Andrea Melandri

The candidate gravitational wave (GW) event S250818k was one of only three non-retracted LIGO-Virgo-KAGRA public alerts issued during the fourth observing run of the network (O4) with a binary neutron star (BNS) merger classification probability exceeding one percent. This triggered a prompt search for a potential electromagnetic (EM) counterpart in the large localization error region (949 deg ² projected in the sky at the 90% credible level). The transient supernova (SN) 2025ulz discovered by the Zwicky Transient Facility (ZTF) during the search, attracted a great deal of attention due to a potential spatial and temporal coincidence, and due to its initial fast decay and featureless spectrum. Here, we report on the follow-up of this transient by the Electromagnetic counterparts of gravitational wave sources at the Very Large Telescope (ENGRAVE) Collaboration. We conducted an extensive multi-wavelength observational campaign, which led to the spectral classification of the transient as a type IIb SN, indicating that it is unrelated to the candidate GW event. In this article, we describe our observing strategies, data reduction, and interpretation. All of our results confirm and strengthen our classification of the source, and also show that shock-cooling tails associated with type IIb SNe are one of the most prominent contaminants in kilonova searches.

Context. The long gamma-ray burst GRB 180728A at a redshift of z = 0.1171 stands out due to its high isotropic energy of E _γ, iso ≈ 2.5 × 10 ⁵¹ erg, in contrast with most events at redshift z < 0.2, but it is comparable to the bulk of luminous bursts more common at higher redshift. Aims. We aim to study the properties of GRB 180728A’s prompt emission, afterglow, and associated supernova (SN 2018fip), comparing them with other GRB-SN events. Methods. This study employs a dense photometric and spectroscopic follow-up of the afterglow and the SN up to 80 days after the burst. We used image subtraction to remove the presence of a nearby bright star, and modelled both the afterglow and the supernova. Results. This event lies on the E _p, i – E _γ, iso plane occupied by classical collapsar events, and the prompt emission is one of the most energetic at z < 0.2 after GRB 030329 and GRB 221009A. The afterglow of GRB 180728A is less luminous than that of most long GRBs, showing a shallow early phase that steepens after about 5 hours (0.2 days). The GRB exploded in an irregular low-mass blue star-forming galaxy, which is typical of low- z collapsar events. Because of the relatively faint afterglow, the light curve bump of SN 2018fip dominates the optical emission already after approximately 3 days and is one of the best sampled to date. The strong suppression below ∼4000 Å and a largely featureless continuum in the early 6–9 day spectra favour aspherical two-component ejecta with a high-velocity collimated component (> 20 000 km s ⁻¹ ), that is dominant early on and a more massive low-velocity component that dominates at much later epochs. Conclusions. Our findings indicate that asymmetries need to be considered in order to better understand GRB-SNe. In any case, SN 2018fip shares many characteristics with typical GRB-SNe. Its kinetic energy is below the common range of 10 ⁵² –10 ⁵³ erg and does not correlate with the high energy of the GRB, highlighting the complexity and diversity of the GRB-SN energy budget partition.

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.

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

We report the discovery of an extremely high-velocity outflow (EHVO) in the most luminous QSO ( L _Bol ∼ 2.29 × 10 ⁴⁸ erg/s), named SMSS J2157-3602, at z = 4.692. Combined XSHOOTER and NIRES observations reveal that the EHVO reaches a maximum velocity of v _max ∼ 0.13 c and persists over rest-frame timescales of a few months up to one year. SMSS J2157-3602 also exhibits one of the highest balnicity index values discovered for an EHVO so far. In addition, the blueshifted CIV emission traces a high-velocity ( v _CIV ⁵⁰ ∼ 4660 km/s) outflow from the broad-line region (BLR). Thanks to an XMM-Newton observation, we were also able to reveal the X-ray weak nature of this QSO, which likely prevents the overionization of the innermost disk atmosphere and facilitates the efficient launch of the detected EHVO and BLR winds. The extraordinary luminosity of SMSS J2157-3602 and the extreme velocity of the EHVO make it a unique laboratory for testing active galactic nucleus (AGN) driven feedback under extreme conditions. Current uncertainties on the outflow’s location and column density strengthen the case for a dedicated follow-up, which will be essential to assess the full feedback potential of this remarkable quasar.

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.

Early multiwavelength observations offer crucial insights into the nature of the relativistic jets responsible for gamma-ray bursts and their interaction with the surrounding medium. We present data of GRB 240825A from 17 space- and ground-based telescopes/instruments, covering wavelengths from NIR/optical to X-ray and GeV, and spanning from the prompt emission to the afterglow phase triggered by Swift and Fermi. The early afterglow observations were carried out by SVOM/C-GFT, and spectroscopic observations of the afterglow by GTC, VLT, and TNG determined the redshift of the burst (z = 0.659) later. A comprehensive analysis of the prompt emission spectrum observed by Swift-BAT and Fermi-GBM/LAT reveals a rare and significant high-energy cutoff at 76 MeV. Assuming this cutoff is due to γγ absorption allows us to place an upper limit on the initial Lorentz factor, Γ₀ < 245. The optical/NIR and GeV afterglow light curves can be described by the standard external shock model, with early-time emission dominated by a reverse shock (RS) and a subsequent transition to forward shock (FS) emission. Our afterglow modeling yields a consistent estimate of the initial Lorentz factor (Γ₀ ∼ 234). Furthermore, the RS-to-FS magnetic field ratio ( R B ∼ 302 ) indicates that the RS region is significantly more magnetized than the FS region. An isotropic-equivalent kinetic energy of E _k,iso = 5.25 × 10⁵⁴ erg is derived, and the corresponding γ-ray radiation efficiency is estimated to be η _γ = 3.1%. On the other hand, the standard afterglow model cannot reproduce the X-ray light curve of GRB 240825A, calling for improved models to characterize all multiwavelength data.