We present optical, near-infrared, and radio observations of the afterglow of GRB 120521C. By modeling the multi-wavelength dataset, we derive a photometric redshift of z ≈ 6.0, which we confirm with a low signal-to-noise ratio spectrum of the afterglow. We find that a model with a constant-density environment provides a good fit to the afterglow data, with an inferred density of n 0.05 cm −3 . The radio observations reveal the presence of a jet break at t jet ≈ 7 d, corresponding to a jet opening angle of θ jet ≈ 3 • . The beaming-corrected γ-ray and kinetic energies are E γ ≈ E K ≈ 3 × 10 50 erg. We quantify the uncertainties in our results using a detailed Markov Chain Monte Carlo analysis, which allows us to uncover degeneracies between the physical parameters of the explosion. To compare GRB 120521C to other high-redshift bursts in a uniform manner we re-fit all available afterglow data for the two other bursts at z 6 with radio detections (GRBs 050904 and 090423). We find a jet break at t jet ≈ 15 d for GRB 090423, in contrast to previous work. Based on these three events, we find that GRBs at z 6 appear to explode in constant-density environments, and exhibit a wide range of energies and densities that span the range inferred for lower redshift bursts. On the other hand, we find a hint for narrower jets in the z 6 bursts, potentially indicating a larger true event rate at these redshifts. Overall, our results indicate that long GRBs share a common progenitor population at least to z ∼ 8.

We report the discovery of six new magnetar counterpart candidates from deep near-infrared Hubble Space Telescope imaging. The new candidates are among a sample of nineteen magnetars for which we present HST data obtained between 2018–2020. We confirm the variability of previously established near-infrared counterparts, and newly identify candidates for PSR J1622-4950, Swift J1822.3-1606, CXOU J171405.7-381031, Swift J1833-0832, Swift J1834.9-0846 and AX J1818.8-1559 based on their proximity to X-ray localisations. The new candidates are compared with the existing counterpart population in terms of their colours, magnitudes, and near-infrared to X-ray spectral indices. We find two candidates for AX J1818 which are both consistent with previously established counterparts. The other new candidates are likely to be chance alignments, or otherwise have a different origin for their near-infrared emission not previously seen in magnetar counterparts. Further observations and studies of these candidates are needed to firmly establish their nature.

Short gamma-ray bursts (SGRBs) are produced by the coalescence of compact binary systems which are remnants of massive stars. GRB 160410A is classified as a short-duration GRB with extended emission and is currently the farthest SGRB with a redshift determined from an afterglow spectrum and also one of the brightest SGRBs to date. The fast reaction to the Neil Gehrels Swift Observatory alert allowed us to obtain a spectrum of the afterglow using the X-shooter spectrograph at the Very Large Telescope (VLT). The spectrum shows several absorption features at a redshift of z = 1.7177, in addition, we detect two intervening systems at z = 1.581 and z = 1.444. The spectrum shows Lyα in absorption with a column density of log (N(HI)/cm2) = 21.2 ± 0.2 which, together with Fe ii, C ii, Si ii, Al iiand O i, allow us to perform the first study of chemical abundances in a SGRB host galaxy. We determine a metallicity of [X/H] = −2.3 ± 0.2 for Fe ii and −2.5 ± 0.2 for Si ii and no dust depletion. We also find no evidence for extinction in the afterglow Spectral Energy Distribution (SED) modeling. The environment has a low degree of ionisation and the C iv and Si iv lines are completely absent. We do not detect an underlying host galaxy down to deep limits. Additionally, we compare GRB 160410A to GRB 201221D, another high-z short GRB that shows absorption lines at z = 1.045 and an underlying massive host galaxy.

The study of the properties of galaxies in the first billion years after the Big Bang is one of the major topics of current astrophysics. Optical and near-infrared spectroscopy of the afterglows of long gamma-ray bursts (GRBs) provides a powerful diagnostic tool to probe the interstellar medium (ISM) of their host galaxies and foreground absorbers, even up to the highest redshifts. We analyze the VLT/X-shooter afterglow spectrum of GRB 210905A, triggered by the Neil Gehrels Swift Observatory, and detect neutral hydrogen, low-ionization, high-ionization, and fine-structure absorption lines from a complex system at z = 6.3118, which we associate with the GRB host galaxy. We use them to study the ISM properties of the host system, revealing the metallicity, kinematics, and chemical abundance pattern of its gas along the GRB line of sight. We also detect absorption lines from at least two foreground absorbers at z = 5.7390 and z = 2.8296. The total metallicity of the z ∼ 6.3 system is [M/H]tot = −1.72 ± 0.13, after correcting for dust depletion and taking α-element enhancement into account, as suggested by our analysis. This is consistent with the values found for the other two GRBs at z ∼ 6 with spectroscopic data showing metal absorption lines (GRB 050904 and GRB 130606A), and it is at the higher end of the metallicity distribution of quasar damped Lyman-α systems (QSO-DLAs) extrapolated to such a high redshift. In addition, we determine the overall amount of dust and dust-to-metal mass ratio (DTM) ([Zn/Fe]fit = 0.33 ± 0.09 and DTM = 0.18 ± 0.03). We find indications of nucleosynthesis due to massive stars and, for some of the components of the gas clouds, we find evidence of peculiar nucleosynthesis, with an overabundance of aluminum (as also found for GRB 130606A). From the analysis of fine-structure lines, we determine distances of several kiloparsecs for the low-ionization gas clouds closest to the GRB. Those are farther distances than usually found for GRB host absorption systems, possibly due to the very high number of ionizing photons produced by the GRB that could ionize the line of sight up to several hundreds of parsecs. Using the HST/F140W image of the GRB field, we show the GRB host galaxy (with a possible afterglow contamination) as well as multiple objects within 2″ from the GRB position. We discuss the galaxy structure and kinematics that could explain our observations, also taking into account a tentative detection of Lyman-α emission at z = 6.3449 (∼1200 km s−1 from the GRB redshift in velocity space), and the observational properties of Lyman-α emitters at very high redshift. This study shows the amazing potential of GRBs to access detailed information on the properties (metal enrichment, gas kinematic, dust content, nucleosynthesis...) of very high-redshift galaxies, independently of the galaxy luminosity. Deep spectroscopic observations with VLT/MUSE and JWST will offer the unique possibility of combining the information presented in this paper with the properties of the ionized gas, with the goal of better understanding how galaxies in the reionization era form and evolve.

It is well established that magnetars are neutron stars with extreme magnetic fields and young ages, but the evolutionary pathways to their creation are still uncertain. Since most massive stars are in binaries, if magnetars are a frequent result of core-collapse supernovae, some fraction are expected to have a bound companion at the time of observation. In this paper, we utilize literature constraints, including deep Hubble Space Telescope imaging, to search for bound stellar companions to magnetars. The magnitude and colour measurements are interpreted in the context of binary population synthesis predictions. We find two candidates for stellar companions associated with CXOU J171405.7–381031 and SGR 0755–2933, based on their J-H colours and H-band absolute magnitudes. Overall, the proportion of the Galactic magnetar population with a plausibly stellar near-infrared counterpart candidate, based on their magnitudes and colours, is between 5 and 10 per cent. This is consistent with a population synthesis prediction of 5 per cent, for the fraction of core-collapse neutron stars arising from primaries which remain bound to their companion after the supernova. These results are therefore consistent with magnetars being drawn in an unbiased way from the natal core-collapse neutron star population, but some contribution from alternative progenitor channels cannot be ruled out.