Since 2005, the blazar 3C 454.3 has shown remarkable flaring activity at all frequencies, and during the last four years it has exhibited more than one γ -ray flare per year, becoming the most active γ -ray blazar in the sky. We present for the first time the multi-wavelength AGILE, Swift, INTEGRAL, and GASP-WEBT data collected in order to explain the extraordinary γ -ray flare of 3C 454.3 which occurred in 2010 November. On 2010 November 20 (MJD 55520), 3C 454.3 reached a peak flux (E >100 MeV) of F p γ = (6.8 ± 1.0) × 10 −5 photons cm −2 s −1 on a timescale of about 12 hr, more than a factor of six higher than the flux of the brightest steady γ -ray source, the Vela pulsar, and more than a factor of three brighter than its previous super-flare on 2009 December 2-3. The multi-wavelength data make possible a thorough study of the present event: the comparison with the previous

Context. Blazars are the most luminous and variable active galactic nuclei (AGNs), and thus excellent probes of accretion and emission processes close to the central engine.Aims. We concentrate here on PKS 1510-089 (z = 0.36), a member of the Flat-Spectrum Radio Quasar variety of blazars, an extremely powerful gamma-ray source and one of the brightest in the Fermi LAT catalog, to study the complex variability of its bright multiwavelength spectrum, identify the physical parameters responsible for the variations and the time scales of possible recurrence and quasi-periodicity at high energies.Methods. PKS 1510-089 was observed twice in hard X-rays with the IBIS instrument onboard INTEGRAL during the flares of Jan 2009 and Jan 2010, and simultaneously with Swift and the Nordic Optical Telescope (NOT), in addition to the constant Fermi monitoring. We also measured the optical polarization in several bands on 18 Jan 2010 at the NOT. Using these and archival data we constructed historical light curves at gamma-to-radio wavelengths covering nearly 20 years and applied tests of fractional and correlated variability. We also assembled spectral energy distributions (SEDs) based on these data and compared them with those at two previous epochs, by applying a model based on synchrotron and inverse Compton radiation from blazars. Results. The modeling of the SEDs suggests that the physical quantities that undergo the largest variations are the total power injected into the emitting region and the random Lorentz factor of the electron distribution cooling break, that are higher in the higher gammaray states. This suggests a correlation of the injected power with enhanced activity of the acceleration mechanism. The cooling likely takes place at a distance of ∼1000 Schwarzschild radii (∼0.03 pc) from the central engine, i.e much smaller than the broad line region (BLR) radius. The emission at a few hundred GeV can be reproduced with inverse Compton scattering of highly relativistic electrons off far-infrared photons if these are located much farther than the BLR, i.e., around 0.2 pc from the AGN, presumably in a dusty torus. We determine a luminosity of the thermal component due to the inner accretion disk of L d 5.9 × 10 45 erg s −1 , a BLR luminosity of L BLR 5.3 × 10 44 erg s −1 , and a mass of the central black hole of M BH 3 × 10 8 M . The fractional variability as a function of wavelength follows the trend expected if X-and gamma-rays are produced by the same electrons as radio and optical photons, respectively. Discrete Correlation Function (DCF) analysis between the long-term Steward observatory optical V-band and gamma-ray Fermi LAT light curves yields a good correlation with no measurable delay. Marginal correlation where X-ray photons lag both optical and gamma-ray ones by time lags between 50 and 300 days is found with the DCF. Our time analysis of the RXTE PCA and Fermi LAT light curves reveals no obvious (quasi-)periodicities, at least up to the maximum time scale (a few years) probed by the light curves, which ar...

Aims. Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy γ-rays, through the investigation of the prototypical blazar PKS 2155−304, which shows complex behaviour. Methods. We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. Results. We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Γ 2 ∼ 2.4 harder than Γ 1 ∼ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.

Aims. Multiwavelength variability of blazars offers indirect, but very effective, insight into their powerful engines and on the mechanisms through which energy is propagated from the centre down the jet. The BL Lac object Mkn 421 is a TeV emitter, a bright blazar at all wavelengths, and therefore an excellent target for variability studies. Methods. We activated INTEGRAL observations of Mkn 421 in an active state on 16-21 April 2013, and complemented them with Fermi-LAT data. Results. We obtained well sampled optical, soft, and hard X-ray light curves that show the presence of two flares and time-resolved spectra in the 3.5-60 keV (JEM-X and IBIS/ISGRI) and 0.1-100 GeV (Fermi-LAT) ranges. The average flux in the 20-100 keV range is 9.1 × 10 −11 erg s −1 cm −2 (∼4.5 mCrab) and the nuclear average apparent magnitude, corrected for Galactic extinction, is V 12.2. In the time-resolved X-ray spectra, which are described by broken power laws and, marginally better, by log-parabolic laws, we see a hardening that correlates with flux increase, as expected in refreshed energy injections in a population of electrons that later cool via synchrotron radiation. The hardness ratios between the JEM-X fluxes in two different bands and between the JEM-X and IBIS/ISGRI fluxes confirm this trend. During the observation, the variability level increases monotonically from the optical to the hard X-rays, while the large LAT errors do not allow a significant assessment of the MeV-GeV variability. The cross-correlation analysis during the onset of the most prominent flare suggests a monotonically increasing delay of the lower frequency emission with respect to that at higher frequency, with a maximum time-lag of about 70 min, that is however not well constrained. The spectral energy distributions from the optical to the TeV domain were compared to homogeneous models of blazar emission based on synchrotron radiation and synchrotron self-Compton scattering. They represent a satisfactory description, except in the state corresponding to the LAT softest spectrum and highest flux. Conclusions. Multiwavelength variability of Mkn 421 can be very complex, with patterns changing from epoch to epoch down to intra-day timescales, depending on the emission state. This makes accurate monitoring of this source during bright hard X-ray states necessary and calls for the elaboration of multicomponent, multizone, time-dependent models.

Aims. We study the blazar nature of the high-redshift Flat-Spectrum Radio Quasar PKS 2149-306 (z = 2.345) by investigating its long-term behavior. Methods. We analyzed all publicly available optical-to-X-ray observations performed by XMM-Newton, Swift, and INTEGRAL. Results. PKS 2149-306 is one of four blazars at z > 2 that have been observed in the hard-X-ray regime with both the BAT and ISGRI instruments. Observations acquired almost 1 year apart in the 60−300 keV energy band in the object rest frame, exhibit no noticeable change in spectral slope associated with a flux variation of more than a factor of two. Swift data appear to show a roll-off below ∼1 keV, which becomes increasingly evident during a ∼3-day time-frame, that can be explained as the natural spectral break caused by the Inverse Compton onset. The broad-band spectra allow us to identify two different states. The SED modeling suggests that they can be interpreted by only a change in the bulk Lorentz factor of the jet.