The supernovae (SNe) associated with gamma-ray bursts (GRBs) are generally seen as a homogenous population, but at least one exception exists, both in terms of luminosity as well as Spectral Energy ...Distribution (SED). However, this event, SN 2011kl, was associated with an ultra-long GRB 111209A. Do such outliers also exist for more typical GRBs? Within the context of a systematic analysis of photometric signatures of GRB-associated SNe, we found an anomalous bump in the late-time transient following GRB 140506A. We hereby aim to show this bump is significantly more luminous and blue than usual SNe following GRBs. We compile all available data from the literature, and add a full analysis of the Swift UVOT data, which allows us to trace the light curve from the first minutes all the way to the host galaxy, as well as construct a broad SED of the afterglow that extends the previous SED analysis based on ground-based spectroscopy. We find robust evidence for a late-time bump/plateau following the afterglow which shows evidence for a strong colour change, with the spectral slope becoming flatter in the blue region of the spectrum. This bump can be interpreted as a luminous SN bump which is spectrally dissimilar to typical GRB-SNe. Correcting it for the large line-of-sight extinction results in extreme values which make the SN associated with GRB 140506A the most luminous detected so far. Even so, it would be in agreement with a luminosity-duration relation of GRB-SNe. While not supported by spectroscopic evidence, it is likely the blue bump following GRB 140506A is the signature of a SN which is spectrally dissimilar to classical GRB-SNe and more similar to SN 2011kl -- while being associated with an average GRB, indicating the GRB-SN population is more diverse than thought so far, and can reach luminosities comparable to those of superluminous SNe.
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 \(\times\) 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 \(\sim 4\%\) that goes to zero at the time of the break, and then increases again to \(\sim 2\%\) with a change of the position angle of \(54 \pm 9\) 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.
GRB~230812B is a bright and relatively nearby (\(z =0.36\)) long gamma-ray burst (GRB) that has generated significant interest in the community and has thus been observed over the entire ...electromagnetic spectrum. We report over 80 observations in X-ray, ultraviolet, optical, infrared, and sub-millimeter bands from the GRANDMA (Global Rapid Advanced Network for Multi-messenger Addicts) network of observatories and from observational partners. Adding complementary data from the literature, we then derive essential physical parameters associated with the ejecta and external properties (i.e. the geometry and environment) of the GRB and compare with other analyses of this event. We spectroscopically confirm the presence of an associated supernova, SN2023pel, and we derive a photospheric expansion velocity of v \(\sim\) 17\(\times10^3\) km s\(^{-1}\). We analyze the photometric data first using empirical fits of the flux and then with full Bayesian Inference. We again strongly establish the presence of a supernova in the data, with a maximum (pseudo-)bolometric luminosity of \(5.75 \times 10^{42}\) erg/s, at \(15.76^{+0.81}_{-1.21}\) days (in the observer frame) after the trigger, with a half-max time width of 22.0 days. We compare these values with those of SN1998bw, SN2006aj, and SN2013dx. Our best-fit model favours a very low density environment (\(\log_{10}({n_{\rm ISM}/{\rm cm}^{-3}}) = -2.38^{+1.45}_{-1.60}\)) and small values for the jet's core angle \(\theta_{\rm core} = 1.54^{+1.02}_{-0.81} \ \rm{deg}\) and viewing angle \(\theta_{\rm obs} = 0.76^{+1.29}_{-0.76} \ \rm{deg}\). GRB 230812B is thus one of the best observed afterglows with a distinctive supernova bump.
We present a detailed analysis of short GRB 201221D lying at redshift \(z= 1.045\). We analyse the high-energy data of the burst and compare it with the sample of short gamma-ray bursts (SGRBs). The ...prompt emission characteristics are typical of those seen in the case of other SGRBs except for the peak energy (\(E_{\rm p}\)), which lies at the softer end (generally observed in the case of long bursts). We estimate the host galaxy properties by utilising the {\sc Python}-based software {\sc Prospector} to fit the spectral energy distribution of the host. The burst lies at a high redshift relative to the SGRB sample with a median redshift of \(z=0.47\). We compare the burst characteristics with other SGRBs with known redshifts along with GRB 200826A (SGRB originated from a collapsar). A careful examination of the characteristics of SGRBs at different redshifts reveals that some of the SGRBs lying at high redshifts have properties similar to long GRBs indicating they might have originated from collapsars. Further study of these GRBs can help to explore the broad picture of progenitor systems of SGRBs.
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 ly-alpha in absorption with a column density of log N(HI)=21.2+/-0.2 cm\(^{-2}\) which, together with FeII, CII, SiII, AlII and OI, 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 FeII and -2.5+/-0.2 for SiII 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 CIV and SiIV 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.
We present JWST and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, ...obtained with JWST/NIRSPEC (0.6-5.5 micron) and MIRI (5-12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power-law, with \(F_{\nu} \propto \nu^{-\beta}\), we obtain \(\beta \approx 0.35\), modified by substantial dust extinction with \(A_V = 4.9\). This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post jet-break model, with electron index \(p<2\), and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/nIR to X-shooter spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disc-like host galaxy, viewed close to edge-on, that further complicates the isolation of any supernova component. The host galaxy appears rather typical amongst long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.
Here, we report the discovery of a kilonova associated with the nearby (350 Mpc) minute-duration GRB 211211A. In tandem with deep optical limits that rule out the presence of an accompanying ...supernova to \(M_I > -13\) mag at 17.7 days post-burst, the identification of a kilonova confirms that this burst's progenitor was a compact object merger. While the spectrally softer tail in GRB 211211A's gamma-ray light curve is reminiscent of previous extended emission short GRBs (EE-SGRBs), its prompt, bright spikes last \(\gtrsim 12\) s, separating it from past EE-SGRBs. GRB 211211A's kilonova has a similar luminosity, duration and color to AT2017gfo, the kilonova found in association with the gravitational wave (GW)-detected binary neutron star (BNS) merger GW170817. We find that the merger ejected \(\approx 0.04 M_{\odot}\) of r-process-rich material, and is consistent with the merger of two neutron stars (NSs) with masses close to the canonical \(1.4 M_{\odot}\). This discovery implies that GRBs with long, complex light curves can be spawned from compact object merger events and that a population of kilonovae following GRBs with durations \(\gg 2\) s should be accounted for in calculations of the NS merger r-process contribution and rate. At 350 Mpc, the current network of GW interferometers at design sensitivity would have detected the merger precipitating GRB 211211A, had it been operating at the time of the event. Further searches for GW signals coincident with long GRBs are therefore a promising route for future multi-messenger astronomy.
The hosts of long duration gamma-ray bursts are predominantly starburst galaxies at subsolar metallicity. At redshifts z<1, this implies that most of them are low-mass galaxies similar to the ...populations of blue compact dwarfs and dwarf irregulars. What triggers the massive star-formation (SF) needed for producing a GRB progenitor is still largely unknown, as are the resolved gas properties and kinematics of these galaxies and their formation history. Here we present a sample of six spatially resolved GRB hosts at z<0.3 observed with 3D spectroscopy at high spectral resolution (R=8,000-13,000) using FLAMES/VLT. We analyzed the resolved gas kinematics of the full sample and the abundances in a subsample. Only two galaxies show a regular disk-like rotation field, another two are dispersion-dominated, the remaining two have a double emission component associated with different parts of the galaxy, which might indicate a recent merger. All galaxies show evidence for broad components underlying the main emission peak (sigma = 50-110 km/s). This broad component is more metal-rich than the narrow components, it is blueshifted in most cases, and it follows a different velocity structure. We find a weak correlation between the SF rate and the width of the broad component, its flux compared to the narrow component, and the maximum outflow velocity of the gas, but we do not find any correlation with the SF density, metallicity or stellar mass. We associate this broad component with a metal-rich outflow from star-forming regions. The GRB is not located in the brightest region of the host, but is always associated with some star-forming region showing a clear wind component. Our study shows the potential of 3D spectroscopy to study the SF processes in galaxies hosting extreme transients, the need for high S/N, and the perils using unresolved or only partially resolved data for these kinds of studies.
Context: The extreme luminosity of gamma-ray bursts (GRBs) makes them powerful beacons for studies of the distant Universe. The most luminous bursts are typically detected at moderate/high redshift, ...where the volume for seeing such rare events is maximized and the star-formation activity is greater than at z = 0. For distant events, not all observations are feasible, such as at TeV energies. Aims: Here we present a spectroscopic redshift measurement for the exceptional GRB 221009A, the brightest GRB observed to date with emission extending well into the TeV regime. Methods: We used the X-shooter spectrograph at the ESO Very Large Telescope (VLT) to obtain simultaneous optical to near-IR spectroscopy of the burst afterglow 0.5 days after the explosion. Results: The spectra exhibit both absorption and emission lines from material in a host galaxy at z = 0.151. Thus GRB 221009A was a relatively nearby burst with a luminosity distance of 745 Mpc. Its host galaxy properties (star-formation rate and metallicity) are consistent with those of LGRB hosts at low redshift. This redshift measurement yields information on the energy of the burst. The inferred isotropic energy release, \(E_{\rm iso} > 5 \times 10^{54}\) erg, lies at the high end of the distribution, making GRB 221009A one of the nearest and also most energetic GRBs observed to date. We estimate that such a combination (nearby as well as intrinsically bright) occurs between once every few decades to once per millennium.
GRBs produced by the collapse of massive stars are usually found near the most prominent star-forming regions of star-forming galaxies. GRB 171205A happened in the outskirts of a spiral galaxy, a ...peculiar location in an atypical GRB host. In this paper we present a highly-resolved study of the molecular gas of this host, with CO(1-0) observations from ALMA. We compare with GMRT atomic HI observations, and with data at other wavelengths to provide a broad-band view of the galaxy. The ALMA observations have a spatial resolution of 0.2" and a spectral resolution of 10 km/s, observed when the afterglow had a flux density of ~53 mJy. This allowed a molecular study both in emission and absorption. The HI observations allowed to study the host galaxy and its extended environment. The CO emission shows an undisturbed spiral structure with a central bar, and no significant emission at the location of the GRB. Our CO spectrum does not reveal any CO absorption, with a column density limit of < 10^15 cm^-2. This argues against the progenitor forming in a massive molecular cloud. The molecular gas traces the galaxy arms with higher concentration in the regions dominated by dust. The HI gas does not follow the stellar light or the molecular gas and is concentrated in two blobs, with no emission towards the centre of the galaxy, and is slightly displaced towards the southwest of the galaxy, where the GRB exploded. Within the extended neighbourhood of the host galaxy, we identify another prominent HI source at the same redshift, at a projected distance of 188 kpc. Our observations show that the progenitor of this GRB is not associated to a massive molecular cloud, but more likely related to low-metallicity atomic gas. The distortion in the HI gas field is indicator of an odd environment that could have triggered star formation and could be linked to a past interaction with the companion galaxy.
