It is well accepted that unabsorbed as well as absorbed AGN are needed to explain the nature and the shape of the Cosmic X-ray background, even if the fraction of highly absorbed objects (dubbed Compton-thick sources) substantially still escapes detection. We derive and analyze the absorption distribution using a complete sample of AGN detected by Swift-BAT in the first three years of the survey. The fraction of Compton-thick AGN represents only 4.6% of the total AGN population detected by Swift-BAT. However, we show that once corrected for the bias against the detection of very absorbed sources the real intrinsic fraction of Compton-thick AGN is 20 +9 −6 %. We proved for the first time (also in the BAT band) that the anti-correlation of the fraction of absorbed AGN and luminosity it tightly connected to the different behavior of the luminosity functions (XLFs) of absorbed and unabsorbed AGN. This points towards a difference between the two subsamples of objects with absorbed AGN being, on average, * Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.

We estimate the bulk Lorentz factor Γ 0 of 31 GRBs using the measured peak time of their afterglow light curves. We consider two possible scenarios for the estimate of Γ 0 : the case of a homogeneous circumburst medium or a wind density profile. The values of Γ 0 are broadly distributed between few tens and several hundreds with average values ∼138 and ∼66 for the homogeneous and wind density profile, respectively. We find that the isotropic energy and luminosity correlate in a similar way with Γ 0 , i.e. E iso ∝Γ 0 2 and L iso ∝Γ 0 2 , while the peak energy E peak ∝Γ 0 . These correlations are less scattered in the wind density profile than in the homogeneous case. We then study the energetics, luminosities and spectral properties of our bursts in their comoving frame. The distribution of L ′ iso is very narrow with a dispersion of less than a decade in the wind case, clustering around L ′ iso ∼ 5 × 10 48 erg s −1 . Peak photon energies cluster around E ′ peak ∼ 6 keV. The newly found correlations involving Γ 0 offer a general interpretation scheme for the spectral-energy correlation of GRBs. The E peak − E iso and E peak − L iso correlations are due to the different Γ 0 factors and the collimation-corrected correlation, E peak − E γ (obtained by correcting the isotropic quantities for the jet opening angle θ j ), can be explained if θ 2 j Γ 0 = constant. Assuming the E peak − E γ correlation as valid, we find a typical value of θ j Γ 0 ∼ 6-20, in agreement with the predictions of magnetically accelerated jet models.

Surveys above 10 keV represent one of the best resources to provide an unbiased census of the population of active galactic nuclei (AGNs). We present the results of 60 months of observation of the hard X-ray sky with Swift/Burst Alert Telescope (BAT). In this time frame, BAT-detected (in the 15-55 keV band) 720 sources in an all-sky survey of which 428 are associated with AGNs, most of which are nearby. Our sample has negligible incompleteness and statistics a factor of ∼2 larger over similarly complete sets of AGNs. Our sample contains (at least) 15 bona fide Compton-thick AGNs and 3 likely candidates. Compton-thick AGNs represent ∼5% of AGN samples detected above 15 keV. We use the BAT data set to refine the determination of the log N-log S of AGNs which is extremely important, now that NuSTAR prepares for launch, toward assessing the AGN contribution to the cosmic X-ray background. We show that the log N-log S of AGNs selected above 10 keV is now established to ∼10% precision. We derive the luminosity function of Compton-thick AGNs and measure a space density of 7.9 +4.1 −2.9 × 10 −5 Mpc −3 for objects with a de-absorbed luminosity larger than 2 × 10 42 erg s −1 . As the BAT AGNs are all mostly local, they allow us to investigate the spatial distribution of AGNs in the nearby universe regardless of absorption. We find concentrations of AGNs that coincide spatially with the largest congregations of matter in the local ( 85 Mpc) universe. There is some evidence that the fraction of Seyfert 2 objects is larger than average in the direction of these dense regions.

We study a sample of gamma-ray bursts detected by the Swift satellite with known redshift which show a precursor in the Swift BAT light curve. We analyze the spectra of the precursors and compare them with the time-integrated spectra of the prompt emission. We find neither a correlation between the two slopes nor a tendency for the precursors spectra to be systematically harder or softer than the prompt ones. The energetics of the precursors are large: on average, they are just a factor of a few less energetic (in the source rest-frame energy range 15-150 keV) than the entire bursts. These properties do not depend on the quiescent time between the end of the precursor and the start of the main event. These results suggest that what has been called a "precursor" is not a phenomenon distinct from the main event, but is tightly connected with it, even if, in some case, the quiescent time intervals can be longer than 100 s.

We present the clustering measurement of hard X-ray selected active galactic nuclei (AGNs) in the local universe. We used a sample of 199 sources spectroscopically confirmed, detected by Swift-BAT in its 15-55 keV all-sky survey. We measured the real space projected autocorrelation function (ACF) and detected a signal significant on projected scales lower than 200 Mpc h −1 . We measured a correlation length of r 0 = 5.56−0.43 Mpc h −1 and a slope γ = 1.64−0.08 −0.07 . We also measured the ACF of Type I and Type II AGNs and found higher correlation length for Type I AGNs. We have a marginal evidence of luminosity dependent clustering of AGNs, as we detected a larger correlation length of luminous AGNs than that of low-luminosity sources. The corresponding typical host dark matter halo masses of Swift-BAT are ∼ log(M DMH )∼ 12-14 h −1 M/M , depending on the subsample. For the whole sample, we measured log(M DMH )∼ 13.15 h −1 M/M which is the typical mass of a galaxy group. We estimated that the local AGN population has a typical lifetime τ AGN ∼0.7 Gyr, it is powered by supermassive black hole with mass M BH ∼ (1-10) × 10 8 M and accreting with very low efficiency, log( )∼ −2.0. We also conclude that local AGN host galaxies are typically red-massive galaxies with stellar mass of the order (2-80) × 10 10 h −1 M . We compared our results with clustering predictions of merger-driven AGN triggering models and found a good agreement.

The jet opening angle θ jet and the bulk Lorentz factor Γ 0 are crucial parameters for the computation of the energetics of Gamma Ray Bursts (GRBs). From the ∼30 GRBs with measured θ jet or Γ 0 it is known that: (i) the real energetic E γ , obtained by correcting the isotropic equivalent energy E iso for the collimation factor ∼ θ 2 jet , is clustered around 10 50 -10 51 erg and it is correlated with the peak energy E p of the prompt emission and (ii) the comoving frame E ′ p and E ′ γ are clustered around typical values. Current estimates of Γ 0 and θ jet are based on incomplete data samples and their observed distributions could be subject to biases. Through a population synthesis code we investigate whether different assumed intrinsic distributions of Γ 0 and θ jet can reproduce a set of observational constraints Assuming that all bursts have the same E ′ p and E ′ γ in the comoving frame, we find that Γ 0 and θ jet cannot be distributed as single power-laws. The best agreement between our simulation and the available data is obtained assuming (a) log-normal distributions for θ jet and Γ 0 and (b) an intrinsic relation between the peak values of their distributions, i.e θ jet 2.5 Γ 0 =const. On average, larger values of Γ 0 (i.e. the "faster" bursts) correspond to smaller values of θ jet (i.e. the "narrower"). We predict that ∼6% of the bursts that point to us should not show any jet break in their afterglow light curve since they have sin θ jet < 1/Γ 0 . Finally, we estimate that the local rate of GRBs is ∼0.3% of all local SNIb/c and ∼4.3% of local hypernovae, i.e. SNIb/c with broad-lines.

We investigated the rest frame spectral lags of two complete samples of bright long (50) and short (6) gamma-ray bursts (GRB) detected by Swift. We analysed the Swift /BAT data through a discrete cross-correlation function (CCF) fitted with an asymmetric Gaussian function to estimate the lag and the associated uncertainty. We find that half of the long GRBs have a positive lag and half a lag consistent with zero. All short GRBs have lags consistent with zero. The distributions of the spectral lags for short and long GRBs have different average values. Limited by the small number of short GRBs, we cannot exclude at more than 2 σ significance level that the two distributions of lags are drawn from the same parent population. If we consider the entire sample of long GRBs, we do not find evidence for a lag-luminosity correlation, rather the lag-luminosity plane appears filled on the left hand side, thus suggesting that the lagluminosity correlation could be a boundary. Short GRBs are consistent with the long ones in the lag-luminosity plane.

Context. After the launch of the Swift satellite, the gamma-ray burst (GRB) optical light-curve smoothness paradigm has been questioned thanks to the faster and better sampled optical follow-up, which has unveiled a very complex behaviour. This complexity is triggering the interest of the whole GRB community. The GROND multi-channel imager is used to study optical and near-infrared (NIR) afterglows of GRBs with unprecedented optical and near-infrared temporal and spectral resolution. The GRB 081029 has a very prominent optical rebrightening event and is an outstanding example of the application of the multi-channel imager to GRB afterglows. Aims. Here we exploit the rich GROND multi-colour follow-up of GRB 081029 combined with XRT observations to study the nature of late-time rebrightenings that appear in the optical-NIR light-curves of some GRB afterglows. Methods. We analyse the optical and NIR observations obtained with the seven-channel Gamma-Ray burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPI/ESO telescope and the X-ray data obtained with the XRT telescope on board the Swift observatory. The multi-wavelength temporal and spectral evolution is discussed in the framework of different physical models. Results. The extremely steep optical and NIR rebrightening observed in GRB 081029 cannot be explained in the framework of the standard forward shock afterglow model. The absence of a contemporaneous X-ray rebrightening and the evidence of a strong spectral evolution in the optical-NIR bands during the rise suggest two separate components that dominate in the early and late-time lightcurves, respectively. The steepness of the optical rise cannot be explained even in the framework of the alternative scenarios proposed in the literature unless a late-time activity of the central engine is assumed.