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.
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