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Known for their efficiency in analysing large data sets, machine learning-based classifiers have been widely used in wide-field sky survey pipelines. The upcoming Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will generate millions of real-time alerts every night, enabling the discovery of large samples of rare events. Identifying such objects soon after explosion will be essential to study their evolution. Using ∼5400 transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey as training and test data, we develop NEEDLE (NEural Engine for Discovering Luminous Events), a novel hybrid (convolutional neural network + dense neural network) classifier to select for two rare classes with strong environmental preferences: superluminous supernovae (SLSNe) preferring dwarf galaxies, and tidal disruption events (TDEs) occurring in the centres of nucleated galaxies. The input data includes (i) cutouts of the detection and reference images, (ii) photometric information contained directly in the alert packets, and (iii) host galaxy magnitudes from Pan-STARRS (Panoramic Survey Telescope and Rapid Response System). Despite having only a few tens of examples of the rare classes, our average (best) completeness on an unseen test set reaches 73 per cent (86 per cent) for SLSNe and 80 per cent (87 per cent) for TDEs. While very encouraging for completeness, this may still result in relatively low purity for the rare transients, given the large class imbalance in real surveys. However, the goal of NEEDLE is to find good candidates for spectroscopic classification, rather than to select pure photometric samples. Our system will be deployed as an annotator on the UK alert broker, Lasair , to provide predictions of real-time alerts from ZTF and LSST to the community. [ABSTRACT FROM AUTHOR]

We present a detailed study on SN2019szu, a Type I superluminous supernova at z  = 0.213 that displayed unique photometric and spectroscopic properties. Pan-STARRS and ZTF forced photometry show a pre-explosion plateau lasting ∼40 d. Unlike other SLSNe that show decreasing photospheric temperatures with time, the optical colours show an apparent temperature increase from ∼15 000 to ∼20 000 K over the first 70 d, likely caused by an additional pseudo-continuum in the spectrum. Remarkably, the spectrum displays a forbidden emission line (likely attributed to λλ7320,7330) visible 16 d before maximum light, inconsistent with an apparently compact photosphere. This identification is further strengthened by the appearances of [O  iii ] λλ4959, 5007, and [O  iii ] λ4363 seen in the spectrum. Comparing with nebular spectral models, we find that the oxygen line fluxes and ratios can be reproduced with ∼0.25 M⊙ of oxygen-rich material with a density of |$\sim 10^{-15}\, \rm {g\, cm}^{-3}$|⁠. The low density suggests a circumstellar origin, but the early onset of the emission lines requires that this material was ejected within the final months before the terminal explosion, consistent with the timing of the precursor plateau. Interaction with denser material closer to the explosion likely produced the pseudo-continuum bluewards of ∼5500 Å. We suggest that this event is one of the best candidates to date for a pulsational pair-instability ejection, with early pulses providing the low density material needed for the formation of the forbidden emission line, and collisions between the final shells of ejected material producing the pre-explosion plateau. [ABSTRACT FROM AUTHOR]

An unusually long burst of γ-rays zapped Earth in December 2011, lasting 4 hours. The cause of this burst is now proposed to be a peculiar supernova produced by a [...]

Author(s): Pilar Ruiz-Lapuente (corresponding author) [1, 2]; Fernando Comeron [3]; Javier Méndez [1, 4]; Ramon Canal [1]; Stephen J. Smartt [5]; Alexei V. Filippenko [6]; Robert L. Kurucz [7]; Ryan [...]

This work aims to study different probes of Type Ia supernova (SN Ia) progenitors that have been suggested to be linked to the presence of circumstellar material (CSM). In particular, we have investigated, for the first time, the link between narrow blueshifted Na  i  D absorption profiles and the presence and strength of the broad high-velocity Ca  ii near-infrared triplet absorption features seen in SNe Ia around maximum light. With the probes exploring different distances from the SN; Na  i  D > 1017cm, high-velocity Ca  ii features < 1015cm. For this, we have used a new intermediate-resolution X-shooter spectral sample of 15 SNe Ia. We do not identify a link between these two probes, implying either that, one (or both) is not physically related to the presence of CSM or that the occurrence of CSM at the distance explored by one probe is not linked to its presence at the distance probed by the other. However, the previously identified statistical excess in the presence of blueshifted (over redshifted) Na  i  D absorption is confirmed in this sample at high significance and is found to be stronger in SNe Ia hosted by late-type galaxies. This excess is difficult to explain as being from an interstellar-medium origin as has been suggested by some recent modelling, as such an origin is not expected to show a bias for blueshifted absorption. However, a circumstellar origin for these features also appears unsatisfactory based on our new results, given the lack of link between the two probes of CSM investigated. [ABSTRACT FROM AUTHOR]

This paper describes the rapidly evolving and unusual supernova LSQ13ddu, discovered by the La Silla-QUEST survey. LSQ13ddu displayed a rapid rise of just 4.8 ± 0.9 d to reach a peak brightness of −19.70 ± 0.02 mag in the LSQgr band. Early spectra of LSQ13ddu showed the presence of weak and narrow |$\mathrm{ He}\, {\small I}$| features arising from interaction with circumstellar material (CSM). These interaction signatures weakened quickly, with broad features consistent with those seen in stripped-envelope SNe becoming dominant around two weeks after maximum. The narrow |$\mathrm{ He}\, {\small I}$| velocities are consistent with the wind velocities of luminous blue variables but its spectra lack the typically seen hydrogen features. The fast and bright early light curve is inconsistent with radioactive 56Ni powering but can be explained through a combination of CSM interaction and an underlying 56Ni decay component that dominates the later time behaviour of LSQ13ddu. Based on the strength of the underlying broad features, LSQ13ddu appears deficient in He compared to standard SNe Ib. [ABSTRACT FROM AUTHOR]

Astronomers thrive in unexplored territory--whether that is the depths of dust clouds in our Galaxy, the immediate surroundings of a black hole or the farthest reaches of the Universe. For [...]

Large modern surveys require efficient review of data in order to find transient sources such as supernovae, and to distinguish such sources from artefacts and noise. Much effort has been put into the development of automatic algorithms, but surveys still rely on human review of targets. This paper presents an integrated system for the identification of supernovae in data from Pan-STARRS1, combining classifications from volunteers participating in a citizen science project with those from a convolutional neural network. The unique aspect of this work is the deployment, in combination, of both human and machine classifications for near real-time discovery in an astronomical project. We show that the combination of the two methods outperforms either one used individually. This result has important implications for the future development of transient searches, especially in the era of Large Synoptic Survey Telescope and other large-throughput surveys. [ABSTRACT FROM AUTHOR]

Host galaxy properties provide strong constraints on the stellar progenitors of superluminous supernovae. By comparing a sample of 19 low-redshift (z < 0.3) superluminous supernova hosts to galaxy populations in the local Universe, we show that sub-solar metallicities seem to be a requirement. All superluminous supernovae in hosts with high measured gas-phase metallicities are found to explode at large galactocentric radii, indicating that the metallicity at the explosion site is likely lower than the integrated host value. We found that superluminous supernova hosts do not always have star formation rates higher than typical star-forming galaxies of the same mass. However, we confirm that high absolute specific star formation rates are a feature of superluminous supernova host galaxies, but interpret this as simply a consequence of the anticorrelation between gas-phase metallicity and specific star formation rate and the requirement of on-going star formation to produce young, massive stars greater than ∼10-20 M☉. Based on our sample, we propose an upper limit of ∼0.5 Z☉ for forming superluminous supernova progenitors (assuming an N2 metallicity diagnostic and a solar oxygen abundance of 8.69). Finally, we show that if magnetar powering is the source of the extreme luminosity, then the required initial spins appear to be correlated with metallicity of the host galaxy. This correlation needs further work, but if it applies, it is a powerful link between the supernova parameters and nature of the progenitor population. [ABSTRACT FROM AUTHOR]

We present photometric and spectroscopic observations of the interacting transient SN 2009ip taken during the 2013 and 2014 observing seasons. We characterize the photometric evolution as a steady and smooth decline in all bands, with a decline rate that is slower than expected for a solely 56Co-powered supernova at late phases. No further outbursts or eruptions were seen over a two year period from 2012 December until 2014 December. SN 2009ip remains brighter than its historic minimum from pre-discovery images. Spectroscopically, SN 2009ip continues to be dominated by strong, narrow (≲2000 km s-1) emission lines of H, He, Ca, and Fe. While we make tenuous detections of [Fe II] λ7155 and [O I] λλ6300, 6364 lines at the end of 2013 June and the start of 2013 October, respectively, we see no strong broad nebular emission lines that could point to a core-collapse origin. In general, the lines appear relatively symmetric, with the exception of our final spectrum in 2014 May, when we observe the appearance of a redshifted shoulder of emission at +550 km s-1. The lines are not blueshifted, and we see no significant near- or mid-infrared excess. From the spectroscopic and photometric evolution of SN 2009ip until 820 d after the start of the 2012a event, we still see no conclusive evidence for core-collapse, although whether any such signs could be masked by ongoing interaction is unclear. [ABSTRACT FROM AUTHOR]

Cao et al. reported a possible progenitor detection for the Type Ib supernovae iPTF13bvn for the first time. We find that the progenitor is in fact brighter than the magnitudes previously reported by approximately 0.7-0.2 mag with a larger error in the bluer filters. We compare our new magnitudes to our large set of binary evolution models and find that many binary models with initial masses in the range of 10-20 Mʘ match this new photometry and other constraints suggested from analysing the supernova. In addition, these lower mass stars retain more helium at the end of the model evolution indicating that they are likely to be observed as Type Ib supernovae rather than their more massive, Wolf-Rayet counter parts. We are able to rule out typical Wolf-Rayet models as the progenitor because their ejecta masses are too high and they do not fit the observed SED unless they have a massive companion which is the observed source at the supernova location. Therefore only late-time observations of the location will truly confirm if the progenitor was a helium giant and not a Wolf-Rayet star. [ABSTRACT FROM AUTHOR]

The blue supergiant Sher 25 is surrounded by an asymmetric, hourglass-shaped circumstellar nebula, which shows similarities to the triple-ring structure seen around SN 1987A. From optical spectroscopy over six consecutive nights, we detect periodic radial velocity variations in the stellar spectrum of Sher 25 with a peak-to-peak amplitude of ∼12 km s−1 on a time-scale of about 6 d, confirming the tentative detection of similar variations by Hendry et al. From consideration of the amplitude and time-scale of the signal, coupled with observed line profile variations, we propose that the physical origin of these variations is related to pulsations in the stellar atmosphere, rejecting the previous hypothesis of a massive, short-period binary companion. The radial velocities of two other blue supergiants with similar bipolar nebulae, SBW1 and HD 168625, were also monitored over the course of six nights, but these did not display any significant radial velocity variations. [ABSTRACT FROM AUTHOR]

Knowledge of the nature and mass of the progenitor stars of core-collapse supernovae are critical elements to test theoretical models of stellar evolution and stellar explosions. Here we describe the current limits and restrictions that can be placed on the progenitor stars of type II SNe and those of Ib/c. There are detections of some type II-P SN progenitors but the exploding stars that produce type Ib/c have eluded discovery. We discuss implications of these quantitative limits and the conclusions that we can now draw. [ABSTRACT FROM PUBLISHER]

The progenitor of SN 2005cs, in the galaxy M51, is identified in pre-explosion Hubble Space Telescope ( HST) Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) imaging. Differential astrometry, with post-explosion ACS High Resolution Channel (HRC) F555W images, permitted the identification of the progenitor with an accuracy of 0.006 arcsec. The progenitor was detected in the F814W pre-explosion image with , but was below the detection thresholds of the F435W and F555W images, with and at 5σ. Limits were also placed on the U- and R-band fluxes of the progenitor from pre-explosion HST Wide Field Planetary Camera 2 (WFPC2) F336W and F675W images. Deep images in the infrared from the Near Infra-red Imager (NIRI) on the Gemini North Telescope were taken 2 months prior to explosion, but the progenitor is not clearly detected on these. The upper limits for the JHK magnitudes of the progenitor were and . Despite having a detection in only one band, a restrictive spectral energy distribution of the progenitor star can be constructed and a robust case is made that the progenitor was a red supergiant with spectral type between mid-K and late-M. The spectral energy distribution allows a region in the theoretical Hertzsprung–Russell diagram to be determined that must contain the progenitor star. The initial mass of the star is constrained to be , which is very similar to the identified progenitor of the Type II-P SN 2003gd, and also consistent with upper mass limits placed on five other similar supernovae. The upper limit in the deep K-band image is significant in that it allows us to rule out the possibility that the progenitor was a significantly higher mass object enshrouded in a dust cocoon before core collapse. This is further evidence that the trend for Type II-P supernovae to arise in low- to moderate-mass red supergiants is real. [ABSTRACT FROM AUTHOR]

We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Nubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8[sup+4,sub-2] solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II — plateau supernovae. [ABSTRACT FROM AUTHOR]

Time-domain astronomy has come of age with astronomers now able to monitor the sky at high cadence, both across the electromagnetic spectrum and using neutrinos and gravitational waves. The advent of new observing facilities permits new science, but the ever-increasing throughput of facilities demands efficient communication of coincident detections and better subsequent coordination among the scientific community so as to turn detections into scientific discoveries. To discuss the revolution occurring in our ability to monitor the Universe and the challenges it brings, on 25-26 April 2012, a group of scientists from observational and theoretical teams studying transients met with representatives of the major international transient observing facilities at the Kavli Royal Society International Centre, UK. This immediately followed the Royal Society Discussion Meeting 'New windows on transients across the Universe' held in London. Here, we present a summary of the Kavli meeting at which the participants discussed the science goals common to the transient astronomy community and analysed how to better meet the challenges ahead as ever more powerful observational facilities come on stream. [ABSTRACT FROM AUTHOR]