Extreme BL Lacs: probes for cosmology and UHECR candidates

High-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.Comment: 6 pages, to appear in the Proceedings of the Conference "High-Energy Phenomena and Relativistic Outflows V", held in La Plata, 5-8 October 201

On the detectability of Lorentz invariance violation through anomalous multi-TeV γ\gamma-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.Comment: 9 pages, 9 figures, accepted by A&

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).Comment: 12 pages, 5 figures, accepted for publication in MNRA

Very-high-energy quasars hint at ALPs

One of the mysteries of very-high-energy (VHE) astrophysics is the observation of flat spectrum radio quasars (FSRQs) above about 30 GeV, because at those energies their broad line region should prevent photons produced by the central engine to escape. Although a few astrophysical explanations have been put forward, they are totally ad hoc. We show that a natural explanation emerges within the conventional models of FSRQs provided that photon-ALP oscillations take place inside the source for the model parameters within an allowed range.Comment: 4 pages, 3 figures. Proceeding of the workshop "9th Patras Workshop on Axions, WIMPs and WISPs", 24 - 28 June 2013, Schloss Waldhausen, Mainz, Germany (to be published in the Proceedings

Constraining the location of the emitting region in Fermi blazars through rapid gamma-ray variability

We consider the 1.5 years Fermi Large Area Telescope light curves (E > 100 MeV) of the flat spectrum radio quasars 3C 454.3 and PKS 1510-089, which show high activity in this period of time. We characterise 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 timescales of few hours. We discuss the implications of significant variability on such short timescales, that challenge the scenario recently advanced in which the bulk of the gamma-ray luminosity is produced in regions of the jet at large distances (tens of parsec) from the black hole.Comment: 5 pages, 5 figures, accpted for publication in MNRAS Letters

Blazars distance indications from Fermi and TeV data

A new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes will be presented. The underlying assumption is that the Very High Energy (VHE) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the extrapolation of the gamma-ray spectrum observed by Fermi/LAT. Starting from the observed spectral data at VHE, the EBL-corrected spectra are derived as a function of the redshift z and fitted with power laws. Comparing the redshift dependent VHE slopes with the power law fits to the LAT data an upper limit to the source redshift can be derived. The method is applied to all TeV blazars detected by LAT with known distance and an empirical law describing the relation between the upper limits and the true redshifts is derived. This law can be used to estimate the distance of unknown redshift blazars: as an example, the distance of PKS 1424+240 is inferred.Comment: Contribution to SciNeGHE 2010, Trieste, Italy, September 2010; 4 pages, 2 figur

The red blazar PMN J2345-1555 becomes blue

The Flat Spectrum Radio Quasar PMN J2345-1555 is a bright gamma-ray source, that recently underwent a flaring episode in the IR, UV and gamma-ray bands. The flux changed quasi simultaneously at different frequencies, suggesting that it was produced by a single population of emitting particles, hence by a single and well localized region of the jet. While the overall Spectral Energy Distribution (SED) before the flare was typical of powerful blazars (namely two broad humps peaking in the far IR and below 100 MeV bands, respectively), during the flare the peaks moved to the optical-UV and to energies larger than 1 GeV, to resemble low power BL Lac objects, even if the observed bolometric luminosity increased by more than one order of magnitude. We interpret this behavior as due to a change of the location of the emission region in the jet, from within the broad line region, to just outside. The corresponding decrease of the radiation energy density as seen in the comoving frame of the jet allowed the relativistic electrons to be accelerated to higher energies, and thus produce a "bluer" SED.Comment: 5 pages, 4 figures, MNRAS Letters, in pres

TeV blazars and their distance

Recently, a new method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes has been developed, with the underlying assumption that the Very High Energy (VHE) spectrum corrected for the absorption of TeV photons by the Extragalactic Background Light (EBL) via photon-photon interaction should still be softer than the gamma-ray spectrum observed by Fermi/LAT. The constraints found are related to the real redshifts by a simple linear relation, that has been used to infer the unknown distance of blazars. The sample will be revised with the up-to-date spectra in both TeV and GeV bands, the method tested with the more recent EBL models and finally applied to the unknown distance blazars detected at VHE.Comment: Contribution to "Cosmic Radiation Fields: Sources in the early Universe", Desy, Germany, November 9-12, 2010; 6 pages, 3 figures (revised version

Boosting the performance of the ASTRI SST-2M prototype: reflective and anti-reflective coatings

ASTRI is a Flagship Project of the Italian Ministry of Education, University and Research, led by the Italian National Institute of Astrophysics, INAF. One of the main aims of the ASTRI Project is the design, construction and verification on-field of a dual mirror (2M) end-to-end prototype for the Small Size Telescope (SST) envisaged to become part of the Cherenkov Telescope Array. The ASTRI SST-2M prototype adopts the Schwarzschild-Couder design, and a camera based on SiPM (Silicon Photo Multiplier); it will be assembled at the INAF astronomical site of Serra La Nave on mount Etna (Catania, Italy) within mid 2014, and will start scientific validation phase soon after. The peculiarities of the optical design and of the SiPM bandpass pushed towards specifically optimized choices in terms of reflective coatings for both the primary and the secondary mirror. In particular, multi-layer dielectric coatings, capable of filtering out the large Night Sky Background contamination at wavelengths $\lambda \gtrsim 700$ nm have been developed and tested, as a solution for the primary mirrors. Due to the conformation of the ASTRI SST-2M camera, a reimaging system based on thin pyramidal light guides could be optionally integrated aiming to increase the fill factor. An anti-reflective coating optimized for a wide range of incident angles faraway from normality was specifically developed to enhance the UV-optical transparency of these elements. The issues, strategy, simulations and experimental results are thoroughly presented.Comment: 4 pages, 6 figures. In Proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), Rio de Janeiro (Brazil). All ASTRI contributions at arXiv:1307.463