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Nikolajuk M.,Data Center for Astrophysics | Nikolajuk M.,University of Bialystok | Walter R.,Data Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2012

The nature of weak emission-line quasars (WLQs) is probed by comparing the Baldwin effect (BEff) in WLQs and normal quasars [quasi-stellar objects (QSOs)]. We selected 81 high-redshift (z > 2.2) and two intermediate-redshift (z= 1.66 and 1.89) WLQs. Their rest-frame equivalent widths (EWs) of the Civ emission line and their Eddington ratio were obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) quasar catalogue or from Diamond-Stanic et al. We compare the parameters of WLQs with those of 81 normal quasars from Bright Quasar Survey and 155 radio-quiet and radio-intermediate quasars detected by SDSS and Chandra. The influence of the Eddington ratio, L Bol/L Edd, and the X-ray to optical luminosity ratio, α ox, on the BEff is analysed. We find that WLQs follow a different relationship on the EW(Civ)-L Bol/L Edd plane than normal quasars. This relationship disagrees with the super-Eddington hypothesis. The weakness/absence of emission lines in WLQs does not seem to be caused by their extremely soft ionizing continuum but by low covering factor (Ω/4π) of their broad-line region. Comparison of emission-line intensities indicates that the ratios of high-ionization line and low-ionization line regions (i.e. Ω HIL/Ω LIL) are lower in WLQs than in normal QSOs. The covering factors of the regions producing Civ and Lyα emission lines are similar in both WLQs and QSOs. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


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.


Neronov A.,Data Center for Astrophysics | Neronov A.,Geneva Observatory | Semikoz D.V.,Paris West University Nanterre La Défense | 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.


Neronov A.,Data Center for Astrophysics | Semikoz D.V.,Paris West University Nanterre La Défense | 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 Défense | 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.


Neronov A.,Data Center for Astrophysics | Semikoz D.,Paris West University Nanterre La Défense | Semikoz D.,RAS Institute for Nuclear Research | Taylor A.M.,Data Center for Astrophysics
Astronomy and Astrophysics | Year: 2012

Aims. We investigate the peculiar properties of a large TeV γ-ray flare from Mrk 501 detected during the 2009 multiwavelength campaign. Methods. We identified the counterpart of the flare in the Fermi/LAT telescope data and studied its spectral and timing characteristics. Results. A strong order-of-magnitude increase of the very-high-energy γ-ray flux during the flare was not accompanied by an increase in the X-ray flux, so that the flare was one of the "orphan"-type TeV flares observed in BL Lacs. The flare lasted about one month at energies above 10 GeV. The flaring source spectrum in the 10-200 GeV range was very hard, with a photon index Γ = 1.1 ± 0.2, harder than that observed in any other blazar in the γ-ray band. No simultaneous flaring activity was detected below 10 GeV. Different variability properties of the emission below and above 10 GeV indicate the existence of two separate components in the spectrum. We investigate possible explanations of the very hard flaring component. We consider, among others, the possibility that the flare is produced by an electromagnetic cascade initiated by very-high-energy γ-rays in the intergalactic medium. Within such an interpretation, peculiar spectral and temporal characteristics of the flare could be explained if the magnetic field in the intergalactic medium is on the order of 10 -17-10 -16 G. © 2012 ESO.


Vovk I.,Data Center for Astrophysics | Taylor A.M.,Data Center for Astrophysics | Semikoz D.,Paris West University Nanterre La Défense | 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.


Vovk I.,Data Center for Astrophysics | Neronov A.,Data Center for Astrophysics
Astrophysical Journal | Year: 2013

We investigate the variability properties of blazars in the GeV band using data from the Fermi/Large Area Telescope (LAT) telescope. We find that blazars exhibit variability down to the minimum timescale resolvable by Fermi; this variability is a function of the peak photon count rate in the LAT. This implies that the real minimum variability timescales for the majority of blazars are typically shorter than those resolvable by the LAT. We find that for several blazars these minimum variability timescales reach those associated with the blazar central engine, the supermassive black hole. At the same time, none of the blazars exhibits variability on a timescale shorter than the black hole horizon light-crossing time and/or the period of rotation around the last stable circular orbit. Based on this fact, we argue that the timing properties of the γ-ray signal could be determined by the processes in the direct vicinity of the supermassive black hole. © 2013. The American Astronomical Society. All rights reserved.


Taylor A.M.,Data Center for Astrophysics | Vovk I.,Data Center for Astrophysics | Neronov A.,Data Center for Astrophysics
Astronomy and Astrophysics | Year: 2011

Context. Attenuation of the TeV .-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary .-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade .-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV-TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. Constraints on the EGMF previously derived from the gamma-ray data suffer from an uncertainty related to the non-simultaneity of GeV and TeV band observations. Aims. We investigate constraints on the EGMF derived from observations of blazars for which TeV observations simultaneous with those by Fermi telescope were reported. We study the dependence of the EGMF bound on the hidden assumptions it rests upon. Methods. We select blazar objects for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission have been published. We model the development of electromagnetic cascades along the gamma-ray beams from these sources using Monte Carlo simulations, including the calculation of the temporal delay incurred by cascade photons, relative to the light propagation time of direct .-rays from the source. Results. Constraints on the EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in the GeV band is lower than the flux of the expected cascade component calculated under the assumption of zero EGMF. Assuming that the reason for the suppression of the cascade component is the extended nature of the cascade emission, we find that B B ≳ 10 -15 G (assuming an EGMF correlation length of =1Mpc) is consistent with the data. Alternatively, the assumption that the suppression of the cascade emission is caused by the time delay of the cascade photons the data are consistent with B ≳ 10-17 G for the same correlation length. © 2011 ESO.


Neronov A.,Data Center for Astrophysics | Vovk I.,Data Center for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2011

We report the discovery of fast variability of γ-ray flares from blazar OJ287. This blazar is known to be powered by a binary system of supermassive black holes. The observed variability time-scale Tvar≲ 3-10h is much shorter than the light-crossing time of the more massive (1.8 × 1010M⊙) black hole and is comparable to the light-crossing time of the less massive (1.3 × 108M⊙) black hole. This indicates that γ-ray emission is produced by the relativistic jet ejected by the black hole of smaller mass. Detection of γ-rays with energies in excess of 10GeV during the fast variable flares constrains the Doppler factor of the jet to be larger than 4. The possibility of studying the orbital modulation of emission from the relativistic jet makes OJ287 a unique laboratory for the study of the mechanism(s) of formation of jets by black holes, and in particular the response of the jet parameters to changes in the parameters of the medium from which the black hole accretes and into which the jet expands. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

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