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Publisher's version (útgefin grein) ; We present here a survey of high-ionization absorption lines in the afterglow spectra of long-duration gamma-ray bursts (GRBs) obtained with the VLT/X-shooter spectrograph. Our main goal is to investigate the circumburst medium in the natal regions of GRBs. Our primary focus is on the N Vλλ 1238, 1242 line transitions, but we also discuss other high-ionization lines such as O VI, C IV, and Si IV. We find no correlation between the column density of N V and the neutral gas properties such as metallicity, H I column density, and dust depletion; however, the relative velocity of N V, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of H I. This may be explained if the N V gas is part of an H II region hosting the GRB, where the region's expansion is confined by dense, neutral gas in the GRB's host galaxy. We find tentative evidence (at 2σ significance) that the X-ray derived column density, NH, X, may be correlated with the column density of N V, which would indicate that both measurements are sensitive to the column density of the gas located in the vicinity of the GRB. We investigate the scenario where N V (and also O VI) is produced by recombination after the corresponding atoms have been stripped entirely of their electrons by the initial prompt emission, in contrast to previous models where highly ionized gas is produced by photoionization from the GRB afterglow. ; KEH and PJ acknowledge support by a Project Grant (162948–051) from The Icelandic Research Fund. JK acknowledges financial support from the Danish Council for Independent Research (EU-FP7 under the Marie-Curie grant agreement no. 600207) with reference DFF–MOBILEX–5051–00115. JJ acknowledges support from NOVA and NWO-FAPESP grant for advanced instrumentation in astronomy. AG acknowledges the financial support from the Slovenian Research Agency (research core funding no. P1-0031 and project grant no. J1-8136). AdUP, CCT, and ZC acknowledge support from the Spanish Ministry of Economy and Competitivity under grant number AYA 2014-58381-P. AdUP and CCT acknowledge support from Ramon y Cajal fellowships (RyC-2012-09975 and RyC-2012-09984). AdUP acknowledges support from a grant from the BBVA foundation for researchers and cultural creators. ZC acknowledges support from the Juan de la Cierva Incorporación fellowship IJCI-2014-21669 and from the Spanish research project AYA 2014-58381-P. RS-R acknowledges support from ASI (Italian Space Agency) through the contract no. 2015-046-R.0 and from European Union Horizon 2020 Programme under the AHEAD project (grant agreement no. 654215). ; Peer Reviewed

We present the extensive follow-up campaign on the afterglow of GRB 110715A at 17 different wavelengths, from X-ray to radio bands, starting 81 s after the burst and extending up to 74 d later. We performed for the first time a GRB afterglow observation with the ALMA observatory. We find that the afterglow of GRB 110715A is very bright at optical and radio wavelengths. We use the optical and near-infrared spectroscopy to provide further information about the progenitor's environment and its host galaxy. The spectrum shows weak absorption features at a redshift z = 0.8225, which reveal a host-galaxy environment with low ionization, column density, and dynamical activity. Late deep imaging shows a very faint galaxy, consistent with the spectroscopic results. The broad-band afterglow emission is modelled with synchrotron radiation using a numerical algorithm and we determine the best-fitting parameters using Bayesian inference in order to constrain the physical parameters of the jet and the medium in which the relativistic shock propagates. We fitted our data with a variety of models, including different density profiles and energy injections. Although the general behaviour can be roughly described by these models, none of them are able to fully explain all data points simultaneously. GRB 110715A shows the complexity of reproducing extensive multiwavelength broad-band afterglow observations, and the need of good sampling in wavelength and time and more complex models to accurately constrain the physics of GRB afterglows. ; RSR is grateful to SEPE for financial support while finishing this work and his PhD thesis. RSR, SRO, AJCT, YDH, SJ, and JCT acknowledge the financial support of the Spanish Government projects AYA 2009- 14000-C03-01 and AYA 2012-39727-C03-01. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project No. CE110001020. AdUP and CT acknowledge support from Ramon´ y Cajal fellowships and from the Spanish research project AYA 2014-58381. JJ acknowledges financial contribution from the grant PRIN MIUR 2012 201278X4FL 002 'The Intergalactic Medium as a probe of the growth of cosmic structures'. DAK acknowledges the financial support by MPE Garching and TLS Tautenburg. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). PS and TK acknowledges support through the Sofja Kovalevskaja Award to P. Schady from the Alexander von Humboldt Foundation of Germany. AU is grateful for travel funding support through the Max-Planck Inst. for Extraterrestrial Physics. SK and ANG acknowledge support by DFG grant Kl 766/16-1. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Facilities: This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX) under program 087.F- 9301(A). This paper makes use of the following ALMA data: ADS/JAO.ALMA#2011.0.00001.E. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This publication is based on data acquired with the VLT/Xshooter under program 087.A-0055(C), as well as with VLT/FORS2 under program 091.A-0703(A). ; Peer-reviewed ; Publisher Version

We present Hubble Space Telescope (HST) and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star merger detected via gravitational-wave emission by LIGO and Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z - 0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent (less than or similar to 1 Gyr) "dry" merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with less than or similar to 1% of any light arising from a population with ages <500 Myr. Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius (r(e) similar to 3 kpc), providing an r(e)-normalized offset that is closer than similar to 90% of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy. ; We thank the referee for a prompt and highly constructive report that improved the content and clarity of the manuscript. We also thank the editor, Fred Rasio, for helpful comments. Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 099.D-0668 (A.J.L.), and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with programs GO 14771 (N.R.T.), GO 14804 (A.J.L.), and GO 14850 (E.T.). We thank the staff at ESO and STScI for their excellent support of these observations. A.J.L. acknowledges that this project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No. 725246) A.J.L., D.S., and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T., K.W., P.T.O., J.L.O., and S.R. acknowledge support from STFC. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). A.d.U.P., C.T., Z.C., and D.A.K. acknowledge support from the Spanish project AYA 2014-58381-P. Z.C. also acknowledges support from the Juan de la Cierva Incorporacion fellowship IJCI-2014-21669, and D.A.K. from Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. M.I. was supported by the NRFK grant, No. 2017R1A3A3001362. E.T. acknowledges support from grants GO718062A and HSTG014850001A. S.R. has been supported by the Swedish Research Council (VR) under grant number 2016-03657_3, by the Swedish National Space Board under grant number Dnr. 107/16 and by the research environment grant "Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)" funded by the Swedish Research council (VR) under Dnr 2016-06012. P.A.E. acknowledges UKSA support. ; Peer Reviewed

We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo (GW170817) and as a short gamma-ray burst by Fermi Gamma-ray Burst Monitor (GBM) and Integral SPI-ACS (GRB 170817A). The evolution of the transient light is consistent with predictions for the behavior of a "kilonova/macronova" powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide-dominated ejecta, and the much slower evolution in the near-infrared ${K}_{{\rm{s}}}$-band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the third r-process peak (atomic masses $A\approx 195$). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major—if not the dominant—site of rapid neutron capture nucleosynthesis in the universe. ; HST observations were obtained using programs GO 14771 (PI: Tanvir), GO 14804 (PI: Levan), and GO 14850 (PI: Troja). VLT observations were obtained using programs 099.D-0688, 099.D-0116, and 099.D-0622. N.R.T., K.W., P.T.O., J.L.O., and S.R. acknowledge support from STFC. A.J.L., D.S., and J.D.L. acknowledge support from STFC via grant ST/P000495/1. N.R.T. and A.J.L. have received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (grant agreement No. 725246, TEDE, Levan). Ad.U.P., C.T., Z.C., and D.A.K. acknowledge support from the Spanish project AYA 2014-58381-P. Z.C. also acknowledges support from the Juan de la Cierva Incorporacion fellowship IJCI-2014-21669, and D.A.K. from Juan de la Cierva Incorporacion fellowship IJCI-2015-26153. J.H. is supported by a VILLUM FONDEN Investigator grant (project number 16599). P.D.A., S.C., and A.M. acknowledge support from the ASI grant I/004/11/3. S.R. has been supported by the Swedish Research Council (VR) under grant No. 2016-03657_3, by the Swedish National Space Board under grant No. Dnr. 107/16, and by the research environment grant "Gravitational Radiation and Electromagnetic Astrophysical Transients (GREAT)" funded by the Swedish Research council (VR) under Dnr 2016-06012. P.A.E. acknowledges UKSA support. The VISTA observations were processed by C.G.F. at the Cambridge Astronomy Survey Unit (CASU), which is funded by the UK Science and Technology Research Council under grant ST/N005805/1. This research used resources provided by the Los Alamos National Laboratory Institutional Computing Program, which is supported by the U.S. Department of Energy National Nuclear Security Administration under contract No. DE-AC52-06NA25396. Based on observations made with the Nordic Optical Telescope (program 55-013, PI Pian), operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. ; Peer Reviewed