Time-dependent modelling of PKS 2155-304 in a low state

We apply both leptonic and leptohadronic emission scenarios for modelling the multiwavelength photon spectra and the observed variability in the optical, X-ray, and TeV gamma-ray energy bands of blazar PKS 2155-304 while being in a low state between 25 August and 6 September 2008. We consider three emission models, i.e. a one-component synchrotron self-Compton model (1-SSC), a one-zone proton synchrotron model (LHs), and a two-component SSC model (2-SSC). Only in the first scenario can the emission from the optical up to TeV gamma-rays be attributed to a single particle population from one emission region. Using a time-dependent numerical code that solves the kinetic equations for each particle species, we derived, in all cases, acceptable fits to the time-averaged SED. By imposing variations to one (or more) model parameters according to observed variability pattern in one (or more) frequencies we calculated the respective light curves and compared them with the observations. We show that the 1-SSC model cannot account for the anticorrelation observed between the X-rays and VHE gamma-rays, although it can explain the time-averaged SED. The anticorrelation can be more naturally explained by the two-component emission models. Both of them reproduce satisfactorily the optical, X-ray, and TeV variability but at the cost of additional free parameters, which from four in the 2-SSC model increase to six in the LHs model. Although the results of our time-resolved analysis do not favour one of the aforementioned models, they suggest that a two-component scenario is more adequate for the emission of PKS 2155-304 in the low state of 2008, which agrees with a recent independent analysis. This suggests that the quiescent blazar radiation might result from a superposition of the radiation from different components, while a flare might still be the result of a single component.Comment: 11 pages, 10 figures, 2 tables, 1 appendix, accepted in A&A, shorter abstract than the accepted versio

The X-ray dust scattered rings of the black hole low mass binary V404 Cyg

We report on the first detection of X-ray dust scattered rings from the Galactic low mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on June 30 2015 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg. Follow-up Swift/XRT observations allowed a time-dependent study of the X-ray rings. Assuming that these are the result of small-angle, single X-ray scattering by dust grains along the line of sight, we find that their angular size scales as $\theta \propto\sqrt{t}$ in agreement with theoretical predictions. The dust grains are concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and 1.18 kpc from the observer. These coincide roughly with locations of enhanced extinction as determined by infrared photometry. Assuming that the grain size distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we find that the power-law index of the most distant cloud is $q\sim 4.4$, while $q \sim 3.5-3.7$ in all other clouds. We constrain at a $3\sigma$ level the maximum grain size of the intermediate dust layers in the range $0.16-0.20\,\mu$m and set a lower limit of $\sim 0.2\,\mu$m in the other clouds. Hints of an exponential cutoff at the angular intensity profile of the outermost X-ray ring suggest that the smallest grains have sizes $0.01 \mu{\rm m}\le \alpha_{\min} \lesssim 0.03\,\mu$m. Based on the relative ratios of dust column densities we find the highest dust concentration at $\sim 1.6$ kpc. Our results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.Comment: 17 pages, 13 figures, accepted by MNRA

Spontaneously quenched gamma-ray spectra from compact sources

We study a mechanism for producing intrinsic broken power-law gamma-ray spectra in compact sources. This is based on the principles of automatic photon quenching, according to which, gamma-rays are being absorbed on spontaneously produced soft photons, whenever the injected luminosity in gamma-rays lies above a certain critical value. We derive an analytical expression for the critical gamma-ray compactness in the case of power-law injection. For the case where automatic photon quenching is relevant, we calculate analytically the emergent steady-state gamma-ray spectra. We perform also numerical calculations in order to back up our analytical results. We show that a spontaneously quenched power-law gamma-ray spectrum obtains a photon index 3{\Gamma}/2, where {\Gamma} is the photon index of the power-law at injection. Thus, large spectral breaks of the gamma-ray photon spectrum, e.g. $\Delta \Gamma \gtrsim 1$, can be obtained by this mechanism. We also discuss additional features of this mechanism that can be tested observationally. Finally, we fit the multiwavelength spectrum of a newly discovered blazar (PKS 0447-439) by using such parameters, as to explain the break in the gamma-ray spectrum by means of spontaneous photon quenching, under the assumption that its redshift lies in the range 0.1<z<0.24.Comment: 14 pages, 9 figures, 2 tables, accepted for publication in A&