We present data and initial results from VLT/X-Shooter emission-line spectroscopy of 96 galaxies selected by long γ-ray bursts (GRBs) at 0.1 < z < 3.6, the largest sample of GRB host spectra available to date. Most of our GRBs were detected by Swift and 76% are at 0.5 < z < 2.5 with a median z med ∼ 1.6. Based on Balmer and/or forbidden lines of oxygen, nitrogen, and neon, we measure systemic redshifts, star formation rates (SFR), visual attenuations (A V ), oxygen abundances (12 + log(O/H)), and emission-line widths (σ). We study GRB hosts up to z ∼ 3.5 and find a strong change in their typical physical properties with redshift. The median SFR of our GRB hosts increases from SFR med ∼ 0.6 M yr −1 atO ] at higher redshifts leads to an increasing distance of GRB-selected galaxies to the locus of local galaxies in the Baldwin-Phillips-Terlevich diagram. There is weak evidence for a redshift evolution in A V and σ, with the highest values seen at z ∼ 1.5 (A V ) or z ∼ 2 (σ). Oxygen abundances of the galaxies are distributed between 12 + log(O/H) = 7.9 and 12 + log(O/H) = 9.0 with a median 12 + log(O/H) med ∼ 8.5. The fraction of GRB-selected galaxies with super-solar metallicities is ∼20% at z < 1 in the adopted metallicity scale. This is significantly less than the fraction of total star formation in similar galaxies, illustrating that GRBs are scarce in high metallicity environments. At z ∼ 3, sensitivity limits us to probing only the most luminous GRB hosts for which we derive metallicities of Z 0.5 Z . Together with a high incidence of Z ∼ 0.5 Z galaxies at z ∼ 1.5, this indicates that a metallicity dependence at low redshift will not be dominant at z ∼ 3. Significant correlations exist between the hosts' physical properties. Oxygen abundance, for example, relates to A V (12 + log(O/H) ∝ 0.17 · A V ), line width (12 + log(O/H) ∝ σ 0.6 ), and SFR (12 + log(O/H) ∝ SFR 0.2 ). In the last two cases, the normalization of the relations shift to lower metallicities at z > 2 by ∼0.4 dex. These properties of GRB hosts and their evolution with redshift can be understood in a cosmological context of star-forming galaxies and a picture in which the hosts' properties at low redshift are influenced by the tendency of GRBs to avoid the most metal-rich environments.

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After the initial burst of γ-rays that defines a γ-ray burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, during which a forward shock travels outwards and a reverse shock propagates backwards into the oncoming collimated flow, or 'jet'. Light from the reverse shock should be highly polarized if the jet's magnetic field is globally ordered and advected from the central engine, with a position angle that is predicted to remain stable in magnetized baryonic jet models or vary randomly with time if the field is produced locally by plasma or magnetohydrodynamic instabilities. Degrees of linear polarization of P ≈ 10 per cent in the optical band have previously been detected in the early afterglow, but the lack of temporal measurements prevented definitive tests of competing jet models. Hours to days after the γ-ray burst, polarization levels are low (P < 4 per cent), when emission from the shocked ambient medium dominates. Here we report the detection of P =28(+4)(-4) per cent in the immediate afterglow of Swift γ-ray burst GRB 120308A, four minutes after its discovery in the γ-ray band, decreasing to P = 16(+5)(-4) per cent over the subsequent ten minutes. The polarization position angle remains stable, changing by no more than 15 degrees over this time, with a possible trend suggesting gradual rotation and ruling out plasma or magnetohydrodynamic instabilities. Instead, the polarization properties show that GRBs contain magnetized baryonic jets with large-scale uniform fields that can survive long after the initial explosion.

Aims. Long gamma-ray bursts (LGRBs) are associated with massive stars and are therefore linked to star formation. However, the conditions needed for the progenitor stars to produce LGRBs can affect the relation between the LGRB rate and star formation. By using the power of a complete LGRB sample, our long-term aim is to understand whether such a bias exists and, if it does, what its origin is. Methods. To reach our goal we use the Swift/BAT6 complete sample of LGRBs. In this first paper, we build the spectral energy distribution (SED) of the 14 z < 1 host galaxies of the BAT6 LGRB sample and determine their stellar masses (M ) from SED fitting. To investigate the presence of a bias in the LGRB-star formation relation we compare the stellar mass distribution of the LGRB host galaxies (i) with star-forming galaxies observed in deep surveys (UltraVISTA) within the same redshift limit; (ii) with semi-analytical models of the z < 1 star-forming galaxy population; and (iii) with dedicated numerical simulations of LGRB hosts having different metallicity thresholds for the progenitor star environment. Results. We find that at z < 1, LGRBs tend to avoid massive galaxies and are very powerful for selecting a population of faint low-mass star-forming galaxies, partly below the completeness limits of galaxy surveys. The stellar mass distribution of the hosts is not consistent with that of the UltraVISTA star-forming galaxies weighted by their star formation rate (SFR). This implies that, at least at z < 1, LGRBs are not unbiased tracers of star formation. To make the two distributions consistent, a much steeper faint end of the mass function would be required or a very shallow SFR-mass relation for the low-mass galaxy population. The comparison with the GRB host galaxy simulations indicates that, to reproduce the stellar mass distribution, a metallicity threshold of the order of Z th = 0.3−0.5 Z is necessary to form a LGRB. Models without a metallicity threshold or with an extreme threshold of Z th = 0.1 Z are excluded at z < 1. Under a very basic assumption, we estimate that the LGRB rate can directly trace the SFR starting from z ∼ 4 and above. Conclusions. GRB hosts at z < 1 have lower luminosities and stellar masses than expected if LGRBs were unbiased star formation tracers. The use of the Swift/BAT6 complete sample keeps this result from being affected by possible biases that could have influenced past results based on incomplete samples. The preference for low metallicities (Z 0.5 Z ) inferred by the comparison with the simulations can be a consequence of the particular conditions needed for the progenitor star to produce a GRB.Key words. gamma-ray burst: general -galaxies: star formation -galaxies: photometry Appendix A is available in electronic form at http://www.aanda.orgArticle published by EDP Sciences A102, page 1 of 13 A&A 581, A102 (2015)

2Long-duration Gamma-Ray Bursts (GRBs) are an extremely rare outcome of the collapse of massive stars, and are typically found in the distant Universe.Because of its intrinsic luminosity (L ∼ 3 × 10 53 erg s −1 ) and its relative proximity (z = 0.34), GRB 130427A was a unique event that reached the highest fluence observed in the γ-ray band. Here we present a comprehensive multiwavelength view of GRB 130427A with Swift, the 2-m Liverpool and Faulkes telescopes and by other ground-based facilities, highlighting the evolution of the burst emission from the prompt to the afterglow phase. The properties of GRB 130427A are similar to those of the most luminous, high-redshift GRBs,suggesting that a common central engine is responsible for producing GRBs in both the contemporary and the early Universe and over the full range of GRB isotropic energies.GRB 130427A was the brightest burst detected by Swift (1) as well as by several γ-ray detectors onboard other space missions. It was also the brightest and longest burst detected above 100 MeV, with the most energetic photon detected at 95 GeV (2). It was detected by Fermi-GBM (3) at T 0,GBM = 07:47:06.42 UT on April 27 2013. Hereafter this time will be our reference time T 0 . The Burst Alert Telescope (BAT, (4)) onboard Swift triggered on GRB 130427Aat t = 51.1 s, when Swift completed a pre-planned slew. The Swift slew to the source started at t = 148 s and ended at t = 192 s. The Swift UltraViolet Optical Telescope (UVOT, (5)) began observations at t = 181 s while observations by the Swift X-ray Telescope (XRT, (6)) started at t = 195 s (see (7) for more details). The structure of the γ-ray light curve revealed by the Swift-BAT in the 15-350 keV band ( Fig. 1) can be divided in three main episodes: an initial peak, beginning at t = 0.1 s and peaking at t = 0.5 s; a second large peak showing a complex 3 structure with a duration of ∼ 20 s and a third, much weaker episode, starting at t ∼120 s showing a fast rise/exponential decay behavior. The overall duration of the prompt emission was T 90(15−150 keV) = 276 ± 5 s (i.e. the time containing 90% of the fluence) calculated over the first 1830 s of BAT observation from T 0,GBM . During the early phases of the γ-ray emission strong spectral variability is observed (Fig. 1). A marked spectral hardening is observed during is (2.68 ± 0.01) × 10 −3 erg cm −2 , with a spectrum peaking at E peak = 1028 ± 8 keV, while the fluence of the emission episode at (120 -250 s) is ∼ 9 × 10 −5 erg cm −2 , with a spectrum peaking at ∼240 keV (9).This event was extremely bright also in the optical and it was immediately detected by various robotic telescopes: in particular, the Raptor robotic telescope detected a bright optical counterpart already at t = 0.5 s (10). Optical spectroscopy of the afterglow determined the redshift to be z = 0.34 (11); an UVOT UV grism spectrum (7) was also acquired. At this distance the rest frame 1 keV-10 MeV isotropic energy is E iso = 8.1 × 10 53 erg and the peak luminosity is L iso = 2.7 × 10 53 erg s −1 . Acc...

Aims. Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars and might therefore be a potentially powerful tool for tracing cosmic star formation. However, especially at low redshifts (z < 1.5) LGRBs seem to prefer particular types of environment. Our aim is to study the host galaxies of a complete sample of bright LGRBs to investigate the effect of the environment on GRB formation. Methods. We studied host galaxy spectra of the Swift/BAT6 complete sample of 14 z < 1 bright LGRBs. We used the detected nebular emission lines to measure the dust extinction, star formation rate (SFR), and nebular metallicity (Z) of the hosts and supplemented the data set with previously measured stellar masses M . The distributions of the obtained properties and their interrelations (e.g. mass-metallicity and SFR-M relations) are compared to samples of field star-forming galaxies. Results. We find that LGRB hosts at z < 1 have on average lower SFRs than if they were direct star formation tracers. By directly comparing metallicity distributions of LGRB hosts and star-forming galaxies, we find a good match between the two populations up to 12 + log O H ∼ 8.4−8.5, after which the paucity of metal-rich LGRB hosts becomes apparent. The LGRB host galaxies of our complete sample are consistent with the mass-metallicity relation at similar mean redshift and stellar masses. The cutoff against high metallicities (and high masses) can explain the low SFR values of LGRB hosts. We find a hint of an increased incidence of starburst galaxies in the Swift/BAT6 z < 1 sample with respect to that of a field star-forming population. Given that the SFRs are low on average, the latter is ascribed to low stellar masses. Nevertheless, the limits on the completeness and metallicity availability of current surveys, coupled with the limited number of LGRB host galaxies, prevents us from investigating more quantitatively whether the starburst incidence is such as expected after taking into account the high-metallicity aversion of LGRB host galaxies.

Aims. Long gamma-ray bursts (LGRB) have been proposed as promising tracers of star formation owing to their association with the core-collapse of massive stars. Nonetheless, previous studies we carried out at z < 1 support the hypothesis that the conditions necessary for the progenitor star to produce an LGRB (e.g. low metallicity), were challenging the use of LGRBs as star-formation tracers, at least at low redshift. The goal of this work is to characterise the population of host galaxies of LGRBs at 1 < z < 2, investigate the conditions in which LGRBs form at these redshifts and assess their use as tracers of star formation. Methods. We performed a spectro-photometric analysis to determine the stellar mass, star formation rate, specific star formation rate and metallicity of the complete, unbiased host galaxy sample of the Swift/BAT6 LGRB sample at 1 < z < 2. We compared the distribution of these properties to the ones of typical star-forming galaxies from the MOSDEF and COSMOS2015 Ultra Deep surveys, within the same redshift range. Results. We find that, similarly to z < 1, LGRBs do not directly trace star formation at 1 < z < 2, and they tend to avoid highmass, high-metallicity host galaxies. We also find evidence for an enhanced fraction of starbursts among the LGRB host sample with respect to the star-forming population of galaxies. Nonetheless we demonstrate that the driving factor ruling the LGRB efficiency is metallicity. The LGRB host distributions can be reconciled with the ones expected from galaxy surveys by imposing a metallicity upper limit of 12 + log(O/H)∼8.55. We can determine upper limits on the fraction of super-solar metallicity LGRB host galaxies of ∼ 20%, 10% at z < 1, 1 < z < 2, respectively. Conclusions. Metallicity rules the LGRB production efficiency, which is stifled at Z 0.7 Z . Under this hypothesis we can expectLGRBs to trace star formation at z > 3, once the bulk of the star forming galaxy population are characterised by metallicities below this limit. The role played by metallicity can be explained by the conditions necessary for the progenitor star to produce an LGRB. The moderately high metallicity threshold found is in agreement with the conditions necessary to rapidly produce a fast-rotating Wolf-Rayet stars in close binary systems, and could be accommodated by single star models under chemically homogeneous mixing with very rapid rotation and weak magnetic coupling.

We use a parent sample of 118 gamma-ray burst (GRB) afterglows, with known redshift and host galaxy extinction, to separate afterglows with and without signatures of dominant reverse-shock emission and to determine which physical conditions lead to a prominent reverse-shock emission. We identify 10 GRBs with reverse shock signatures -GRBs 990123, 021004, 021211, 060908, 061126, 080319B, 081007, 090102, 090424 and 130427A. By modeling their optical afterglows with reverse and forward shock analytic light curves and using Monte Carlo simulations, we estimate the parameter space of the physical quantities describing the ejecta and circumburst medium. We find that physical properties cover a wide parameter space and do not seem to cluster around any preferential values. Comparing the rest-frame optical, X-ray and high-energy properties of the larger sample of non-RS-dominated GRBs, we show that the early-time (< 1ks) optical spectral luminosity, X-ray afterglow luminosity and γ-ray energy output of our reverse-shock dominated sample do not differ significantly from the general population at early times. However, the GRBs with dominant reverse shock emission have fainter than average optical forward-shock emission at late time (> 10 ks). We find that GRBs with an identifiable reverse shock component show high magnetization parameter R B = ε B,r /ε B,f ∼ 2 − 10 4 . Our results are in agreement with the mildly magnetized baryonic jet model of GRBs.