X-shooter is the first 2nd generation instrument of the ESO Very Large Telescope (VLT). It is a very efficient, single-target, intermediate-resolution spectrograph that was installed at the Cassegrain focus of UT2 in 2009. The instrument covers, in a single exposure, the spectral range from 300 to 2500 nm. It is designed to maximize the sensitivity in this spectral range through dichroic splitting in three arms with optimized optics, coatings, dispersive elements and detectors. It operates at intermediate spectral resolution (R ∼ 4000−17 000, depending on wavelength and slit width) with fixed échelle spectral format (prism cross-dispersers) in the three arms. It includes a 1.8 × 4 integral field unit as an alternative to the 11 long slits. A dedicated data reduction package delivers fully calibrated two-dimensional and extracted spectra over the full wavelength range. We describe the main characteristics of the instrument and present its performance as measured during commissioning, science verification and the first months of science operations.

Optical and near-infrared observations of the gamma-ray burst GRB 031203, at z = 0.1055, are reported. A very faint afterglow is detected superimposed to the host galaxy in our first infrared JHK observations, carried out ∼ 9 hours after the burst. Subsequently, a rebrightening is detected in all bands, peaking in the R band about 18 rest-frame days after the burst. The rebrightening closely resembles the light curve of a supernova like SN 1998bw, assuming that the GRB and the SN went off almost simultaneously, but with a somewhat slower evolution. Spectra taken close to the maximum of the rebrightening show extremely broad features as in SN 1998bw. The determination of the absolute magnitude of this SN (SN 2003lw) is difficult owing to the large and uncertain extinction, but likely this event was brighter than SN 1998bw by 0.5 mag in the V RI bands, reaching an absolute magnitude M V = −19.75 ± 0.15.

Abstract. We report the high S/N observation on October 3, 2002 with XMM-Newton of the brightest X-ray flare detected so far from Sgr A* with a duration shorter than one hour (∼2.7 ks). The light curve is almost symmetrical with respect to the peak flare, and no significant difference between the soft and hard X-ray range is detected. The overall flare spectrum is well represented by an absorbed power-law with a soft photon spectral index of Γ = 2.5 ± 0.3, and a peak 2-10 keV luminosity of 3.6 +0.3 −0.4 × 10 35 erg s −1 , i.e. a factor 160 higher than the Sgr A* quiescent value. No significant spectral change during the flare is observed. This X-ray flare is very different from other bright flares reported so far: it is much brighter and softer. The present accurate determination of the flare characteristics challenge the current interpretation of the physical processes occuring inside the very close environment of Sgr A* by bringing very strong constraints for the theoretical flare models.

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.

We present comprehensive multiwavelength observations of three gamma-ray bursts (GRBs) with durations of several thousand seconds. We demonstrate that these events are extragalactic transients; in particular we resolve the long-standing conundrum of the distance of GRB 101225A (the "Christmasday burst"), finding it to have a redshift z = 0.847, and showing that two apparently similar events (GRB 111209A and GRB 121027A) lie at z = 0.677 and z = 1.773 respectively. The systems show extremely unusual X-ray and optical lightcurves, very different from classical GRBs, with long lasting highly variable X-ray emission and optical light curves that exhibit little correlation with the behaviour seen in the X-ray. Their host galaxies are faint, compact, and highly star forming dwarf galaxies, typical of "blue compact galaxies". We propose that these bursts are the prototypes of a hitherto largely unrecognized population of ultra-long GRBs, that while observationally difficult to detect may be astrophysically relatively common. The long durations may naturally be explained by the engine driven explosions of stars of much larger radii than normally considered for GRB progenitors which are thought to have compact Wolf-Rayet progenitor stars. However, we cannot unambiguously identify supernova signatures within their light curves or spectra. We also consider the alternative possibility that they arise from the tidal disruption of stars by supermassive black holes.

We report the Swift discovery of the nearby long, soft gamma-ray burst GRB 100316D, and the subsequent unveiling of its low-redshift host galaxy and associated supernova. We derive the redshift of the event to be z = 0.0591 ± 0.0001 and provide accurate astrometry for the gamma-ray burst (GRB) supernova (SN). We study the extremely unusual prompt emission with time-resolved γ -ray to X-ray spectroscopy and find that the spectrum is best modelled with a thermal component in addition to a synchrotron emission component with a low peak energy. The X-ray light curve has a remarkably shallow decay out to at least 800 s. The host is a bright, blue galaxy with a highly disturbed morphology and we use Gemini-South, Very Large Telescope and Hubble Space Telescope observations to measure some of the basic host galaxy properties. We compare and contrast the X-ray emission and host galaxy of GRB 100316D to a subsample of GRB-SNe. GRB 100316D is unlike the majority of GRB-SNe in its X-ray evolution, but resembles rather GRB 060218, and we find that these two events have remarkably similar high energy prompt emission properties. Comparison of the host galaxies of GRB-SNe demonstrates, however, that there is a great diversity in the environments in which GRB-SNe can be found. GRB 100316D is an important addition to the currently sparse sample of spectroscopically confirmed GRB-SNe, from which a better understanding of long GRB progenitors and the GRB-SN connection can be gleaned.

Many X-ray binaries remain undetected in the mid-infrared, a regime where emission from their compact jets is likely to dominate. Here, we report the detection of the black hole binary GX 339-4 with the Widefield Infrared Survey Explorer (WISE) during a very bright, hard accretion state in 2010. Combined with a rich contemporaneous multiwavelength dataset, clear spectral curvature is found in the infrared, associated with the peak flux density expected from the compact jet. An optically-thin slope of ∼-0.7 and a jet radiative power of >6×10 35 erg s −1 (d/8 kpc) 2 are measured. A ∼24 h WISE light curve shows dramatic variations in mid-infrared spectral slope on timescales at least as short as the satellite orbital period ∼95 mins. There is also significant change during one pair of observations spaced by only 11 s. These variations imply that the spectral break associated with the transition from self-absorbed to optically-thin jet synchrotron radiation must be varying across the full wavelength range of ∼3-22 µm that WISE is sensitive to, and more. Based on fourband simultaneous mid-infrared detections, the break is constrained to frequencies of ≈4.6 +3.5 −2.0 ×10 13 Hz in at least two epochs of observation, consistent with a magnetic field B≈1.5(±0.8)×10 4 G assuming a single-zone synchrotron emission region. The observed variability implies that either B, or the size of the acceleration zone above the jet base, are being modulated by factors of ∼10 on relatively-short timescales.

We have initiated a survey using the newly commissioned X‐shooter spectrograph to target candidate relatively metal‐rich damped Lyα absorbers (DLAs). Our rationale is that high‐metallicity DLAs due to the luminosity–metallicity relation likely will have the most luminous galaxy counterparts. In addition, the spectral coverage of X‐shooter allows us to search for not only Lyα emission, but also rest‐frame optical emission lines. We have chosen DLAs where the strongest rest‐frame optical lines ([O ii], [O iii], Hβ and Hα) fall in the near‐infrared atmospheric transmission bands. In this first paper resulting from the survey, we report on the discovery of the galaxy counterpart of the zabs= 2.354 DLA towards the z= 2.926 quasar Q 2222−0946. This DLA is amongst the most metal‐rich z > 2 DLAs studied so far at comparable redshifts and there is evidence for substantial depletion of refractory elements on to dust grains. We measure metallicities from Zn ii, Si ii, Ni ii, Mn ii and Fe ii of −0.46 ± 0.07, − 0.51 ± 0.06, − 0.85 ± 0.06, − 1.23 ± 0.06 and −0.99 ± 0.06, respectively. The galaxy is detected in the Lyα, [O iii]λλ4959, 5007 and Hα emission lines at an impact parameter of about 0.8 arcsec (6 kpc at zabs= 2.354). Based on the Hα line, we infer a star formation rate of 10 M⊙ yr−1, which is a lower limit due to the possibility of slit loss. Compared to the recently determined Hα luminosity function for z= 2.2 galaxies, the DLA‐galaxy counterpart has a luminosity of L∼ 0.1L*Hα. The emission‐line ratios are 4.0 (Lyα/Hα) and 1.2 ([O iii]/Hα). In particular, the Lyα line shows clear evidence for resonant scattering effects, namely an asymmetric, redshifted (relative to the systemic redshift) component and a much weaker blueshifted component. The fact that the blueshifted component is relatively weak indicates the presence of a galactic wind. The properties of the galaxy counterpart of this DLA are consistent with the prediction that metal‐rich DLAs are associated with the most luminous of the DLA‐galaxy counterparts.