Pulsars, transients and relativistic astrophysics

Abstract:

We present optical and near-infrared (NIR) observations of the Type Ib supernova (SN) 2024rbc. Emission from the first CO overtone, resting on a dust continuum at 2.3−2.4 μm, was observed at 62 days postexplosion. The CO band heads are not resolved, and the emission appears broad, lacking sharp spectral features. This is the first observation of CO in the ejecta of a Type Ib SN reported in literature. The spectra of SN 2024rbc exhibit strong He I lines and numerous neutral and ionized metal lines. Comparing the spectral evolution of SN 2024rbc to other Type Ib, Ic, and IIb SNe indicates it is a Type Ib SN. We compare the velocities of key optical lines to examine the evolution of the ejecta. Additionally, fitting SN light-curve models of helium star progenitors computed with the STELLA code to photometric observations indicates a 56Ni mass of 0.07 M⊙ and an ejecta mass of 1.7 M⊙. Fitting an LTE model to the CO overtone implies a CO mass of 5.2 × 10−4 M⊙, a CO temperature of 4040 K, and a CO velocity width of 5905 km s−1. We also fitted a modified blackbody model to the dust continuum, deriving a dust temperature of 910 K and a dust mass of 1.3 × 10−3 M⊙. The CO provides direct evidence for the onset of dust formation, and the observed dust continuum likely originates from newly formed dust in the ejecta. However, robust dust mass estimates require mid-infrared (MIR) observations.

Abstract:

Calcium-strong supernovae (Ca-strong SNe) are a class of thermonuclear transients characterised by rapid photometric evolution (rise times of ≤16 d) and nebular-phase spectra dominated by strong [Ca II] λλ 7291,7324 emission, compared to the strength of [O I] λλ 6300,6364 emission. Despite their unique spectroscopic signatures and association with remote environments at large physical offsets from their host galaxy, their origins remain uncertain. Using new discoveries from surveys such as the Zwicky Transient Facility, supplemented by archival data, we compiled a sample of 46 Ca-strong SNe, including 8 newly classified objects – the largest sample presented to date. We present a comprehensive optical photometric and spectroscopic study of Ca-strong SNe, aimed at better constraining their diversity, progenitor systems, and explosion mechanisms. This includes Gaussian decomposition of the [Ca II] and [O I] nebular emission lines to measure bulk velocities and line profile evolution from peak brightness to deep nebular-phase epochs. Of the 35 Ca-strong SNe with spectra near peak light, we find that 32 objects exhibit significant [Ca II] emission within 10 days of peak. Forbidden [Ca II] emission is typically expected to emerge after the ejecta have expanded, cooled, and reached a sufficiently low density for forbidden transitions to occur. The prevalence of these early features in Ca-strong SNe, however, remains unexplained by current explosion models. In several objects, we identify complex, multi-component line profiles that suggest a transition between two distinct velocity regimes. These transitional features, combined with the common presence of early forbidden emission, lead us to favour a scenario in which extended material surrounds the progenitor and pre-dates the SN itself. Our results suggest that the diverse spectroscopic behaviours of Ca-strong SNe are consistent with white dwarfs exploding in extended environments polluted by prior activity of the progenitor system.

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.