Gregory Corcoran

We present a multiwavelength study of the γ /X-ray transient EP250416a (also designated GRB 250416C), triggered by the Einstein Probe (EP) Wide-field X-ray Telescope as well as by SVOM and Konus-Wind (KW). Observations spanning the γ -ray, X-ray, and optical bands facilitated a detailed analysis of the burst’s prompt emission, afterglow evolution, and physical origin. GRB 250416C exhibits a burst duration of 30 s in the X-ray and 17.7 s in γ rays, and a joint spectral fitting of 0.5−5000 keV data yields E _peak = 342 ₋₂₃₂ ^+0.90 keV. Optical spectroscopy of the afterglow, acquired with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South, yields a redshift of z = 0.963. Accounting for the measured redshift, the isotropic energies are E _X,sio = 2.7 _−0.5 ^+0.9 × 10 ⁵⁰ erg and E _{γ ,sio} = 7.34 _−2.1 ^+5.1 × 10 ⁵¹ erg, aligning with the Amati relation for long GRBs. The fluence ratio S (25−50 keV)/ S (50−100 KeV) = 0.78 _−0.12 ^+0.10 classifies GRB 250416C as an X-ray rich GRB. The X-ray afterglow exhibits initial shallow decay ( α ≈ −0.5) transitioning to a canonical decay phase ( α ≈ −1), with a very late jet break at t ∼ 1.5 × 10 ⁶ s. This corresponds to a jet half-opening angle of θ _j = 10.6 _−1.8 ^+1.9 degrees. GRB 250416C is optically dark, as it shows only a faint r -band detection ( r = 24.16 ± 0.13 mag) from Gemini South-GMOS and a low optical-to-X-ray spectral index β _OX = 0.3. This may be attributed to significant host-galaxy extinction, with a required A _V ^host = 2.7 ± 0.3 mag derived from the extinction curve model.

Science is currently at an age where there is more data than we know how to deal with. Machine learning (ML) is an emerging tool that is useful for drawing valuable science out of incomprehensibly large datasets and identifying complex trends in data that may otherwise be overlooked. Moreover, ML can potentially enhance the quality and quantity of scientific data as they are collected. This paper explores how a new ML method can improve the rate of classification of rare broad-lined Ic (Ic-BL) supernovae (SNe). We introduce new parameters called magnitude rates to train ML models to identify SNe Ic-BL in large datasets and apply this same methodology to a population of SN Ia to test if our ML approach is reproducible. The information we required to train each ML model included three magnitudes, three time differences, two magnitude rates, and the second derivative of these rates using the first three available photometric data points in a single filter. Our initial investigations showed that the random forest algorithm provides a strong foundation for the early classifications SNe Ic-BL and SNe Ia. Testing this model again on an unseen dataset showed that the model can identify upward of 13.6% of the total true SN Ic-BL population, significantly improving on current methods. By implementing a dedicated observation campaign using this model, the number of SN Ic-BL classified and the quality of early-time data collected each year will see considerable growth in the near future.

We present detailed, multi-wavelength analysis of GRB 210704A: a Fermi/GBM discovered and Fermi/LAT detected gamma-ray burst (GRB). The burst is dominated by a short (≈2 s) pulse followed by weaker, softer emission. We line stack our afterglow spectrum and determine the most likely redshift to be z = 2.34. This is corroborated by the photometric redshift of the extended source underlying the GRB. The spectral energy distribution fit parameters, late-time imaging, as well as the GRB’s energetics, spectral lag, and location point to a collapsar nature. Follow-up observations reveal excess optical/infrared emission with respect to a standard afterglow, peaking around T₀ + 7 d (2 d in the rest frame). The excess is extremely luminous (M_r = −22.0 mag) and rapidly evolving. Strikingly, it resembles the emission seen in recently discovered Einstein Probe fast X-ray transients EP241021a and EP240414a, as well as the population of luminous fast blue optical transients (LFBOTs). This provides a link between these sources and GRBs. Fermi/LAT observations imply a high Lorentz factor, making this a case where LFBOT-like emission is also associated with a powerful successfully launched jet. We model the excess as likely coming from an energetic refreshed shock.

We present follow-up observations of the day-long, repeating gamma-ray burst (GRB) GRB 250702B with the Near Infrared Spectrograph on board the James Webb Space Telescope. Through the identification of narrow hydrogen emission lines at a consistent redshift of z = 1.036 ± 0.004, we calibrate the distance scale, and therefore the energetics, of this unique extragalactic transient. At this distance, the resulting γ -ray energy release is at least E _{γ ,iso} = 2.2 × 10 ⁵⁴ erg. We find no evidence for ongoing transient emission at the GRB position and exclude any accompanying supernova (SN) with a luminosity comparable to the Type Ic broad-line SN 2023lcr, though we are unable to rule out a fainter SN counterpart owing to high extinction. The inferred rate of such events, assuming at most one in the lifetime of Fermi, suggests that such bursts are very rare, with volumetric rates over 1000 times lower than normal high-luminosity long GRBs and >10 ⁵ times lower than core-collapse SNe, when corrected for beaming. Furthermore, we find that the host galaxy is unique among GRB host galaxies and extremely rare in the general galaxy population, being extremely large and dusty and with high stellar mass. The identification of such an exotic GRB in such an unusual galaxy raises the possibility that the environment was important in the progenitor channel creating GRB 250702B.

Most long gamma-ray bursts (LGRBs) originate from a rare type of massive stellar explosion. Their afterglows, while rapidly fading, can initially be extremely luminous at optical and near-infrared wavelengths, making them detectable at large cosmological distances. Here we report the detection and observations of GRB 250314A by the SVOM satellite and the subsequent follow-up campaign that led to the discovery of the near-infrared afterglow and spectroscopic measurements of its redshift z ≃ 7.3. This burst occurred when the Universe was only about 5% of its current age. We discuss the signature of these rare events within the context of the SVOM operating model and the ways to optimise their identification with adapted ground follow-up observation strategies.

The majority of long-duration gamma-ray bursts (GRBs) are thought to arise from the collapse of massive stars, making them powerful tracers of star formation across cosmic time. Evidence for this origin comes from the presence of supernovae (SNe) in the aftermath of the GRB event, whose properties in turn link back to those of the collapsing star. In principle, thanks to GRBs, we can study the properties of individual stars in the distant Universe. Here, we present JWST/NIRCAM observations that have been able to detect both the host galaxy and a likely SN in the Space-based multi-band astronomical Variable Objects Monitor (SVOM) detected GRB 250314A. GRB 250314A has a spectroscopically measured redshift of z ≃ 7.3, placing it deep in the era of reionisation. The data are well described by a combination of faint blue host, similar to many z ∼ 7 galaxies, with a SN of similar luminosity to the prototype GRB SN, namely, SN 1998bw. The SN may be somewhat fainter if a larger contribution from the underlying galaxy is present, but SNe brighter than SN 1998bw can be excluded, given the evidence of low dust extinction, based on the blue afterglow colours. These observations suggest that despite disparate physical conditions, the star that created GRB 250314A was not much more massive and it might even resemble the GRB progenitors of the Local Universe.

γ-ray bursts (GRBs) are singular outbursts of high-energy radiation with durations typically lasting from milliseconds to minutes and, in extreme cases, a few hours. They are attributed to the catastrophic outcomes of stellar-scale events and, as such, are not expected to recur. Here, we present observations of the exceptional GRB 250702B (formerly GRB 250702BDE) which triggered the Fermi GRB monitor on three occasions over several hours, and which was detected in soft X-rays by the Einstein Probe several hours before the γ-ray triggers (EP 250702a). We present the discovery of an extremely red infrared counterpart of the event with the Very Large Telescope, as well as radio observations from MeerKAT. Hubble Space Telescope observations pinpoint the source to a nonnuclear location in a host galaxy with complex morphology, implying GRB 250702B is an extragalactic event. The multiwavelength counterpart is well described with standard afterglow models at a relatively low redshift z ∼ 0.3, but the prompt emission does not readily fit within the expectations for either collapsar or merger-driven GRBs. Indeed, a striking feature of the multiple prompt outbursts is that the third occurs at an integer multiple of the interval between the first two. Although not conclusive, this could be indicative of periodicity in the progenitor system. We discuss several possible scenarios to explain the exceptional properties of the burst, which suggest that either a very unusual collapsar or the tidal disruption of a white dwarf by an intermediate-mass black hole are plausible explanations for this unprecedented GRB.