arXiv subjects

A. Rossi

Publications and source records attributed to A. Rossi.

At least 55 records · Page 3Linked to original sources

arXiv2024

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 $\times$ 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.

astro-ph.HE↗

arXiv2024

Physical properties of asteroid Dimorphos as derived from the DART impact

On September 26, 2022, NASA's Double Asteroid Redirection Test (DART) mission successfully impacted Dimorphos, the natural satellite of the binary near-Earth asteroid (65803) Didymos. Numerical simulations of the impact provide a means to explore target surface material properties and structures, consistent with the observed momentum deflection efficiency, ejecta cone geometry, and ejected mass. Our simulation, which best matches observations, indicates that Dimorphos is weak, with a cohesive strength of less than a few pascals (Pa), similar to asteroids (162173) Ryugu and (101955) Bennu. We find that a bulk density of Dimorphos, rhoB, lower than 2400 kg/m3, and a low volume fraction of boulders (<40 vol%) on the surface and in the shallow subsurface, are consistent with measured data from the DART experiment. These findings suggest Dimorphos is a rubble pile that might have formed through rotational mass shedding and re-accumulation from Didymos. Our simulations indicate that the DART impact caused global deformation and resurfacing of Dimorphos. ESA's upcoming Hera mission may find a re-shaped asteroid, rather than a well-defined crater.

astro-ph.EP↗

arXiv2024

Fires in the deep: The luminosity distribution of early-time gamma-ray-burst afterglows in light of the Gamow Explorer sensitivity requirements

Gamma-ray bursts (GRBs) are ideal probes of the Universe at high redshift (z > 5), pinpointing the locations of the earliest star-forming galaxies and providing bright backlights that can be used to spectrally fingerprint the intergalactic medium and host galaxy during the period of reionization. Future missions such as Gamow Explorer are being proposed to unlock this potential by increasing the rate of identification of high-z GRBs to rapidly trigger observations from 6-10 m ground telescopes, JWST, and the Extremely Large Telescopes. Gamow was proposed to the NASA 2021 Medium-Class Explorer (MIDEX) program as a fast-slewing satellite featuring a wide-field lobster-eye X-ray telescope (LEXT) to detect and localize GRBs, and a 30 cm narrow-field multi-channel photo-z infrared telescope (PIRT) to measure their photometric redshifts using the Lyman-alpha dropout technique. To derive the PIRT sensitivity requirement we compiled a complete sample of GRB optical-near-infrared afterglows from 2008 to 2021, adding a total of 66 new afterglows to our earlier sample, including all known high-z GRB afterglows. We performed full light-curve and spectral-energy-distribution analyses of these afterglows to derive their true luminosity at very early times. For all the light curves, where possible, we determined the brightness at the time of the initial finding chart of Gamow, at different high redshifts and in different NIR bands. We then followed the evolution of the luminosity to predict requirements for ground and space-based follow-up. We find that a PIRT sensitivity of 15 micro-Jy (21 mag AB) in a 500 s exposure simultaneously in five NIR bands within 1000s of the GRB trigger will meet the Gamow mission requirement to recover > 80% of all redshifts at z > 5.

astro-ph.HE↗

arXiv2024

Highly luminous supernovae associated with gamma-ray bursts II. The luminous blue bump in the afterglow of GRB 140506A

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.

astro-ph.HE↗

arXiv2024

GRB 201015A: from seconds to months of optical monitoring and supernova discovery

We present full photometric coverage and spectroscopic data for soft GRB 201015A with a redshift z = 0.426. Our data spans a time range of 85 days following the detection of GRB. These observations revealed an underlying supernova SN 201015A with a maximum at $8.54 \pm $1.48 days (rest frame) and an optical peak absolute magnitude $-19.45_{-0.47}^{+0.85}$ mag. The supernova stands out clearly, since the contribution of the afterglow at this time is not dominant, which made it possible to determine SN's parameters. A comparison of these parameters reveals that the SN 201015A is the earliest (the minimum $T_{max}$) known supernova associated with gamma-ray bursts. Spectroscopic observations during the supernova decay stage showed broad lines, indicating a large photospheric velocity, and identified this supernova as a type Ic-BL. Thus, the SN 201015A associated with the GRB 201015A becomes the 27th SN/GRB confirmed by both photometric and spectroscopic observations. Using the results of spectral analysis based on the available data of Fermi-GBM experiment, the parameters $E_\text{p,i} = 20.0 \pm 8.5$ keV and $E_\text{iso} = (1.1 \pm 0.2) \times 10^{50}$ erg were obtained. According to the position of the burst on the $E_\text{p,i}$-$E_\text{iso}$ correlation, GRB 201015A was classified as a Type II (long) gamma-ray burst, which was also confirmed by the $T_\text{90,i}$-$EH$ diagram.

astro-ph.HE↗

arXiv2023

A search for the afterglows, kilonovae, and host galaxies of two short GRBs: GRB 211106A and GRB 211227A

Context: GRB 211106A and GRB 211227A are recent gamma-ray bursts (GRBs) with initial X-ray positions suggesting associations with nearby galaxies (z < 0.7). Their prompt emission characteristics indicate GRB 211106A is a short-duration GRB and GRB 211227A is a short GRB with extended emission, likely originating from compact binary mergers. However, classifying solely based on prompt emission can be misleading. Aims: These short GRBs in the local Universe offer opportunities to search for associated kilonova (KN) emission and study host galaxy properties in detail. Methods: We conducted deep optical and NIR follow-up using ESO-VLT FORS2, HAWK-I, and MUSE for GRB 211106A, and ESO-VLT FORS2 and X-Shooter for GRB 211227A, starting shortly after the X-ray afterglow detection. We performed photometric analysis to look for afterglow and KN emissions associated with the bursts, along with host galaxy imaging and spectroscopy. Optical/NIR results were compared with Swift X-Ray Telescope (XRT) and other high-energy data. Results: For both GRBs we placed deep limits to the optical/NIR afterglow and KN emission. Host galaxies were identified: GRB 211106A at photometric z = 0.64 and GRB 211227A at spectroscopic z = 0.228. Host galaxy properties aligned with typical short GRB hosts. We also compared the properties of the bursts with the S-BAT4 sample to further examined the nature of these events. Conclusions: Study of prompt and afterglow phases, along with host galaxy analysis, confirms GRB 211106A as a short GRB and GRB 211227A as a short GRB with extended emission. The absence of optical/NIR counterparts is likely due to local extinction for GRB 211106A and a faint kilonova for GRB 211227A.

astro-ph.HE↗

arXiv2023

The cosmic build-up of dust and metals. Accurate abundances from GRB-selected star-forming galaxies at $1.7 < z < 6.3$

The chemical enrichment of dust and metals in the interstellar medium (ISM) of galaxies throughout cosmic time is one of the key driving processes of galaxy evolution. Here we study the evolution of the gas-phase metallicities, dust-to-gas (DTG), and dust-to-metal (DTM) ratios of 36 star-forming galaxies at $1.7 < z < 6.3$ probed by gamma-ray bursts (GRBs). We compile all GRB-selected galaxies with intermediate (R=7000) to high (R>40,000) resolution spectroscopic data for which at least one refractory (e.g. Fe) and one volatile (e.g. S or Zn) element have been detected at S/N>3. This is to ensure that accurate abundances and dust depletion patterns can be obtained. We first derive the redshift evolution of the dust-corrected, absorption-line based gas-phase metallicity [M/H]$_{\rm tot}$ in these galaxies, for which we determine a linear relation with redshift ${\rm [M/H]_{tot}}(z) = (-0.21\pm 0.04)z -(0.47\pm 0.14)$. We then examine the DTG and DTM ratios as a function of redshift and through three orders of magnitude in metallicity, quantifying the relative dust abundance both through the direct line-of-sight visual extinction $A_V$ and the derived depletion level. We use a novel method to derive the DTG and DTM mass ratios for each GRB sightline, summing up the mass of all the depleted elements in the dust-phase. We find that the DTG and DTM mass ratios are both strongly correlated with the gas-phase metallicity and show a mild evolution with redshift as well. While these results are subject to a variety of caveats related to the physical environments and the narrow pencil-beam sightlines through the ISM probed by the GRBs, they provide strong implications for studies of dust masses to infer the gas and metal content of high-redshift galaxies, and particularly demonstrate the large offset from the average Galactic value in the low-metallicity, high-redshift regime.

astro-ph.GA↗

arXiv2023

The ultra-long GRB 220627A at z=3.08

GRB 220627A is a rare burst with two distinct gamma-ray emission episodes separated by almost 1000 s that triggered the Fermi Gamma-ray Burst Monitor twice. High-energy GeV emission was detected by the Fermi Large Area Telescope coincident with the first emission episode but not the second. The discovery of the optical afterglow with MeerLICHT led to MUSE observations which secured the burst redshift to z=3.08, making this the most distant ultra-long gamma-ray burst (GRB) detected to date. The progenitors of some ultra-long GRBs have been suggested in the literature to be different to those of normal long GRBs. Our aim is to determine whether the afterglow and host properties of GRB 220627A agree with this interpretation. We performed empirical and theoretical modelling of the afterglow data within the external forward shock framework, and determined the metallicity of the GRB environment through modelling the absorption lines in the MUSE spectrum. Our optical data show evidence for a jet break in the light curve at ~1.2 days, while our theoretical modelling shows a preference for a homogeneous circumburst medium. Our forward shock parameters are typical for the wider GRB population, and we find that the environment of the burst is characterised by a sub-solar metallicity. Our observations and modelling of GRB 220627A do not suggest that a different progenitor compared to the progenitor of normal long GRBs is required. We find that more observations of ultra-long GRBs are needed to determine if they form a separate population with distinct prompt and afterglow features, and possibly distinct progenitors.

astro-ph.HE↗

arXiv2023

JWST detection of heavy neutron capture elements in a compact object merger

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe.

astro-ph.HE↗

arXiv2023

Panning for gold, but finding helium: discovery of the ultra-stripped supernova SN2019wxt from gravitational-wave follow-up observations

We present the results from multi-wavelength observations of a transient discovered during the follow-up of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN2019wxt, a young transient in a galaxy whose sky position (in the 80\% GW contour) and distance ($\sim$150\,Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude ($M_i \sim -16.7$\,mag) and the $r-$band decline rate of $\sim 1$\,mag per 5\,days appeared suggestive of a compact binary merger. However, SN2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of $\sim 0.1\,M_\odot$, with $^{56}$Ni comprising $\sim 20\%$ of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitors that could give rise to the observed properties of SN2019wxt, and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling electromagnetic counterparts to GW events from transients such as SN2019wxt is challenging: in a bid to characterise the level of contamination, we estimated the rate of events with properties comparable to those of SN2019wxt and found that $\sim 1$ such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500\,Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.

astro-ph.HE↗

arXiv2023

First Experimental Evidence of a Beam-Beam Long-Range Compensation Using Wires in the Large Hadron Collider

In high intensity and high energy colliders such as the CERN Large Hadron Collider and its future High Luminosity upgrade, interactions between the two beams around the different Interaction Points impose machine performance limitations. In fact, their effect reduces the beam lifetime and therefore the collider's luminosity reach. Those interactions are called Beam-Beam Long-Range interactions and a possible mitigation of their effect using DC wires was proposed for the first time in the early 2000's. This solution is currently being studied as an option for enhancing the HL-LHC performance. In 2017 and 2018, four demonstrators of wire compensators have been installed in the LHC. A two-year long experimental campaign followed in order to validate the possibility to mitigate the BBLR interactions in the LHC. During this campaign, a proof-of-concept was completed and motivated an additional set of experiments, successfully demonstrating the mitigation of BBLR interactions effects in beam conditions compatible with the operational configuration. This paper reports in detail the preparation of the experimental campaign, the obtained results and draws some perspectives for the future.

physics.acc-ph↗

arXiv2023

The first JWST spectrum of a GRB afterglow: No bright supernova in observations of the brightest GRB of all time, GRB 221009A

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 $F_ν \propto ν^{-β}$, we obtain $β\approx 0.35$, 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.

astro-ph.HE↗

arXiv2023

Dissecting the interstellar medium of a z=6.3 galaxy: X-shooter spectroscopy and HST imaging of the afterglow and environment of the Swift GRB 210905A

The study of the properties of galaxies in the first billion years after the Big Bang is one of the major topic of current astrophysics. Optical/near-infrared spectroscopy of the afterglows of long Gamma-ray bursts (GRBs) provide a powerful diagnostic tool to probe the interstellar medium (ISM) of their host galaxies and foreground absorbers, even up to the highest redshifts. We analyze the VLT/X-shooter afterglow spectrum of GRB 210905A, triggered by the Swift Neil Gehrels Observatory, and detect neutral-hydrogen, low-ionization, high-ionization, and fine-structure absorption lines from a complex system at z=6.3118, that we associate with the GRB host galaxy. We study the ISM properties of the host system, revealing the metallicity, kinematics and chemical abundance pattern. The total metallicity of the z~6.3 system is [M/H]=-1.72+/-0.13, after correcting for dust-depletion and taking into account alpha-element enhancement. In addition, we determine the overall amount of dust and dust-to-metal mass ratio (DTM) ([Zn/Fe]_fit=0.33+/-0.09, DTM=0.18+/-0.03). We find indications of nucleosynthesis due to massive stars and evidence of peculiar over-abundance of aluminium. From the analysis of fine-structure lines, we determine distances of several kpc for the low-ionization gas clouds closest to the GRB. Those farther distances are possibly due to the high number of ionizing photons. Using the HST/F140W image of the GRB field, we show the GRB host galaxy as well as multiple objects within 2" from the GRB. We discuss the galaxy structure and kinematics that could explain our observations, also taking into account a tentative detection of Lyman-alpha emission. Deep spectroscopic observations with VLT/MUSE and JWST will offer the unique possibility of combining our results with the ionized-gas properties, with the goal of better understanding how galaxies in the reionization era form and evolve.

astro-ph.GA↗

arXiv2023

GRB 160410A: the first Chemical Study of the Interstellar Medium of a Short GRB

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.

astro-ph.HE↗

arXiv2022

Letter of Intent: the NA60+ experiment

We propose a new fixed-target experiment for the study of electromagnetic and hard probes of the Quark-Gluon Plasma (QGP) in heavy-ion collisions at the CERN SPS. The experiment aims at performing measurements of the dimuon spectrum from threshold up to the charmonium region, and of hadronic decays of charm and strange hadrons. It is based on a muon spectrometer, which includes a toroidal magnet and six planes of tracking detectors, coupled to a vertex spectrometer, equipped with Si MAPS immersed in a dipole field. High luminosity is an essential requirement for the experiment, with the goal of taking data with 10$^6$ incident ions/s, at collision energies ranging from $\sqrt{s_{\rm NN}} = 6.3$ GeV ($E_{\rm lab}= 20$ A GeV) to top SPS energy ($\sqrt{s_{\rm NN}} = 17.3$ GeV, $E_{\rm lab}= 158$ A GeV). This document presents the physics motivation, the foreseen experimental set-up including integration and radioprotection studies, the current detector choices together with the status of the corresponding R&D, and the outcome of physics performance studies. A preliminary cost evaluation is also carried out.

nucl-ex↗

arXiv2022

A short gamma-ray burst from a proto-magnetar remnant

The contemporaneous detection of gravitational waves and gamma rays from the GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron-star mergers as progenitors of short-duration gamma-ray bursts (GRBs), and cosmic sources of heavy r-process nuclei. However, the nature (and lifespan) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with the short GRB 180618A with extended gamma-ray emission, with ultraviolet and optical multicolour observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission at 15 minutes post-burst, which fades abruptly and chromatically (flux density $F_ν \propto t^{-α}$, $α=4.6 \pm 0.3$) just 35 minutes after the GRB. Our observations from gamma rays to optical wavelengths are consistent with a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly-spinning and highly magnetized neutron star (i.e. a millisecond magnetar), whose rotational energy is released at a rate $L_{\rm th} \propto t^{-(2.22\pm 0.14)}$ to reheat the unbound merger-remnant material. These results suggest such neutron stars can survive the collapse to a black hole on timescales much larger than a few hundred milliseconds after the merger, and power the GRB itself through accretion. Bright thermal optical counterparts to binary merger gravitational wave sources may be common in future wide-field fast-cadence sky surveys.

astro-ph.HE↗

arXiv2022

A Kilonova Following a Long-Duration Gamma-Ray Burst at 350 Mpc

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.

astro-ph.HE↗

arXiv2022

A blast from the infant Universe: the very high-z GRB 210905A

We present a detailed follow-up of the very energetic GRB 210905A at a high redshift of z = 6.312 and its luminous X-ray and optical afterglow. We obtained a photometric and spectroscopic follow-up in the optical and near-infrared (NIR), covering both the prompt and afterglow emission from a few minutes up to 20 Ms after burst. With an isotropic gamma-ray energy release of Eiso = 1.27E54 erg, GRB 210905A lies in the top ~7% of gamma-ray bursts (GRBs) in terms of energy released. Its afterglow is among the most luminous ever observed. It starts with a shallow evolution that can be explained by energy injection, and it is followed by a steeper decay, while the spectral energy distribution is in agreement with slow cooling in a constant-density environment within the standard fireball theory. A jet break at ~ 46.2+-16.3 d (~6.3 d rest-frame) has been observed in the X-ray light curve; however, it is hidden in the H band due to the contribution from the likely host galaxy, the fourth GRB host at z > 6 known to date. We derived a half-opening angle of 8.4+-1.0 degrees, which is the highest ever measured for a z>6 burst, but within the range covered by closer events. The resulting collimation-corrected gamma-ray energy release of 1E52 erg is also among the highest ever measured. The moderately large half-opening angle argues against recent claims of an inverse dependence of the half-opening angle on the redshift. The total jet energy is likely too large to be sustained by a standard magnetar, and it suggests that the central engine of this burst was a newly formed black hole. Despite the outstanding energetics and luminosity of both GRB 210905A and its afterglow, we demonstrate that they are consistent with those of less distant bursts, indicating that the powering mechanisms and progenitors do not evolve significantly with redshift.

astro-ph.HE↗