The research field of the gamma-ray astronomy is new and rapidly grown, providing remarkable and promising scientific results.
In the last decades, the development of new gamma-ray detectors on board satellites as the AGILE and the Fermi observatories, or groud-based instruments as the Imaging Atmospheric Cherenkov Technique telescope MAGIC, lead up to a big increase in the number of discovered gamma-ray emitters.
Different types of astrophysical sources have been identified as gamma-ray emitter classes, of Galactic and extragalactic origin, such as Supernova Remants, Pulsars, Microquasars and Active Galactic Nuclei.
Moreover, it has been suggested that gamma-ray radiation can be also related to annihilation/decay processes of Dark Matter particles expected in several celestial objects with high DM density as the Galactic Center, the Galaxy clusters, the Dwarf Spheroidal Satellite Galaxies of the Milky Way and finally the clumps of DM overdensities within the DM halo of our Galaxy.
The aim of this PhD Thesis is both searching for gamma-ray signals from DM particles and combining the time-progressive all-sky Fermi survey with dedicated pointed observations performed by the Cherenkov telescopes.
To perform this purpose, this Thesis work followed two different, but deeply related paths.
The first part of this Thesis has adopted the Second Catalogue of the Fermi Gamma Ray Observatory (2FGL) as the main dataset for searches of new classes of extragalactic sources, DM objects candidates or unexpected high-energy phenomena.
The Fermi mission has carried out a survey of the all sky at the gamma-ray energies from 30 MeV to 100 GeV, making use of the large area and field of view of the LAT instrument.
In the 2FGL catalogue, 1873 points sources are detected: 1297 have been associated as known class of gamma-ray emitters, while the remaining 576 objects, the so-called Unassociated Fermi Objects (UFO), still lack a plausible identification and offer the best chance to search for DM sources.
The population of the UFOs is the second major component of the gamma-ray sky and we selected a sub-sample of 183 UFOs of high Galactic latitude with |b|>20° (avoiding the confusion effects in the Galactic plane) with the aim to classify these sources and to determine if, among them, there are new types of AGNs, DM object candidates or unexpected high-energy phenomena.
For each UFO of our sample we search for counterparts in optical, X-ray and radio band, in order to determine a possible association with a suitable set of sources of other astronomical catalogs. Then, through new tools based on multiwavelength approaches, we suggest a rather secure classification for most of them.
Our UFO association procedure is primarily based on the use of available X-ray Swift satellite data that cover the FERMI error-box (typically of a few arcmins) associated to the gamma-ray detection. If an X-ray bright source is revealed within the FERMI error-box and if other sources at different wavelengths (from radio, IR and optical catalogues) are positionally coincident with the X-ray counterpart, we consider them as an unique source associated to the UFO. In addition we perform a dedicated X-ray data analysis for the X-ray counterpart found in order to determine the spectral shape in this energy band.
Finally we build the UFO broad-band Spectral Energy Distribution (SED) combining the available measured fluxes at different wavelengths recovered as explained before.
At this point, we perform the characterization of the SED of each our UFO, identifying it as belonging to a class of the main gamma-ray emitters.
In particular for the blazar identification tool, we build a code based on the use of a library of bona-fide multi-frequency SED templates of known objects belonging to the four blazar categories: High-peaked (HBL), Intermediate-peaked (IBL), Low-peaked BL Lac (LBL), and Flat Spectrum radio Quasar (FSQR). Therefore, through the adoption of these SED template and choosing the one that models the data maximizing the likelihood, this code determines if a given UFO of our sample can be identified as a blazar object. In the same time, it provides an estimate of the redshift, still in the absence of spectroscopic observations (that is a well known issue for this kind of sources).
From this procedure we obtain a reasonable classification as blazar objects for about 50 UFOs of our sample, belonging to all of the classes and with redshifts spanning from 0.2 to 1.5.
This tool has been also applied on two well-known blazars, 1ES 1011+496 and PG 1553+113, used as test sources to prove the efficacy in recognizing AGN sources. Notably, we found results compatible with the real blazar class and redshift of the two test sources.
Although in this PhD thesis we study UFOs of high Galactic latitude, the other category of gamma-ray emitters that we decided to consider as possible identification was also the class of microquasars. We have been encouraged to pursue this task because, during the procedure of association for some UFOs, we find set of MWL counterparts resulting in a peculiar broad-band SED similar to the those typically observed for microquasar objects.
To achieve this interpretation, we built a diagnostic plot superimposing the luminosity data points of four known microquasars. If the SED points of a given UFO are located inside the regions defined by the known microquasar points, we suggest a microquasar identification for the studied object.
About 15 UFOs of our sample are turned out to likely be Galactic sources, as microquasars, high galactic latitude Neutron Stars and pulsars. This could be a finding of great interest, given the small number of such objects in the 2FGL catalogue. For this reason, further investigations are ongoing.
We note that for ten objects no X-ray sources in the available Swift/XRT data has been found within the Fermi error-box and, although we highlight that it might be dependent on the quality of the X-ray observations, they could be considered as the best candidates to perform DM searches.
Finally, for the remaining UFOs, we cannot provide a clear identification since we found multiple sets of associations. Further observations, especially in the X-ray and radio bands, will allow us to disentangle this issue.
The second line of investigation of this Thesis is in the field of very high energy (VHE) observations of Fermi sources with Cherenkov telescopes on ground, in particular with the MAGIC telescope, one of the largest IACT stereo systems and situated on the Canarian island La Palma, at 2245 m a.s.l.
Such VHE data offer an invaluable astrophysical information on the sources.
In this Thesis a detailed data analysis of the VHE radiation emitted by two HBL blazars, PG 1553+113 and 1ES 1011+496, is provided.
Both sources have been observed in stereoscopic mode with MAGIC for a total of 12 hours and 21 hours, respectively, in order to perform a complete temporal evolution analysis of the integral flux and a detailed study of the VHE differential energy spectrum.
Futhermore, the MAGIC observations were performed in the framework of dedicated multiwavelength campaigns, and for PG 1553+113 the campaign was planned in collaboration with the WEBT team involving several instruments of the north hemisphere operating at different energy bands.
Simultaneous data, with a good temporal coverage, were collected from radio to VHE regimes, allowing us to sample the whole broad-band SED. The typical doubled bump shape, due to syncrothron and IC mechanisms, is showed by both sources, in agreement with the current Synchrotron Self-Compton blazar models.
In addition both sources provide a modest (or any) activity in the HE, VHE and radio bands, while a clear variability in the UV and Optical regimes, related to the synchrotron bump frequencies, is present, especially for PG 1553+113. However any evidence of flare episodes have been detected.
For this reasons, and thanks to high quality multiwavelength sampling of their SEDs, PG 1553+113 and 1ES 1011+496 were used as test sources to verify the efficency of our blazar identification tool (explained before) in recognizing AGN sources
