TeV observations of blazars and constraints on their redshifts: a detailed study of PG 1553+113 and PKS 1424+240 with MAGIC

In this thesis recent results achieved in the young field of extragalactic Very High Energy (VHE) γ–ray astrophysics are presented. This is a promising discipline which is extending the frontier of our knowledge of the emission of distant sources to the upper edge of the electromagnetic spectrum. Almost all of the 46 sources detected up to now in the energy range between 100GeV to some tens of TeV (also known as TeV emitters) belong to the class of radio–loud Active Galactic Nuclei (AGNs). They are supermassive black holes accreting material and showing two narrow jets of relativistic particles. In particular, the majority of the sources are blazars, i.e. radio loud AGNs with a jet almost aligned to the line of sight. It is interesting to note that more than a half of these sources have been detected as TeV emitters in the last three years (between 2008 to 2010) thanks to the last generation of Cherenkov telescopes: MAGIC, H.E.S.S. and VERITAS. These discoveries were made possible not only thanks to the improved sensitivity and increased energy range of the instruments, but also thanks to the cooperation between instruments operating at different frequencies, such as optical and γ–rays below 100GeV. A peculiar characteristics of the detected AGNs is their relative short distance: the most distant source known so far is 3C 279, located at redshift z = 0.536. The reason for this limited distribution of the distances is believed to originate from the interaction of VHE photons with the optical–infrared light filling the Universe (the Extragalactic Background Light, EBL). This radiation is composed of the light emitted by stars and partially reprocessed by dust and redshifted by the expansion of the Universe along the history. Part of the VHE radiation emitted by distant blazars (the intrinsic spectrum) is, in fact, absorbed and the TeV spectrum observed is significantly deformed. The amount of this deformation is an increasing function of the energy of the energetic photon and of the distance of the emitter. Actually, it is possible to define a VHE γ–ray horizon, beyond which the Universe becomes opaque to the TeV radiation. Another feature of the sample, related to the characteristics of the emitted radiation, is that a number of sources have uncertain or unknown redshift. This thesis is focused on the characterization of the distances of blazars, starting from the properties of the detected TeV emission. In particular, the research activity presented may be divided into an experimental and a phenomenological part, the former carried out as a member of the MAGIC Collaboration. In the first part of the work, a detailed analysis of the VHE γ–ray radiation emitted by the two blazars with uncertain redshift PG 1553+113 and PKS 1424+240 observed with MAGIC is presented. MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACTs), located in the Canary island of La Palma at ∼2240 m asl. It observes VHE photons coming from space taking advantage of the Cherenkov light emitted by particle showers induced by VHE γ–rays. For each source, the differential energy spectrum and the temporal evolution of the detected integral flux are studied. For the case of PG 1553+113, the new sample presented, composed of the data collected from 2007 to 2009, is combined with previous observations (2005/06). This large sample makes PG 1553+113 one of the longest studied sources at energies above 100GeV. Concerning PKS 1424+240, the analysis of 2009 and 2010 datasets is presented. The main feature of 2010 sample is that it is collected in stereoscopic mode, using the upgraded MAGIC stereoscopic system. Both sources reveal a modest variability in the VHE γ–rays, and a steep spectrum of power law index ∼4. The results obtained are, in both cases, combined with partially simultaneous observations carried out in other frequencies, from optical to γ–rays. Correlation studies between the optical and the TeV integral flux suggest a connection between these two extreme components, especially for the case of PG 1553+113. Conversely, the small significance of the signal detected from PKS 1424+240 prevents any definitive conclusion about eventual correlation of TeV photons with optical, X–ray and γ–ray data. Finally, the mean spectra measured at VHE are combined with archival data available for other wavelengths. In both cases a clear two bump structure arises, in agreement with current models of blazar emission. Furthermore, for the case of PG 1553+113, the mean spectral energy distribution is modeled with a one– zone SSC (Synchrotron Self Compton) model, and the main physical parameters governing the emission in the blazar jet are derived. In the second part of the thesis the results of the phenomenological work are reported, aimed to set constraints on the TeV blazars distances. This work is of particular interest for the current VHE γ–rays astrophysics, since, as mentioned above, many sources have unknown/uncertain redshifts. First the existing techniques are presented. Such techniques make use of reasonable hypotheses on the VHE γ–rays intrinsic spectrum emitted by blazars to set an upper limit on their distances. The methods are then applied to the PG 1553+113 and PKS 1424+240 observed spectra. In the first case, the requirement that the spectrum corrected for EBL absorption (deabsorbed) does not show a pile up at high energies leads to the most stringent limit on the source distance of z < 0.67 at a two sigma level. In the second case, on the contrary, it is the demand of a spectral index softer than 1.5 (also called maximum hardness criterion) to better constrain the source redshift at z < 0.81. Starting from these methods, a new method is developed based on combined GeV and TeV observations. This technique can be seen as a sort of experimental version of the maximum hardness criterion, in which instead of assuming a limiting slope for the deabsorbed spectrum given by theory, the slope measured by Fermi/LAT at lower energies, in the High Energy (HE) regime (0.2–100GeV), is used. Since almost all of these slopes are above the limiting value 1.5, the upper limits that can be set with this method are below the previous limits. Therefore, in principle, this technique is more constraining. In order to check its validity, it is tested on a wide sample of TeV blazars detected also at lower energies by Fermi, using different EBL models. The results obtained are satisfactory, and it can be concluded that, for a TeV blazar, the redshift, z∗, at which the deabsorbed slope equals the slope measured by Fermi/LAT at lower energies can be considered as an upper limit on the source distance, at least with mean and low density EBL models. Adopting a mean density EBL model for the blazars PG 1553+113 and PKS 1424+240, the redshift values of z∗ = 0.75 ± 0.07 and z∗ = 0.45 ± 0.15 are obtained, respectively, which correspond to the 2 σ upper limits z < 0.89 and z < 0.75. As a spin–off, the same procedure is then applied to the two uncertain redshift sources S5 0716+714 and 3C 66A, recently observed at VHE γ–rays. The values z∗ = 0.22 ± 0.10 and z∗ = 0.38 ± 0.05 are obtained, respectively, in partial contradiction with the (uncertain) redshifts resulting from optical measurements. Following previous works, finally the possibility of a linear relation between the z∗ estimates and the real distances of the sources is checked. A linear fit describes quite well the results, independently of the EBL model considered. The relation obtained suggests the use of the z∗ estimate not only as an upper limit on a blazar redshift, but also, via the inverse formula, as an evaluation of this distance. This method is demonstrated to be statistically consistent; therefore, it can be used to make a first estimation of the distance of TeV emitting blazars. The method applied to PG 1553+113 returns the value of 0.43 ± 0.05 for the reconstructed redshift, in agreement with both upper and lower limits estimated with other methods. For PKS 1424+240 the evaluated distance is 0.26 ± 0.05. Regarding the uncertain redshift sources, the value of the redshift of S5 0716+714 that results with this method is 0.12 ± 0.05, where the error quoted is the σ of the z distribution. For 3C 66A, the same procedure leads to a redshift estimate of 0.22 ± 0.05. In conclusion, we have determined, through a detailed study of their TeV emission, new constraints on the distances of the blazars PG 1553+113 and PKS 1424+240. Furthermore we have developed a new technique, based on the comparison among GeV and TeV blazars spectra, which allows to give an estimate, and not only to set a limit, on the distance of TeV emitting blazars. We applied this technique to PG 1553+113 and PKS 1424+240, and obtained for the first time a measure of their redshifts. Moreover, we applied it on other uncertain redshifts blazars. The method developed uses combined information at the highest energies of the electromagnetic spectrum (HE and VHE γ–rays) and takes advantage of the interaction of such photons with the optical–IR light filling the universe. Hence, in this interaction, where the most powerful shining objects of the Universe meet their past, two distinct branches of modern astrophysics, VHE γ–rays astrophysics and observational cosmology, overlap each other granting us a new tool for the measurement of previously unresolved blazars distances.