The TeV AGN Portfolio: extending Fermi LAT analysis into the CTA realm

The extragalactic γ-ray sky is completely dominated by active galax- ies, where by active we mean that a significant fraction of the emitted energy is not due to the standard components of a galaxy: stars, gases and interstellar dust. Every detected active galaxy seems to be powered by a compact region at their center; explaining why active galaxies are often referred as Active Galactic Nuclei (AGNs). About 1% of all galaxies are AGNs, believed to be fueled by the accretion onto a supermassive black hole, the central engine of the active galaxy. In addition, about 10% of AGNs display powerful jets of particles and radiation. The current model of AGNs is highly anisotropic and many of the observational characteristics of AGNs can be attributed to the way we are observing it and, in particular, to the orientation of the relativistic jets with respect to the observer. Among AGNs, blazars, which host a jet oriented at a small angle to the line of sight, are of particular interest for γ-ray astrophysics. The emission from these objects is dominated by relativistic beaming effects, which dramatically boosts the observed photon energies and luminosity, the reason why we expect that the observation of blazars at γ-ray energies should be the most fruitful. To confirm our guess, after the launch of the Fermi Gamma-ray Space Telescope, bearing on-board the Large Area Telescope (LAT), which provides virtually continuous observation of blazars between 20 MeV and 300 GeV, many new discoveries refined the current modeling of blazars, by providing useful insights into jets and other AGN features. On the other hand, at the same energies, other observations found puzzling results, bewildering astronomers and astrophysicists. In addition to the LAT, Imaging Atmospheric Cherenkov (IAC) telescopes (namely MAGIC, HESS and VERITAS) provided a good coverage at even higher energies (typically above 30 GeV) and the benefit of simultaneous observations was apparent just after the first broadband paper about PKS 2155−304 (Aharonian et al., 2009). More insights should be gained when the Cherenkov Telescope Array (CTA) will become operational, as it will cover an extended energy window with respect to operating IAC telescopes and will reduce the sensitivity threshold. In addition, CTA will have a huge energy overlap with the LAT, allowing for the first time a reliable way to correlate data obtained by the two detectors. In this Thesis, we present in-depth studies of LAT γ-ray observations of blazars, complemented by multifrequencies observations which are an essential tool to model their behavior. On one hand, we will discuss the characterization of a TeV blazars sample that were simultaneously observed both by Fermi and MAGIC instruments. The joint observations and the ideal coverage provided by the synergy of the two instruments naturally motivates the extrapolation of Fermi spectra to MAGIC energies, with the aim, in the near future, to extend this effort to CTA realm. On the other hand, we will discuss a flux-limited sample of bright blazars detected by Fermi in the first 3.5 years of operations. These objects, displaying extreme outbursts, make up less than 10% of the sources detected by Fermi in its second source catalog. We discuss the characteristics of the sample with respect to the entire catalog of AGNs detected by Fermi and adding some considerations with respect to previous γ-ray observation carried out by EGRET. At the end of this work, we will then focus on one of these objects, that met particular attention for being a gravitationally lensed system, PKS 1830−211

Fermi-LAT Observations of the 2014 May-July outburst from 3C 454.3

A prominent outburst of the flat spectrum radio quasar 3C~454.3 was observed in 2014 June with the \emph{Fermi} Large Area Telescope. This outburst was characterized by a three-stage light-curve pattern---plateau, flare and post-flare---that occurred from 2014 May to July, in a similar pattern as observed during the exceptional outburst in 2010 November. The highest flux of the outburst reported in this paper occurred during 2014 June 7--29, showing a multiple-peak structure in the light-curves. The average flux in these 22 days was found to be $F[E > 100~\mathrm{MeV}] = (7.2 \pm 0.2) \times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$, with a spectral index, for a simple power law, of $\Gamma = 2.04 \pm 0.01$. That made this outburst the first $\gamma$-ray high state of 3C~454.3 ever to be detected by \emph{Fermi} with such a hard spectrum over several days. The highest flux was recorded on 2014 June 15, in a 3 hr bin, at MJD 56823.5625, at a level of $F[E > 100~\mathrm{MeV}] = (17.6 \pm 1.9) \times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. The rise time of one of the short subflares was found to be $T_r= 1200 \pm 700$~s at MJD = 56827, when the flux increased from 4 to 12 $\times 10^{-6}$~ph~cm$^{-2}$~s$^{-1}$. Several photons above 20 GeV were collected during this outburst, including one at 45 GeV on MJD 56827, constraining the $\gamma$-ray emission region to be located close to the outer boundary of the broad-line region, leading to fast flux variability.Comment: Accepted for publication in {\sc the astrophysical journal}: 2016 July 12}; 15 pages, 7 figures, 6 table

Ornstein-Uhlenbeck parameter extraction from light curves of Fermi-LAT observed blazars

Context. Monthly-binned gamma-ray light curves of 236 bright gamma-ray sources, particularly blazars, selected from a sample of 2278 high-galactic latitude objects observed with Fermi-LAT, show flux variability characterized by power spectral densities consisting of a single power-law component, ranging from Brownian to white noise. Aims. The main goal here is to assess the Ornstein-Uhlenbeck (OU) model by studying the range of its three parameters that reproduces these statistical properties. Methods. We develop procedures for extracting values of the three OU model parameters (mean flux, correlation length, and random amplitude) from time series data, and apply them to compare numerical integrations of the OU process with the Fermi-LAT data. Results. The OU process fully describes the statistical properties of the flux variations of the 236 blazars. The distributions of the extracted OU parameters are narrowly peaked about well-defined values (sigma, mu, theta) = (0.2, -8.4, 0.5) with variances (0.004, 0.07, 0.13). The distributions of rise and decay time scales of flares in the numerical simulations, i.e. major flux variations fulfilling pre-defined criteria, are in agreement with the observed ones. The power spectral densities of the synthetic light curves are statistically indistinguishable from those of the measured light curves. Conclusions. Long-term gamma-ray flux variability of blazars on monthly time scales is well described by a stochastic model involving only three parameters. The methods described here are powerful tools to study randomness in light curves and thereby constrain the physical mechanisms responsible for the observed flux variations.Comment: 13 pages, 9 figure

Beginning a journey across the universe: the discovery of extragalactic neutrino factories

Neutrinos are the most elusive particles in the Universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them. Amongst the candidate sources of neutrinos there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed towards the Earth. Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at unprecedented level of confidence, i.e. chance probability of 6 x 10^{-7}. Our statistical analysis provides the observational evidence that blazars are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators.Comment: Published in ApJ

Constraining the PG 1553+113 binary hypothesis: interpreting a new, 22-year period

PG 1553+113 is a well-known blazar exhibiting evidence of a $\sim\! 2.2$-year quasi-periodic oscillation in radio, optical, X-ray, and $\gamma$-ray bands. We present evidence of a new, longer oscillation of $21.8 \pm 4.7$ years in its historical optical light curve covering 100 years of observation. On its own, this $\sim\! 22$-year period has a statistical significance of $1.9\sigma$ when accounting for the look-elsewhere effect. However, the probability of both the $2.2$- and $22$-year periods arising from noise is $\sim0.02\%$ ($3.5\sigma$). The next peak of the 22-year oscillation should occur around July 2025. We find that the $\sim\,$10:1 relation between these two periods can arise in a plausible supermassive black hole binary model. Our interpretation of PG 1553+113's two periods suggests that the binary engine has a mass ratio $\gtrsim 0.2$, an eccentricity $\lesssim 0.1$, and accretes from a disk with characteristic aspect ratio $\sim 0.03$. The putative supermassive black hole binary radiates nHz gravitational waves, but the amplitude is $\sim10-100$ times too low for detection by foreseeable pulsar timing arrays.Comment: 18 pages, 13 figures, 1 tabl

Hadronic processes at work in 5BZB J0630-2406

Recent observations are shedding light on the important role that active galactic nuclei (AGN) play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630-2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7-yr IceCube observations. Modelling the quasi-simultaneous, broad-band spectral energy distribution, we explore various scenarios from purely leptonic to lepto-hadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino-blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630-2406 belongs to the relatively rare sub-class of high-power flat-spectrum radio quasars (FSRQs). Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.Comment: Accepted for publication. 18 pages, 10 figure

The first GeV flare of the radio-loud narrow-line Seyfert 1 galaxy PKS 2004-447

On 2019 October 25, the Fermi-Large Area Telescope observed the first gamma-ray flare from the radio-loud narrow-line Seyfert 1 (NLSy 1) galaxy PKS 2004$-$447 ($z=0.24$). We report on follow-up observations in the radio, optical-UV, and X-ray bands that were performed by ATCA, the Neil Gehrels Swift observatory, XMM-Newton, and NuSTAR, respectively, and our multi-wavelength analysis. We study the variability across all energy bands and additionally produce $\gamma$-ray light curves with different time binnings to study the variability on short timescales during the flare. We examine the X-ray spectrum from 0.5$-$50 keV by describing the spectral shape with an absorbed power law. We analyse multi-wavelength datasets before, during, and after the flare and compare these with a low activity state of the source by modelling the respective SEDs with a one-zone synchrotron inverse Compton radiative model. Finally, we compare our results to gamma-ray flares previously observed from other $\gamma$-loud NLSy 1 galaxies. At gamma-ray energies (0.1$-$300 GeV) the flare reached a total maximum flux of $(2.7\pm0.6)\times10^{-6}$~ph~cm$^{-2}$~s$^{-1}$ in 3-hour binning. With a photon index of $\Gamma_{0.1-300\mathrm{GeV}}=2.42\pm0.09$ during the flare, this corresponds to an isotropic gamma-ray luminosity of $(2.9\pm0.8)\times10^{47}\,\mathrm{erg}\,\mathrm{s}^{-1}$. The $\gamma$-ray, X-ray, and optical-UV light curves covering the end of September to the middle of November show significant variability, and we find indications for flux-doubling times of $\sim 2.2$~hours at $\gamma$-ray energies. During the flare, the SED exhibits large Compton dominance. While the increase in the optical-UV range can be explained by enhanced synchrotron emission, the elevated $\gamma$-ray flux can be accounted for by an increase in the bulk Lorentz factor of the jet, similarly observed for flaring gamma-ray blazars.Comment: 17 pages, 7 figures. Accepted for publication in Astronomy & Astrophysic