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Neronov A.,Data Center for Astrophysics | Semikoz D.V.,Paris West University Nanterre La Defense | Semikoz D.V.,RAS Institute for Nuclear Research
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We consider a possibility of identification of sources of cosmic rays (CR) of the energy above 1 TeV via observation of degree-scale extended γ-ray emission which traces the locations of recent sources in the Galaxy. Such emission in the energy band above 100 GeV is produced by CR nuclei and electrons released by the sources and spreading into the interstellar medium. We use the data from the Fermi γ-ray telescope to locate the degree-scale 100 GeV γ-ray sources. We find that the number of such sources and their overall power match to those expected when CRs injection events happen every ∼100yr in portions of ∼1050erg. We find that most of the sources are associated to pulsars with spin-down age less than ∼30kyr and hence to the recent supernova explosions. This supports the hypothesis of supernova origin of Galactic CRs. We notice that the degree-scale extended emission does not surround shell-like supernova remnants without pulsars. Based on this observation, we argue that the presence of the pulsar is essential for the CR acceleration process. We expect that a significant fraction of the degree-scale sources should be detectable as extended sources with km3-scale neutrino detectors. © 2012 American Physical Society.

Neronov A.,Data Center for Astrophysics | Semikoz D.V.,Paris West University Nanterre La Defense | Semikoz D.V.,RAS Institute for Nuclear Research
Astrophysical Journal | Year: 2012

We study the origin of the extragalactic diffuse gamma-ray background using the data from the Fermi telescope. To estimate the background level, we count photons at high Galactic latitudes |b| > 60°. Subtracting photons associated with known sources and the residual cosmic-ray and Galactic diffuse backgrounds, we estimate the extragalactic gamma-ray background (EGB) flux. We find that the spectrum of EGB in the very high energy band above 30 GeV follows the stacked spectrum of BL Lac objects. Large Area Telescope data reveal the positive (1 + z)k, 1 < k < 4 cosmological evolution of the BL Lac source population consistent with that of their parent population, Fanaroff-Riley type I radio galaxies. We show that EGB at E > 30 GeV could be completely explained by emission from unresolved BL Lac objects if k≃ 3. © 2012. The American Astronomical Society. All rights reserved.

Ishibashi W.,Institute of Astronomy | Courvoisier T.J.-L.,Data Center for Astrophysics | Courvoisier T.J.-L.,University of Geneva
Astronomy and Astrophysics | Year: 2012

X-ray variability of active galactic nuclei (AGN) and black hole binaries can be analysed by means of the power spectral density (PSD). The break observed in the power spectrum defines a characteristic variability timescale of the accreting system. The empirical variability scaling that relates characteristic timescale, black hole mass, and accretion rate (T B ∝ M BH 2.1/Ṁ 0.98) extends from supermassive black holes in AGN down to stellar-mass black holes in binary systems. We suggest that the PSD break timescale is associated with the cooling timescale of electrons in the Comptonisation process at the origin of the observed hard X-ray emission. We find that the Compton cooling timescale directly leads to the observational scaling and naturally reproduces the functional dependence on black hole mass and accretion rate (t C ∝ M 2 BH/Ṁ). This result simply arises from general properties of the emission mechanism and is independent of the details of any specific accretion model. © 2012 ESO.

Vovk I.,Data Center for Astrophysics | Taylor A.M.,Data Center for Astrophysics | Semikoz D.,Paris West University Nanterre La Defense | Semikoz D.,RAS Institute for Nuclear Research | Neronov A.,Data Center for Astrophysics
Astrophysical Journal Letters | Year: 2012

We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one of the primary sources used to put constraints on the extragalactic background light (EBL) and extragalactic magnetic field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. in 2008 is allowed if the EGMF is strong enough. On the other hand, an EGMF strength as low as 6 × 10-18G is allowed if the EBL density is at the level of the lower bound from the direct source counts. We present the combined EBL and EGMF limits as an exclusion plot in two-dimensional parameter space: EGMF strength versus EBL density. © 2012 The American Astronomical Society. All rights reserved.

Neronov A.,Data Center for Astrophysics | Semikoz D.V.,Paris West University Nanterre La Defense | Taylor A.M.,Data Center for Astrophysics
Physical Review Letters | Year: 2012

Measurements of the low-energy spectrum of Galactic cosmic rays (GCRs) by detectors on or near Earth are affected by solar modulation. To overcome this difficulty, we consider nearby molecular clouds as GCR detectors outside the Solar System. Using γ-ray observations of the clouds by the Fermi telescope, we derive the spectrum of GCRs in the clouds from the observed γ-ray emission spectrum. We find that the GCR spectrum has a low-energy break with the spectral slope hardening by ΔΓ=1.1±0.3 at an energy of E=9±3GeV. Detection of a low-energy break enables a measurement of GCR energy density in the interstellar space U=0.9±0.3eV/cm3. © 2012 American Physical Society.

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