Half of all the elements in the universe heavier than iron were created by rapid neutron capture. The theory for this astrophysical 'r-process' was worked out six decades ago and requires an enormous neutron flux to make the bulk of these elements. 1 Where this happens is still debated. 2 A key piece of missing evidence is the identification of freshly-synthesised r-process elements in an astrophysical site. Current models 3-5 and circumstantial evidence 6 point to neutron star mergers as a probable r-process site, with the optical/infrared 'kilonova' emerging in the days after the merger a likely place to detect the spectral signatures of newly-created neutron-capture elements. 7-9 The kilonova, AT2017gfo, emerging from the gravitational-wave-discovered neutron star merger, GW170817, 10 was the first kilonova where detailed spectra were recorded. When these spectra were first reported 11, 12 it was argued that they were broadly consonant with an outflow of radioactive heavy elements, however, there was no robust identification of any element. Here we report the identification of the neutron-capture element strontium in a re-analysis of these spectra. The detection of a neutron-capture element associated with the collision of two extreme-density stars establishes the origin of r-process elements in neutron star mergers, and demonstrates that neutron stars comprise neutron-rich matter 13 .The most detailed information available for a kilonova comes from a series of spectra of AT2017gfo taken over several weeks with the medium resolution, ultraviolet (320 nm) to near-infrared (2,480 nm) spectrograph, X-shooter, mounted at the Very Large Telescope at the European Southern Observatory. These spectra 11, 12 , allow us to track the evolution of the kilonova's primary electromagnetic output from 1.5 days until 10 days after the event. Detailed modelling of these spectra has yet to be done owing to the limited understanding of the phenomenon and the expectation that a very large number of moderate to weak lanthanide lines with unknown oscillator strengths would dominate the spectra 14,15 . Despite the expected complexity, we sought to identify individual elements in the early spectra because these spectra are well-reproduced by relatively simple models 11 .The first epoch spectrum can be reproduced over the entire observed spectral range with a single-temperature blackbody with an observed temperature 4, 800 K. The two major deviations short of 1 µm from a pure blackbody are due to two very broad (∼ 0.2c) absorption components. These components are observed centred at about 350 nm and 810 nm (Fig. 1). The shape of the ultraviolet absorption component is not well constrained because it lies close to the edge of our sensitivity limit and may simply be cut off below about 350 nm. The presence of the absorption feature at 810 nm at this epoch has been noted in earlier publications 11,12 .The fact that the spectrum is very well reproduced by a single temperature blackbody in the first epoch suggests a population of states 0.3...

We present new ALMA observations and physical properties of a Lyman Break Galaxy at z = 7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, M UV ≈ −22.4, and has been spectroscopically identified in Lyα with a small rest-frame equivalent width of ≈ 4Å. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [Oiii] 88 µm, [Cii] 158 µm, and their underlying dust continuum emission. In the whole system of B14-65666, the [Oiii] and [Cii] lines have consistent redshifts of 7.1520 ± 0.0003, and the [Oiii] luminosity, (34.4 ± 4.1) × 10 8 L ⊙ , is about three times higher than the [Cii] luminosity, (11.0 ± 1.4) × 10 8 L ⊙ . With our two continuum flux densities, the dust temperature is constrained to be T d ≈ 50 − 60 K under the assumption of the dust emissivity index of β d = 2.0 − 1.5, leading to a large total infrared luminosity of L TIR ≈ 1 × 10 12 L ⊙ . Owing to our high spatial resolution data, we show that the [Oiii] and [Cii] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by ≈ 200 km s −1 . We also find these two clumps have comparable UV, infrared, [Oiii], and [Cii] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR = 260 +119 −57 Gyr −1 , inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [Oiii]-to-[Cii] luminosity ratio.

THESEUS is a space mission concept aimed at exploiting Gamma-Ray Bursts for investigating the early Universe and at providing a substantial advancement of multi-messenger and time-domain astrophysics. These goals will be achieved through a unique combination of instruments allowing GRB and X-ray transient detection over a broad field of view (more than 1sr) with 0.5-1 arcmin localization, an energy band extending from several MeV down to 0.3 keV and high sensitivity to transient sources in the soft X-ray domain, as well as on-board prompt (few minutes) followup with a 0.7 m class IR telescope with both imaging and spectroscopic capabilities. THESEUS will be perfectly suited for addressing the main open issues in cosmology such as, e.g., star formation rate and metallicity evolution of the inter-stellar and intra-galactic medium up to redshift ∼10, signatures of Pop III stars, sources and physics of reionization, and the faint end of the galaxy luminosity function. In addition, it will provide unprecedented capability to monitor the X-ray variable sky, thus detecting, localizing, and identifying the electromagnetic counterparts to sources of gravitational radiation, which may be routinely detected in the late '20s / early '30s by next generation facilities like aLIGO/ aVirgo, eLISA, KAGRA, and Einstein Telescope. THESEUS will also provide powerful synergies with the next generation of multi-wavelength observatories (e.g., LSST, ELT, SKA, CTA, ATHENA).

In this work we present spectra of all γ-ray burst (GRB) afterglows that have been promptly observed with the X-shooter spectrograph until 31/03/2017. In total, we obtained spectroscopic observations of 103 individual GRBs observed within 48 hours of the GRB trigger. Redshifts have been measured for 97 per cent of these, covering a redshift range from 0.059 to 7.84. Based on a set of observational selection criteria that minimize biases with regards to intrinsic properties of the GRBs, the follow-up effort has been focused on producing a homogeneous sample of 93 afterglow spectra for GRBs discovered by the Swift satellite. We here provide a public release of all the reduced spectra, including continuum estimates and telluric absorption corrections. For completeness, we also provide reductions for the 18 late-time observations of the underlying host galaxies. We provide an assessment of the degree of completeness with respect to the parent GRB population, in terms of the X-ray properties of the bursts in the sample and find that the sample presented here is representative of the full Swift sample. We constrain the fraction of dark bursts to be < 28 per cent and we confirm previous results that higher optical darkness is correlated with increased X-ray absorption. For the 42 bursts for which it is possible, we provide a measurement of the neutral hydrogen column density, increasing the total number of published HI column density measurements by ∼ 33 per cent. This dataset provides a unique resource to study the ISM across cosmic time, from the local progenitor surroundings to the intervening universe.Article number, page 19 of 43 A&A proofs: manuscript no. XSGRB_sample_arxiv Cité, 10, Rue Alice Domon et Léonie Duquet, 75205,