We report the discovery of GRB 221009A, the highest flux gamma-ray burst (GRB) ever observed by the Fermi Gamma-ray Burst Monitor (Fermi-GBM). This GRB has continuous prompt emission lasting more than 600 s, which smoothly transitions to afterglow emission visible in the Fermi-GBM energy range (8 keV–40 MeV), and total energetics higher than any other burst in the Fermi-GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the Fermi-GBM trigger time (t ₀; 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock breakout, with significant emission up to ∼15 MeV. We characterize the onset of external shock at t ₀ + 600 s and find evidence of a plateau region in the early-afterglow phase, which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the Fermi-GBM sample and find that this GRB has the highest total isotropic-equivalent energy (E _γ,iso = 1.0 × 10⁵⁵ erg) and second highest isotropic-equivalent luminosity (L _γ,iso = 9.9 × 10⁵³ erg s^–1) based on its redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of Fermi-GBM from the beginning of the prompt emission phase through the onset of early afterglow.

The X-ray to radio afterglow emission of GRB 170817A / GW 170817 so far scales as F_ν ∝ ν^−0.6t^0.8 with observed frequency and time, consistent with a single power-law segment of the synchrotron spectrum from the external shock going into the ambient medium. This requires the effective isotropic equivalent afterglow shock energy in the visible region to increase as ∼t^1.7. The two main channels for such an energy increase are (i) radial: more energy carried by slower material (in the visible region) gradually catches up with the afterglow shock and energizes it, and (ii) angular: more energy in relativistic outflow moving at different angles to our line of sight, whose radiation is initially beamed away from us but its beaming cone gradually reaches our line of sight as it decelerates. One cannot distinguish between these explanations (or combinations of them) using only the X-ray to radio F_ν(t). Here, we demonstrate that the most promising way to break this degeneracy is through afterglow imaging and polarization, by calculating the predicted evolution of the afterglow image (its size, shape, and flux centroid) and linear polarization Π(t) for different angular and/or radial outflow structures that fit F_ν(t). We consider two angular profiles – a Gaussian and a narrow core with power-law wings in energy per solid angle, as well as a (cocoon motivated) (quasi-) spherical flow with radial velocity profile. For a jet viewed off-axis (and a magnetic field produced in the afterglow shock) Π(t) peaks when the jet’s core becomes visible, at ≈2t_p where the light-curve peaks at t_p, and the image can be elongated with aspect ratios ≳ 2. A quasi-spherical flow has an almost circular image and a much lower Π(t) (peaking at ≈t_p) and flux centroid displacement θ_fc (a spherical flow has Π(t) = θ_fc = 0 and a perfectly circular image).

The main hard pulse of prompt gamma-ray emission in GRB 170817A had a duration of ∼0.5 s, and its onset was delayed with respect to the gravitational-wave chirp signal by . Detailed follow-up of the subsequent broadband kilonova emission revealed a two-component ejecta—a lanthanide-poor ejecta with mass M _ej,blue ≈ 0.025 M _⊙ that powered the early but rapidly fading blue emission and a lanthanide-rich ejecta with mass that powered the longer-lasting redder emission. Both the prompt gamma-ray onset delay and the existence of the blue ejecta with a modest electron fraction, 0.2 ≲ Y _e ≲ 0.3, can be explained if the collapse to a black hole (BH) was delayed by the formation of a hypermassive neutron star. Here we determine the survival time of the merger remnant by combining two different constraints, namely, the time needed to produce the requisite blue-ejecta mass and that necessary for the relativistic jet to bore its way out of the expanding ejecta. In this way, we determine that the remnant of GW170817 must have collapsed to a BH after . We also discuss how future detections and the delays between the gravitational and electromagnetic emissions can be used to constrain the properties of the merged object.

The first, long-awaited, detection of a gravitational-wave (GW) signal from the merger of a binary neutron star (NS–NS) system was finally achieved (GW170817) and was also accompanied by an electromagnetic counterpart—the short-duration gamma-ray burst (GRB) 170817A. It occurred in the nearby ( Mpc) elliptical galaxy NGC 4993 and showed optical, IR, and UV emission from half a day up to weeks after the event, as well as late-time X-ray (at days) and radio (at days) emission. There was a delay of between the GW merger chirp signal and the prompt GRB emission onset, and an upper limit of was set on the viewing angle w.r.t the jet’s symmetry axis from the GW signal. In this letter we examine some of the implications of these groundbreaking observations. The delay sets an upper limit on the prompt GRB emission radius, , for a jet with sharp edges at an angle . GRB 170817A’s relatively low isotropic equivalent γ-ray energy output may suggest a viewing angle slightly outside the jet’s sharp edge, , but its peak photon energy and afterglow emission suggest instead that the jet does not have sharp edges and the prompt emission was dominated by less energetic material along our line of sight, at . Finally, we consider the type of remnant that is produced by the NS–NS merger and find that a relatively long-lived ( s) massive NS is strongly disfavored, while a hyper-massive NS of lifetime appears to be somewhat favored over the direct formation of a black hole.

The short-duration (≲ 2 s) GRB 170817A in the nearby (D ≈ 40 Mpc) elliptical galaxy NGC 4993 is the first electromagnetic counterpart of the first gravitational wave detection of a binary neutron-star (NS–NS) merger. It was followed by optical, IR, and UV emission from half a day up to weeks after the event, as well as late time X-ray to radio emission. The early UV, optical, and IR emission showed a quasi-thermal spectrum suggestive of radioactive-decay powered kilonova-like emission. The late onset of the X-ray (8.9 d) and radio (16.4 d) afterglow emission, together with the low isotropic equivalent γ-ray energy output (E_{γ, iso} ≈ 5 × 10⁴⁶ erg), strongly suggest emission from a narrow relativistic jet viewed off-axis, initially dominated by low-energy material along our line of sight and gradually overtaken by the more energetic parts of the jet near its core. Here, we set up a general framework for off-axis GRB jet afterglow emission, comparing analytic and numerical approaches, and showing their general predictions for short-hard GRBs that accompany binary NS mergers. The prompt GRB emission suggests a viewing angle well outside the jet’s core, and we compare the afterglow light curves expected in such a case to the X-ray, optical, and radio emission from GRB 170817A. We fit the data using a simulation-based off-axis relativistic jet afterglow model featuring an initially top-hat jet, and find a satisfactory fit for a viewing angle to initial jet half-opening angle ratio of θ_obs/θ₀ ≈ 3, or θ_obs ≈ 17°(θ₀/0.1).

We analyzed broadband X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a blackbody+power-law (BB+PL) or 2BB model during all three outbursts. Nuclear Spectroscopic Telescope Array observations revealed a hard X-ray tail, with a PL photon index Γ = 0.9, extending up to 50 keV, with flux comparable to the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5–10 keV flux from SGR J1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of ∼7 following its strongest 2016 June outburst. A Swift/X-Ray Telescope observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of 25 compared to quiescence, while the cold BB component kT = 0.47 keV remained more or less constant. The 2014 and 2015 outbursts decayed quasi-exponentially with timescales of ∼40 days, while the stronger 2016 May and June outbursts showed a quick short-term decay with timescales of about four days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 μJy for the 4.6 GHz observations and 7 μJy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models.