A fireworks model for Gamma-Ray Bursts

The energetics of the long duration GRB phenomenon is compared with models of a rotating Black Hole (BH) in a strong magnetic field generated by an accreting torus. A rough estimate of the energy extracted from a rotating BH with the Blandford-Znajek mechanism is obtained with a very simple assumption: an inelastic collision between the rotating BH and the torus. The GRB energy emission is attributed to an high magnetic field that breaks down the vacuum around the BH and gives origin to a e+- fireball. Its subsequent evolution is hypothesized, in analogy with the in-flight decay of an elementary particle, to evolve in two distinct phases. The first one occurs close to the engine and is responsible of energizing and collimating the shells. The second one consists of a radiation dominated expansion, which correspondingly accelerates the relativistic photon--particle fluid and ends at the transparency time. This mechanism simply predicts that the observed Lorentz factor is determined by the product of the Lorentz factor of the shell close to the engine and the Lorentz factor derived by the expansion. An anisotropy in the fireball propagation is thus naturally produced, whose degree depends on the bulk Lorentz factor at the end of the collimation phase.Comment: Accepted for publication in MNRA

Probing variability patterns of the Fe K line complex in bright nearby AGNs

The unprecedented sensitivity of current X-ray telescopes allows for the first time to address the issue of the Fe K line complex variability patterns in bright, nearby AGNs. We examine XMM-Newton observations of the brightest sources of the FERO sample of radio-quiet type 1 AGNs with the aim of characterizing the temporal behaviour of Fe K complex features. A systematic mapping of residual flux above and below the continuum in the 4-9 keV range is performed in the time vs energy domain, with the purpose of identifying interesting spectral features in the three energy bands: 5.4-6.1 keV, 6.1-6.8 keV and 6.8-7.2 keV, corresponding respectively to the redshifted, rest frame and blueshifted or highly ionized Fe Kalpha line bands. The variability significance is assessed by extracting light curves and comparing them with MonteCarlo simulations. The time-averaged profile of the Fe K complex revealed spectral complexity in several observations. Red- and blue-shifted components (either in emission or absorption) were observed in 30 out of 72 observations, with an average ~90 eV for emission and ~ -30 eV for absorption features. We detected significant line variability (with confidence levels ranging between 90% and 99.7%) within at least one of the above energy bands in 26 out of 72 observations on time scales of ~6-30 ks. Reliability of these features has been carefully calculated using this sample and has been assessed at ~3sigma confidence level. This work increases the currently scanty number of detections of variable, energy shifted, Fe lines and confirms the reliability of the claimed detections. We found that the distribution of detected features is peaked at high variability significances in the red- and blue-shifted energy bands, suggesting an origin in a relativistically modified accretion flow.Comment: Accepted for publication in Astronomy & Astrophysic

Compton rockets and the minimum power of relativistic jets

The power of a relativistic jet depends on the number of leptons and protons carried by the jet itself. We have reasons to believe that powerful gamma-ray flat spectrum radio sources emit most of their radiation where radiative cooling is severe. This helps to find the minimum number of emitting leptons needed to explain the radiation we see. The number of protons is more uncertain. If there is one proton per electron, they dominate the jet power, but they could be unimportant if the emission is due to electron-positron pairs. In this case the total jet power could be much smaller. However, if the gamma-ray flux is due to inverse Compton scattering with seed photons produced outside the jet, the radiation is anisotropic also in the comoving frame, making the jet to recoil. This Compton rocket effect is strong for light, electron-positron jets, and negligible for heavy, proton dominated jets. No significant deceleration, required by fast superluminal motion, requires a minimum number of protons per lepton, and thus a minimum jet power. We apply these ideas to the blazar 3C 454.3, to find a robust lower limit to its total jet power: if the viewing angle theta_v ~ 1/Gamma the jet power is larger than the accretion luminosity L_d for any bulk Lorentz factor Gamma. For theta_v =0, instead, the minimum jet power can be smaller than L_d for Gamma<25. No more than ~10 pairs per proton are allowed.Comment: 5 pages, 2 figures, accepted for publication as a letter to MNRA

Stochastic wake field particle acceleration in Gamma-Ray Bursts

Gamma-Ray Burst (GRB) prompt emission can, for specific conditions, be so powerful and short-pulsed to strongly influence any surrounding plasma. In this paper, we briefly discuss the possibility that a very intense initial burst of radiation produced by GRBs satisfy the intensity and temporal conditions to cause stochastic wake-field particle acceleration in a surrounding plasma of moderate density. Recent laboratory experiments clearly indicate that powerful laser beam pulses of tens of femtosecond duration hitting on target plasmas cause efficient particle acceleration and betatron radiation up to tens of MeV. We consider a simple but realistic GRB model for which particle wake-field acceleration can first be excited by a very strong low-energy precursor, and then be effective in producing the observed prompt X-ray and gamma-ray GRB emission. We also briefly discuss some of the consequences of this novel GRB emission mechanism.Comment: 5 pages, 1 figure, submitted to MNRA

Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT

We report on a detailed investigation of the high-energy gamma-ray emission from NGC\,1275, a well-known radio galaxy hosted by a giant elliptical located at the center of the nearby Perseus cluster. With the increased photon statistics, the center of the gamma-ray emitting region is now measured to be separated by only 0.46' from the nucleus of NGC1275, well within the 95% confidence error circle with radius ~1.5'. Early Fermi-LAT observations revealed a significant decade-timescale brightening of NGC1275 at GeV photon energies, with a flux about seven times higher than the one implied by the upper limit from previous EGRET observations. With the accumulation of one-year of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral variations of this source on month timescales, as reported in this paper. The average >100 MeV gamma-ray spectrum of NGC1275 shows a possible deviation from a simple power-law shape, indicating a spectral cut-off around an observed photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X 10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest gamma-ray flaring event was observed in April--May 2009 and was accompanied by significant spectral variability above E > 1-2 GeV. The gamma-ray activity of NGC1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the gamma-ray source was detected at the very end of the observation, with the observed energy of E = 67.4GeV and an angular separation of about 2.4' from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed gamma-ray spectral evolution of NGC1275 in the context of gamma-ray blazar sources in general.Comment: 20 pages, 6 figures, accepted for publication in the Ap

Jet and accretion power in the most powerful Fermi blazars

Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the three months of survey had an average gamma-ray luminosity above 1e48 erg/s. For 17 out of the 23 sources we found and analysed X-ray and optical-UV data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high energy states of single sources, like 0528+134 and 3C 454.3, for which multiple good sets of multi-wavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disk emission. We can then constrain the black hole mass and the disk luminosity. Both are large (i.e. masses equal or greater than 1e9 solar masses and disk luminosities above 0.1 Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disk luminosity, and proportional to the mass accretion rate.Comment: 15 pages, 14 figures, accepted for publication in MNRA