G. Bonnoli

The γ-ray brightest days of the blazar 3C 454.3

In the first week of 2009 December, the blazar 3C 454.3 became the brightest high-energy source in the sky. Its photon flux reached and surpassed the level of 10 −5 ph cm −2 s −1 above 100 MeV. The Swift satellite observed the source several times during the period of high γ -ray flux, and we can construct really simultaneous spectral energy distributions (SEDs) before, during and after the luminosity peak. Our main findings are (i) the optical, X-ray and γ -ray fluxes correlate; (ii) the γ -ray flux varies quadratically (or even more) with the optical flux; (iii) a simple one-zone synchrotron inverse Compton model can account for all the considered SED; (iv) in this framework the γ -ray versus optical flux correlation can be explained if the magnetic field is slightly fainter when the overall jet luminosity is stronger and (v) the power that the jet spent to produce the peak γ -ray luminosity is of the same order, or larger, than the accretion disc luminosity.

Extreme TeV blazars and the intergalactic magnetic field

We study the four BL Lac objects (RGBJ0152+017, IES 0229+200, 1ES 0347―121 and PKS 0548-322) detected in the TeV band but not present in the first Fermi-LAT (1FGL) catalogue of the Fermi/Large Area Telescope (LAT). We analyse the 24 months of LAT data deriving γ-ray fluxes or upper limits that we use to assemble their spectral energy distributions (SED). We model the SEDs with a standard one-zone leptonic model, also including the contribution of the reprocessed radiation in the multi-GeV band, emitted by the pairs produced through the conversion of the primary TeV emission by interaction with the cosmic optical-infrared background. For simplicity, in the calculation of this component we adopt an analytical approach including some simplifying assumptions, in particular (i) the blazar high-energy emission is considered on average stable over times of the order of 10 7 yr and (ii) the observer is exactly on-axis. We compare the physical parameters derived by the emission model with those of other high-energy emitting BL Lacs, confirming that TeV BL Lacs with a rather small GeV flux are characterized by extremely low values of the magnetic field and large values of the electron energies. The comparison between the flux in the GeV band and that expected from the reprocessed TeV emission allows us to confirm and strengthen the lower limit of B ≳ 10 ―15 G for the intergalactic magnetic field using a theoretically motivated spectrum for the primary high-energy photons.

On the origin of theγ-ray emission from the flaring blazar PKS 1222+216

The flat-spectrum radio quasar PKS 1222+216 (4C+21.35, z = 0.432) was detected in the very high energy γ-ray band by MAGIC during a highly active γ-ray phase following an alert by the Large Area Telescope (LAT) onboard Fermi. Its relatively hard spectrum (70–400 GeV photon index Γ= 2.7± 0.3) without a cut off, together with its observed variability on a timescale of ∼10 min challenges standard emission models. In particular, if the emission originates in a portion of the relativistic jet located inside the broad line region (BLR), severe absorption of γ rays above a few tens of GeV is expected to be caused by the γγ → e ± process. These observations therefore imply that there is a very compact (Rb ∼ 5 × 10 14 cm) and rapidly moving blob located far beyond the BLR radius (to avoid the gamma-ray absorption through pair production) that is responsible for the rapidly varying high energy flux. However, the long-term (day-week) coherent evolution of the GeV flux recorded by LAT indicates that there could also be a substantial contribution from another, larger emission region. We model the spectral energy distribution of PKS 1222+216 during the epoch of the MAGIC detection assuming three different scenarios, namely: (1) a one-zone model considering only the emission from a compact blob outside the BLR; (2) a two-zone model consisting of a compact blob plus an emitting region encompassing the whole jet cross-section located outside the BLR; and (3) a two-zone model with the jet emitting region inside the BLR. In all cases we find that the high-energy emission from the compact blob is dominated by the inverse Compton scattering of the infrared thermal radiation of the dusty torus. Furthermore, both regions are matter-dominated, with the Poynting flux providing a negligible contribution to the total jet power. These results do not support models in which the compact blob is the result of reconnection events inside the jet or “needles” of highenergy electrons accelerated close to the BH. The observational framework and our radiative models might instead be compatible with scenarios in which the jet is re-collimated and focussed at large distances from the central BH.

The intergalactic magnetic field constrained by Fermi/LAT observations of the TeV blazar 1ES 0229+200

TeV photons from blazars at relatively large distances, interacting with the optical-IR cosmic background, are efficiently converted into electron-positron pairs. The produced pairs are extremely relativistic (Lorentz factors of the order of 1e6 1e7 and promptly loose their energy through inverse Compton scatterings with the photons of the microwave cosmic background, producing emission in the GeV band. The spectrum and the flux level of this reprocessed emission is critically dependent on the intensity of the intergalactic magnetic field, B, that can deflect the pairs diluting the intrinsic emission over a large solid angle. We derive a simple relation for the reprocessed spectrum expected from a steady source. We apply this treatment to the blazar 1ES 0229+200, whose intrinsic very hard TeV spectrum is expected to be approximately steady. Comparing the predicted reprocessed emission with the upper limits measured by the Fermi/Large Area Telescope, we constrain the value of the intergalactic magnetic field to be larger than

Search for the shortest variability at gamma rays in flat-spectrum radio quasars

B \simeq 5\times 10^{-15}

Constraining blazar distances with combined Fermi and TeV data: an empirical approach

Gauss, depending on the model of extragalactic background light.

The radio/gamma-ray connection in Fermi-blazars

We report about the search for short-term variability in the high-energy gamma-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222+216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2 s^-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (< 2-3 hours) -- the smallest measured to date at MeV energies --, which can constrain the size of the gamma-ray emitting region. The results obtained in the present work favor the hypothesis that gamma rays are generated inside the broad-line region.

Radio-to-[gamma]-ray monitoring of the narrow-line Seyfert 1 galaxy PMN J0948+0022 from 2008 to 2011

We discuss a method to constrain the distance of blazars with unknown redshift using combined observations in the GeV and TeV regimes. We assume that the Very High Energies (VHE) spectrum corrected for the absorption through the interaction with the extragalactic background light cannot be harder than the spectrum in the Fermi/Large Area Telescope (LAT) band. Starting from the observed VHE spectral data we derive the EBL-corrected spectra as a function of the redshift z and fit them with power laws to be compared with power-law fits to the LAT data. We apply the method to all TeV blazars detected by LAT with known distance and derive an empirical law describing the relation between the upper limits and the true redshifts that can be used to estimate the distance of unknown redshift blazars. Using different EBL models leads to systematic changes in the derived upper limits. Finally, we use this relation to infer the distance of the unknown redshift blazar PKS 1424+240.

Fermi/LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089

We study the correlation between the gamma-ray flux F_g, averaged over the first 11 months of Fermi survey and integrated above 100 MeV, and the radio flux density (F_r at 20 GHz) of Fermi sources associated with a radio counterpart in the AT20G survey. Considering the blazars detected in both bands, the correlation is highly significant and it is F_g~F_r^0.85+-0.04, similar for BL Lac and FSRQ sources. However, only a small fraction (~1/15) of the AT20G radio sources with flat radio spectrum, are detected by Fermi. To understand if this correlation is real, we examine the selection effects introduced by the flux limits of the radio and gamma-ray surveys, and the importance of variability of the gamma-ray flux. We find that the radio/gamma-ray flux correlation is real, but its slope is steeper than the observed one, i.e. F_g~F_r^delta with delta in the range 1.25-1.5. The observed F_g-F_r correlation and the fraction of radio sources detected by Fermi is reproduced assuming a long term gamma-ray flux variability following a log-normal probability distribution with standard deviation sigma>0.5 (corresponding to F_g varying by at least a factor 3). Such a variability is compatible with what observed when comparing, for the sources in common, the EGRET and the Fermi gamma-ray fluxes (even if the Fermi fluxes are averaged over ~1 year). Another indication of variability is the non detection of 12 out of 66 EGRET blazars by Fermi, despite its higher sensitivity.We also study the strong linear correlation between the gamma-ray and the radio luminosity of the 144 AT20G-Fermi associations with known redshift and show that it is statistically robust. Two possible implications of these correlations are discussed: the contribution of blazars to the extragalactic gamma-ray background and the prediction of blazars that might undergo extremely high states of gamma-ray emission in the next years.

Does the gamma-ray flux of the blazar 3C 454.3 vary on subhour time-scales?

We present more than three years of observations at different frequencies, from radio to high-energy γ-rays, of the Narrow-Line Seyfert 1 (NLS1) Galaxy PMN J0948+0022 (z = 0.585). This source is the first NLS1 detected at energies above 100 MeV and therefore can be considered the prototype of this emerging new class of γ-ray emitting active galactic nuclei (AGN). The observations performed from 2008 August 1 to 2011 December 31 confirmed that PMN J0948+0022 generates a powerful relativistic jet, which is able to develop an isotropic luminosity at γ-rays of the order of 10 48 erg s −1 , at the level of powerful quasars. The evolution of the radiation emission of this source in 2009 and 2010 followed the canonical expectations of relativistic jets with correlated multiwavelength variability (γ-rays followed by radio emission after a few months), but it was difficult to retrieve a similar pattern in the light curves of 2011. The comparison of γ-ray spectra before and including 2011 data suggested that there was a softening of the highenergy spectral slope. We selected five specific epochs to be studied by modelling the broad-band spectrum, which are characterised by an outburst at γ-rays or very low/high flux at other wavelengths. The observed variability can largely be explained by changes in the injected power, the bulk Lorentz factor of the jet, or the electron spectrum. The characteristic time scale of doubling/halving flux ranges from a few days to a few months, depending on the frequency and the sampling rate. The shortest doubling time scale at γ-rays is 2.3 ± 0.5 days. These small values underline the need of highly sampled multiwavelength campaigns to better understand the physics of these sources.