Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (greater than two seconds) duration are produced by the core-collapse of massive ...stars
, and those of short (less than two seconds) duration by the merger of compact objects, such as two neutron stars
. A third class of events with hybrid high-energy properties was identified
, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions
, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented
. Here we report observations of the exceptionally bright GRB 211211A, which classify it as a hybrid event and constrain its distance scale to only 346 megaparsecs. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (approximately 10
erg per second) kilonova possibly formed in the ejecta of a compact object merger.
The recent discovery of a gamma-ray burst (GRB) coincident with the gravitational-wave (GW) event GW170817 revealed the existence of a population of low-luminosity short duration gamma-ray transients ...produced by neutron star mergers in the nearby Universe. These events could be routinely detected by existing gamma-ray monitors, yet previous observations failed to identify them without the aid of GW triggers. Here we show that GRB150101B is an analogue of GRB170817A located at a cosmological distance. GRB150101B is a faint short burst characterized by a bright optical counterpart and a long-lived X-ray afterglow. These properties are unusual for standard short GRBs and are instead consistent with an explosion viewed off-axis: the optical light is produced by a luminous kilonova, while the observed X-rays trace the GRB afterglow viewed at an angle of ~13°. Our findings suggest that these properties could be common among future electromagnetic counterparts of GW sources.
ABSTRACT
A significant fraction (30 per cent) of well-localized short gamma-ray bursts (sGRBs) lack a coincident host galaxy. This leads to two main scenarios: (i) that the progenitor system merged ...outside of the visible light of its host, or (ii) that the sGRB resided within a faint and distant galaxy that was not detected by follow-up observations. Discriminating between these scenarios has important implications for constraining the formation channels of neutron star mergers, the rate and environments of gravitational wave sources, and the production of heavy elements in the Universe. In this work, we present the results of our observing campaign targeted at 31 sGRBs that lack a putative host galaxy. Our study effectively doubles the sample of well-studied sGRB host galaxies, now totaling 72 events of which $28{{\ \rm per\ cent}}$ lack a coincident host to deep limits (r ≳ 26 or F110W ≳ 27 AB mag), and represents the largest homogeneously selected catalogue of sGRB offsets to date. We find that 70 per cent of sub-arcsecond localized sGRBs occur within 10 kpc of their host’s nucleus, with a median projected physical offset of 5.6 kpc. Using this larger population, we discover an apparent redshift evolution in their locations: bursts at low-z occur at 2 × larger offsets compared to those at z > 0.5. This evolution could be due to a physical evolution of the host galaxies themselves or a bias against faint high-z galaxies. Furthermore, we discover a sample of hostless sGRBs at z ≳ 1 that are indicative of a larger high-z population, constraining the redshift distribution and disfavoring lognormal delay time models.
ABSTRACT We report the results of our observing campaign on GRB 140903A, a nearby (z = 0.351) short-duration (T90 ∼ 0.3 s) gamma-ray burst discovered by Swift. We monitored the X-ray afterglow with ...Chandra up to 15 days after the burst and detected a steeper decay of the X-ray flux after tj 1 day. Continued monitoring at optical and radio wavelengths showed a similar decay in flux at nearly the same time, and we interpret it as evidence of a narrowly collimated jet. By using the standard fireball model to describe the afterglow evolution, we derive a jet opening angle θj 5° and a collimation-corrected total energy release E 2 erg. We further discuss the nature of the GRB progenitor system. Three main lines disfavor a massive star progenitor: the properties of the prompt gamma-ray emission, the age and low star formation rate of the host galaxy, and the lack of a bright supernova. We conclude that this event likely originated from a compact binary merger.
We present the results of a multiwavelength campaign targeting FRB 20201124A, the third closest repeating fast radio burst (FRB), which was recently localized in a nearby (
z
= 0.0978) galaxy. Deep ...VLA observations led to the detection of quiescent radio emission, which was also marginally visible in X-rays with
Chandra
. Imaging at 22 GHz allowed us to resolve the source on a scale of ≳1″ and locate it at the position of the FRB, within an error of 0.2″. The EVN and e-MERLIN observations sampled small angular scales, from 2 to 100 mas, providing tight upper limits on the presence of a compact source and evidence for diffuse radio emission. We argue that this emission is associated with enhanced star formation activity in the proximity of the FRB, corresponding to a star formation rate (SFR) of ≈10
M
⊙
yr
−1
. The surface SFR at the location of FRB 20201124A is two orders of magnitude larger than what is typically observed in other precisely localized FRBs. Such a high SFR is indicative of this FRB source being a newborn magnetar produced from a supernova explosion of a massive star progenitor. Upper limits to the X-ray counterparts of 49 radio bursts observed in our simultaneous FAST, SRT, and
Chandra
campaign are consistent with a magnetar scenario.
A long-standing paradigm in astrophysics is that collisions- or mergers-of two neutron stars form highly relativistic and collimated outflows (jets) that power Y-ray bursts of short (less than two ...seconds) duration. The observational support for this model, however, is only indirect. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with Y-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a 'kilonova') from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta, observations at X-ray and, later, radio frequencies are consistent with a short Y-ray burst viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short Y-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the Y-ray-burst emission.
ABSTRACT
We present a detailed multiwavelength analysis of two short gamma-ray bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at $z$ = 0.483 and GRB 200522A at $z$ = ...0.554. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated with a late-type galaxy with an old stellar population (≈3 Gyr) and moderate ongoing star formation (≈1 M⊙ yr−1). Hubble and Gemini limits on optical/near-infrared emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavouring large masses of wind ejecta (≲0.03 M⊙). In contrast, observations of GRB 200522A uncovered a luminous (LF125W ≈ 1042 erg s−1 at 2.3 d) and red (r − H ≈ 1.3 mag) counterpart. The red colour can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 M⊙ ≲ M ≲ 0.1 M⊙) or moderate extinction, E(B − V) ≈ 0.1−0.2 mag, along the line of sight. The location of this sGRB in the inner regions of a young (≈0.1 Gyr) star-forming (≈2−6 M⊙ yr−1) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red colour.
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•Podiform chromitites are abundant in the Matysken Ural-Alaskan type complex.•Multiphase silicate inclusions are found in isoferroplatinum from lode chromitites.•Multiphase inclusions ...are in total disequilibrium with any possible ultramafic melt.•Non-magmatic models of podiform platiniferous chromitites formation are advocated.
The origin of large platinum-group mineral (PGM) nuggets inside chromitite schlieren in some Ural-Alaskan type ultramafic complexes has been assigned to orthomagmatic processes. A promising phenomenon which may provide new insights into the processes responsible for PGE mineralization formation, or at least place constraints on existing models, are the multiphase inclusions hosted in PGM which have been previously found in platinum nuggets from alluvial deposits. For the first time, this study examines multiphase inclusions hosted in isoferroplatinum nuggets from lode chromitite schlieren in almost unaltered dunite from the Matysken Ural-Alaskan type complex (Koryak Highlands, Far East Russia). These multiphase inclusions are comprised of diopside, hydrous silicates, apatite, plagioclase, K-feldspar, silica and other minerals that are distinctly different from the host dunite mineralogy. Taken together with similar mineral assemblages in Cr-spinel-hosted inclusions, these inclusions cannot be crystallization products of a mafic/ultramafic melt, but instead require an alternative explanation. We critically evaluate different genetic models and conclude that a range of fluid-assisted metamorphic processes should be involved in producing massive PGM within chromitite schlieren in dunites.
We present observations of the possible short GRB 180418A in γ-rays, X-rays, and in the optical. Early optical photometry with the TAROT and RATIR instruments shows a bright peak ( 14.2 AB mag) ...between T + 28 and T + 90 s that we interpret as the signature of a reverse shock. Later observations can be modeled by a standard forward shock model and show no evidence of a jet break, allowing us to constrain the jet collimation to θj > 7°. Using deep late-time optical observations, we place an upper limit of r > 24 AB mag on any underlying host galaxy. The detection of the afterglow in the Swift UV filters constrains the GRB redshift to z < 1.3 and places an upper bound on the γ-ray isotropic equivalent energy Eγ,iso < 3 × 1051 erg. The properties of this GRB (e.g., duration, hardness ratio, energy, and environment) lie at the intersection between short and long bursts, and we cannot conclusively identify its type. We estimate that the probability that it is drawn from the population of short GRBs is 10%-30%.
Measurement of polarized light provides a direct probe of magnetic fields in collimated outflows (jets) of relativistic plasma from accreting stellar-mass black holes at cosmological distances. These ...outflows power brief and intense flashes of prompt gamma-rays known as Gamma Ray Bursts (GRBs), followed by longer-lived afterglow radiation detected across the electromagnetic spectrum. Rapid-response polarimetric observations of newly discovered GRBs have probed the initial afterglow phase. Linear polarization degrees as high as Π∼30% are detected minutes after the end of the prompt GRB emission, consistent with a stable, globally ordered magnetic field permeating the jet at large distances from the central source. In contrast, optical and gamma-ray observations during the prompt phase led to discordant and often controversial results, and no definitive conclusions on the origin of the prompt radiation or the configuration of the magnetic field could be derived. Here we report the detection of linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt gamma-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the GRB jet, closer to the central source, and show that the prompt GRB phase is produced via fast cooling synchrotron radiation in a large-scale magnetic field advected from the central black hole and distorted from dissipation processes within the jet.
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