Pulsars, transients and relativistic astrophysics

Abstract:

Type Ibn supernovae (SNe) are a class of interacting SNe characterised by narrow helium lines in their spectra. We present an extensive observational dataset of the Type Ibn SN 2025kzr at 51 Mpc, including the discovery of a precursor outburst with a peak brightness of M r ≈ − 13.6 mag beginning ≈55 days before explosion. Our photometry indicates the SN was discovered within the first day of explosion, and shows fast-rising, ultraviolet-bright emission peaking at M r = −19.26 ± 0.09 mag and a peak blackbody temperature of T ≈29 000 K, consistent with shock breakout within a region of dense and confined circumstellar material (CSM). Our high-cadence spectroscopic dataset spanning 1.9–58.5 days post-explosion shows flash-ionised emission features during the first ten days. In our SALT spectrum at 3.8 days, we observe a pronounced blueshift of the He  II lines by 460 km s −1 compared to the He  I lines at zero velocity, while a Pickering-decrement analysis reveals a CSM that is fully hydrogen free. The timing of the disappearance of the flash features combined with the CSM velocity of 1500 km s −1 imply a mass-loss event ≈66 days before explosion, in close agreement with the timing of the precursor observed 55 days before explosion and strongly suggestive of a physical link. We derive a CSM mass of 0.03–1.7 M ⊙ and a corresponding high mass-loss rate of ≳10 −1 M ⊙ yr −1 . The precursor timescale and energetics suggest an extreme mass-loss event that might be explained by wave-driven mass loss during the late stages of nuclear burning, in particular the oxygen-burning phase. Overall, we favour a single massive Wolf-Rayet progenitor with M ZAMS ~30–40 M ⊙ to explain SN 2025kzr, although a binary origin cannot be excluded.

Abstract:

Previous S IROCCO radiative-transfer models of gas-cocooned active galactic nuclei (AGNs) predicted lower-column counterparts to little red dots (LRDs): compact, X-ray-weak sources with bluer continuum slopes and Balmer jumps rather than Balmer breaks. The recently identified population of little blue dots (LBDs) closely resembles this predicted phase. Here we explore these lower-column-density cocoons in which nebular recombination emission remains visible while strong Balmer-continuum absorption is avoided. We find that a sequence of increasing column density connects more classical AGN spectra, Balmer-jump LBD-like spectra at N H ∼ few × 10 24 cm −2 , and Balmer-break LRD-like spectra at higher columns. Along this sequence, electron scattering produces exponential line wings and suppresses X-ray emission before strong Balmer absorption features emerge. The models predict that LBDs should share X-ray weakness and exponential line wings with LRDs, while being distinguished by Balmer jumps in emission, weak or absent absorption features, potential He II λ 4686 emission, lower H α luminosities, and narrower H α lines. We compare with spectra of eight LBDs and find that significant Balmer-jump signatures are common. We therefore suggest that LBDs are lower-column analogues of LRDs within a common gas-cocooned AGN sequence.

Abstract:

Some of the most extreme galactic black hole transients exhibit a nebular phase at the end of their outbursts. This stage was first identified in the 2015 outburst of V404 Cygni, when it dominated the optical spectrum for ∼4 days. It is characterised by exceptionally strong optical emission lines, together with a significant increase in the Balmer decrement. The emission features show broad wings reaching velocities consistent with outflow speeds derived from prominent P-Cygni profiles that preceded the nebular phase. They are thus interpreted as arising from previously launched ejecta that expand and recombine as X-ray irradiation declines. In this work, we modelled this phase using the photoionisation code C LOUDY . Adopting simple assumptions for the irradiating continuum and emitting gas, we quantitatively reproduced key observables, such as the Balmer decrement and the H α equivalent width, and qualitatively reproduced the observed spectra. Our simulations show that the Balmer decrement remains roughly stable within the observed range across orders-of-magnitude changes in the density and ionisation parameter, spanning n H  ∼ 10 10.5 –10 12.5 cm −3 and −1.5 ≲ log  ξ  ≲ 1. This supports the scenario in which the nebular phase can persist through moderate ejecta expansion and cooling, consistent with observations. Finally, we used different approaches to constrain the general properties of the nebular phase, which we propose is produced by clumpy, partially ionised ejecta whose total mass likely exceeds the mass accreted during the outburst.

Abstract:

There is a general consensus that Type Ia supernovae (SNe Ia) comprise the thermonuclear explosions of a C/O white dwarf (WD) in a binary system. However, the nature of their progenitors and explosion mechanism still remains elusive. Their early light curves can offer strong constraints on these parameters, as they trace the outermost layers and provide evidence of their initial conditions. In this work, we study the Zwicky Transient Facility (ZTF) data release 2 sample of SNe Ia in search for objects with early flux excess (EEx SNe Ia). Two main approaches were used: i) finding deviations between early-time observations and models without early excess and ii) finding an agreement between observations and models with early excess. For the former approach, different methods, such as power-law and SALT2 fits, were used to identify EEx SN Ia candidates. For the second approach, we used double-detonation (DD) and companion-interaction (CI) models, finding 145 and 199 candidates, respectively. In general, there is a disagreement between the methods in the exact objects identified as EEx SNe Ia as well as in their number. By combining multiple methods, we found 17 strong EEx SN Ia candidates, 6 of which are of the overluminous subtype (five 91T-like and one 03 fg-like). The most prominent excesses reach ∼10–17% of peak flux (up to ∼−17 mag). We found that the best EEx SN Ia candidates have higher stretch on average compared to the rest of the SNe Ia. They also seem to prefer lower mass and bluer host galaxies, potentially linked to younger stellar populations. Additionally, we estimated the fraction of EEx SNe Ia to be ≲25% of all SNe Ia, in agreement with previous works, with overluminous subtypes having a higher relative fraction compared to normal SNe Ia. When studying the parameter distribution of the DD models for the EEx SN Ia candidates, we see a preference for intermediate WD core mass (1 M ⊙ ), a lower fraction of shell burning (20%), and heavier He-burning products ( 56 Ni). For the CI models, we see no trends with any of the parameters. We are unable to draw strong conclusions on whether one model is preferred over the other as a viable description of SN Ia explosions Finally, we highlight the challenge of identifying EEx SNe Ia in real time, making the triggering of early-time spectroscopic observations for these objects a difficult task.