Physics from Time Variability of the VHE Blazar PKS 2155-304

Blazars are the principal extragalactic sources of very high energy gamma-ray emission in the Universe. These objects constitute a sub-class of Active Galactic Nuclei whose emission is dominated by Doppler boosted non-thermal radiation from plasma outflowing at relativistic speeds from the central engine. This plasma outflow happens in the form of large-scale collimated structures called jets, which can extend for Mpc in length and transport energy from the central engine of the galaxy to the larger scale intergalac- tic medium. Over thirty such sources have been discovered to date by ground-based gamma-ray telescopes such as H.E.S.S., and PKS 2155-304 is the prototypical southern-hemisphere representative of this population of objects. In this thesis I have studied in detail some aspects of the temporal variability of the jet emission from PKS 2155-304, combining coordinated observations across the electromagnetic spectrum, from optical polarimetric measurements to X-ray and ground-based gamma-ray data. The temporal properties of the dataset allowed us to derive important physical information about the structure and emission mechanisms of the source and put constraints to the location of the sites of VHE emission and particle acceleration within the jet. We have also derived a sensitive statistical measure, called Kolmogorov distance, which we applied to the large outburst observed from PKS 2155-304 in July 2006, to derive the most stringent constraints to date on limits for the violation of Lorentz invariance induced by quantum-gravity effects from AGN measurements

Future developments in ground-based gamma-ray astronomy

Ground-based gamma-ray astronomy is a powerful tool to study cosmic-ray physics, providing a diagnostic of the high-energy processes at work in the most extreme astrophysical accelerators of the universe. Ground-based gamma-ray detectors apply a number of experimental techniques to measure the products of air showers induced by the primary gamma-rays over a wide energy range, from about 30 GeV to few PeV. These are based either on the measurement of the atmospheric Cherenkov light induced by the air showers, or the direct detection of the shower's secondary particles at ground level. Thanks to the recent development of new and highly sensitive ground-based gamma-ray detectors, important scientific results are emerging which motivate new experimental proposals, at various stages of implementation. In this chapter we will present the current expectations for future experiments in the field.Comment: To appear in "Handbook of X-ray and Gamma-ray Astrophysics" by Springer (Eds. C. Bambi and A. Santangelo) - 59 p

Very high energy sky monitoring with the Southern Widefield Gamma-ray Observatory

The Southern Wide-field Gamma-ray Observatory (SWGO) is the proposal for a new ground-based γ-ray instrument in the Southern Hemisphere, which will use an array of water-Cherenkov particle detectors to provide continuous monitoring of a large portion of the sky at the very- and ultra-high-energies (VHE and UHE, respectively). At the low energy side, SWGO aims to push the observational range of wide-field ground-based γ-ray facilities down to a few hundred GeV, thus bridging the gap between space and ground-based facilities in the monitoring of the VHE sky. In the high energy domain, on the contrary, it will benefit from the optimal coverage of the Galactic Plane to map the distribution of UHE sources in the inner parts of the Galactic disk and close to the Galactic Center, leading to an extraordinary improvement in our ability to identify their most likely counterparts. In this contribution, we describe the concept of SWGO and its potential to constrain the physics of VHE emission and particle acceleration in γ-ray sources powered by relativistic jets and energetic shocks. We finally discuss its role within the global network of multi-messenger facilities

Centaurus A: The Nearest Blazar?

Abstract. Centaurus A (NGC5128), at a distance of 3.4 Mpc is the nearest Active Galaxy, classified as a low luminosity Fanaroff-Riley class I object. Although the central source is completely obscured at optical wavelengths, VLBI studies at radio frequencies show an unresolved core and an asymmetric jet at sub-parsec scales. Kinematical studies of the jet components show subluminal expansion velocities, which together with the jet-counterjet intensity ratio implies that the jet direction forms a large angle with the line of sight (50 o to 80 o ). The nuclear emission is highly variable at all wavelengths, from radio to γ-rays. Single dish radio observations showed that the stronger, long duration outbursts (months to years) present a correlation at radio and X-rays, although it is not clear whether the emission mechanism is synchrotron radiation at both frequencies or if the inverse Compton process dominates at high energies. Moreover, no information is available about the correlation between the emission at these two frequencies at shorter timescales (days and hours), due to the lack of short term monitoring at radio frequencies. In this work we report 43 GHz monitoring of Cantaurus A at the Itapetinga Radio Observatory during the last year, with daily resolution during a three-month period. We found very large variations (factor of two) within a few days, which puts Centaurus A in the blazar category. These variations were superimposed to a continuous rise in flux density that lasted until the end of 2003, when it started a fast decline. No apparent correlation with the All Sky Monitor (ASM/RXTE) data was found at these short timescales

Time-Evolving SED of MKN421: A Multi-Band View and Polarimetric Signatures

The most detailed temporal studies of the emission from blazars are usually done by trying to obtain a dense, simultaneous coverage of the source over a large multi-wavelenght observational campaign. Although it is well know that correlations between multi-band emission present sizeable temporal lags in their correlated variability, such properties are usually neglected in the majority of observational studies, which model the evolution of source parameters over time by either building time-averaged SEDs (when data is scarce) or considering strictly simultaneous snapshots of the source along the spectrum. By making use of the resources and large database made available through the ASI Science Data Center (ASDC), we present a novel approach to the modelling of blazar emission whereby the multi-epoch SEDs for Mkn 421 are modelled considering the temporal lags between bands (both in short and long-timescales), as obtained by a detailed cross-correlation analysis, spanning data from radio to VHE gamma-rays from 2008 to 2014. In addition to that, long-term optical polarization data is used to aid and complement our physical interpretation of the state and evolution of the source

LATTES: a novel detector concept for a gamma-ray experiment in the Southern hemisphere

The Large Array Telescope for Tracking Energetic Sources (LATTES), is a novel concept for an array of hybrid EAS array detectors, composed of a Resistive Plate Counter array coupled to a Water Cherenkov Detector, planned to cover gamma rays from less than 100 GeV up to 100 TeVs. This experiment, to be installed at high altitude in South America, could cover the existing gap in sensitivity between satellite and ground arrays. The low energy threshold, large duty cycle and wide field of view of LATTES makes it a powerful tool to detect transient phenomena and perform long term observations of variable sources. Moreover, given its characteristics, it would be fully complementary to the planned Cherenkov Telescope Array (CTA) as it would be able to issue alerts. In this talk, a description of its main features and capabilities, as well as results on its expected performance, and sensitivity, will be presented