87,650 research outputs found
On the detectability of Lorentz invariance violation through anomalies in the multi-TeV γ -ray spectra of blazars
No full textContext. 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
On the distribution of fluxes of gamma-ray blazars: hints for a stochastic process?
No full textWe 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
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
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
Stochastic acceleration in extreme TeV BL Lacs through MCMC
No full textContext. Extreme TeV BL Lacs are a class of blazars with unique spectral and temporal features that are not easily reproducible using standard one-zone models based on single shock acceleration. To account for their peculiar properties, we elaborated a two-step acceleration model in which a recollimation shock and the subsequent downstream turbulence energize non-thermal electrons. Aims. We applied the model to a sample of extreme TeV BL Lacs with well-characterized spectral energy distributions. Since we used several sources, we automatized the exploration of the parameter space. This allowed us to derive the parameter distributions and study the correlations among them. Methods. We numerically solved a system of two coupled nonlinear differential equations to obtain the non-thermal particles and turbulence spectra. We calculated the spectral energy distribution via the synchrotron self-Compton emission model. The automatization of the parameter space exploration is possible through a Markov chain Monte Carlo (MCMC) ensemble sampler, in our case emcee. Results. We derived well-defined posterior distributions for the parameters, showing that the model is well constrained by available data and demonstrating the suitability of our method. The cross-correlations among some of the physical parameters are not trivial. Therefore, we conclude that MCMC sampling is a key instrument for characterizing the complexity of our multiparameter phenomenological model
Extreme BL Lacs: probes for cosmology and UHECR candidates
No full textHigh-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
Extreme blazars: the result of unstable recollimated jets?
No full textABSTRACT
Extreme BL Lacs (EHBL) form a subclass of blazars, which challenge standard emission scenarios. In a recent study it has been argued that their peculiar properties can be explained if emitting electrons are accelerated in a series of oblique shocks induced by the recollimation of the relativistic jet. However, new 3 D simulations of recollimated, weakly magnetized jets reveal that, in correspondence with the first recollimation shock, the flow develops a rapidly growing instability, becomes highly turbulent and decelerates, effectively hampering the formation of the multiple shock structure routinely observed in 2 D simulations. Building on these new findings, we propose here a revised scenario for EHBL, in which the emission is produced by electrons accelerated at the recollimation shock and subsequently further energized through stochastic acceleration in the turbulent downstream flow. We apply a simple version of this scenario to the prototypical EHBL 1ES 0229-200, showing that the spectral energy distribution (SED) can be satisfactorily reproduced with standard values of the main physical parameters.</jats:p
Extreme TeV blazars and the intergalactic magnetic field
No full textWe study the four BL Lac objects (RGBJ0152+017, 1ES0229+200, 1ES0347-121 and PKS0548-322) 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≳ 10-15G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
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
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