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M. H. Salamon

Bio: M. H. Salamon is an academic researcher from University of Utah. The author has contributed to research in topics: Blazar & Intergalactic travel. The author has an hindex of 10, co-authored 13 publications receiving 1132 citations.

Papers
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Journal ArticleDOI
TL;DR: The gamma-ray spectrum of 3C 279 during 1991 June exhibited a near-perfect power law between 50 MeV and over 5 GeV with a differential spectral index of -(2.02 +/- 0.07).
Abstract: The gamma-ray spectrum of 3C 279 during 1991 June exhibited a near-perfect power law between 50 MeV and over 5 GeV with a differential spectral index of -(2.02 +/- 0.07). If extrapolated, the gamma-ray spectrum of 3C 279 should be easily detectable with first-generation air Cerenkov detectors operating above about 0.3 TeV provided there is no intergalactic absorption. However, by using model-dependent lower and upper limits for the extragalactic infrared background radiation field, a sharp cutoff of the 3C 279 spectrum is predicted at between about 0.1 and about 1 TeV. The sensitivity of present air Cerenkov detectors is good enough to measure such a cutoff, which would provide the first opportunity to obtain a measurement of the extragalactic background infrared radiation field.

491 citations

Journal ArticleDOI
TL;DR: In this article, the absorption of high-energy γ-rays in intergalactic space has been studied by calculating the absorption properties of 10-500 GeV γ rays at high redshifts.
Abstract: In this paper, we extend previous work on the absorption of high-energy γ-rays in intergalactic space by calculating the absorption of 10-500 GeV γ-rays at high redshifts. This calculation requires the determination of the high-redshift evolution of the intergalactic starlight photon field, including its spectral energy distribution out to frequencies beyond the Lyman limit. To estimate this evolution, we have followed a recent analysis by Fall, Charlot, & Pei, which reproduces the redshift dependence of the starlight background emissivity obtained by the Canada-France Redshift Survey group. We give our results for the γ-ray opacity as a function of redshift out to a redshift of z = 3. We also give predicted γ-ray spectra for selected blazars and extend our calculations of the extragalactic γ-ray background from blazars to an energy of 500 GeV with absorption effects included. Our results indicate that the extragalactic γ-ray background spectrum from blazars should steepen significantly above 20 GeV, owing to extragalactic absorption. Future observations of a such a steepening would thus provide a test of the blazar origin hypothesis for the γ-ray background radiation. We also note that our absorption calculations can be used to place limits on the redshifts of γ-ray bursts; for example, our calculated opacities indicate that the 1994 February 17 burst observed by EGRET most probably originated at z ≤ ~2.

207 citations

Journal ArticleDOI
TL;DR: In this article, the authors suggest that low-redshift X-ray-selected BL Lacertae objects (XBLs) may be the only extragalactic γ-ray sources observable at TeV energies.
Abstract: We suggest that low-redshift X-ray-selected BL Lacertae objects (XBLs) may be the only extragalactic γ-ray sources observable at TeV energies. We use simple physical considerations involving synchrotron and Compton component spectra for blazars to suggest why the observed TeV sources are XBLs, whereas mostly radio-selected BL objects (RBLs) and flat spectrum radio quasars (FSRQs) are seen at GeV energies. These considerations indicate that the differences between XBLs and RBLs cannot be explained purely as relativistic jet orientation effects. We note that the only extragalactic TeV sources that have been observed are XBLs and that a nearby RBL with a very hard spectrum in the GeV range has not been seen at TeV energies. We also note that of the 14 BL Lac objects observed by EGRET, 12 are RBLs, whereas only two are XBLs. We give a list of nearby XBLs that we consider to be good candidate TeV sources and predict estimated TeV fluxes for these objects.

96 citations

Journal ArticleDOI
TL;DR: In this article, the authors 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 radiation luminosity for these sources.
Abstract: 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.

75 citations

Journal ArticleDOI
01 May 1994-Nature
TL;DR: In this article, the authors used ground-based observations of high-energy γ-rays to probe the intergalactic infrared radiation field (IIRF) in the early Universe.
Abstract: THE magnitude of the intergalactic infrared radiation field (IIRF) is of fundamental importance to investigations of the evolution of galaxies. Bursts of star formation, which seem to be critically important to galaxy evolution, are characterized by large infrared luminosities1, yet we have little knowledge of the frequency and intensity of starbursts in the early Universe. Unfortunately, direct observational determinations of the IIRF are plagued by the difficulty of separating the extragalactic emission from Galactic foreground radiation and the zodiacal light in the Solar System, and have so far produced only upper limits that are far above theoretical expectations2. We have previously proposed3 a way to use ground-based observations of high-energy γ-rays to probe the IIRF, and here we apply our method to the spectrum4,5 of the BL Lac object Mrk421. The intensity we derive for the IIRF is consistent with the conversion of at least 30% of the energy from stellar nucleosynthesis to infrared radiation, both from emission by cool stars and as a result of absorption and re-emission by interstellar dust grains.

71 citations


Cited by
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Journal Article
TL;DR: For the next few weeks the course is going to be exploring a field that’s actually older than classical population genetics, although the approach it’ll be taking to it involves the use of population genetic machinery.
Abstract: So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

9,847 citations

Journal ArticleDOI
TL;DR: In this article, the authors have modelled the extragalactic optical and infrared backgounds using available information on cosmic sources in the universe from far-UV to sub-millimeter wavelengths over a wide range of cosmic epochs, and applied their photon-photon opacity estimates to the analysis of spectral data at TeV energies on a few BLAZARs of particular interest.
Abstract: Context. The background radiation in the optical and the infrared cause energy loss in the propagation of high energy particles through space. In particular, TeV observations with Cherenkov telescopes of extragalactic sources are influenced by the opacity effects due to the interaction of the very high-energy source photons with the background light. Aims. With the aim of assessing with the best possible detail these opacity terms, we have modelled the extragalactic optical and infrared backgounds using available information on cosmic sources in the universe from far-UV to sub-millimeter wavelengths over a wide range of cosmic epochs. Methods. We have exploited the relevant cosmological survey data – including number counts, redshift distributions, luminosity functions – from ground-based observatories in the optical, near-IR, and sub-millimeter, as well as multi-wavelength information coming from the HST, ISO and Spitzer space telescopes. Additional constraints have been used from direct measurements or upper limits on the extragalactic backgrounds by dedicated missions (COBE). All data were fitted and interpolated with a multi-wavelength backward evolutionary model, allowing us to estimate the background photon density and its redshift evolution. From the redshift-dependent background spectrum, the photon-photon opacities for sources of high-energy emission at any redshifts were then computed. The same results can also be used to compute the optical depths for any kind of processes in the intergalactic space involving interactions with background photons (like scattering of cosmic-ray particles). Results. We have applied our photon-photon opacity estimates to the analysis of spectral data at TeV energies on a few BLAZARs of particular interest. The opacity-corrected TeV spectra are entirely consistent with standard photon-generation processes and show photon indices steeper than Γintrinsic = 1.6. Contrary to some previous claims, but in agreement with other reports, we find no evidence for any truly diffuse background components in addition to those from resolved sources. We have tested in particular the effects of a photon background originating at very high redshifts, as would be the emissions by a primeval population of Population III stars around z ∼ 10. We could not identify any opacity features in our studied BLAZAR spectra consistent with such an emission and place a stringent limit on such a diffuse photon intensity of ∼ 6n W/m 2 /sr between 1 and 4 μm. Conclusions. TeV observations of BLAZARs are consistent with background radiation contributed by resolved galaxies in the optical and IR, and exclude prominent additional components from very high-z unresolved sources.

989 citations

Journal ArticleDOI
TL;DR: In this article, the authors proposed the "accretion disk plus wind" model for radio-quiet active galactic nuclei (AGN) and blazars, which is the most appropriate model at present.
Abstract: ▪ Abstract A large collective effort to study the variability of active galactic nuclei (AGN) over the past decade has led to a number of fundamental results on radio-quiet AGN and blazars. In radio-quiet AGN, the ultraviolet (UV) bump in low-luminosity objects is thermal emission from a dense medium, very probably an accretion disk, irradiated by the variable X-ray source. The validity of this model for high-luminosity radio-quiet AGN is unclear because the relevant UV and X-ray observations are lacking. The broad-line gas kinematics appears to be dominated by virialized motions in the gravity field of a black hole, whose mass can be derived from the observed motions. The “accretion disk plus wind” model explains most of the variability (and other) data and appears to be the most appropriate model at present. Future investigations are outlined. In blazars, rapid variability at the highest energies (gamma-rays) implies that the whole continuum is relativistically boosted along the line of sight. The gener...

876 citations

Journal ArticleDOI
TL;DR: The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang as discussed by the authors, with additional constraints coming from studies of the attenuation of TeV γ-rays.
Abstract: ▪ Abstract The cosmic infrared background records much of the radiant energy released by processes of structure formation that have occurred since the decoupling of matter and radiation following the Big Bang. In the past few years, data from the Cosmic Background Explorer (COBE) mission provided the first measurements of this background, with additional constraints coming from studies of the attenuation of TeV γ-rays. At the same time, there has been rapid progress in resolving a significant fraction of this background with the deep galaxy counts at infrared wavelengths from the Infrared Space Observatory (ISO) instruments and at submillimeter wavelengths from the Submillimeter Common User Bolometer Array (SCUBA) instrument. This article reviews the measurements of the infrared background and sources contributing to it and discusses the implications for past and present cosmic processes.

850 citations

Journal ArticleDOI
Fermi-LAT1, Asas-Sn1, Hawc1, Kanata1, Kiso1, Kapteyn1, Liverpool Telescope1, NuSTAR1, Vla1, B teams1 
13 Jul 2018-Science
TL;DR: The discovery of an extraterrestrial diffuse flux of high-energy neutrinos, announced by IceCube in 2013, has characteristic properties that hint at contributions from extragalactic sources, although the individual sources remain as yet unidentified.
Abstract: INTRODUCTION Neutrinos are tracers of cosmic-ray acceleration: electrically neutral and traveling at nearly the speed of light, they can escape the densest environments and may be traced back to their source of origin. High-energy neutrinos are expected to be produced in blazars: intense extragalactic radio, optical, x-ray, and, in some cases, γ-ray sources characterized by relativistic jets of plasma pointing close to our line of sight. Blazars are among the most powerful objects in the Universe and are widely speculated to be sources of high-energy cosmic rays. These cosmic rays generate high-energy neutrinos and γ-rays, which are produced when the cosmic rays accelerated in the jet interact with nearby gas or photons. On 22 September 2017, the cubic-kilometer IceCube Neutrino Observatory detected a ~290-TeV neutrino from a direction consistent with the flaring γ-ray blazar TXS 0506+056. We report the details of this observation and the results of a multiwavelength follow-up campaign. RATIONALE Multimessenger astronomy aims for globally coordinated observations of cosmic rays, neutrinos, gravitational waves, and electromagnetic radiation across a broad range of wavelengths. The combination is expected to yield crucial information on the mechanisms energizing the most powerful astrophysical sources. That the production of neutrinos is accompanied by electromagnetic radiation from the source favors the chances of a multiwavelength identification. In particular, a measured association of high-energy neutrinos with a flaring source of γ-rays would elucidate the mechanisms and conditions for acceleration of the highest-energy cosmic rays. The discovery of an extraterrestrial diffuse flux of high-energy neutrinos, announced by IceCube in 2013, has characteristic properties that hint at contributions from extragalactic sources, although the individual sources remain as yet unidentified. Continuously monitoring the entire sky for astrophysical neutrinos, IceCube provides real-time triggers for observatories around the world measuring γ-rays, x-rays, optical, radio, and gravitational waves, allowing for the potential identification of even rapidly fading sources. RESULTS A high-energy neutrino-induced muon track was detected on 22 September 2017, automatically generating an alert that was distributed worldwide within 1 min of detection and prompted follow-up searches by telescopes over a broad range of wavelengths. On 28 September 2017, the Fermi Large Area Telescope Collaboration reported that the direction of the neutrino was coincident with a cataloged γ-ray source, 0.1° from the neutrino direction. The source, a blazar known as TXS 0506+056 at a measured redshift of 0.34, was in a flaring state at the time with enhanced γ-ray activity in the GeV range. Follow-up observations by imaging atmospheric Cherenkov telescopes, notably the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes, revealed periods where the detected γ-ray flux from the blazar reached energies up to 400 GeV. Measurements of the source have also been completed at x-ray, optical, and radio wavelengths. We have investigated models associating neutrino and γ-ray production and find that correlation of the neutrino with the flare of TXS 0506+056 is statistically significant at the level of 3 standard deviations (sigma). On the basis of the redshift of TXS 0506+056, we derive constraints for the muon-neutrino luminosity for this source and find them to be similar to the luminosity observed in γ-rays. CONCLUSION The energies of the γ-rays and the neutrino indicate that blazar jets may accelerate cosmic rays to at least several PeV. The observed association of a high-energy neutrino with a blazar during a period of enhanced γ-ray emission suggests that blazars may indeed be one of the long-sought sources of very-high-energy cosmic rays, and hence responsible for a sizable fraction of the cosmic neutrino flux observed by IceCube.

813 citations