The Square Kilometre Array (SKA)

Abstract:

Wide-field optical transient surveys collectively discover ∼50 new transients per night, making rapid, multi-filter and/or spectroscopic follow-up of all but the most exceptional events unfeasible. Consequently, the (early) light-curve properties of most supernovae (SNe), particularly sub-luminous events, are often poorly characterised. Here, we showcase recovery of key parameters by combining publicly available data from multiple, contemporaneous surveys. We built a catalogue of 125 SNe by cross-matching the MeerLICHT (ML) transient survey with the Transient Name Server list of spectroscopically classified SNe detected between 2017 August and 2022 October. For a subset of 14 SNe with good multi-survey photometric coverage, we combined ML-q, ZTF-g, r, and ATLAS-c, o data to construct composite early-time light curves. We then applied an expanding fireball (t2) model to constrain their explosion dates and rest-frame rise times. We find that this simple model provides a reliable description for the early emission of most SNe in our sample and successfully constrains their rise time, a key parameter for characterising SN categories and probing progenitor origins. This multi-survey approach, only using standard survey products, provides a powerful method to characterise large numbers of SNe that would otherwise be lost, a technique that will be essential for maximising the scientific return from current and future surveys such as GOTO, BlackGEM, the Vera C. Rubin Observatory’s LSST, and Global Open Transient Telescope Array (GOTTA).

Abstract:

We present the results from an extensive broadband (radio to X-rays) observing campaign of the engine-driven Type Ib SN 2012au in the first 13 yr of evolution. The early time (δt ≤ 190 days) radio and X-ray evolution is well described by conventional models of a forward shock interacting with a wind-like circumstellar medium (CSM; ρCSM ∝ r−2). However, starting at δt ≈ 6.7 yr, we detect a significant radio rebrightening. This late-time emission is dominated by a luminous component characterized by a broad and rapidly evolving spectral peak and a shallow optically thin spectral slope, Fν ∝ ν−0.31 ± 0.02. These properties imply a compact emitting region (R ≲ 1016 cm) expanding at a remarkably slow velocity (vs ≲ 500 km s−1) into a high-density environment (ne ≥ 104 cm−3), accompanied by a hard electron power-law index p ≈ 1.6. No soft or hard X-ray emission is detected at any epoch, indicating that high-energy radiation is either strongly absorbed or intrinsically absent. In the context of aspherical shock–CSM interaction models, these observations imply extreme properties of the CSM (geometry, density, and total mass) that lack clear astrophysical motivation. Instead, we show that the emergence of radiation from a newborn pulsar wind nebula (PWN) naturally explains the radio spectral evolution and high-energy limits, where the emission is governed by the adiabatic expansion of a relic pair plasma. We conclude that SN 2012au represents the most compelling candidate for a young, newborn PWN discovered to date, a scenario that can be directly tested with pending very long baseline interferometry observations.

Abstract:

Context . Compact galaxies represent a key population for understanding galaxy evolution, as their high stellar densities are closely linked to early formation and/or stripping processes. However, the relative role of internal structure and environment in shaping their origin and evolution remains unclear. Aims . We aim to investigate how the abundance and quenching of compact galaxies depend on environment, and to determine whether their formation is governed by a single evolutionary pathway or multiple channels. Methods . We analyse a large sample of galaxies in the nearby Universe ( z < 0.05) with stellar masses in the range 8.2 < log( M ⋆ /M ⊙ ) < 10.5, comparing compact and control populations as a function of local overdensity by measuring the fraction of galaxies above a given overdensity ( δ ) threshold. Results . We find that the environmental dependence of compact galaxies is non-monotonic, with two characteristic overdensities, the transition overdensity ( δ t ) and the critical overdensity ( δ c ), defining three distinct regimes. Below δ t , compact galaxies are relatively more abundant than the control population, while between δ t and δ c their relative abundance decreases significantly. Above δ c , compact galaxies become increasingly overabundant again, although this behaviour strongly depends on stellar mass. Low-mass (8.2 < log( M/M ⊙ ) < 9.0) compact galaxies exhibit the strongest deficit at intermediate overdensities. In contrast, high-mass compact galaxies (9.0 < log( M/M ⊙ ) < 10.5) dominate the excess population above δ c . In addition, we find that the fraction of red galaxies increases with local overdensity in control and compact samples. However, the red galaxy fraction is systematically higher in compact galaxies than in the control sample, indicating that both environment and internal structure (compactness) play a role in galaxy quenching. The relative importance of these effects strongly depends on stellar mass, with environmental quenching being more prominent in low-mass compact galaxies, while internal structural properties may dominate in the high-mass compact population. Conclusions . Our results support a unified evolutionary framework in which compact galaxies do not arise from a single evolutionary channel, but instead reflect multiple pathways whose relative importance depends strongly on stellar mass and environment. Low-mass compact galaxies appear to be closely linked to environmentally driven transformation and quenching processes acting in groups, filaments, and cluster infall regions. In contrast, high-mass compact galaxies are found across all environments, but become increasingly overabundant relative to the control population in the densest regions. Their uniformly high red galaxy fractions and stellar surface mass densities indicate that they survive efficiently in high-density environments as structurally dense systems, similar to compact relic galaxies.