverage.
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EGRET Observations of Gamma‐Ray Bursts
Annals of the New York Academy of Sciences, 1995Co-Authors: E. J. Schneid, Gottfried Kanbach, D J Thompson, C. E. Fichtel, D. L. Bertsch, R. C. Hartman, Stanley D. Hunter, H. A. Mayer-hasselwander, Yiing Lin, P. F. MichelsonAbstract:The Energetic Gamma-Ray Experiment Telescope (EGRET) has observed Gamma-Rays bursts with the highest energy Gamma-Rays and the longest high energy emission to date. EGRET measures the high energy Gamma-Rays with its large NaI scintillator (1 to 200 MeV) and its spark chamber (30 MeV to 30 GeV). The spark chamber also measures time and arrival directions of individual photons allowing locations for the energetic bursts to be determined. Since the Compton Gamma Ray Observatory launch in 1991, EGRET has observed five bursts in the spark chamber with several having Gamma-Ray energies grater than 1 GeV. The recording breaking burst, GRB940217, had Gamma-Rays up to 18 GeV and lasted over 5000 seconds. The results for the energetic bursts are presented. The high energies observed from these Gamma-Ray bursts set constraints for the burst distances.
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Observation of Gamma Ray bursts and flares by the EGRET telescope on the Compton Gamma Ray Observatory
AIP Conference Proceedings, 1991Co-Authors: E. J. Schneid, P. Sreekumar, D J Thompson, C. E. Fichtel, D. L. Bertsch, R. C. Hartman, Stanley D. Hunter, P. W. Kwok, J. R. Mattox, Gottfried KanbachAbstract:The Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray observatory has observed energetic Gamma Ray bursts and flares. On May 3, 1991, EGRET detected a Gamma Ray burst both in the energy measuring NaI (Tl) scintillator and independently in the spark chamber imaging assembly. The NaI spectra were accumulated by a special BURST mode of EGRET. The spectra were measured over a range from 1 to 200 MeV, in three sequential spectra of 1, 2, and 4 seconds. During the peak of the burst, six individual Gamma Rays were detected in the spark chamber, allowing a determination of the burst arrival direction. The intense flares of June were also detected. A solar flare on June 4 was observed to last for several minutes and for a brief time, less than a minute, had significant emission of Gamma Rays exceeding 150 MeV.
Guillaume Dubus - One of the best experts on this subject based on the ideXlab platform.
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Sizing up the population of Gamma-Ray binaries
Astronomy and Astrophysics - A&A, 2017Co-Authors: Guillaume Dubus, Nicolas Guillard, Pierre-olivier Petrucci, Pierrick MartinAbstract:Context. Gamma-Ray binaries are thought to be composed of a young pulsar in orbit around a massive O or Be star with their Gamma-Ray emission powered by pulsar spin-down. The number of such systems in our Galaxy is not known. Aims: We aim to estimate the total number of Gamma-Ray binaries in our Galaxy and to evaluate the prospects for new detections in the GeV and TeV energy range, taking into account that their Gamma-Ray emission is modulated on the orbital period. Methods: We modelled the population of Gamma-Ray binaries and evaluated the fraction of detected systems in surveys with the Fermi-LAT (GeV), H.E.S.S., HAWC and CTA (TeV) using observation-based and synthetic template light curves. Results: The detected fraction depends more on the orbit-average flux than on the light-curve shape. Our best estimate for the number of Gamma-Ray binaries is 101-52+89 systems. A handful of discoveries are expected by pursuing the Fermi-LAT survey. Discoveries in TeV surveys are less likely. However, this depends on the relative amounts of power emitted in GeV and TeV domains. There could be as many as ≈ 200 HESS J0632+057-like systems with a high ratio of TeV to GeV emission compared to other Gamma-Ray binaries. Statistics allow for as many as three discoveries in five years of HAWC observations and five discoveries in the first two years of the CTA Galactic Plane survey. Conclusions: We favour continued Fermi-LAT observations over ground-based TeV surveys to find new Gamma-Ray binaries. Gamma-Ray observations are most sensitive to short orbital period systems with a high spin-down pulsar power. Radio pulsar surveys (SKA) are likely to be more efficient in detecting long orbital period systems, providing a complementary probe into the Gamma-Ray binary population.
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Sizing up the population of Gamma-Ray binaries
Astron.Astrophys., 2017Co-Authors: Guillaume Dubus, Nicolas Guillard, Pierre-olivier Petrucci, Pierrick MartinAbstract:Context. Gamma-Ray binaries are thought to be composed of a young pulsar in orbit around a massive O or Be star with their Gamma-Ray emission powered by pulsar spin-down. The number of such systems in our Galaxy is not known.Aims. We aim to estimate the total number of Gamma-Ray binaries in our Galaxy and to evaluate the prospects for new detections in the GeV and TeV energy range, taking into account that their Gamma-Ray emission is modulated on the orbital period.Methods. We modelled the population of Gamma-Ray binaries and evaluated the fraction of detected systems in surveys with the Fermi-LAT (GeV), H.E.S.S., HAWC and CTA (TeV) using observation-based and synthetic template light curves.Results. The detected fraction depends more on the orbit-average flux than on the light-curve shape. Our best estimate for the number of Gamma-Ray binaries is 101 systems. A handful of discoveries are expected by pursuing the Fermi-LAT survey. Discoveries in TeV surveys are less likely. However, this depends on the relative amounts of power emitted in GeV and TeV domains. There could be as many as ≈ 200 HESS J0632+057-like systems with a high ratio of TeV to GeV emission compared to other Gamma-Ray binaries. Statistics allow for as many as three discoveries in five years of HAWC observations and five discoveries in the first two years of the CTA Galactic Plane survey.Conclusions. We favour continued Fermi-LAT observations over ground-based TeV surveys to find new Gamma-Ray binaries. Gamma-Ray observations are most sensitive to short orbital period systems with a high spin-down pulsar power. Radio pulsar surveys (SKA) are likely to be more efficient in detecting long orbital period systems, providing a complementary probe into the Gamma-Ray binary population. Key words: surveys / pulsars: general / Galaxy: stellar content / Gamma Rays: stars / X-Rays: binaries
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Gamma-Ray binaries and related systems
The Astronomy and Astrophysics Review, 2013Co-Authors: Guillaume DubusAbstract:After initial claims and a long hiatus, it is now established that several binary stars emit high- (0.1–100 GeV) and very high-energy (>100 GeV) Gamma Rays. A new class has emerged called “Gamma-Ray binaries”, since most of their radiated power is emitted beyond 1 MeV. Accreting X-Ray binaries, novae and a colliding wind binary ( η Car) have also been detected—“related systems” that confirm the ubiquity of particle acceleration in astrophysical sources. Do these systems have anything in common? What drives their high-energy emission? How do the processes involved compare to those in other sources of Gamma Rays: pulsars, active galactic nuclei, supernova remnants? I review the wealth of observational and theoretical work that have followed these detections, with an emphasis on Gamma-Ray binaries. I present the current evidence that Gamma-Ray binaries are driven by rotation-powered pulsars. Binaries are laboratories giving access to different vantage points or physical conditions on a regular timescale as the components revolve on their orbit. I explain the basic ingredients that models of Gamma-Ray binaries use, the challenges that they currently face, and how they can bring insights into the physics of pulsars. I discuss how Gamma-Ray emission from microquasars provides a window into the connection between accretion–ejection and acceleration, while η Car and novae raise new questions on the physics of these objects—or on the theory of diffusive shock acceleration. Indeed, explaining the Gamma-Ray emission from binaries strains our theories of high-energy astrophysical processes, by testing them on scales and in environments that were generally not foreseen, and this is how these detections are most valuable.
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High energy Gamma-Ray emission from binaries
New Astronomy Review, 2008Co-Authors: Guillaume DubusAbstract:The current Cherenkov telescopes together with GLAST are opening up a new window into the physics at work close to black holes and rapidly rotating neutron stars with great breakthrough potential. Very high energy Gamma-Ray emission up to 10 TeV is now established in several binaries. The radiative output of Gamma-Ray binaries is in fact dominated by emission above 1 10 MeV. Most are likely powered by the rotational spindown of a young neutron star that generates a highly relativistic wind. The interaction of this pulsar wind with the companion's stellar wind is responsible for the high energy Gamma-Ray emission. There are hints that microquasars, accretion-powered binaries emitting relativistic jets, also emit Gamma-Ray flares that may be linked to the accretion ejection process. Studying high energy Gamma-Ray emission from binaries offers good prospects for the study of pulsar winds physics and may bring new insights into the link between accretion and ejection close to black holes.
Gerald J. Fishman - One of the best experts on this subject based on the ideXlab platform.
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Gamma-Ray Bursts
2013Co-Authors: Gerald J. FishmanAbstract:The present status of Gamma-Ray burst research is reviewed, with an emphasis on recent observations of their temporal, spectral, and global distribution properties. The observed sky distribution of weak Gamma-Ray bursts constrains the allowable geometrical models to sources in either a giant spherical galactic halo or to sources at cosmological distances. Observations of time dilation consistent with the latter have been reported. Extensive searches for a counterpart to Gamma-Ray bursts in other wavelength regions have thus far proved negative. In spite of the abundance of new observations of Gamma-Ray bursts, their energy source and emission mechanism remain highly speculative. New, rapid counterpart search efforts and several new space-borne experiments may provide the needed observations to make progress in the field
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Stellar flares and Gamma‐Ray bursts
AIP Conference Proceedings, 2008Co-Authors: P Li, K. Hurley, Gerald J. Fishman, C KouveliotouAbstract:We have searched for Gamma‐Ray bursts during stellar flares using Ulysses and BATSE/CGRO data. A total of five stellar flares were identified (two from AD Leo and three from AU Mic), but neither BATSE nor Ulysses observed any Gamma‐Ray bursts which could be attributed to them. Using the BATSE trigger threshold, and the known distances to these flare stars, upper limits to the Gamma‐Ray luminosity were obtained. The conditions under which stellar flare Gamma‐Ray bursts could be detected by BATSE as weak events were studied. In particular, we found that if weak events (10−7 ergs cm−2 s−1) are to be explained by flares as recently suggested, the stellar X‐Ray emission (≳25 keV) must be comparable to its optical emission, and the ratio of Lx/Lopt must be higher than that for solar flares by at least 4 orders of magnitude. The stellar flare logN‐logS and spatial distributions are studied and their implications for Gamma‐Ray bursts are discussed.
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The Mystery of Gamma-Ray Bursts
1998Co-Authors: Gerald J. FishmanAbstract:Gamma-Ray bursts remain one of the greatest mysteries in astrophysics. Observations of Gamma-Ray bursts made by the BATSE experiment on the Compton Gamma-Ray Observatory will be described. Most workers in the field now believe that they originate from cosmological distances. This view has been reinforced by observations this year of several optical afterglow counterparts to Gamma-Ray bursts. A summary of these recent discoveries will be presented, along with their implications for models of the burst emission mechanism and the energy source of the bursts.
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Gamma-Ray Burst Observations with Batse
The Hot Universe, 1998Co-Authors: Gerald J. FishmanAbstract:Gamma-Ray bursts (GRBs) will be recorded as one of the outstanding new phenomena discovered in astronomy this century. About once per day, a burst of Gamma Rays appears from a random direction on the sky. Often, the burst outshines all other sources of Gamma-Rays in the sky, combined. This paper reviews some of the key observed phenomenon of bursts in the hard x-Ray/Gamma-Ray region, as observed with the BATSE experiment [4] on the Compton Gamma Ray Observatory. The observed time profiles,spectral properties and durations of Gamma-Ray bursts cover a wide range. Recent breakthroughs in the observation of Gamma-Ray burst counterparts and afterglows in other wavelenth regions have marked the beginning of a new era in Gamma-Ray burst research. Those observations are described in following papers in these proceedings.
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Gamma-Ray Bursts: An Overview
Publications of the Astronomical Society of the Pacific, 1995Co-Authors: Gerald J. FishmanAbstract:A history and overview of the observed properties of Gamma-Ray bursts are presented. The phenomenon of Gamma-Ray bursts is without precedence in astronomy, having no observed property that would be a direct indicator of their distance and no counterpart object in another wavelength region. Their brief, random appearance only in the Gamma-Ray region has made their study difficult. The observed time profiles, spectral properties and durations of Gamma-Ray bursts cover a wide range. All proposed models for their origin must be considered speculative. It is humbling to think that even after 25 years since their discovery, the distance scale of Gamma-Ray bursts is still very much debatable.
F. W. Stecker - One of the best experts on this subject based on the ideXlab platform.
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The Blazar Gamma-Ray Luminosity Function and the Diffuse Extragalactic Gamma-Ray Background
The Astrophysical Journal, 1994Co-Authors: M. H. Salamon, F. W. SteckerAbstract:We have used the data from the new EGRET catalog on 'grazars' (blazers which are observed to be high-energy Gamma-Ray sources), together with radio data, to construct a new relation between radio and Gamma-Ray luminosity for these sources. Using this relation to construct a grazar Gamma-Ray luminosity function, we then calculate the contribution of unresolved grazars to the cosmic Gamma-Ray background radiation. We derive the energy spectrum of this background component above 100 MeV and the angular fluctuations in this background implied by our model.
