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

GRB070125 and the environments of spectral-line poor afterglow absorbers

GRB 070125 is among the most energetic bursts detected and the most extensively observed so far. Nevertheless, unresolved issues are still open in the literature on the physics of the afterglow and on the gamma-ray burst (GRB) environment. In particular, GRB 070125 was claimed to have exploded in a galactic halo environment, based on the uniqueness of the optical spectrum and the non-detection of an underlying host galaxy. In this work we collect all publicly available data and address these issues by modelling the near-infrared to X-ray spectral energy distribution (SED) and studying the high signal-to-noise ratio Very Large Telescope/FOcal Reducer/low dispersion Spectrograph afterglow spectrum in comparison with a larger sample of GRB absorbers. The SED reveals a synchrotron cooling break in the ultraviolet, low equivalent hydrogen column density and little reddening caused by a Large Magellanic Cloud type or Small Magellanic Cloud type extinction curve. From the weak Mg ii absorption at z= 1.5477 in the spectrum, we derived log N(Mg ii) = 12.96+0.13− 0.18 and upper limits on the ionic column density of several metals. These suggest that the GRB absorber is most likely a Lyman limit system with a 0.03 < Z < 1.3 Z⊙ metallicity. The comparison with other GRB absorbers places GRB 070125 at the low end of the absorption-line equivalent width distribution, confirming that weak spectral features and spectral-line poor absorbers are not so uncommon in afterglow spectra. Moreover, we show that the effect of photoionization on the gas surrounding the GRB, combined with a low N(H i) along a short segment of the line of sight within the host galaxy, can explain the lack of spectral features in GRB 070125. Finally, the non-detection of an underlying galaxy is consistent with a faint GRB host galaxy, well within the GRB host brightness distribution. Thus, the possibility that GRB 070125 is simply located in the outskirts of a gas-rich, massive star-forming region inside its small and faint host galaxy seems more likely than a gas-poor, halo environment origin

Investigating the nature of the INTEGRAL gamma-ray bursts and sub-threshold triggers with Swift follow-up

We explore the potential of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) to improve our understanding of the low-fluence regime for explosive transients, such as Gamma-ray Bursts (GRBs). We probe the nature of the so-called 'WEAK' INTEGRAL triggers, when the gamma-ray instruments record intensity spikes that are below the usual STRONG significance thresholds. In a targeted Swift follow-up campaign, we observed 15 WEAK triggers.We find six of these can be classified as GRBs. This includes GRB 150305A, a GRB discovered from our campaign alone. We also identified a source coincident with one trigger, IGRW151019, as a candidate active galactic nucleus.We show that real events such as GRBs exist within the INTEGRAL Burst Alert System (IBAS) WEAK trigger population. A comparison of the fluence distributions of the full INTEGRAL IBAS and Swift-BAT GRB samples showed that the two are similar.We also find correlations between the prompt gamma-ray and X-ray properties of the two samples, supporting previous investigations.We find that both satellites reach similar, low fluence levels regularly, although Swift is more sensitive to short, low-fluence GRBs

A case of mistaken identity? GRB 060912A and the nature of the long-short GRB divide

We investigate the origin of the GRB 060912A, which has observational properties that make its classification as either a long or short burst ambiguous. Short-duration gamma-ray bursts (SGRBs) are thought to have typically lower energies than long-duration bursts, can be found in galaxies with populations of all ages and are likely to originate from different progenitors to the long-duration bursts. However, it has become clear that duration alone is insufficient to make a distinction between the two populations in many cases, leading to a desire to find additional discriminators of burst type. GRB 060912A had a duration of 6 s and occurred only ∼10 arcsec from a bright, low-redshift (z= 0.0936) elliptical galaxy, suggesting that this may have been the host, which would favour it being a short burst. However, our deep optical imaging and spectroscopy of the location of GRB 060912A using the Very Large Telescope (VLT) shows that GRB 060912A more likely originates in a distant star-forming galaxy at z= 0.937, and is most likely a long burst. This demonstrates the risk in identifying bright, nearby galaxies as the hosts of given gamma-ray bursts (GRBs) without further supporting evidence. Further, it implies that, in the absence of secure identifications, ‘host’ type, or more broadly discriminators that rely on galaxy redshifts, may not be good indicators of the true nature of any given GRB

Liverpool Telescope follow-up of candidate electromagnetic counterparts during the first run of Advanced LIGO

The first direct detection of gravitational waves was made in late 2015 with the Advanced LIGO detectors. By prior arrangement, a worldwide collaboration of electromagnetic follow-up observers were notified of candidate gravitational wave events during the first science run, and many facilities were engaged in the search for counterparts. No counterparts were identified, which is in line with expectations given that the events were classified as black hole - black hole mergers. However these searches laid the foundation for similar follow-up campaigns in future gravitational wave detector science runs, in which the detection of neutron star merger events with observable electromagnetic counterparts is much more likely. Three alerts were issued to the electromagnetic collaboration over the course of the first science run, which lasted from September 2015 to January 2016. Two of these alerts were associated with the gravitational wave events since named GW150914 and GW151226. In this paper we provide an overview of the Liverpool Telescope contribution to the follow-up campaign over this period. Given the hundreds of square degree uncertainty in the sky position of any gravitational wave event, efficient searching for candidate counterparts required survey telescopes with large (~degrees) fields-of-view. The role of the Liverpool Telescope was to provide follow-up classification spectroscopy of any candidates. We followed candidates associated with all three alerts, observing 1, 9 and 17 candidates respectively. We classify the majority of the transients we observed as supernovae

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

Variable Lyα sheds light on the environment surrounding GRB 090426

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionization state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z= 2.609, whose optical spectrum indicates an almost fully ionized medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Lyα absorption line. Photoionization modelling shows that we are probing material from the vicinity of the burst (∼80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies

The host of GRB 060206: Kinematics of a distant galaxy

Context. GRB afterglow spectra are sensitive probes of interstellar matter along the line-of-sight in their host galaxies, as well as in intervening galaxies. The rapid fading of GRBs makes it very difficult to obtain spectra of sufficient resolution and S/N to allow for these kinds of studies. Aims. We investigate the state and properties of the interstellar medium in the host of GRB 060206 at z= 4.048 with a detailed study of groundstate and finestructure absorption lines in an early afterglow spectrum. This allows us to derive conclusions on the nature and origin of the absorbing structures and their connection to the host galaxy and/or the GRB. Methods. We used early (starting 1.6 h after the burst) WHT/ISIS optical spectroscopy of the afterglow of the gamma-ray burst GRB 060206 detecting a range of metal absorption lines and their finestructure transitions. Additional information is provided by the afterglow lightcurve. The resolution and wavelength range of the spectra and the bright afterglow have facilitated a detailed study and fitting of the absorption line systems in order to derive column densities. We also used deep imaging to detect the host galaxy and probe the nature of an intervening system at z = 1.48 seen in absorption in the afterglow spectra. Results. We detect four discrete velocity systems in the resonant metal absorption lines, best explained by shells within and/or around the host created by starburst winds. The finestructure lines have no less than three components with strengths decreasing from the redmost components. We therefore suggest that the finestructure lines are best explained as being produced by UV pumping from which follows that the redmost component is the one closest to the burst where \ion{N}{v} was detected as well. The host is detected in deep HST imaging with F814WAB = 27.48 $\pm$ 0.19 mag and a 3$\sigma$ upper limit of H = 20.6 mag (Vega) is achieved. A candidate counterpart for the intervening absorption system is detected as well, which is quite exceptional for an absorber in the sightline towards a GRB afterglow. The intervening system shows no temporal evolution as claimed by Hao et al. (2007, ApJ, 659, 99), which we prove from our WHT spectra taken before and Subaru spectra taken during those observations

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

Spectroscopy and multiband photometry of the afterglow of intermediate duration γ-ray burst GRB 040924 and its host galaxy

Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB 040924. This GRB had a rather short duration of T90 ~2.4 s, and a well sampled optical afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor. Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data. Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ~0.03 days. We determine the redshift z = 0.858 $\pm$ 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the [Ne III] emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their [Ne III] emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter. Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star