Explore the vast range of titles available.

Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. The duration and hardness distribution of GRBs has two clusters, now understood to reflect (at least) two different progenitors. Short-hard GRBs (SGRBs; T(sub 90) < 2 s) arise from compact binary mergers, and long-soft GRBs (LGRBs; T(sub 90) > 2 s) have been attributed to the collapse of peculiar massive stars (collapsars). The discovery of SN 1998bw/GRB 980425 marked the first association of an LGRB with a collapsar, and AT 2017gfo/GRB 170817A/GW170817 marked the first association of an SGRB with a binary neutron star merger, which also produced a gravitational wave. Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi satellite and the Interplanetary Network localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova, but which is consistent with being the supernova. Although the GRB duration is short (rest-frame T90 of 0.65 s), our panchromatic follow-up data confirm a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.

While the dominant radiation mechanism of gamma-ray bursts (GRBs) remains a question of debate, synchrotron emission is one of the foremost candidates to describe the multi-wavelength afterglow observations. As such, it is expected that GRBs should present some degree of polarization across their evolution—presenting a feasible means of probing these bursts’ energetic and angular properties. Although obtaining polarization data is difficult due to the inherent complexities regarding GRB observations, advances are being made, and theoretical modeling of synchrotron polarization is now more relevant than ever. In this manuscript, we present the polarization for a fiduciary model, where the synchrotron FS emission evolving in the radiative–adiabatic regime is described by a radially stratified off-axis outflow. This is parameterized with a power-law velocity distribution and decelerated in a constant-density and wind-like external environment. We apply this theoretical polarization model for two select GRBs, presenting upper limits in their polarization—GRB 170817A, a known off-axis GRB with radio polarization upper limits, and GRB 190014C, an on-axis case, where the burst was seen from within the half-opening angle of the jet, with observed optical polarization—in an attempt to constrain their magnetic field geometry in the emitting region.

Short gamma-ray bursts (GRBs) are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum 1 . Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10–300 ms before collapsing to a black hole 2 , 3 . Numerical relativity simulations across different groups consistently show broad power spectral features in the 1–5-kHz range in the post-merger gravitational-wave signal 4 – 14 , which is inaccessible by current gravitational-wave detectors but could be seen by future third-generation ground-based detectors in the next decade 15 – 17 . This implies the possibility of quasiperiodic modulation of the emitted gamma rays in a subset of events in which a neutron star is formed shortly before the final collapse to a black hole 18 – 21 . Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival Burst and Transient Source Experiment (BATSE) data, which are compatible with the predictions from numerical relativity. Two signals identified in short gamma-ray bursts from archival Burst and Transient Source Experiment data show kilohertz quasiperiodic oscillations, implying the ringing of a hypermassive neutron star before collapsing to a black hole.

Neutrinos interact only very weakly with matter, but giant detectors have succeeded in detecting small numbers of astrophysical neutrinos. Aside from a diffuse background, only two individual sources have been identified: the Sun and a nearby supernova in 1987. A multiteam collaboration detected a high-energy neutrino event whose arrival direction was consistent with a known blazar—a type of quasar with a relativistic jet oriented directly along our line of sight. The blazar, TXS 0506+056, was found to be undergoing a gamma-ray flare, prompting an extensive multiwavelength campaign. Motivated by this discovery, the IceCube collaboration examined lower-energy neutrinos detected over the previous several years, finding an excess emission at the location of the blazar. Thus, blazars are a source of astrophysical neutrinos. Science , this issue p. 147 , p. eaat1378 A high-energy neutrino was emitted by a blazar during a flare, prompting observations across the electromagnetic spectrum. Previous detections of individual astrophysical sources of neutrinos are limited to the Sun and the supernova 1987A, whereas the origins of the diffuse flux of high-energy cosmic neutrinos remain unidentified. On 22 September 2017, we detected a high-energy neutrino, IceCube-170922A, with an energy of ~290 tera–electron volts. Its arrival direction was consistent with the location of a known γ-ray blazar, TXS 0506+056, observed to be in a flaring state. An extensive multiwavelength campaign followed, ranging from radio frequencies to γ-rays. These observations characterize the variability and energetics of the blazar and include the detection of TXS 0506+056 in very-high-energy γ-rays. This observation of a neutrino in spatial coincidence with a γ-ray–emitting blazar during an active phase suggests that blazars may be a source of high-energy neutrinos.

Observationally, kilonovae are astrophysical transients powered by the radioactive decay of nuclei heavier than iron, thought to be synthesized in the merger of two compact objects 1 – 4 . Over the first few days, the kilonova evolution is dominated by a large number of radioactive isotopes contributing to the heating rate 2 , 5 . On timescales of weeks to months, its behaviour is predicted to differ depending on the ejecta composition and the merger remnant 6 – 8 . Previous work has shown that the kilonova associated with gamma-ray burst 230307A is similar to kilonova AT2017gfo (ref. 9 ), and mid-infrared spectra revealed an emission line at 2.15 micrometres that was attributed to tellurium. Here we report a multi-wavelength analysis, including publicly available James Webb Space Telescope data 9 and our own Hubble Space Telescope data, for the same gamma-ray burst. We model its evolution up to two months after the burst and show that, at these late times, the recession of the photospheric radius and the rapidly decaying bolometric luminosity ( L bol  ∝  t −2.7±0.4 , where t is time) support the recombination of lanthanide-rich ejecta as they cool. A modelling analysis shows that an unusually long gamma-ray burst gave rise to a lanthanide-rich kilonova following the merger of a neutron star–neutron star or of a neutron star–black hole.

Short gamma-ray bursts are associated with binary neutron star mergers, which are multimessenger astronomical events that have been observed both in gravitational waves and in the multiband electromagnetic spectrum. Depending on the masses of the stars in the binary and on details of their largely unknown equation of state, a dynamically evolving and short-lived neutron star may be formed after the merger, existing for approximately 10-300 ms before collapsing to a black hole. Numerical relativity simulations across different groups consistently show broad power spectral features in the 1-5 kHz range in the post-merger gravitational wave signal, which is inaccessible by current gravitational-wave detectors but could be seen by future third generation ground-based detectors in the next decade. This implies the possibility of quasiperiodic modulation of the emitted gamma-rays in a subset of events where a neutron star is formed shortly prior to the final collapse to a black hole. Here we present two such signals identified in the short bursts GRB 910711 and GRB 931101B from archival BATSE data, which are compatible with the predictions from numerical relativity.

The gamma-ray burst (GRB) GRB~211211A is believed to have occurred due to the merger of two neutron stars or a neutron star and a black hole, despite its duration of more than a minute. Subsequent analysis has revealed numerous interesting properties including the possible presence of a \\(\\sim 22\\)~Hz quasiperiodic oscillation (QPO) during precursor emission. Here we perform timing analysis of Fermi and Swift gamma-ray data on GRB~211211A and, although we do not find a strong QPO during the precursor, we do find an extremely significant 19.5~Hz flux oscillation, which has higher fractional amplitude at higher energies, in a \\(\\sim 0.2\\)~second segment beginning \\(\\sim 1.6\\)~seconds after the start of the burst. After presenting our analysis we discuss possible mechanisms for the oscillation.

We use a sample of 27 GRBs at redshift \\(z=2-6\\) to probe the outflows in their respective host galaxies (\\(\\mathrm{log(M_*/M_{\\odot})}~\\sim~9-11\\)) and search for possible relations between the outflow properties and those of the host galaxies such as \\(\\mathrm{M_*}\\), SFR, and specific SFR. First, we consider three outflow properties \\(-\\) outflow column density (\\(\\mathrm{N_{out}}\\)), maximum outflow velocity (\\(\\mathrm{V_{max}}\\)), and normalized maximum velocity (\\(\\mathrm{V_{norm}}\\) = \\(\\mathrm{V_{max}/V_{circ, halo}}\\), where \\(\\mathrm{V_{circ,halo}}\\) is the halo circular velocity). We observe clear trends of \\(\\mathrm{N_{out}}\\) and \\(\\mathrm{V_{max}}\\) with increasing SFR in high-ion-traced outflows, with a stronger (\\(>~3\\sigma\\)) \\(\\mathrm{V_{max}}-\\)SFR correlation. We find that the estimated mass outflow rate and momentum flux of the high-ion outflows scale with SFR and can be supported by the momentum imparted by star formation (supernovae and stellar winds). The kinematic correlations of high-ion-traced outflows with SFR are similar to those observed for star-forming galaxies at low redshifts. The correlations with SFR are weaker in low-ions. This, along with the lower detection fraction in low-ions, indicates that the outflow is primarily high-ion dominated. We also observe a strong (\\(>~3\\sigma\\)) trend of normalized velocity (\\(\\mathrm{V_{norm}}\\)) decreasing with halo mass and increasing with sSFR, suggesting that outflows from low-mass halos and high-sSFR galaxies are most likely to escape and enrich the outer CGM and IGM with metals. By comparing the CGM-GRB stacks with those of starbursts at \\(z\\sim2\\) and \\(z\\sim0.1\\), we find that over a broad redshift range, the outflow strength strongly depends on the main-sequence offset at the respective redshifts rather than simply the SFR.

Energetic GeV photons expected from the closest and the most energetic Gamma-ray bursts (GRBs) provide an unique opportunity to study the very-high-energy emission as well as the possible correlations with lower energy bands in realistic GRB afterglow models. In the standard GRB afterglow model, the relativistic homogeneous shock is usually considered to be fully adiabatic, however, it could be partially radiative. Based on the external forward-shock scenario in both stellar wind and constant-density medium. We present a radiative-adiabatic analytical model of the synchrotron self-Compton (SSC) and synchrotron processes considering an electron energy distribution with a power-law index of 1 < p < 2 and 2 \\(\\leq\\) p. We show that the SSC scenario plays a relevant role in the radiative parameter \\(\\epsilon\\), leading to a prolonged evolution during the slow cooling regime. In a particular case, we derive the Fermi/LAT light curves together with the photons with energies \\(\\geq\\) 100 MeV in a sample of nine bursts from the second Fermi/LAT GRB catalog that exhibited temporal and spectral indices with \\(\\geq\\) 1.5 and \\(\\approx\\) 2, respectively. These events can hardly be described with closure relations of the standard synchrotron afterglow model, and also exhibit energetic photons above the synchrotron limit. We have modeled the multi-wavelength observations of our sample to constrain the microphysical parameters, the circumburst density, the bulk Lorentz factor and the mechanism responsible for explaining the energetic GeV photons.