The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process)^1,2,3. Such transients, named ‘macronovae’ or ‘kilonovae’^4,5,6,7, are believed to be centres of production of rare elements such as gold and platinum⁸. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst^9,10 at redshift z = 0.356, although findings indicating bluer events have been reported¹¹. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source¹² GW170817 and γ-ray burst^13,14 GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models^15,16. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.

We studied all blazars of known redshift detected by the Fermi satellite during its first 3-month survey. For the majority of them, pointed Swift obs

We consider all bursts with known redshift and $\nu F_\nu$ peak energy, $E^{obs}_{peak}$. For a good fraction of them an estimate of the jet opening angle is available from the achromatic break of their afterglow light curve. This allows the derivation of the collimation--corrected energy of the bursts, $E_\gamma$. The distribution of the values of $E_\gamma$ is more spread with respect to previous findings, covering about two orders of magnitude. We find a surprisingly tight correlation between $E_\gamma$ and the source frame $E_{peak}$: $E^{obs}_{peak}(1+z) \propto E_\gamma^{0.7}$. This correlation can shed light on the still uncertain radiation processes for the prompt GRB emission. More importantly, if the small scatter of this newly found correlation will be confirmed by forthcoming data, it will be possible to use it for cosmological purposes.Comment: 25 pages, 3 figures, accepted for publication in Ap

Merging produced a structured jet: The binary neutron star merger event GW170817 was observed with gravitational waves and across the electromagnetic spectrum. However, the physical processes that produced that emission remain poorly understood, particularly the late-time x-ray and radio emission. Ghirlanda et al. observed the radio afterglow with an interferometric array of 32 radio telescopes spread across the globe. The size and position of the radio source are not compatible with a uniformly expanding cocoon, as some have suggested. Instead, the data indicate that GW170817 produced a structured jet of material that escaped the surrounding ejecta and is now expanding into the interstellar medium at relativistic speeds. Science , this issue p. 968

TeV photons from blazars at relatively large distances, interacting with the optical–infrared cosmic background, are efficiently converted into electron–positron pairs. The produced pairs are extremely relativistic (Lorentz factors of the order of 106– 107) and promptly lose their energy through inverse Compton scatterings with the photons of the microwave cosmic background, producing emission in the GeV band. The spectrum and the flux level of this reprocessed emission are critically dependent on the intensity of the intergalactic magnetic field, B, that can deflect the pairs diluting the intrinsic emission over a large solid angle. We derive a simple relation for the reprocessed spectrum expected from a steady source. We apply this treatment to the blazar 1ES0229+200, whose intrinsic, very hard TeV spectrum is expected to be approximately steady. Comparing the predicted reprocessed emission with the upper limits measured by the Fermi/Large Area Telescope, we constrain the value of the intergalactic magnetic field to be larger than B≃ 5 × 10−15 G, depending on the model of extragalactic background light.

We study the BL Lac objects detected in the 1-year all-sky survey of the Fermi satellite, with an energy spectral slope αγ in the 0.1–100 GeV band gr