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

Low energy cut-offs and hard X-ray spectra in high-z radio-loud quasars: the Suzaku view of RBS315

We present the results from the Suzaku observation of the powerful radio-loud quasar RBS315 (z=2.69), for which a previous XMM-Newton observation showed an extremely flat X-ray continuum up to 10 keV (photon index Gamma=1.26) and indications of strong intrinsic absorption (N_H~10^22 cm^{-2} assuming neutral gas). The instrument for hard X-rays HXD/PIN allows us a detection of the source up to 50 keV. The broad-band continuum (0.5-50 keV) can be well modeled with a power-law with slope Gamma=1.5 (definitively softer than the continuum measured by XMM-Newton) above 1 keV with strong deficit of soft photons. The low-energy cut-off can be well fitted either with intrinsic absorption (with column density N_H~10^22 cm^{-2} in the quasar rest frame) or with a break in the continuum, with an extremely hard (Gamma =0.7) power-law below 1 keV. We construct the Spectral Energy Distribution of the source, using also optical-UV measurements obtained through a quasi-simultaneous UVOT/SWIFT observation. The shape of the SED is similar to that of other Flat Spectrum Radio Quasars (FSRQs) with similar power, making this source an excellent candidate for the detection in gamma-rays by GLAST. We model the SED with the synchrotron-Inverse Compton model usually applied to FSRQs, showing that the deficit of soft photons can be naturally interpreted as due to an intrinsic curvature of the spectrum near the low energy end of the IC component rather than to intrinsic absorption, although the latter possibility cannot be ruled out. We propose that in at least a fraction of the radio-loud QSOs at high redshift the cut-off in the soft X-ray band can be explained in a similar way. Further studies are required to distinguish between the two alternatives.Comment: Accepted for publication in Ap

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

Cosmological implications of Compton tails in long duration GRB

The recent suggestion of the possible presence of a significant amount of material (Thomson optical depth ∼ 1) at rest and at a typical distance of ∼ 1015 cm with respect to the GRB is presented. The relevance of such interpretation for GRB energetics and its cosmological implications is outlined

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