Gamma-ray bursts (GRBs) are classified into long and short events. Long GRBs (LGRBs) are associated with the end states of very massive stars, while short GRBs (SGRBs) are linked to the merger of compact objects. GRB 200826A was a peculiar event, because by definition it was an SGRB, with a rest-frame duration of ∼0.5 s. However, this event was energetic and soft, which is consistent with LGRBs. The relatively low redshift (z = 0.7486) motivated a comprehensive, multiwavelength follow-up campaign to characterize its host, search for a possible associated supernova (SN), and thus understand the origin of this burst. To this aim we obtained a combination of deep near-infrared (NIR) and optical imaging together with spectroscopy. Our analysis reveals an optical and NIR bump in the light curve whose luminosity and evolution are in agreement with several SNe associated to LGRBs. Analysis of the prompt GRB shows that this event follows the E _p,i–E _iso relation found for LGRBs. The host galaxy is a low-mass star-forming galaxy, typical of LGRBs, but with one of the highest star formation rates, especially with respect to its mass ( log M * / M ⊙ = 8.6 , SFR ∼ 4.0 M _⊙ yr⁻¹). We conclude that GRB 200826A is a typical collapsar event in the low tail of the duration distribution of LGRBs. These findings support theoretical predictions that events produced by collapsars can be as short as 0.5 s in the host frame and further confirm that duration alone is not an efficient discriminator for the progenitor class of a GRB.

The results of observations of the gravitational-wave (GW) event S190425z recorded by the LIGO/Virgo detectors with the anti-coincidence shield (ACS) of the SPI gamma-ray spectrometer onboard the INTEGRAL observatory are presented. With a high probability (>99%) it was associated with a neutron star (NS) merger in a close binary system. This is only the second event of such a type in the history of gravitational-wave observations (after GW170817). A weak gamma-ray burst, GRB190425, consisting of two pulses ∼0.5 and ∼5.9 s after the NS merger in the event S190425z with an a priori significance of 3.5 and 4.4σ (taken together 5.5σ) was detected by SPI-ACS. Analysis of the SPI-ACS count rate history recorded on these days (a total of ∼125 ks of observations) has shown that the rate of random occurrence of two close spikes with the characteristics of GRB190425 does not exceed 6.4 × 10⁻⁵ s⁻¹ (i.e., such events occur by chance, on average, every ∼4.3 hours). Note that the time profile of GRB190425 has much in common with the profile of GRB170817A accompanying the event GW170817, that both NS mergers were the nearest (≤150 Mpc) of all the events recorded by the LIGO/Virgo detectors, and that no significant excesses of the gamma-ray flux above the background were detected in any of ∼30 black hole merger events recorded to date by these detectors. No bursts of hard radiation were detected in the field of view of the SPI and IBIS/ISGRI gamma-ray telescopes onboard INTEGRAL. This, along with the absence of detection of gamma-ray emission from GRB190425 by the GBM gamma-ray burst monitor of the Fermi observatory suggesting its occultation by the Earth, allows the localization region for the source of this GWevent to be reduced significantly. The parameters E_iso and E_p for GRB190425 are estimated and compared with those for GRB170817A.

We present multiband observations and analysis of EP240801a, a low-energy, extremely soft gamma-ray burst (GRB) discovered on 2024 August 1 by the Einstein Probe (EP) satellite with a weak contemporaneous signal also detected by the Fermi Gamma-ray Burst Monitor (GBM). Optical spectroscopy of the afterglow, obtained by Gran Telescopio Canarias and Keck, identified the redshift of z = 1.6734. EP240801a exhibits a burst duration of 148 s in X-rays and 22.3 s in gamma rays, with X-rays leading by 80.61 s. Spectral lag analysis indicates that the gamma-ray signal arrived 8.3 s earlier than the X-rays. Joint spectral fitting of EP Wide-field X-ray Telescope and Fermi/GBM data yields an isotropic energy E γ , iso = ( 5.5 7 − 0.50 + 0.54 ) × 1 0 51 erg , a peak energy E peak = 14.9 0 − 4.71 + 7.08 keV , and a fluence ratio S(25–50 keV)/S ( 50 – 100 keV ) = 1.6 7 − 0.46 + 0.74 , classifying EP240801a as an X-ray flash (XRF). The host-galaxy continuum spectrum, inferred using Prospector, was used to correct its contribution for the observed outburst optical data. Unusual early R-band behavior and EP Follow-up X-ray Telescope observations suggest multiple components in the afterglow. Three models are considered: a two-component jet model, a forward-reverse shock model, and a forward shock model with energy injection. All three provide reasonable explanations. The two-component jet model and the energy injection model imply a relatively small initial energy and velocity of the jet in the line of sight, while the forward-reverse shock model remains typical. Under the two-component jet model, EP240801a may resemble GRB 221009A (BOAT) if the bright narrow beam is viewed on-axis. Therefore, EP240801a can be interpreted as an off-beam (narrow) jet or an intrinsically weak GRB jet. Our findings provide crucial clues for uncovering the origin of XRFs.

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For gamma-ray burst 100901A, we have obtained Gemini-North and Very Large Telescope optical afterglow spectra at four epochs: 1 h, 1 d, 3 d and 1 week after the burst, thanks to the afterglow remaining unusually bright at late times. Apart from a wealth of metal resonance lines, we also detect lines arising from fine-structure levels of the ground state of Fe ii, and from metastable levels of Fe ii and Ni ii at the host redshift (z = 1.4084). These lines are found to vary significantly in time. The combination of the data and modelling results shows that we detect the fall of the Ni ii ⁴F_9/2 metastable level population, which to date has not been observed. Assuming that the population of the excited states is due to the ultraviolet radiation of the afterglow, we estimate an absorber distance of a few hundred pc. This appears to be a typical value when compared to similar studies. We detect two intervening absorbers (z = 1.3147, 1.3179). Despite the wide temporal range of the data, we do not see significant variation in the absorption lines of these two intervening systems.

We present a brief history and the current status of research on gamma-ray bursts (GRBs), including observations at various GRB phases: the prompt emission, afterglow, supernova/kilonova, and the host galaxy. An overview of the known phenomenological features and theoretical models for GRBs is given. Problems, the solution of which is necessary for a successful study of the phenomenon of GRB, are discussed.