Extreme BL Lac Object Research Articles

Particle Acceleration and Synchrotron Self-Compton Emission in Blazar Jets. I. An Application to Quiescent Emission

Abstract There are still some important unanswered questions about the detailed particle acceleration and escape occurring during quiescent epochs. As a result, the particle distribution that is adopted in the blazar quiescent spectral model has numerous unconstrained shapes. To help remedy this problem, we introduce an analytical particle transport model to reproduce the quiescent broadband spectral energy distribution of blazars. In this model, the exact electron distribution is solved from a generalized transport equation that contains the terms describing the first-order and second-order Fermi acceleration, the escape of particles due to both advection and spatial diffusion, and energy losses due to synchrotron emission and inverse-Compton scattering of an assumed soft photon field. We suggest that advection is a significant escape mechanism in blazar jets. We find that in our model, the advection process tends to harden the particle distribution, which enhances the high-energy components of the resulting synchrotron and synchrotron self-Compton spectra from the jets. Our model is able to roughly reproduce the observed spectra of the extreme BL Lac object 1ES 0414+009 with reasonable assumptions about the physical parameters.

Expected signatures from hadronic emission processes in the TeV spectra of BL Lacertae objects

Context. The wealth of recent data from Imaging Air Cherenkov telescopes (IACTs), ultra-high energy cosmic-ray experiments and neutrino telescopes have fuelled a renewed interest in hadronic emission models for γ-loud blazars. Aims. We explore physically plausible solutions for a lepto-hadronic interpretation of the stationary emission from high-frequency peaked BL Lac objects (HBLs). The modelled spectral energy distributions are then searched for specific signatures at very high energies that could help to distinguish the hadronic origin of the emission from a standard leptonic scenario. Methods. By introducing a few basic constraints on parameters of the model, such as assuming the co-acceleration of electrons and protons, we significantly reduced the number of free parameters. We then systematically explored the parameter space of the size of the emission region and its magnetic field for two bright γ-loud HBLs, PKS 2155-304 and Mrk 421. For all solutions close to equipartition between the energy densities of protons and of the magnetic field, and with acceptable jet power and light-crossing timescales, we inspected the spectral hardening in the multi-TeV domain from proton-photon induced cascades and muon-synchrotron emission inside the source. Very-high-energy spectra simulated with the available instrument functions from the future Cherenkov Telescope Array (CTA) were evaluated for detectable features as a function of exposure time, source redshift, and flux level. Results. A range of hadronic scenarios are found to provide satisfactory solutions for the broad band emission of the sources under study. The TeV spectrum can be dominated either by proton-synchrotron emission or by muon-synchrotron emission. The solutions for HBLs cover a parameter space that is distinct from the one found for the most extreme BL Lac objects in an earlier study. Over a large range of model parameters, the spectral hardening due to internal synchrotron-pair cascades, the “cascade bump”, should be detectable for acceptable exposure times with the future CTA for a few nearby and bright HBLs.

On the detectability of Lorentz invariance violation through anomalies in the multi-TeVγ-ray spectra of blazars

Cosmic opacity for very high-energy gamma rays ($E>10$ TeV) due to the interaction with the extragalactic background light can be strongly reduced because of possible Lorentz-violating terms in the particle dispersion relations expected, e.g., in several versions of quantum gravity theories. We discuss the possibility to use very high energy observations of blazars to detect anomalies of the cosmic opacity induced by LIV, considering in particular the possibility to use -- besides the bright and close-by BL Lac Mkn 501 -- extreme BL Lac objects. We derive the modified expression for the optical depth of $\gamma$ rays considering also the redshift dependence and we apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides the nearby and well studied BL Lac Mkn 501 -- especially in high state --, suitable targets are extreme BL Lac objects, characterized by quite hard TeV intrinsic spectra likely extending at the energies relevant to detect LIV features.

An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?

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 vs. radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, profiting of publicly available archival observations performed by the Swift, Galex and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected VHE 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 (CTA).

X-ray and TeV observations of the extreme BL Lac AGN H1426+428 during 2002

Between 7 March 2002 and 15 June 2002, an intensive multiwavelength campaign was carried out on the extreme BL Lac object H1426+428. Previous measurements have indicated that the synchrotron peak of H1426+428 lies in excess of 100 keV. At a redshift of 0.129, this active galactic nucleus (AGN) provides us with an excellent opportunity to study the properties of the extragalactic medium through which the radiation must propagate, as well as the properties of the AGN acceleration mechanism itself. By taking frequent observations over a four-month campaign, which included ∼450 ksec of RXTE time and >107 h of Whipple on-source time, it was possible to obtain data that can be used to search for correlations on multiple timescales. The 3–24 keV X-ray spectra exhibited variability, signifying variability in the location of the first peak of the spectral energy distribution, while the TeV emission above ∼390 GeV remained relatively low.

X‐Ray Spectral Variability of Extreme BL Lacertae Active Galactic Nucleus H1426+428

Between 2002 March 7 and 2002 June 15, intensive X-ray observations were carried out on the extreme BL Lac object H1426+428 with instruments on board the Rossi X-Ray Timing Explorer (RXTE). These instruments provide measurements of H1426+428 in the crucial energy range that characterizes the first peak of its spectral energy distribution. This peak, which is almost certainly due to synchrotron emission, has previously been inferred to be in excess of 100 keV. By taking frequent observations over a four-month campaign, which included ~450 ks of RXTE time, studies of flux and spectral variability on multiple timescales were performed, along with studies of spectral hysteresis. The 3-24 keV X-ray flux and spectra exhibited significant variability, implying variability in the location of the first peak of the spectral energy distribution. Hysteresis patterns were observed, and their characteristics have been discussed within the context of emission models.