The Gamma-Ray Background from Blazars: A New Look

Stecker, F. W.; Salamon, M. H.

doi:10.1086/177348

Cited by 222 publications

(285 citation statements)

References 13 publications

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“…One of the main results here is the evidence for a VHE γ-ray quiescent-state emission, where the variations in the flux are found to have a lognormal distribution. The existence of such a state was postulated by Stecker & Salamon (1996) in order to explain the extragalactic γ-ray background at 0.03−100 GeV detected by EGRET (Fichtel 1996;Sreekumar et al 1998) as coming from quiescent-state unresolved blazars. Such a background has not yet been detected in the VHE range, as it is technically difficult with the atmospheric Cherenkov technique to find an isotropic extragalactic emission and even more to distinguish it from the cosmic-ray electron flux (Aharonian et al 2008b).…”

Section: Discussionmentioning

confidence: 99%

VHEγ-ray emission of PKS 2155–304: spectral and temporal variability

Abramowski¹,

Acero²,

Aharonian³

et al. 2010

A&A

101

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Context. Observations of very high-energy γ-rays from blazars provide information about acceleration mechanisms occurring in their innermost regions. Studies of variability in these objects lead to a better understanding of the mechanisms in play. Aims. To investigate the spectral and temporal variability of VHE (>100 GeV) γ-rays of the well-known high-frequency-peaked BL Lac object PKS 2155−304 with the HESS imaging atmospheric Cherenkov telescopes over a wide range of flux states. Methods. Data collected from 2005 to 2007 were analyzed. Spectra were derived on time scales ranging from 3 years to 4 min. Light curve variability was studied through doubling timescales and structure functions and compared with red noise process simulations. Results. The source was found to be in a low state from 2005 to 2007, except for a set of exceptional flares that occurred in July 2006. The quiescent state of the source is characterized by an associated mean flux level of (4.32 ± 0.09 stat ± 0.86 syst ) × 10 −11 cm −2 s −1 above 200 GeV, or approximately 15% of the Crab Nebula, and a power-law photon index of Γ = 3.53 ± 0.06 stat ± 0.10 syst . During the flares of July 2006, doubling timescales of ∼2 min are found. The spectral index variation is examined over two orders of magnitude in flux, yielding different behavior at low and high fluxes, which is a new phenomenon in VHE γ-ray emitting blazars. The variability amplitude characterized by the fractional rms F var is strongly energy-dependent and is ∝E 0.19±0.01 . The light curve rms correlates with the flux. This is the signature of a multiplicative process that can be accounted for as a red noise with a Fourier index of ∼2. Conclusions. This unique data set shows evidence of a low-level γ-ray emission state from PKS 2155−304 that possibly has a different origin than the outbursts. The discovery of the light curve lognormal behavior might be an indicator of the origin of aperiodic variability in blazars.

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Section: Discussionmentioning

confidence: 99%

VHEγ-ray emission of PKS 2155–304: spectral and temporal variability

Abramowski¹,

Acero²,

Aharonian³

et al. 2010

A&A

101

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“…The points with errorbars correspond to the EGRET measurements of the extragalactic gamma background as given in Sreekumar et al (1998). The solid horizontal line, which is used in the following as an upper bound for the level of diffuse γ-ray background, represents the EGRET measurements reduced by a factor of three to approximately account for the following: (i) dependent on the modeling details, it is predicted that from 25% up to 100% of the diffuse extragalactic γ-ray background is due to unresolved AGNs (Stecker & Salamon 1996;Chiang & Mukherjee 1998;Mukherjee & Chiang 1999;Mücke & Pohl 2000); (ii) the improved model for the Galactic contribution further suppresses the measurements (Strong et al 2004); (iii) there is an additional component due to the DM annihilation produced in our own Galactic halo (see the relevant calculation by Kawasaki et al 2009) 4 . Note that due to relatively small fraction of energy released in the form of prompt photons ∼2-4%, except for the τ − + τ + channel where the prompt photon part reaches up to ∼17%, the inverse Compton part of the spectrum has a higher amplitude than the prompt portion.…”

Section: Radiative Transfer Extragalactic Gamma Background Reionizamentioning

confidence: 99%

Constraints on leptonically annihilating dark matter from reionization and extragalactic gamma background

Hütsi

Hektor

Raidal

2009

A&A

106

134

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Context. The PAMELA, Fermi and HESS experiments (PFH) have shown anomalous excesses in the cosmic positron and electron fluxes. A very exciting possibility is that those excesses are due to annihilating dark matter (DM). Aims. In this paper we calculate constraints on leptonically annihilating DM using observational data on diffuse extragalactic γ-ray background and measurements of the optical depth to the last-scattering surface, and compare those with the PFH favored region in the m DM − σ A υ plane. Methods. Having specified the detailed form of the energy input with PYTHIA Monte Carlo tools we solve the radiative transfer equation which allows us to determine the amount of energy being absorbed by the cosmic medium and also the amount left over for the diffuse gamma background. Results. We find that the constraints from the optical depth measurements are able to rule out the PFH favored region fully for the τ − + τ + annihilation channel and almost fully for the μ − + μ + annihilation channel. It turns out that those constraints are quite robust with almost no dependence on low redshift clustering boost. The constraints from the γ-ray background are sensitive to the assumed halo concentration model and, for the power law model, rule out the PFH favored region for all leptonic annihilation channels. We also find that it is possible to have models that fully ionize the Universe at low redshifts. However, those models produce too large free electron fractions at z 100 and are in conflict with the optical depth measurements. Also, the magnitude of the annihilation cross-section in those cases is larger than suggested by the PFH data.

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“…It has been argued (Dermer & Schlickeiser 1992;Stecker et al 1993;Padovani et al 1993;Salamon & Stecker 1994;Setti & Woltjer 1994;Chiang et al 1995;Erlykin & Wolfendale 1995;Stecker & Salamon 1996;Chiang & Mukherjee 1998;Weferling & Schlickeiser 1999;Mucke & Pohl 2000) that the bulk of this emission most likely originates from unresolved blazars, whose properties are similar to those detected by EGRET (von Montigny et al 1995;Hartman et al1999). However, as the internal parameters and cosmological distribution of EGRET-like blazar sources are not precisely known, the origin of the diffuse -ray background is still being debated, and new classes of objects emitting GeV photons can be analyzed in this context (e.g., Loeb & Waxman 2000;Gabici & Blasi 2003).…”

Section: Introductionmentioning

confidence: 96%

Kiloparsec‐Scale Jets in FR I Radio Galaxies and the γ‐Ray Background

Stawarz

Kneiske

Kataoka

2006

ApJ

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We discuss the contribution of kiloparsec-scale jets in FR I radio galaxies to the diffuse -ray background radiation. The analyzed -ray emission comes from inverse-Compton scattering of starlight photon fields by the ultrarelativistic electrons whose synchrotron radiation is detected from such sources at radio, optical, and X-ray energies. We find that these objects, under the minimum-power hypothesis (corresponding to a magnetic field of 300 G in the brightest knots of these jets), can contribute about one percent to the extragalactic -ray background measured by EGRET. We point out that this result already indicates that the magnetic fields in kiloparsec-scale jets of low-power radio galaxies are not likely to be smaller than 10 G on average, as otherwise the extragalactic -ray background would be overproduced.

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The Gamma-Ray Background from Blazars: A New Look

Cited by 222 publications

References 13 publications

VHEγ-ray emission of PKS 2155–304: spectral and temporal variability

VHEγ-ray emission of PKS 2155–304: spectral and temporal variability

Constraints on leptonically annihilating dark matter from reionization and extragalactic gamma background

Kiloparsec‐Scale Jets in FR I Radio Galaxies and the γ‐Ray Background

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