Calibration of EFOSC2 Broadband Linear Imaging Polarimetry

The European Southern Observatory Faint Object Spectrograph and Camera v2 is one of the workhorse instruments on ESO’s New Technology Telescope, and is one of the most popular instruments at La Silla observatory. It is mounted at a Nasmyth focus, and therefore exhibits strong, wavelength and pointing-direction-dependent instrumental polarisation. In this document, we describe our efforts to calibrate the broadband imaging polarimetry mode, and provide a calibration for broadband B, V, and R filters to a level that satisfies most use cases (i.e. polarimetric calibration uncertainty ~0.1%). We make our calibration codes public. This calibration effort can be used to enhance the yield of future polarimetric programmes with the European Southern Observatory Faint Object Spectrograph and Camera v2, by allowing good calibration with a greatly reduced number of standard star observations. Similarly, our calibration model can be combined with archival calibration observations to post-process data taken in past years, to form the European Southern Observatory Faint Object Spectrograph and Camera v2 legacy archive with substantial scientific potential

The late peaking afterglow of GRB 100418A

GRB 100418A is a long gamma-ray burst (GRB) at redshift z = 0.6235 discovered with the Swift Gamma-ray Burst Explorer with unusual optical and X-ray light curves. After an initial short-lived, rapid decline in X-rays, the optical and X-ray light curves observed with Swift are approximately flat or rising slightly out to at least ~7 × 10[superscript: 3] s after the trigger, peak at ~5 × 10[superscript: 4] s, and then follow an approximately power-law decay. Such a long optical plateau and late peaking is rarely seen in GRB afterglows. Observations with Rapid Eye Mount during a gap in the Swift coverage indicate a bright optical flare at ~2.5 × 10[superscript: 4] s. The long plateau phase of the afterglow is interpreted using either a model with continuous injection of energy into the forward shock of the burst or a model in which the jet of the burst is viewed off-axis. In both models the isotropic kinetic energy in the late afterglow after the plateau phase is ≥10[superscript: 2] times the 10[superscript: 51] erg of the prompt isotropic gamma-ray energy release. The energy injection model is favored because the off-axis jet model would require the intrinsic T [subscript: 90] for the GRB jet viewed on-axis to be very short, ~10 ms, and the intrinsic isotropic gamma-ray energy release and the true jet energy to be much higher than the typical values of known short GRBs. The non-detection of a jet break up to t ~ 2 × 10[superscript: 6] s indicates a jet half-opening angle of at least ~14°, and a relatively high-collimation-corrected jet energy of E [subscript: jet] ≥ 10[superscript: 52] erg

Accessing the population of high-redshift Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are a powerful probe of the high-redshift Universe. We present a tool to estimate the detection rate of high-z GRBs by a generic detector with defined energy band and sensitivity. We base this on a population model that reproduces the observed properties of GRBs detected by Swift, Fermi and CGRO in the hard X-ray and γ-ray bands. We provide the expected cumulative distributions of the flux and fluence of simulated GRBs in different energy bands. We show that scintillator detectors, operating at relatively high energies (e.g. tens of keV to the MeV), can detect only the most luminous GRBs at high redshifts due to the link between the peak spectral energy and the luminosity (Epeak–Liso) of GRBs. We show that the best strategy for catching the largest number of high-z bursts is to go softer (e.g. in the soft X-ray band) but with a very high sensitivity. For instance, an imaging soft X-ray detector operating in the 0.2–5 keV energy band reaching a sensitivity, corresponding to a fluence, of ∼10−8 erg cm−2 is expected to detect ≈40 GRBs yr−1 sr−1 at z ≥ 5 (≈3 GRBs yr−1 sr−1 at z ≥ 10). Once high-z GRBs are detected the principal issue is to secure their redshift. To this aim we estimate their NIR afterglow flux at relatively early times and evaluate the effectiveness of following them up and construct usable samples of events with any forthcoming GRB mission dedicated to explore the high-z Universe

Varying linear polarisation in the dust-free GRB 210610B

Long gamma ray bursts (GRBs) are produced by the collapse of some very massive stars, which emit ultra-relativistic jets. When the jets collide with the interstellar medium they decelerate and generate the so-called afterglow emission, which has been observed to be polarised. In this work we study the polarimetric evolution of GRB 210610B afterglow, at z=1.1341. This allows to evaluate the role of geometric and/or magnetic mechanisms in the GRB afterglow polarisation. We observed GRB 210610B using imaging polarimetry with CAFOS on the 2.2 m Calar Alto Telescope and FORS2 on the 4 × 8.1 m Very Large Telescope. Complementary optical spectroscopy was obtained with OSIRIS on the 10.4 m Gran Telescopio Canarias. We study the GRB light-curve from X-rays to optical bands and the Spectral Energy Distribution (SED). This allows us to strongly constrain the line-of-sight extinction. Finally, we study the GRB host galaxy using optical/NIR data to fit the SED and derive its integrated properties. GRB 210610B had a bright afterglow with a negligible line-of-sight extinction. Polarimetry was obtained at three epochs: during an early plateau phase, at the time when the light curve breaks, and after the light curve steepened. We observe an initial polarisation of ∼4% that goes to zero at the time of the break, and then increases again to ∼2% with a change of the position angle of 54±9 deg. The spectrum show features with very low equivalent widths, indicating a small amount of material in the line-of-sight within the host. The lack of dust and the low amount of material on the line-of-sight to GRB 210610B allow us to study the intrinsic polarisation of the GRB optical afterglow. We find the GRB polarisation signals are consistent with ordered magnetic fields in refreshed shock or/and hydrodynamics-scale turbulent fields in the forward shock.</p

The Infra-Red Telescope on board the THESEUS mission

The Infra-Red Telescope (IRT) on board the Transient High Energy Sky and Early Universe Surveyor (THESEUS) ESA M5 candidate mission will play a key role in identifying and characterizing moderate to high redshift Gamma-Ray Bursts afterglows. The IRT is the enabling instrument on board THESEUS for measuring autonomously the redshift of the several hundreds of GRBs detected per year by the Soft X-ray Imager (SXI) and the X- and Gamma-Ray Imaging Spectrometer (XGIS), and thus allowing the big ground based telescopes to be triggered on a redshift pre-selected sample, and finally fulfilling the cosmological goals of the mission. The IRT will be composed by a primary mirror of 0.7 m of diameter coupled to a single camera in a Cassegrain design. It will work in the 0.7-1.8 {\mu}m wavelength range, and will provide a 10x10 arc min imaging field of view with sub-arc second localization capabilities, and, at the same time, a 5x5 arc min field of view with moderate (R up to ~500) spectroscopic capabilities. Its sensitivity, mainly limited by the satellite jitter, is adapted to detect all the GRBs, localized by the SXI/XGIS, and to acquire spectra for the majority of them

Mass and metallicity scaling relations of high redshift star-forming galaxies selected by GRBs

We present a comprehensive study of the relations between gas kinematics, metallicity, and stellar mass in a sample of 82 GRB-selected galaxies using absorption and emission methods. We find the velocity widths of both emission and absorption profiles to be a proxy of stellar mass. We also investigate the velocity-metallicity correlation and its evolution with redshift and find the correlation derived from emission lines to have a significantly smaller scatter compared to that found using absorption lines. Using 33 GRB hosts with measured stellar mass and metallicitiy, we study the mass-metallicity relation for GRB host galaxies in a stellar mass range of $10^{8.2} M_{\odot}$ to $10^{11.1} M_{\odot}$ and a redshift range of $z\sim 0.3-3.4$. The GRB-selected galaxies appear to track the mass-metallicity relation of star forming galaxies but with an offset of 0.15 towards lower metallicities. This offset is comparable with the average error-bar on the metallicity measurements of the GRB sample and also the scatter on the MZ relation of the general population. It is hard to decide whether this relatively small offset is due to systematic effects or the intrinsic nature of GRB hosts. We also investigate the possibility of using absorption-line metallicity measurements of GRB hosts to study the mass-metallicity relation at high redshifts. Our analysis shows that the metallicity measurements from absorption methods can significantly differ from emission metallicities and assuming identical measurements from the two methods may result in erroneous conclusions

The luminous, massive and solar metallicity galaxy hosting the Swift gamma-ray burst, GRB 160804A at z = 0.737

We here present the spectroscopic follow-up observations with VLT/X-shooter of the Swift long-duration gamma-ray burst GRB 160804A atz = 0.737. Typically, GRBs are found in lowmass, metal-poor galaxies that constitute the sub-luminous population of star-forming galaxies. For the host galaxy of the GRB presented here, we derive a stellar mass of log (M∗/ M) = 9.80 ± 0.07, a roughly solar metallicity (12 + log (O/H) = 8.74 ± 0.12) based on emission line diagnostics, and an infrared luminosity of M3.6/(1 + z) = −21.94 mag, but find it to be dust-poor (E(B − V) < 0.05 mag). This establishes the galaxy hosting GRB 160804A as one of the most luminous, massive and metal-rich GRB hosts at z < 1.5. Furthermore, the gasphase metallicity is found to be representative of the physical conditions of the gas close to the explosion site of the burst. The high metallicity of the host galaxy is also observed in absorption, where we detect several strong Fe II transitions as well as Mg II and Mg I. Although host galaxy absorption features are common in GRB afterglow spectra, we detect absorption from strong metal lines directly in the host continuum (at a time when the afterglow was contributing to <15 per cent). Finally, we discuss the possibility that the geometry and state of the absorbing and emitting gas are indicative of a galactic scale outflow expelled at the final stage of two merging galaxies

GRB 050410 and GRB 050412: Are they really dark gamma-ray bursts?

Aims.We present a detailed analysis of the prompt and afterglow emission of GRB 050410 and GRB 050412 detected by Swift for which no optical counterpart was observed. Methods.We analysed data from the prompt emission detected by the Swift BAT and from the early phase of the afterglow obtained by the Swift narrow field instrument XRT. Results.The 15-150 keV energy distribution of the GRB 050410 prompt emission shows a peak energy at 53 -21+40 keV. The XRT light curve of this GRB decays as a power law with a slope of $\alpha=$ 1.06 $\pm$ 0.04. The spectrum is well reproduced by an absorbed power law with a spectral index $\Gamma_{\rm x}=2.4$ $\pm$ 0.4 and a low energy absorption $N_{\rm H}$ = 4 +3-2 $\times$ 1021 cm-2 which is higher than the Galactic value. The 15-150 keV prompt emission in GRB 050412 is modelled with a hard ($\Gamma$ = 0.7 $\pm$ 0.2) power law. The XRT light curve follows a broken power law with the first slope $\alpha_1$ = 0.7 $\pm$ 0.4, the break time $T_{\rm break}$ = 254 -41+79 s and the second slope $\alpha_2$ = 2.8 -0.8+0.5. The spectrum is fitted by a power law with spectral index $\Gamma_{\rm x}=1.3$ $\pm$ 0.2 which is absorbed at low energies by the Galactic column. Conclusions.The GRB 050410 afterglow light curve reveals the expected characteristics of the third component of the canonical Swift light curve. Conversely, a complex phenomenology was detected in the GRB 050412 because of the presence of the very early break. The light curve in this case can be interpreted as being the last peak of the prompt emission. The two bursts present tight upper limits for the optical emission, however, neither of them can be clearly classified as dark. For GRB 050410, the suppression of the optical afterglow could be attributed to a low density interstellar medium surrounding the burst. For GRB 050412, the evaluation of the darkness is more difficult due to the ambiguity in the extrapolation of the X-ray afterglow light curve

Highly-ionized metals as probes of the circumburst gas in the natal regions of gamma-ray bursts

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 NV 1238,1242 line transitions, but we also discuss other high-ionization lines such as OVI, CIV and SiIV. We find no correlation between the column density of NV and the neutral gas properties such as metallicity, HI column density and dust depletion, however the relative velocity of NV, typically a blueshift with respect to the neutral gas, is found to be correlated with the column density of HI. This may be explained if the NV gas is part of an HII 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 2-sigma significance) that the X-ray derived column density, N_H,X, may be correlated with the column density of NV, 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 NV (and also OVI) 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

A Precise Distance to the Host Galaxy of the Binary Neutron Star Merger GW170817 Using Surface Brightness Fluctuations

The joint detection of gravitational waves (GWs) and electromagnetic radiation from the binary neutron star (BNS) merger GW170817 has provided unprecedented insight into a wide range of physical processes: heavy element synthesis via the r-process; the production of relativistic ejecta; the equation of state of neutron stars and the nature of the merger remnant; the binary coalescence timescale; and a measurement of the Hubble constant via the "standard siren" technique. In detail, all of these results depend on the distance to the host galaxy of the merger event, NGC 4993. In this Letter we measure the surface brightness fluctuation (SBF) distance to NGC 4993 in the F110W and F160W passbands of the Wide Field Camera 3 Infrared Channel (WFC3/IR) on the Hubble Space Telescope (HST). For the preferred F110W passband we derive a distance modulus of $(m-M)\,=33.05\pm 0.08\pm 0.10$ mag, or a linear distance d = 40.7 ± 1.4 ± 1.9 Mpc (random and systematic errors, respectively); a virtually identical result is obtained from the F160W data. This is the most precise distance to NGC 4993 available to date. Combining our distance measurement with the corrected recession velocity of NGC 4993 implies a Hubble constant H 0 = 71.9 ± 7.1 km s−1 Mpc−1. A comparison of our result to the GW-inferred value of H 0 indicates a binary orbital inclination of i gsim 137°. The SBF technique can be applied to early-type host galaxies of BNS mergers to ~100 Mpc with HST and possibly as far as ~300 Mpc with the James Webb Space Telescope, thereby helping to break the inherent distance-inclination degeneracy of the GW data at distances where many future BNS mergers are likely to be detected