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SN 2023tsz is a Type Ibn supernova (SNe Ibn) discovered in an extremely low-mass host. SNe Ibn are an uncommon subtype of stripped-envelope core-collapse SNe. They are characterised by narrow helium emission lines in their spectra and are believed to originate from the collapse of massive Wolf-Rayet (WR) stars, though their progenitor systems still remain poorly understood. In terms of energetics and spectrophotometric evolution, SN 2023tsz is largely a typical example of the class, although line profile asymmetries in the nebular phase are seen, which may indicate the presence of dust formation or unshocked circumstellar material. Intriguingly, SN 2023tsz is located in an extraordinarily low-mass host galaxy that is in the 2nd percentile for SESN host masses and star formation rates (SFR). The host has a radius of 1.0 kpc, a $g$-band absolute magnitude of $-12.73$, and an estimated metallicity of $\log(Z_{*}/Z_{\odot}$) = $-1.56$. The SFR and metallicity of the host galaxy raise questions about the progenitor of SN 2023tsz. The low SFR suggests that a star with sufficient mass to evolve into a WR would be uncommon in this galaxy. Further, the very low-metallicity is a challenge for single stellar evolution to enable H and He stripping of the progenitor and produce a SN Ibn explosion. The host galaxy of SN 2023tsz adds another piece to the ongoing puzzle of SNe Ibn progenitors, and demonstrates that they can occur in hosts too faint to be observed in contemporary sky surveys at a more typical SN Ibn redshift., Comment: 14 pages, 10 figures, submitted to MNRAS

Time-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratised, distributed approaches to training sets for machine learning classifiers are crucial to make the most of this torrent of discovery -- with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the $\textit{Kilonova Seekers}$ citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. $\textit{Kilonova Seekers}$ launched in July 2023 and received over 600,000 classifications from approximately 2,000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a `gold-standard' training set of 17,682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development., Comment: 20 pages, 15 figures. Accepted in MNRAS

Multiple explosion mechanisms have been proposed to explain type Ia supernovae (SNe Ia). Empirical modelling tools have also been developed that allow for fast, customised modelling of individual SNe and direct comparisons between observations and explosion model predictions. Such tools have provided useful insights, but the subjective nature with which empirical modelling is performed makes it difficult to obtain robust constraints on the explosion physics or expand studies to large populations of objects. Machine learning accelerated tools have therefore begun to gain traction. In this paper, we present riddler, a framework for automated fitting of SNe Ia spectral sequences up to shortly after maximum light. We train a series of neural networks on realistic ejecta profiles predicted by the W7 and N100 explosion models to emulate full radiative transfer simulations and apply nested sampling to determine the best-fitting model parameters for multiple spectra of a given SN simultaneously. We show that riddler is able to accurately recover the parameters of input spectra and use it to fit observations of two well-studied SNe Ia. We also investigate the impact of different weighting schemes when performing quantitative spectral fitting and show that best-fitting models and parameters are highly dependent on the assumed weighting schemes and priors. As spectroscopic samples of SNe Ia continue to grow, automated spectral fitting tools such as riddler will become increasingly important to maximise the physical constraints that can be gained in a quantitative and consistent manner., Comment: 28 pages, 4 appendices, 18 figures, 4 tables. Accepted for publication in MNRAS. The riddler code is publicly available at https://github.com/MarkMageeAstro/Riddler

We present optical photometric and spectroscopic observations of the 02es-like type Ia supernova (SN) 2022ywc. The transient occurred in the outskirts of an elliptical host galaxy and showed a striking double-peaked light curve with an early excess feature detected in the ATLAS orange and cyan bands. The early excess is remarkably luminous with an absolute magnitude $\sim -19$, comparable in luminosity to the subsequent radioactively-driven second peak. The spectra resemble the hybrid 02es-like SN 2016jhr, that is considered to be a helium shell detonation candidate. We investigate different physical mechanisms that could power such a prominent early excess and rule out massive helium shell detonation, surface $^{56}$Ni distribution and ejecta-companion interaction. We conclude that SN ejecta interacting with circumstellar material (CSM) is the most viable scenario. Semi-analytical modelling with MOSFiT indicates that SN ejecta interacting with $\sim 0.05\,$M$_{\odot}$ of CSM at a distance of $\sim 10^{14}$ cm can explain the extraordinary light curve. A double-degenerate scenario may explain the origin of the CSM, either by tidally-stripped material from the secondary white dwarf, or disk-originated matter launched along polar axes following the disruption and accretion of the secondary white dwarf. A non-spherical CSM configuration could suggest that a small fraction of 02es-like events viewed along a favourable line of sight may be expected to display a very conspicuous early excess like SN 2022ywc., Comment: Accepted to ApJL after minor revision

Strong gravitational lensing of supernovae is exceedingly rare. To date, only a handful of lensed supernovae are known. Despite this, lensed supernovae have emerged as a promising method for measuring the current expansion rate of the Universe and breaking the Hubble tension. We present an extensive search for gravitationally lensed supernovae within the Zwicky Transient Facility (ZTF) public survey, covering 15,215 transients with good light curves discovered during four years of observations. We crossmatch a catalogue of known and candidate lens galaxies with our sample and find three coincident sources, which were due to chance alignment. To search for supernovae magnified by unknown lenses, we test multiple methods suggested in the literature for the first time on real data. This includes selecting objects with extremely red colours, those that appear inconsistent with the host galaxy redshift, and those with bright absolute magnitudes inferred from the host galaxy redshift. We find a few hundred candidates, most of which are due to contamination from activate galactic nuclei, bogus detections, or unlensed supernovae. The false positive rate from these methods presents significant challenges for future surveys. In total, 132 unique transients were identified across all of our selection methods that required detailed manual rejection, which would be infeasible for larger samples. Overall, we do not find any compelling candidates for lensed supernovae, which is broadly consistent with previous estimates for the rate of lensed supernovae from the ZTF public survey alone and the number expected to pass the selection cuts we apply., Comment: Updated selection criteria. 20 pages, 13 figures, and 3 tables including the appendices

The exact nature of the luminous fast blue optical transient AT 2018cow is still debated. In this first of a two-paper series, we present a detailed analysis of three Hubble Space Telescope (HST) observations of AT 2018cow covering $\sim$50--60 days post-discovery in combination with other observations throughout the first two months, and derive significantly improved constraints of the late thermal properties. By modeling the spectral energy distributions (SEDs), we confirm that the UV-optical emission over 50--60 days was still a smooth blackbody (i.e., optically thick) with a high temperature ($T_{\mathrm{BB}}\sim15000\,\mathrm{K}$) and small radius ($R_{\mathrm{BB}}\lesssim1000\,R_\odot$). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of $L_{\mathrm{BB}}\propto t^{-2.40}$, but faded much faster at $t^{-3.06}$ after day 13. Reexamining possible late-time power sources, we disfavor significant contributions from radioactive decay based on the required $^{56}$Ni mass and lack of UV line blanketing in the HST SEDs. We argue that the commonly proposed interaction with circumstellar material may face significant challenges in explaining the late thermal properties, particularly the effects of the optical depth. Alternatively, we find that continuous outflow/wind driven by a central engine can still reasonably explain the combination of a receding photosphere, optically thick and rapidly fading emission, and intermediate-width lines. However, the rapid fading may have further implications on the power output and structure of the system. Our findings may support the hypothesis that AT 2018cow and other "Cow-like transients" are powered mainly by accretion onto a central engine., Comment: 22 pages, 10 figures. This is paper I of a two-paper series. v2: updated to the accepted version

In this second of a two-paper series, we present a detailed analysis of three HST observations taken $\sim$2--4 years post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at $\sim$2--3 years post-discovery revealed an exceptionally blue ($L_\nu\propto \nu^{1.99}$) underlying source with relatively stable optical brightness, fading in the NUV was observed at year 4, indicating flattening in the spectrum (to $L_\nu\propto \nu^{1.64}$). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with the known radio-producing circumstellar material as well as magnetar spin down with $B\sim10^{15}\,\mathrm{G}$ as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., previously ejected hydrogen envelope) or a magnetar with $B\lesssim10^{14}\,\mathrm{G}$. Finally, we find that a highly-inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source., Comment: 24 pages, 12 figures. This is paper II of a two-paper series. v2: updated to the accepted version. Note a mistake in v1 in the caption of Figure 6 is now fixed: removed the word "dereddened" because the plotted observed magnitudes are directly from Table 1 which are not extinction-corrected (the analyses are not affected by this)

Samples of young type Ia supernovae have shown `early excess' emission in a few cases. Similar excesses are predicted by some explosion and progenitor scenarios and hence can provide important clues regarding the origin of thermonuclear supernovae. They are however, only predicted to last up to the first few days following explosion. It is therefore unclear whether such scenarios are intrinsically rare or if the relatively small sample size simply reflects the difficulty in obtaining sufficiently early detections. To that end, we perform toy simulations covering a range of survey depths and cadences, and investigate the efficiency with which young type Ia supernovae are recovered. As input for our simulations, we use models that broadly cover the range of predicted luminosities. Based on our simulations, we find that in a typical three day cadence survey, only $\sim$10% of type Ia supernovae would be detected early enough to rule out the presence of an excess. A two day cadence however, should see this increase to $\sim$15%. We find comparable results from more detailed simulations of the Zwicky Transient Facility surveys. Using the recovery efficiencies from these detailed simulations, we investigate the number of young type Ia supernovae expected to be discovered assuming some fraction of the population come from scenarios producing an excess at early times. Comparing the results of our simulations to observations, we find the intrinsic fraction of type Ia supernovae with early flux excesses is $\sim28^{+13}_{-11}%$%., Comment: 16 pages, 15 figures, 2 tables. Accepted for publication in MNRAS

In the new era of time-domain surveys Type Ia supernovae are being caught sooner after explosion, which has exposed significant variation in their early light curves. Two driving factors for early time evolution are the distribution of nickel in the ejecta and the presence of flux excesses of various causes. We perform an analysis of the largest young SN Ia sample to date. We compare 115 SN Ia light curves from the Zwicky Transient Facility to the turtls model grid containing light curves of Chandrasekhar-mass explosions with a range of nickel masses, nickel distributions and explosion energies. We find that the majority of our observed light curves are well reproduced by Chandrasekhar-mass explosion models with a preference for highly extended nickel distributions. We identify six SNe Ia with an early-time flux excess in our g- and r-band data (four `blue' and two `red' flux excesses). We find an intrinsic rate of 18+/-11 per cent of early flux excesses in SNe Ia at z < 0.07, based on three detected flux excesses out of 30 (10 per cent) observed SNe Ia with a simulated efficiency of 57 per cent. This is comparable to rates of flux excesses in the literature but also accounts for detection efficiencies. Two of these events are mostly consistent with CSM interaction, while the other four have longer lifetimes in agreement with companion interaction and nickel-clump models. We find a higher frequency of flux excesses in 91T/99aa-like events (44+/-13 per cent).

[Abridged] We classify AT 2020zso as a TDE based on the blackbody evolution inferred from UV/optical photometric observations, and spectral line content and evolution. We identify transient, double-peaked Bowen (N III), He I, He II and Halpha emission lines. We model medium resolution optical spectroscopy of the He II (after careful deblending of the N III contribution) and Halpha lines during the rise, peak and early decline of the light curve using relativistic, elliptical accretion disk models. We find that the spectral evolution before peak can be explained by optical depth effects consistent with an outflowing, optically thick Eddington envelope. Around peak the envelope reaches its maximum extent (approximately 10^15 or 3000-6000 gravitational radii for an inferred black hole mass of 5-10 10^5) and becomes optically thin. The Halpha and He II emission lines at and after peak can be reproduced with a highly inclined (i=85+-5 degrees), highly elliptical (e=0.97+-0.01) and relatively compact (Rin = several 100 Rg and Rout = several 1000 Rg ) accretion disk. Overall, the line profiles suggest a highly elliptical geometry for the new accretion flow, consistent with theoretical expectations of newly formed TDE disks. We quantitatively confirm, for the first time, the high inclination nature of a Bowen (and X-ray dim) TDE, consistent with the unification picture of TDEs where the inclination largely determines the observational appearance. Rapid line profile variations rule out the binary SMBH hypothesis as the origin of the eccentricity; these results thus provide a direct link between a TDE in an AGN and the eccentric accretion disk. We illustrate for the first time how optical spectroscopy can be used to constrain the black hole spin, through (the lack of) disk precession signatures (changes in inferred inclination) - and rule out high black hole spin values (a < 0.8)., Comment: 20 pages, 15 figures and 5 tables. Accepted for publication in A&A. The spectra will be made publicly available through WISErep

Investigations of some type Iax supernovae have led to the suggestion that their ejecta must be layered to some degree. Such an ejecta structure has been argued as inconsistent with the well-mixed composition predicted by pure deflagrations. Based on explosion models, we create toy models in which the ejecta are artificially stratified and progressively mixed until a uniform composition is obtained. We find that models that are heavily mixed, containing burned and unburned material at all velocities, produce reasonably good agreement with SN 2012Z, for which a layered structure has been suggested. We also discuss how existing ejecta compositions determined for type Iax supernovae do not necessarily contradict pure deflagration models and may be consistent with a steeper density profile. We investigate previous claims that differences in line profile shapes may be due to strong blending, by presenting a series of models with different plasma states. These models indicate that blending could indeed explain differences in the observed profiles. Alternatively, stratification could also explain such differences, however all of our models indicate that this does not necessarily require stratification in abundance. Sufficient stratification in ionisation state can be achieved even for a well-mixed model. Based on our analysis, we demonstrate that there is insufficient evidence to suggest the ejecta of type Iax supernovae must be layered and therefore argue the pure deflagration scenario is not ruled out, even for the brightest type Iax supernovae. Our analysis does not indicate the ejecta cannot be layered to some degree, but observations within days of explosion are necessary to determine the extent to which the outer ejecta could be layered., Comment: 19 pages, 12 figures, 8 tables. Accepted for publication in MNRAS

The division between stripped-envelope supernovae (SE-SNe) and superluminous supernovae (SLSNe) is not well defined in either photometric or spectroscopic space. While a sharp luminosity threshold has been suggested, there remains an increasing number of transitional objects that reach this threshold without the spectroscopic signatures common to SLSNe. In this work we present data and analysis on four SNe transitional between SE-SNe and SLSNe; the He-poor SNe 2019dwa and 2019cri, and the He-rich SNe 2019hge and 2019unb. Each object displays long-lived and variable photometric evolution with luminosities around the SLSN threshold of $M_r < -19.8$ mag. Spectroscopically however, these objects are similar to SE-SNe, with line velocities lower than either SE-SNe and SLSNe, and thus represent an interesting case of rare transitional events., Comment: Accepted for publication in MNRAS

The detonation of a helium shell on top of a carbon-oxygen white dwarf has been argued as a potential explosion mechanism for type Ia supernovae (SNe~Ia). The ash produced during helium shell burning can lead to light curves and spectra that are inconsistent with normal SNe~Ia, but may be viable for some objects showing a light curve bump within the days following explosion. We present a series of radiative transfer models designed to mimic predictions from double detonation explosion models. We consider a range of core and shell masses, and systematically explore multiple post-explosion compositions for the helium shell. We find that a variety of luminosities and timescales for early light curve bumps result from those models with shells containing $^{56}$Ni, $^{52}$Fe, or $^{48}$Cr. Comparing our models to SNe~Ia with light curve bumps, we find that these models can reproduce the shapes of almost all of the bumps observed, but only those objects with red colours around maximum light ($B-V \gtrsim 1$) are well matched throughout their evolution. Consistent with previous works, we also show that those models in which the shell does not contain iron-group elements provide good agreement with normal SNe~Ia of different luminosities from shortly after explosion up to maximum light. While our models do not amount to positive evidence in favour of the double detonation scenario, we show that provided the helium shell ash does not contain iron-group elements, it may be viable for a wide range of normal SNe~Ia., Comment: 21 pages, 25 figures, 1 table. Accepted for publication in MNRAS. Model light curves available at https://github.com/MarkMageeAstro/TURTLS-Light-curves

An excess of flux (i.e. a bump) in the early light curves of type Ia supernovae has been observed in a handful of cases. Multiple scenarios have been proposed to explain this. It has been shown that for at least one object (SN~2018oh) the excess emission observed could be the result of a large amount of $^{56}$Ni in the outer ejecta ($\sim$0.03~$M_{\rm{\odot}}$). We present a series of model light curves and spectra for ejecta profiles containing $^{56}$Ni shells of varying masses (0.01, 0.02, 0.03, and 0.04~$M_{\rm{\odot}}$) and widths. We find that even for our lowest mass $^{56}$Ni shell, an increase of \textgreater2 magnitudes is produced in the bolometric light curve at one day after explosion relative to models without a $^{56}$Ni shell. We show that the colour evolution of models with a $^{56}$Ni shell differs significantly from those without and shows a colour inversion similar to some double-detonation explosions. Spectra of our $^{56}$Ni shell models show that strong suppression of flux between $\sim$3\,700 -- 4\,000~$\AA$ close to maximum light appears to be a generic feature for this class of model. Comparing our models to observations of SNe~2017cbv and 2018oh, we show that a $^{56}$Ni shell of 0.02 -- 0.04~$M_{\rm{\odot}}$ can match shapes of the early optical light curve bumps, but the colour and spectral evolution are in disagreement. This would indicate that an alternative origin for the flux excess is necessary. Based on existing explosion scenarios, producing such a $^{56}$Ni shell in the outer ejecta as required to match the light curve shape, without the presence of additional short-lived radioactive material, may prove challenging. Given that only a small amount of $^{56}$Ni in the outer ejecta is required to produce a bump in the light curve, such non-monotonically decreasing $^{56}$Ni distributions in the outer ejecta must be rare, if they were to occur at all., Comment: 14 pages, 11 figures. Updated to match version published in A&A. Model light curves available at https://github.com/MarkMageeAstro/TURTLS-Light-curves

Early observations of Type Ia supernovae (SNe$\,$Ia) provide essential clues for understanding the progenitor system that gave rise to the terminal thermonuclear explosion. We present exquisite observations of SN$\,$2019yvq, the second observed SN$\,$Ia, after iPTF$\,$14atg, to display an early flash of emission in the ultraviolet (UV) and optical. Our analysis finds that SN$\,$2019yvq was unusual, even when ignoring the initial flash, in that it was moderately underluminous for an SN$\,$Ia ($M_g \approx -18.5\,$mag at peak) yet featured very high absorption velocities ($v \approx 15,000\,\mathrm{km\,s}^{-1}$ for Si II $\lambda$6355 at peak). We find that many of the observational features of SN$\,$2019yvq, aside from the flash, can be explained if the explosive yield of radioactive $^{56}\mathrm{Ni}$ is relatively low (we measure $M_{^{56}\mathrm{Ni}} = 0.31 \pm 0.05\,M_\odot$) and it and other iron-group elements are concentrated in the innermost layers of the ejecta. To explain both the UV/optical flash and peak properties of SN$\,$2019yvq we consider four different models: interaction between the SN ejecta and a nondegenerate companion, extended clumps of $^{56}\mathrm{Ni}$ in the outer ejecta, a double-detonation explosion, and the violent merger of two white dwarfs. Each of these models has shortcomings when compared to the observations; it is clear additional tuning is required to better match SN$\,$2019yvq. In closing, we predict that the nebular spectra of SN$\,$2019yvq will feature either H or He emission, if the ejecta collided with a companion, strong [Ca II] emission, if it was a double detonation, or narrow [O I] emission, if it was due to a violent merger., Comment: 27 pages, 14 figures, accepted in ApJ

We present DES16C3cje, a low-luminosity, long-lived type II supernova (SN II) at redshift 0.0618, detected by the Dark Energy Survey (DES). DES16C3cje is a unique SN. The spectra are characterized by extremely narrow photospheric lines corresponding to very low expansion velocities of $\lesssim1500$ km s$^{-1}$, and the light curve shows an initial peak that fades after 50 days before slowly rebrightening over a further 100 days to reach an absolute brightness of M$_r\sim -15.5$ mag. The decline rate of the late-time light curve is then slower than that expected from the powering by radioactive decay of $^{56}$Co but is comparable to that expected from accretion power. Comparing the bolometric light curve with hydrodynamical models, we find that DES16C3cje can be explained by either i) a low explosion energy (0.11 foe) and relatively large $^{56}$Ni production of 0.075 M$_{\odot}$ from a $\sim15$ M$_{\odot}$ red supergiant progenitor typical of other SNe II, or ii) a relatively compact $\sim40$ M$_{\odot}$ star, explosion energy of 1 foe, and 0.08 M$_{\odot}$ of $^{56}$Ni. Both scenarios require additional energy input to explain the late-time light curve, which is consistent with fallback accretion at a rate of $\sim0.5\times{10^{-8}}$ M$_{\odot}$ s$^{-1}$., Comment: Accepted in MNRAS. 17 pages, 6 figures

Recent studies have shown how the distribution of $^{56}$Ni within the ejecta of type Ia supernovae can have profound consequences on the observed light curves. Observations at early times can therefore provide important details on the explosion physics in thermonuclear supernovae. We present a series of radiative transfer calculations that explore variations in the $^{56}$Ni distribution. Our models also show the importance of the density profile in shaping the light curve, which is often neglected in the literature. Using our model set, we investigate the observations that are necessary to determine the $^{56}$Ni distribution as robustly as possible within the current model set. We find that this includes observations beginning at least $\sim$14 days before $B$-band maximum, extending to approximately maximum light with a high ($\lesssim$3 day) cadence, and in at least one blue and one red band are required (such as $B$ and $R$, or $g$ and $r$). We compare a number of well-observed type Ia supernovae that meet these criteria to our models and find that the light curves of $\sim$70-80\% of objects in our sample are consistent with being produced solely by variations in the $^{56}$Ni distributions. The remaining supernovae show an excess of flux at early times, indicating missing physics that is not accounted for within our model set, such as an interaction or the presence of short-lived radioactive isotopes. Comparing our model light curves and spectra to observations and delayed detonation models demonstrates that while a somewhat extended $^{56}$Ni distribution is necessary to reproduce the observed light curve shape, this does not negatively affect the spectra at maximum light. Investigating current explosion models shows that observations typically require a shallower decrease in the $^{56}$Ni mass towards the outer ejecta than is produced for models of a given $^{56}$Ni mass., Comment: 31 pages, 32 figures, 2 tables. Accepted for publication in A&A. Model light curves available at https://github.com/MarkMageeAstro/TURTLS-Light-curves

This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely host. Its light curves rise to maximum light in $5-6$ d and then display a decline of $\Delta m_{15} \sim5$ mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and `ultra-stripped' type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10,000 -- 12,000 km/s. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of $0.2 - 0.4$ M$_\odot$ and a low kinetic energy of $ (2-4)\times 10^{50}$ erg, giving a specific kinetic energy $\sim1$ [$10^{51}$ erg]/M$_\odot$. The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of \ArII, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events., Comment: Accepted for publication in Astronomy & Astrophysics. Minor changes to section 4.3, some minor discussion added regarding opacities and line identification

Recent studies have argued that the progenitor system of type Iax supernovae must consist of a carbon-oxygen white dwarf accreting from a helium star companion. Based on existing explosion models invoking the pure deflagration of carbon-oxygen white dwarfs, we investigate the likelihood of producing spectral features due to helium in type Iax supernovae. From this scenario, we select those explosion models producing ejecta and $^{56}$Ni masses that are broadly consistent with those estimated for type Iax supernovae (0.014 - 0.478~$M_{\odot}$ and $\sim0.003$ - 0.183~$M_{\odot}$, respectively). To this end, we present a series of models of varying luminosities ($-18.4 \lesssim M_{\rm{V}} \lesssim -14.5$~mag) with helium abundances accounting for up to $\sim$36\% of the ejecta mass, and covering a range of epochs beginning a few days before B$-$band maximum to approximately two weeks after maximum. We find that the best opportunity for detecting \ion{He}{i} features is at near-infrared wavelengths, and in the post-maximum spectra of the fainter members of this class. We show that the optical spectrum of SN~2007J is potentially consistent with a large helium content (a few 10$^{-2}~M_{\odot}$), but argue that current models of accretion and material stripping from a companion struggle to produce compatible scenarios. We also investigate the presence of helium in all objects with near-infrared spectra. We show that SNe~2005hk, 2012Z, and 2015H contain either no helium or their helium abundances are constrained to much lower values ($\lesssim$10$^{-3}~M_{\odot}$). Our results demonstrate the differences in helium content among type Iax supernovae, perhaps pointing to different progenitor channels. Either SN~2007J is an outlier in terms of its progenitor system, or it is not a true member of the type Iax supernova class., Comment: 15 pages, 12 figures, 2 tables. Accepted for publication in Astronomy and Astrophysics

We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity ($\sim$1.7 $\times$ 10$^{44}$ erg s$^{-1}$), rapidly evolving light curves ($>$5 mag rise to peak in $\sim$3.5 days), and hot blackbody spectra, peaking at $\sim$27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of $0.1-0.4$ \msol, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but remained smooth and featureless. Broad and shallow emission lines appear after about 20 days, and we tentatively identify them as He I although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data., Comment: Accepted for publication in ApJL, updated to reflect the published version

Recent studies have demonstrated the diversity in type Ia supernovae (SNe Ia) at early times and highlighted a need for a better understanding of the explosion physics as manifested by observations soon after explosion. To this end, we present a Monte Carlo code designed to model the light curves of radioactively driven, hydrogen-free transients from explosion to approximately maximum light. In this initial study, we have used a parametrised description of the ejecta in SNe Ia, and performed a parameter study of the effects of the $^{56}$Ni distribution on the observed colours and light curves for a fixed $^{56}$Ni mass of 0.6 $M_\odot$. For a given density profile, we find that models with $^{56}$Ni extending throughout the entirety of the ejecta are typically brighter and bluer shortly after explosion. Additionally, the shape of the density profile itself also plays an important role in determining the shape, rise time, and colours of observed light curves. We find that the multi-band light curves of at least one SNe Ia (SN 2009ig) are inconsistent with less extended $^{56}$Ni distributions, but show good agreement with models that incorporate $^{56}$Ni throughout the entire ejecta. We further demonstrate that comparisons with full $UVOIR$ colour light curves are powerful tools in discriminating various $^{56}$Ni distributions, and hence explosion models., Comment: 14 pages, 8 figures, 2 tables. Minor changes in notation to match published version in Astronomy & Astrophysics

Gravitational waves were discovered with the detection of binary black hole mergers and they should also be detectable from lower mass neutron star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal called a kilonova. The gravitational wave source GW170817 arose from a binary neutron star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC4993, which is spatially coincident with GW170817 and a weak short gamma-ray burst. The transient has physical parameters broadly matching the theoretical predictions of blue kilonovae from neutron star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 +/- 0.01 Msol, with an opacity of kappa <= 0.5 cm2/gm at a velocity of 0.2 +/- 0.1c. The power source is constrained to have a power law slope of beta = -1.2 +/- 0.3, consistent with radioactive powering from r-process nuclides. We identify line features in the spectra that are consistent with light r-process elements (90 < A < 140). As it fades, the transient rapidly becomes red, and emission may have contribution by a higher opacity, lanthanide-rich ejecta component. This indicates that neutron star mergers produce gravitational waves, radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements., Comment: Nature, in press, DOI 10.1038/nature24303. Data files will be made available at http://www.pessto.org

In this study, we present observations of a type Iax supernova, PS1-12bwh, discovered during the Pan-STARRS1 3$\pi$-survey. Our analysis was driven by previously unseen pre-maximum, spectroscopic heterogeneity. While the light curve and post-maximum spectra of PS1-12bwh are virtually identical to those of the well-studied type Iax supernova, SN 2005hk, the $-$2 day spectrum of PS1-12bwh does not resemble SN 2005hk at a comparable epoch; instead, we found it to match a spectrum of SN 2005hk taken over a week earlier ($-$12 day). We are able to rule out the cause as being incorrect phasing, and argue that it is not consistent with orientation effects predicted by existing explosion simulations. To investigate the potential source of this difference, we performed radiative transfer modelling of both supernovae. We found that the pre-maximum spectrum of PS1-12bwh is well matched by a synthetic spectrum generated from a model with a lower density in the high velocity ($\gtrsim$6000 km~s$^{-1}$) ejecta than SN 2005hk. The observed differences between SN 2005hk and PS1-12bwh may therefore be attributed primarily to differences in the high velocity ejecta alone, while comparable densities for the lower velocity ejecta would explain the nearly identical post-maximum spectra. These two supernovae further highlight the diversity within the SNe Iax class, as well as the challenges in spectroscopically identifying and phasing these objects, especially at early epochs., Comment: 12 pages, 8 figures. Accepted for publication in Astronomy & Astrophysics

When a star passes within the tidal radius of a supermassive black hole, it will be torn apart. For a star with the mass of the Sun ($M_\odot$) and a non-spinning black hole with a mass $<10^8 M_\odot$, the tidal radius lies outside the black hole event horizon and the disruption results in a luminous flare. Here we report observations over a period of 10 months of a transient, hitherto interpreted as a superluminous supernova. Our data show that the transient rebrightened substantially in the ultraviolet and that the spectrum went through three different spectroscopic phases without ever becoming nebular. Our observations are more consistent with a tidal disruption event than a superluminous supernova because of the temperature evolution, the presence of highly ionised CNO gas in the line of sight and our improved localisation of the transient in the nucleus of a passive galaxy, where the presence of massive stars is highly unlikely. While the supermassive black hole has a mass $> 10^8 M_\odot$, a star with the same mass as the Sun could be disrupted outside the event horizon if the black hole were spinning rapidly. The rapid spin and high black hole mass can explain the high luminosity of this event., Comment: Modified after proofs to match published version as much as possible. 56 pages, 12 figures, 2 tables, including Methods and Supplementary Information. A photometry file is linked here as ancillary file. All data (photometry and spectra) are also publicly available from WISeREP

We present results based on observations of SN 2015H which belongs to the small group of objects similar to SN 2002cx, otherwise known as type Iax supernovae. The availability of deep pre-explosion imaging allowed us to place tight constraints on the explosion epoch. Our observational campaign began approximately one day post-explosion, and extended over a period of about 150 days post maximum light, making it one of the best observed objects of this class to date. We find a peak magnitude of M$_r$ = -17.27 $\pm$ 0.07, and a ($\Delta m_{15})_r$ = 0.69 $\pm$ 0.04. Comparing our observations to synthetic spectra generated from simulations of deflagrations of Chandrasekhar mass carbon-oxygen white dwarfs, we find reasonable agreement with models of weak deflagrations that result in the ejection of ~0.2 M$_{\odot}$ of material containing ~0.07 M$_{\odot}$ of 56Ni. The model light curve however, evolves more rapidly than observations, suggesting that a higher ejecta mass is to be favoured. Nevertheless, empirical modelling of the pseudo-bolometric light curve suggests that $\lesssim$0.6 M_sun of material was ejected, implying that the white dwarf is not completely disrupted, and that a bound remnant is a likely outcome., Comment: 19 pages, 12 figures, accepted for publication in Astronomy & Astrophysics

We present our findings on a supernova (SN) impostor, SNHunt248, based on optical and near-IR data spanning $\sim$15 yrs before discovery, to $\sim$1 yr post-discovery. The light curve displays three distinct peaks, the brightest of which is at $M_{R} \sim -15.0$ mag. The post-discovery evolution is consistent with the ejecta from the outburst interacting with two distinct regions of circumstellar material. The 0.5 - 2.2 $\mu$m spectral energy distribution at -740 d is well-matched by a single 6700 K blackbody with $\log(L/L_\odot) \sim 6.1$. This temperature and luminosity support previous suggestions of a yellow hypergiant progenitor; however, we find it to be brighter than the brightest and most massive Galactic late-F to early-G spectral type hypergiants. Overall the historical light curve displays variability of up to $\sim \pm1$ mag. At current epochs ($\sim$1 yr post-outburst), the absolute magnitude ($M_{R} \sim -9$ mag) is just below the faintest observed historical absolute magnitude $\sim$10 yrs before discovery., Comment: 7 pages, 3 figures, accepted for A&A Letters

Using imaging from the Pan-STARRS1 survey, we identify a precursor outburst at epochs 287 and 170 days prior to the reported explosion of the purported Type IIn supernova (SN) 2011ht. In the Pan-STARRS data, a source coincident with SN 2011ht is detected exclusively in the \zps\ and \yps-bands. An absolute magnitude of M$_z\simeq$-11.8 suggests that this was an outburst of the progenitor star. Unfiltered, archival Catalina Real Time Transient survey images also reveal a coincident source from at least 258 to 138 days before the main event. We suggest that the outburst is likely to be an intrinsically red eruption, although we cannot conclusively exclude a series of erratic outbursts which were observed only in the redder bands by chance. This is only the fourth detection of an outburst prior to a claimed SN, and lends credence to the possibility that many more interacting transients have pre-explosion outbursts, which have been missed by current surveys., Comment: Revised version following referee report