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  • 1
    Electronic Resource
    Electronic Resource
    SSC models for compact sources
    Amsterdam : Elsevier
    Advances in Space Research 8 (1988), S. 579-581 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(88)90462-0
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    High energy emission from AGN
    Amsterdam : Elsevier
    Advances in Space Research 13 (1993), S. 587-596 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(93)90168-B
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  • 3
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    CMB quenching of high-redshift radio-loud AGNs (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-06
    Description: The very existence of more than a dozen of high-redshift ( z   4) blazars indicates that a much larger population of misaligned powerful jetted active galactic nucleus (AGN) was already in place when the Universe was 1.5 Gyr old. Such parent population proved to be very elusive, and escaped direct detection in radio surveys so far. High-redshift blazars themselves seem to be failing in producing extended radio lobes, raising questions about the connection between such class and the vaster population of radio galaxies. We show that the interaction of the jet electrons with the intense cosmic microwave background (CMB) radiation explains the lack of extended radio emission in high-redshift blazars and in their parent population, helping to explain the apparently missing misaligned counterparts of high-redshift blazars. On the other hand, the emission from the more compact and more magnetized hotspots are less affected by the enhanced CMB energy density. By modelling the spectral energy distribution of blazar lobes and hotspots, we find that most of them should be detectable by low-frequency deep radio observations, e.g. by LOw-Frequency ARray for radio astronomy and by relatively deep X-ray observations with good angular resolution, e.g. by the Chandra satellite. At high redshifts, the emission of a misaligned relativistic jet, being debeamed, is missed by current large sky area surveys. The isotropic flux produced in the hotspots can be below ~1 mJy and the isotropic lobe radio emission is quenched by the CMB cooling. Consequently, even sources with very powerful jets can go undetected in current radio surveys, and misclassified as radio-quiet AGNs.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes? (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-31
    Description: High-energy observations of extreme BL Lac objects, such as 1ES 0229+200 or 1ES 0347–121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. However, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates starting from the BL Lac sample of Plotkin et al. ( 2011 ), considering those undetected (or only barely detected) by the Large Area Telescope onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multiwavelength spectral energy distribution of the resulting nine sources, profiting of publicly available archival observations performed by Swift , GALEX , and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected very high energy flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming Cherenkov Telescope Array.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 5
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    Light curves and spectra from off-axis gamma-ray bursts (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-24
    Description: If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time-scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles view jet + –1 , the pulse broadening significantly smears out the light-curve variability. This is largely independent of geometry and emission processes. To explore a specific case, we set up a simple model of a single pulse under the assumption that the pulse rise and decay are dominated by the shell curvature effect. We show that such a pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a different hardness–intensity correlation with respect to the same pulse seen on-axis. For each of these effects, we provide an intuitive physical explanation. We then show how a synthetic light curve made by a superposition of pulses changes with increasing viewing angle. We find that a highly variable light curve (as seen on-axis) becomes smooth and apparently single-pulsed (when seen off-axis) because of pulse overlap. To test the relevance of this fact, we estimate the fraction of off-axis gamma-ray bursts detectable by Swift as a function of redshift, finding that a sizeable fraction (between 10 per cent and 80 per cent) of nearby ( z 〈 0.1) bursts are observed with view jet + –1 . Based on these results, we argue that low-luminosity gamma-ray bursts are consistent with being ordinary bursts seen off-axis.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 6
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    Dark bubbles around high-redshift radio-loud active galactic nucleus (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-04
    Description: At redshift larger than 3 there is a disagreement between the number of blazars (whose jet is pointing at us) and the number of expected parents (whose jet is pointing elsewhere). Now we strengthen this claim because (i) the number of blazars identified within the Sloan Digital Sky Survey (SDSS)+Faint Images of the Radio Sky at Twenty-cm (FIRST) survey footprint increased, demanding a more numerous parent population, and (ii) the detected blazars have a radio flux large enough to be above the FIRST flux limit even if the jet is slightly misaligned. The foreseen number of these slightly misaligned jets, in principle detectable, is much larger than the radio-detected sources in the FIRST+SDSS survey (at redshift larger than 4). This argument is independent of the presence of an isotropic radio component, such as the hotspot or the radio lobe, and does not depend on the bulk Lorentz factor . We propose a scenario that ascribes the lack of slightly misaligned sources to an overobscuration of the nucleus by a ‘bubble’ of dust, possibly typical of the first high-redshift quasars.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    SDSS J013127.34-032100.1: a candidate blazar with an 11 billion solar mass black hole at z = 5.18 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-08
    Description: The radio-loud quasar SDSS J013127.34–032100.1 at a redshift z  = 5.18 is one of the most distant radio-loud objects. The radio to optical flux ratio (i.e. the radio-loudness) of the source is large, making it a promising blazar candidate. Its overall spectral energy distribution, completed by the X-ray flux and spectral slope derived through Target of Opportunity Swift /X-ray Telescope observations, is interpreted by a non-thermal jet plus an accretion disc and molecular torus model. We estimate that its black hole mass is (1.1 ± 0.2) 10 10 M for an accretion efficiency  = 0.08, scaling roughly linearly with . Although there is a factor 2 of systematic uncertainty, this black hole mass is the largest found at these redshifts. We derive a viewing angle between 3 and 5 deg. This implies that there must be other (hundreds) sources with the same black hole mass of SDSS J013127.34–032100.1, but whose jets are pointing away from Earth. We discuss the problems posed by the existence of such large black hole masses at such redshifts, especially in jetted quasars. In fact, if they are associated with rapidly spinning black holes, the accretion efficiency is high, implying a slower pace of black hole growth with respect to radio-quiet quasars.
    Print ISSN: 1745-3925
    Electronic ISSN: 1745-3933
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    Unveiling the nature of the {gamma}-ray emitting active galactic nucleus PKS 0521-36 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-21
    Description: PKS 0521–36 is an active galactic nucleus (AGN) with uncertain classification. We investigate the properties of this source from radio to -rays. The broad emission lines in the optical and ultraviolet bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521–36 shows a knotty structure similar to misaligned AGN. The core dominance and the -ray properties are similar to those estimated for other SSRQ and BLRG detected in -rays, suggesting an intermediate viewing angle with respect to the observer. In this context the flaring activity detected from this source by Fermi -Large Area Telescope between 2010 June and 2012 February is very intriguing. We discuss the -ray emission of this source in the framework of the structured jet scenario, comparing the spectral energy distribution (SED) of the flaring state in 2010 June with that of a low state. We present three alternative models corresponding to three different choices of the viewing angles v  = 6°, 15°, and 20°. We obtain a good fit for the first two cases, but the SED obtained with v  = 15° if observed at a small angle does not resemble that of a typical blazar since the synchrotron emission should dominate by a large factor (~100) the inverse Compton component. This suggests that a viewing angle between 6° and 15° is preferred, with the rapid variability observed during -ray flares favouring a smaller angle. However, we cannot rule out that PKS 0521–36 is the misaligned counterpart of a synchrotron-dominated blazar.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Structure of gamma-ray burst jets: intrinsic versus apparent properties (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-20
    Description: With this paper we introduce the concept of apparent structure of a gamma-ray burst (GRB) jet, as opposed to its intrinsic structure. The latter is customarily defined specifying the functions () (the energy emitted per jet unit solid angle) and () (the Lorentz factor of the emitting material); the apparent structure is instead defined by us as the isotropic equivalent energy E iso ( v ) as a function of the viewing angle v . We show how to predict the apparent structure of a jet given its intrinsic structure. We find that a Gaussian intrinsic structure yields a power-law apparent structure: this opens a new viewpoint on the Gaussian (which can be understood as a proxy for a realistic narrow, well-collimated jet structure) as a possible candidate for a quasi-universal GRB jet structure. We show that such a model (a) is consistent with recent constraints on the observed luminosity function of GRBs; (b) implies fewer orphan afterglows with respect to the standard uniform model; (c) can break out the progenitor star (in the collapsar scenario) without wasting an unreasonable amount of energy; (d) is compatible with the explanation of the Amati correlation as a viewing angle effect; (e) can be very standard in energy content, and still yield a very wide range of observed isotropic equivalent energies.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    High-energy cosmic neutrinos from spine-sheath BL Lac jets (2015)
    Oxford University Press
    Details
    Publication Date: 2015-06-07
    Description: We recently proposed that structured (spine-sheath) jets associated with BL Lac objects could offer a suitable environment for the production of the extragalactic high-energy ( E  〉 100 TeV) neutrino recently revealed by IceCube. Our previous analysis was limited to low-power BL Lac objects. We extend our preliminary study to the entire BL Lac population, assuming that the entire diffuse emission is accounted for by these sources. The neutrino output from a single source depends on a relatively large number of parameters. However, for several of them we have constraints coming from observations and previous application of the structured jet model to blazar and radiogalaxy emission. The observed neutrino spectrum then fixes the remaining free parameters. We assume that the power of cosmic rays as well as the radiative luminosity of the sheath depends linearly on the jet power. In turn, we assume that the latter is well traced by the -ray luminosity. We exploit the BL Lac -ray luminosity function and its cosmic evolution as recently inferred from Fermi -LAT data to derive the expected neutrino cumulative intensity from the entire BL Lac population. When considering only the low-power BL Lacs, a large cosmic ray power for each source is required to account for the neutrino flux. Instead, if BL Lacs of all powers produce neutrinos, the power demand decreases, and the required cosmic ray power becomes of the same order of the radiative jet power. In our scheme, the maximum energy of cosmic rays is constrained to be  few PeV by the lack of events above few PeV. Although such a value is obtained through a fine-tuning with the data, we show that it could be possibly related to the equilibrium between cooling and acceleration processes for high-energy cosmic rays. We also discuss the prospects for the direct association of IceCube events with BL Lacs, providing an estimate of the expected counts for the most promising sources.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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