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Abstract:

Supernovae (SNe) IIn are terminal explosions of massive stars that are surrounded by a dense circumstellar medium (CSM). Among SNe IIn, a notable subset is the SN 2009ip-like, which exhibits an initial, fainter peak attributed to stellar variability in the late evolutionary stages, followed by a brighter peak, interpreted as the SN explosion itself. In this context, we analysed the spectrophotometric evolution of SN 2024hpj, an object with a triple-peaked light curve and spectra typical of a SN IIn but with a complex line profile composed of broad P-Cygni features topped by narrow emissions. Comparing it with other SN 2009ip-like events in the literature, as well as with other unpublished objects (SNe 2019mry, 2022ytx, 2024uzf, and 2025csc), we identify star-forming regions as their preferred formation environment. On the other hand, the diversity of spectrophotometric features within the sample suggests that variations in CSM mass and distribution may influence the observed characteristics. We identify four sub-classes based on the luminosity and rapidity of the light curve evolution, which provides insights into possible differences in the progenitors, while a statistical analysis of their observed rate indicates progenitor masses around 25 − 31 M ⊙ or lower.

Abstract:

The blazar PKS 0903 - 57 has recently been classified as a flat spectrum radio quasar at a redshift of z = 0.2621 . In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy γ rays with the flux increasing by  ∼ 2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy γ rays for six nights with a total observation time of 13.1 h, resulting in the discovery of PKS 0903 - 57 in this energy band with an average flux of 1.5 × 10 − 10 ph cm − 2 s − 1 above an energy threshold of  ∼ 180 GeV corresponding to 60% of the Crab Nebula flux above the same threshold. The very-high-energy γ-ray flux was strongly variable. X-ray and optical data were collected with Swift and ATOM, and also indicate significant variability. The observed multiwavelength flux and spectral variability during the H.E.S.S. observation window suggest variability time scales on the order of a few hours and reveal complex correlation patterns. The lack of absorption beyond that of the extragalactic background light in the γ-ray domain suggests that the emission region was located outside of the broad-line region. A leptonic one-zone modeling of the six H.E.S.S. observation nights using the dusty torus as seed photons for the inverse-Compton scattering, results in a low magnetization of the emission region. This implies that shock acceleration is likely the main driver during the event.

Abstract:

Context. Recent observational efforts using imaging atmospheric Cherenkov telescopes (IACTs) have led to firm detections of very-high-energy (VHE) signals from bright gamma-ray bursts (GRBs), often at moderate redshifts. Aims. This work presents 15 years of H.E.S.S. GRB observations and examines their implications through population comparisons and selected modelling cases. Methods. GRBs are a key science target of the High Energy Stereoscopic System (H.E.S.S.). With a low-energy threshold (≲100 GeV) and rapid repointing capabilities, H.E.S.S. can begin follow-up observations within tens of seconds after a GRB trigger, covering the late prompt or early afterglow phases. Results. We report GRB follow-up observations with H.E.S.S. from 2004 to 2019, which resulted in no significant VHE signals (aside from the detections of GRB 180720B and GRB 190829A). The resulting upper limits comprise the largest set available for GRBs at VHE. Conclusions. A subset of bursts with favourable conditions were selected for X-ray analysis and emission modelling. Population studies were performed to compare detected and non-detected GRBs. The results indicate that VHE-detected GRBs are not a distinct population, but tend to feature luminous X-ray emission and favourable redshift and observing conditions. This highlights the potential of next-generation IACTs such as the Cherenkov Telescope Array Observatory (CTAO), whose lower energy threshold will enhance the detection of fainter and more distant GRBs.