Context. Cosmic opacity for very high energy γ rays (E> 10 TeV) that result from the interaction with the extragalactic background light can be strongly reduced. This is because of possible ...Lorentz-violating terms in the dispersion relations for particles expected for several versions of quantum gravity theories. Aims. We discuss the possibility of using very high-energy observations of blazars to detect anomalies in the cosmic opacity that are induced by Lorentz invariance violation (LIV), taking the possibility of using extreme BL Lacertae (BL Lac) objects into particular consideration, as well as the bright and nearby BL Lac Mkn 501. Methods. We derive the modified expression for the optical depth of γ rays by also taking redshift dependence into consideration and, by applying this, we derive the expected high-energy spectrum above 10 TeV of Mkn 501, in high and low states, and of the extreme BL Lac 1ES 0229+200. Results. Along with the nearby and well studied BL Lac Mkn 501 especially in high state, other suitable targets are extreme BL Lac objects, characterized by quite hard TeV intrinsic spectra which probably extend at the energies relevant when detecting LIV features.
TeV photons from blazars at relatively large distances, interacting with the optical–infrared cosmic background, are efficiently converted into electron–positron pairs. The produced pairs are ...extremely relativistic (Lorentz factors of the order of 106– 107) and promptly lose their energy through inverse Compton scatterings with the photons of the microwave cosmic background, producing emission in the GeV band. The spectrum and the flux level of this reprocessed emission are critically dependent on the intensity of the intergalactic magnetic field, B, that can deflect the pairs diluting the intrinsic emission over a large solid angle. We derive a simple relation for the reprocessed spectrum expected from a steady source. We apply this treatment to the blazar 1ES0229+200, whose intrinsic, very hard TeV spectrum is expected to be approximately steady. Comparing the predicted reprocessed emission with the upper limits measured by the Fermi/Large Area Telescope, we constrain the value of the intergalactic magnetic field to be larger than B≃ 5 × 10−15 G, depending on the model of extragalactic background light.
The NuSTAR view on hard-TeV BL Lacs Costamante, L; Bonnoli, G; Tavecchio, F ...
Monthly notices of the Royal Astronomical Society,
07/2018, Letnik:
477, Številka:
3
Journal Article
We study the four BL Lac objects (RGB J0152+017, 1ES 0229+200, 1ES 0347−121 and PKS 0548−322) detected in the TeV band but not present in the first Fermi-LAT (1FGL) catalogue of the Fermi/Large Area ...Telescope (LAT). We analyse the 24 months of LAT data deriving γ-ray fluxes or upper limits that we use to assemble their spectral energy distributions (SED). We model the SEDs with a standard one-zone leptonic model, also including the contribution of the reprocessed radiation in the multi-GeV band, emitted by the pairs produced through the conversion of the primary TeV emission by interaction with the cosmic optical-infrared background. For simplicity, in the calculation of this component we adopt an analytical approach including some simplifying assumptions, in particular (i) the blazar high-energy emission is considered on average stable over times of the order of 107 yr and (ii) the observer is exactly on-axis. We compare the physical parameters derived by the emission model with those of other high-energy emitting BL Lacs, confirming that TeV BL Lacs with a rather small GeV flux are characterized by extremely low values of the magnetic field and large values of the electron energies. The comparison between the flux in the GeV band and that expected from the reprocessed TeV emission allows us to confirm and strengthen the lower limit of B≳ 10−15 G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons.
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.
ABSTRACT
We examine a model for the observed temporal variability of powerful blazars in the γ-ray band in which the dynamics is described in terms of a stochastic differential equation, including ...the contribution of a deterministic drift and a stochastic term. The form of the equation is motivated by the current astrophysical framework, accepting that jets are powered through the extraction of the rotational energy of the central supermassive black hole mediated by magnetic fields supported by a so-called magnetically arrested accretion disc. We apply the model to the γ-ray light curves of several bright blazars and we infer the parameters suitable to describe them. In particular, we examine the differential distribution of fluxes (dN/dFγ) and we show that the predicted probability density function for the assumed stochastic equation naturally reproduces the observed power-law shape at large fluxes $\mathrm{ d}N/\mathrm{ d}F_{\gamma } \propto F_{\gamma }^{-\alpha }$ with α > 2.
We consider the 1.5-yr Fermi/Large Area Telescope (LAT) light curves (E>100 MeV) of the flat spectrum radio quasars 3C454.3 and PKS1510−089, which show high activity in this period of time. We ...characterize the duty cycle of the source by comparing the time spent by the sources at different flux levels. We consider in detail the light curves covering periods of extreme flux. The large number of high-energy photons collected by LAT in these events allows us to find evidence of variability on time-scales of a few hours. We discuss the implications of significant variability on such short time-scales that challenge the scenario recently advanced in which the bulk of the γ-ray luminosity is produced in regions of the jet at large distances (tens of parsec) from the black hole.
We report about the search for short-term variability in the high-energy γ-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222 + 216), whose flux at E > 100 MeV exceeded ...the value of 10-5 ph cm-2 s-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (<2–3 h) – the smallest measured to date at MeV energies –, which can constrain the size of the γ-ray emitting region. The results obtained in the present work favor the hypothesis that γ rays are generated inside the broad-line region.
ABSTRACT
Blazar S5 0716+714 is well-known for its short-term variability, down to intraday time-scales. We here present the 2-min cadence optical light curve obtained by the TESS space telescope in ...2019 December–2020 January and analyse the object fast variability with unprecedented sampling. Supporting observations by the Whole Earth Blazar Telescope Collaboration in B, V, R, and I bands allow us to investigate the spectral variability during the TESS pointing. The spectral analysis is further extended in frequency to the UV and X-ray bands with data from the Neil Gehrels Swift Observatory. We develop a new method to unveil the shortest optical variability time-scales. This is based on progressive de-trending of the TESS light curve by means of cubic spline interpolations through the binned fluxes, with decreasing time bins. The de-trended light curves are then analysed with classical tools for time-series analysis (periodogram, autocorrelation, and structure functions). The results show that below 3 d there are significant characteristic variability time-scales of about 1.7, 0.5, and 0.2 d. Variability on time-scales $\lesssim 0.2$ d is strongly chromatic and must be ascribed to intrinsic energetic processes involving emitting regions, likely jet substructures, with dimension less than about 10−3 pc. In contrast, flux changes on time-scales $\gtrsim 0.5$ d are quasi-achromatic and are probably due to Doppler factor changes of geometric origin.
The γ-ray brightest days of the blazar 3C 454.3 Bonnoli, G.; Ghisellini, G.; Foschini, L. ...
Monthly notices of the Royal Astronomical Society,
01/2011, Letnik:
410, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In the first week of 2009 December, the blazar 3C 454.3 became the brightest high-energy source in the sky. Its photon flux reached and surpassed the level of 10−5 ph cm−2 s−1 above 100 MeV. The ...Swift satellite observed the source several times during the period of high γ-ray flux, and we can construct really simultaneous spectral energy distributions (SEDs) before, during and after the luminosity peak. Our main findings are (i) the optical, X-ray and γ-ray fluxes correlate; (ii) the γ-ray flux varies quadratically (or even more) with the optical flux; (iii) a simple one-zone synchrotron inverse Compton model can account for all the considered SED; (iv) in this framework the γ-ray versus optical flux correlation can be explained if the magnetic field is slightly fainter when the overall jet luminosity is stronger and (v) the power that the jet spent to produce the peak γ-ray luminosity is of the same order, or larger, than the accretion disc luminosity.
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