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Compact binary mergers detectable in gravitational waves can be accompanied by a kilonova, an electromagnetic transient powered by radioactive decay of newly synthesised r-process elements. A few kilonova candidates have been observed during short gamma-ray burst follow-up, and one found associated with a gravitational wave detection, GW170817. However, robust kilonova candidates are yet to be found in un-triggered, wide-field optical surveys; a search not requiring an initial gravitational wave or gamma-ray burst trigger. Here we present the first observing run for the Kilonova and Transients Programme (KNTraP) using the Dark Energy Camera. The first KNTraP run ran for 11 nights, covering 31 fields at a nightly cadence in two filters. The programme is non-disruptive, can detect transients beyond the LIGO/Virgo/KAGRA horizon, is agnostic to the merger orientation, avoids the Sun and/or Galactic plane, and produces high cadence multiwavelength light curves. The data were processed nightly in real-time for rapid identification of transient candidates, allowing for follow-up of interesting candidates before they faded away. Three fast-rising candidates were identified in real-time, however none had the characteristics of the kilonova AT2017gfo associated with GW170817 or with the expected evolution for kilonovae from our fade-rate models. After the run, the data were reprocessed, then subjected to stringent filtering and model fitting to search for kilonovae offline. Multiple KNTraP runs (3+) are expected to detect kilonovae via this optical-only search method. No kilonovae were detected in this first KNTraP run using our selection criteria, constraining the KN rate to $R < 1.8\times 10^{5}$ Gpc$^{-3}$ yr$^{-1}$.

GRB 220831A is a gamma-ray burst (GRB) with a duration and spectral peak energy that places it at the interface between the distribution of long-soft and short-hard GRBs. In this paper, we present the multiwavelength follow-up campaign to GRB 220831A and its optical, near-infrared, X-ray and radio counterparts. Our deep optical and near-infrared observations do not reveal an underlying host galaxy, and establish that GRB 220831A is observationally hostless to depth, $m_i\gtrsim 26.6$ AB mag. Based on the Amati relation and the non-detection of an accompanying supernova, we find that this GRB is most likely to have originated from a collapsar at $z\gt 2$, but it could also possibly be a compact object merger at $z\lt 0.4$ with a large separation distance from its host galaxy. Regardless of its origin, we show that its optical and near-infrared counterpart departs from the evolution expected from a dominated synchrotron afterglow, exhibiting a steep post-break temporal power-law index of $-3.83^{+0.62}_{-0.79}$, too steep to be the jet-break. By analysing a range of models, we find that the observed steep departure from forward shock closure relations is likely due to an internal process producing either a flare or a plateau.

We present a detailed analysis of nearly two decades of optical/UV and X-ray data to study the multi-wavelength pre-explosion properties and post-explosion X-ray properties of nearby SN2023ixf located in M101. We find no evidence of precursor activity in the optical to UV down to a luminosity of$\lesssim$$1.0\times10^{5}\, \textrm{L}_{\odot}$, while X-ray observations covering nearly 18 yr prior to explosion show no evidence of luminous precursor X-ray emission down to an absorbed 0.3–10.0 keV X-ray luminosity of$\sim$$6\times10^{36}$erg s$^{-1}$. ExtensiveSwiftobservations taken post-explosion did not detect soft X-ray emission from SN2023ixf within the first$\sim$3.3 days after first light, which suggests a mass-loss rate for the progenitor of$\lesssim$$5\times10^{-4}\,\textrm{M}_{\odot}$yr$^{-1}$or a radius of$\lesssim$$4\times10^{15}$cm for the circumstellar material. Our analysis also suggests that if the progenitor underwent a mass-loss episode, this had to occur$>$0.5–1.5 yr prior to explosion, consistent with previous estimates.Swiftdetected soft X-rays from SN2023ixf$\sim$$4.25$days after first light, and it rose to a peak luminosity of$\sim10^{39}$erg s$^{-1}$after 10 days and has maintained this luminosity for nearly 50 days post first light. This peak luminosity is lower than expected, given the evidence that SN2023ixf is interacting with dense material. However, this might be a natural consequence of an asymmetric circumstellar medium. X-ray spectra derived from merging allSwiftobservations over the first 50 days are best described by a two-component bremsstrahlung model consisting of a heavily absorbed and hotter component similar to that found usingNuSTAR, and a less-absorbed, cooler component. We suggest that this soft component arises from cooling of the forward shock similar to that found in Type IIn SN2010jl.

Several sources of repeating coherent bursts of radio emission with periods of many minutes have now been reported in the literature. These ‘ultralong period’ (ULP) sources have no clear multiwavelength counterparts and challenge canonical pulsar emission models, leading to debate regarding their nature. In this work, we report the discovery of a bright, highly polarized burst of radio emission at low Galactic latitude as part of a wide-field survey for transient and variable radio sources. ASKAP J175534.9$-$252749.1 does not appear to repeat, with only a single intense two-minute $\sim$200-mJy burst detected from 60 h of observations. The burst morphology and polarization properties are comparable to those of classical pulsars but the duration is more than one hundred times longer, analogous to ULPs. Combined with the existing ULP population, this suggests that these sources have a strong Galactic latitude dependence and hints at an unexplored population of transient and variable radio sources in the thin disc of the Milky Way. The resemblance of this burst with both ULPs and pulsars calls for a unified coherent emission model for objects with spin periods from milliseconds to tens of minutes. However, whether or not these are all neutron stars or have the same underlying power source remains open for debate.

We study the properties of galaxies hosting mid-infrared outbursts in the context of a catalog of 500,000 galaxies from the Sloan Digital Sky Survey. We find that nuclear obscuration, as inferred by the surrounding dust mass, does not correlate with host galaxy type, stellar properties (e.g., total mass and mean age), or with the extinction of the host galaxy as estimated by the Balmer decrement. This implies that nuclear obscuration may not be able to explain any overrepresentation of tidal disruption events in particular host galaxies. We identify a region in the galaxy catalog parameter space that contains all unobscured tidal disruption events but only harbors ≲11% of the mid-infrared outburst hosts. We find that mid-infrared outburst hosts appear more centrally concentrated and have higher galaxy Sérsic indices than galaxies hosting active galactic nuclei (AGNs) selected using the Baldwin–Phillips–Terlevich classification. We thus conclude that the majority of mid-infrared outbursts are not hidden tidal disruption events but are instead consistent with being obscured AGN that are highly variable, such as changing-look AGN.

We present extensive observations of the Type II supernova (SN II) SN 2023ufx, which is likely the most metal-poor SN II observed to date. It exploded in the outskirts of a low-metallicity (Z_host∼ 0.1Z_⊙) dwarf (M_g= −13.39 ± 0.16 mag,r_proj∼ 1 kpc) galaxy. The explosion is luminous, peaking atM_g≈ −18.5 mag, and shows rapid evolution. Ther-band (pseudobolometric) light curve has a shock-cooling phase lasting 20 (17) days followed by a 19 (23) day plateau. The entire optically thick phase lasts only ≈55 days following explosion, indicating that the red supergiant progenitor had a thinned H envelope prior to explosion. The early spectra obtained during the shock-cooling phase show no evidence for narrow emission features and limit the preexplosion mass-loss rate to$\dot{M}\lesssim {10}^{-3}$M_⊙yr⁻¹. The photospheric-phase spectra are devoid of prominent metal absorption features, indicating a progenitor metallicity of ≲0.1Z_⊙. The seminebular (∼60–130 days) spectra reveal weak Feii, but other metal species typically observed at these phases (Tiii, Scii, and Baii) are conspicuously absent. The late-phase optical and near-infrared spectra also reveal broad (≈10⁴km s⁻¹) double-peaked Hα, Pβ, and Pγemission profiles suggestive of a fast outflow launched during the explosion. Outflows are typically attributed to rapidly rotating progenitors, which also prefer metal-poor environments. This is only the second SN II with ≲0.1Z_⊙and both exhibit peculiar evolution, suggesting a sizable fraction of metal-poor SNe II have distinct properties compared to nearby metal-enriched SNe II. These observations lay the groundwork for modeling the metal-poor SNe II expected in the early Universe.

We present a detailed compilation and analysis of the X-ray phase space of low- to intermediate-redshift (0 ≤z≤ 1) transients that consolidates observed light curves (and theory where necessary) for a large variety of classes of transient/variable phenomena in the 0.3–10 keV energy band. We include gamma-ray burst afterglows, supernovae, supernova shock breakouts and shocks interacting with the environment, tidal disruption events and active galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a comprehensive resource for the examination of these ephemeral events, extending the X-ray duration–luminosity phase space (DLPS) to show luminosity evolution. We use existing observations (both targeted and serendipitous) to characterize the behavior of various transient/variable populations. Contextualizing transient signals in the larger DLPS serves two primary purposes: to identify areas of interest (i.e., regions in the parameter space where one would expect detections, but in which observations have historically been lacking), and to provide initial qualitative guidance in classifying newly discovered transient signals. We find that while the most luminous (largely extragalactic) and least luminous (largely Galactic) part of the phase space is well populated att> 0.1 days, intermediate-luminosity phenomena (L_X= 10³⁴–10⁴²erg s⁻¹) represent a gap in the phase space. We thus identifyL_X= 10³⁴–10⁴²erg s⁻¹andt= 10⁻⁴to 0.1 days as a key discovery phase space in transient X-ray astronomy.

We used Transiting Exoplanet Survey Satellite (TESS) data to identify 29 candidate active galactic nuclei (AGNs) through their optical variability. The high-cadence, high-precision TESS light curves present an opportunity for the identification of AGNs, including those not selected through other methods. Of the candidates, we found that 18 have either previously been identified as AGNs in the literature or could have been selected based on emission-line diagnostics, mid-IR colours, or X-ray luminosity. AGNs in low-mass galaxies offer a unique window into supermassive black hole and galaxy co-evolution and 8 of the 29 candidates have estimated black hole masses ≲ 106 M⊙. The low-mass galaxies NGC 4395 and NGC 4449 are two of our five ‘high-confidence’ candidates. Since our initial sample largely draws from just nine TESS sectors, we expect to identify at least ∼45 more candidates in the TESS primary and extended mission data sets, of which ∼60 per cent will be new AGNs and ∼20 per cent will be in low-mass galaxies.

The nearby type II supernova, SN 2023ixf in M101 exhibits signatures of early time interaction with circumstellar material in the first week postexplosion. This material may be the consequence of prior mass loss suffered by the progenitor, which possibly manifested in the form of a detectable presupernova outburst. We present an analysis of long-baseline preexplosion photometric data in theg,w,r,i,z, andyfilters from Pan-STARRS as part of the Young Supernova Experiment, spanning ∼5000 days. We find no significant detections in the Pan-STARRS preexplosion light curves. We train a multilayer perceptron neural network to classify presupernova outbursts. We find no evidence of eruptive presupernova activity to a limiting absolute magnitude of −7 mag. The limiting magnitudes from the full set ofgwrizy(average absolute magnitude ≈ −8 mag) data are consistent with previous preexplosion studies. We use deep photometry from the literature to constrain the progenitor of SN 2023ixf, finding that these data are consistent with a dusty red supergiant progenitor with luminosity$\log \left(L/L_{\odot }\right)$≈ 5.12 and temperature ≈ 3950 K, corresponding to a mass of 14–20M_⊙.

ASASSN-14ko is a nuclear transient at the center of the AGN ESO 253−G003 that undergoes periodic flares. Optical flares were first observed in 2014 by the All-Sky Automated Survey for Supernovae (ASAS-SN) and their peak times are well-modeled with a period of${115.2}_{-1.2}^{+1.3}$days and period derivative of −0.0026 ± 0.0006. Here we present ASAS-SN, Chandra, HST/STIS, NICER, Swift, and TESS data for the flares that occurred on 2020 December, 2021 April, 2021 July, and 2021 November. These four flares represent flares 18–21 of the total number of flares observed by ASAS-SN so far since 2014. The HST/STIS UV spectra evolve from blueshifted broad absorption features to redshifted broad emission features over ∼10 days. The Swift UV/optical light curves peaked as predicted by the timing model, but the peak UV luminosities that varied between flares and the UV flux in Flare 20 were roughly half the brightness of the other peaks. The X-ray luminosities consistently decreased and the spectra became harder during the UV/optical rise, but apparently without changes in absorption. Finally, two high-cadence TESS light curves from Flare 18 and Flare 12 showed that the slopes during the rising and declining phases changed over time, which indicates some stochasticity in the flare’s driving mechanism. Although ASASSN-14ko remains observationally consistent with a repeating partial tidal disruption event, these rich multi-wavelength data are in need of a detailed theoretical model.