1,721,774 research outputs found
Extreme TeV BL Lacs: a self-consistent stochastic acceleration model
No full textLately, a specific kind of blazars drew the attention of the gamma-ray astronomy community: the extreme TeV BL Lacs, blazars that present an extremely energetic and hard emission at very high-energy. Explaining their features is still an open challenge, in fact the most used phenomenological models have difficulties to satisfactorily reproduce their spectral energy distribution. Based on a scenario we have recently proposed, we suppose that the non-thermal particles are first accelerated by a jet recollimation shock, which induces turbulence in the rest of the jet. Non-thermal particles are further accelerated by the turbulence, which hardens the particle spectra and accordingly the radiative emission. Given the physical properties of the plasma, as inferred by emission models, we expect a strong impact of the accelerating particles on the turbulence. Assuming isotropy and homogeneity, the interaction between non-thermal particles and turbulence and their spectra is modelled solving a system of two non-linear, coupled Fokker–Planck equations, while the radiative emission is calculated through the Synchrotron Self Compton model. The emission predicted by our model is then compared with the prototype extreme TeV BL Lac object 1ES 0229+200 and the parameters obtained to reproduce its SED are in line with the expectations
On the detectability of Lorentz invariance violation through anomalies in the multi-TeV γ -ray spectra of blazars
Full text linkContext. 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
Extreme BL Lacs: probes for cosmology and UHECR candidates
Full text linkHigh-energy observations of extreme BL Lac objects, such as 1ES0229+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. Moreover, their enigmatic properties have been interpreted in a scenario in which their primary high- energy output is through a beam of high-energy hadrons. However, despite their possible important role in all these topics, 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 considering those undetected (or only barely detected) by the LAT onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, using available archival data of 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 CTA
Characterizing the Spectral Energy Distribution of blazars as candidates for neutrino emission
No full textBlazars are potential neutrino emitters as suggested by the detection of a high-energy neutrino in the direction of the flaring Blazar TXS 0506+056, in 2017. However, the origin of neutrinos from this source is still unclear. This work aims at finding and characterizing other Blazars with similar features as TXS 0506+056. We selected sources from the most recent Fermi Catalogue, the 4LAC, finding 4 candidates. They were characterized through a multiwavelength analysis, which revealed some peculiarities in the SED of the candidate
TeV variability in blazars: how fast can it be?
Full text linkRecent Cerenkov observations of the two BL Lac objects PKS 2155-304 and Mkn 501 revealed TeV flux variability by a factor ~2 in just 3-5 minutes. Even accounting for the effects of relativistic beaming, such short timescales are challenging simple and conventional emitting models, and call for alternative ideas. We explore the possibility that extremely fast variable emission might be produced by particles streaming at ultra-relativistic speeds along magnetic field lines and inverse Compton scattering any radiation field already present. This would produce extremely collimated beams of TeV photons. While the probability for the line of sight to be within such a narrow cone of emission would be negligibly small, one would expect that the process is not confined to a single site, but can take place in many very localised regions, along almost straight magnetic lines. A possible astrophysical setting realising these conditions is magneto-centrifugal acceleration of beams of particles. In this scenario, the variability timescale would not be related to the physical dimension of the emitting volume, but might be determined by either the typical duration of the process responsible for the production of these high energy particle beams or by the coherence length of the magnetic field. It is predicted that even faster TeV variability - with no X-ray counterpart - should be observed by the foreseen more sensitive Cerenkov telescope
Clumps in large scale relativistic jets
No full textThe relatively intense X-ray emission from large scale (tens to hundreds kpc) jets discovered with Chandra likely implies that jets (at least in powerful quasars) are still relativistic at that distances from the active nucleus. In this case the emission is due to Compton scattering off seed photons provided by the Cosmic Microwave Background, and this on one hand permits to have magnetic fields close to equipartition with the emitting particles, and on the other hand minimizes the requirements about the total power carried by the jet. The emission comes from compact (kpc scale) knots, and we here investigate what we can predict about the possible emission between the bright knots. This is motivated by the fact that bulk relativistic motion makes Compton scattering off the CMB photons efficient even when electrons are cold or mildly relativistic in the comoving frame. This implies relatively long cooling times, dominated by adiabatic losses. Therefore the relativistically moving plasma can emit, by Compton scattering the microwave seed photons, for a long time. We discuss how the existing radio--to--X-ray observations of large scale jets already pose strong constraints on the structure and dynamics of knots and we present a scenario that can satisfactorily reproduce the observed phenomenology of the jet in 3C273. In this scenario the kiloparsec-scale knots visible with HST, Chandra and VLA are composed of several smaller sub--units, accounting for the fast decrease of the flux outside the large knot. Substructure in the X-ray- emitting knots can also explain the month--year variability timescale reported for the large scale jet in M87
On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?
Full text linkWe 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 dN/dFγ∝F−αγ with α > 2
BL Lac Objects: Laboratories to study the environment and properties of emitting particles in relativistic jets
No full textEnvironment and properties of emitting electrons in blazar jets: Mrk 421 as a laboratory
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