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.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

We study the emission observed at energies >100 MeV of 11 gamma-ray bursts (GRBs) detected by the Fermi–Large Area Telescope (LAT) until 2009 Octo

Knowledge of the bulk Lorentz factor Γ₀ of gamma-ray bursts (GRBs) allows us to compute their comoving frame properties shedding light on their physics. Upon collisions with the circumburst matter, the fireball of a GRB starts to decelerate, producing a peak or a break (depending on the circumburst density profile) in the light curve of the afterglow. Considering all bursts with known redshift and with an early coverage of their emission, we find 67 GRBs (including one short event) with a peak in their optical or GeV light curves at a time t_p. For another 106 GRBs we set an upper limit t_p^UL. The measure of t_p provides the bulk Lorentz factor Γ₀ of the fireball before deceleration. We show that t_p is due to the dynamics of the fireball deceleration and not to the passage of a characteristic frequency of the synchrotron spectrum across the optical band. Considering the t_p of 66 long GRBs and the 85 most constraining upper limits, we estimate Γ₀ or a lower limit Γ₀^LL. Using censored data analysis methods, we reconstruct the most likely distribution of t_p. All t_p are larger than the time T_p,γ when the prompt γ-ray emission peaks, and are much larger than the time T_ph when the fireball becomes transparent, that is, t_p>T_p,γ>T_ph. The reconstructed distribution of Γ₀ has median value ~300 (150) for a uniform (wind) circumburst density profile. In the comoving frame, long GRBs have typical isotropic energy, luminosity, and peak energy ⟨ E_iso ⟩ = 3(8) × 10⁵⁰ erg, ⟨ L_iso ⟩ = 3(15) × 10⁴⁷ erg s^-1, and ⟨ E_peak ⟩ = 1(2) keV in the homogeneous (wind) case. We confirm that the significant correlations between Γ₀ and the rest frame isotropic energy (E_iso), luminosity (L_iso), and peak energy (E_p) are not due to selection effects. When combined, they lead to the observed E_p−E_iso and E_p−L_iso correlations. Finally, assuming a typical opening angle of 5 degrees, we derive the distribution of the jet baryon loading which is centered around a few 10^-6M_⊙.

We present the results of the spectral analysis of the public data of 438 gamma ray bursts (GRBs) detected by the Fermi Gamma ray Burst Monitor (GBM) up to March 2010. For 432 bursts we could fit the time-integrated spectrum. In 318 cases we could reliably constrain the peak energy of their ν F ν spectrum by analyzing their time-integrated spectrum between 8 keV and 35 MeV. Eighty percent of these spectra are fitted by a power-law with an exponential cutoff, and the remaining with the Band function. Among these 318 GRBs, 274 belong to the long GRB class and 44 to the short. Long GRBs have a typical peak energy ~ 160 keV and low-energy spectral index α ~ − 0.92. Short GRBs have a harder peak energy ( ~ 490 keV) and a harder low-energy spectral index (α ~ −0.50) than long bursts. For each Fermi GRB we also analyzed the spectrum corresponding to the peak flux of the burst. On average, the peak spectrum has a harder low-energy spectral index than the corresponding time-integrated spectrum for the same burst, but similar Ṫhe spectral parameters derived in our analysis of Fermi/GBM bursts are globally consistent with those reported in the GRB Cicular Network (GCN) archive after December 2008, while we found systematic differences in the low-energy power-law index for earlier bursts.