We assemble a large set of 2-10 GHz radio flux density measurements and upper limits of 294 different supernovae (SNe), from the literature and our own and archival data. Only 31% of SNe were detected. We characterize the SN radio lightcurves near the peak using a two-parameter model, with t(pk) being the time to rise to a peak and L-pk the spectral luminosity at that peak. Over all SNe in our sample at D < 100 Mpc, we find that t(pk) = 10(1.7 +/- 0.9) days and that L-pk = 10(25.5 +/- 1.6) erg s(-1) Hz(-1), and therefore that generally 50% of SNe will have L-pk L-pk values are similar to 30 times lower than those for only detected SNe. Types Ib/c and II (excluding IIn's) have similar mean values of L-pk but the former have a wider range, whereas Type IIn SNe have similar to 10 times higher values with L-pk = 10(26.5 +/- 1.1) erg s(-1) Hz(-1). As for t(pk), Type Ib/c have t(pk) of only 10(1.1 +/- 0.5) days while Type II have t(pk) = 10(1.6 +/- 1.0) and Type IIn the longest timescales with t(pk) = 10(3.1 +/- 0.7) days. We also estimate the distribution of progenitor mass-loss rates, (M) over dot, and find that the mean and standard deviation of log(10) ((M) over dot[M-circle dot yr(-1)]) are -5.4 +/- 1.2 (assuming v(wind) = 1000 km s(-1)) for Type Ib/c SNe, and -6.9 +/- 1.4 (assuming v(wind) = 10 km s(-1)) for Type II SNe excluding Type IIn.

Young supernova remnants (SNRs) provide a unique perspective on supernova (SN) progenitors and connect the late evolution of SNe with the onset of the SNR phase. Here we study SN 1885A and G1.9+0.3, the most recent thermonuclear SNe in the Local Group (with ages ∼100 yr) with radio data, which provide a sensitive probe of the SN environment and energetics. We reduce and co-add 4–8 GHz observations from the Karl G. Jansky Very Large Array (VLA) to produce the deepest radio image of the central region of M31 (rms noise of 1.3 μ Jy beam −1 at 6.2 GHz). We detect some diffuse emission near SN 1885A at 2.6 σ , but the association with SN 1885A is uncertain because diffuse radio emission pervades M31's central region. Accounting for this diffuse emission, we get a 3 σ upper limit of 11 μ Jy for SN 1885A, about four times less luminous than G1.9+0.3 at 6.2 GHz. Together with Hubble Space Telescope data on SN 1885A, we estimate its ambient density, n 0 < 0.04 cm −3 (±0.03 cm −3 due to systematics), and constrain the joint kinetic energy–ejecta mass parameter space. For G1.9+0.3, archival VLA observations yielded n 0 = 0.18 cm −3 . We synthesize the multi-wavelength data on SN 1885A to argue that it is likely the product of a sub-Chandrasekhar explosion, while G1.9+0.3 appears consistent with both Chandrasekhar and sub-Chandrasekhar SN Ia models.

We report on very long baseline interferometry (VLBI) measurements of supernova 2014C at several epochs between t = 384 and 1057 d after the explosion. SN 2014C was an unusual supernova that initially had Type Ib optical spectrum, but after t = 130 d it developed a Type IIn spectrum with prominent Ha lines, suggesting the onset of strong circumstellar interaction. Our first VLBI observation was at t = 384 d, and we find that the outer radius of SN 2014C was (6.40 +/- 0.26) x 10(16) cm (for a distance of 15.1 Mpc), implying an average expansion velocity of 19 300 +/- 790 km s(-1) up to that time. At our last epoch, SN 2014C was moderately resolved and shows an approximately circular outline but with an enhancement of the brightness on the W side. The outer radius of the radio emission at t = 1057 d is (14.9 +/- 0.6) x 10(16) cm. We find that the expansion between t = 384 and 1057 d is well described by a constant velocity expansion with v = 13 600 +/- 650 km s(-1). SN 2014C had clearly been substantially decelerated by t = 384 d. Our measurements are compatible with a scenario where the expanding shock impacted upon a shell of dense circumstellar material during the first year, as suggested by the observations at other wavelengths, but had progressed through the dense shell by the time of the VLBI observations.

We present the results of an extensive observational campaign on the nearby Type Ibn SN 2015G, including data from radio through ultraviolet wavelengths. SN 2015G was asymmetric, showing late-time nebular lines redshifted by 1000 km/s. It shared many features with the prototypical SN In 2006jc, including extremely strong He I emssion lines and a late-time blue pseudocontinuum. The young SN 2015G showed narrow P-Cygni profiles of He I, but never in its evolution did it show any signature of hydrogen - arguing for a dense, ionized, and hydrogen-free circumstellar medium moving outward with a velocity of 1000 km/s and created by relatively recent mass loss from the progenitor star. Ultraviolet through infrared observations show that the fading SN 2015G (which was probably discovered some 20 days post-peak) had a spectral energy distribution that was well described by a simple, single-component blackbody. Archival HST images provide upper limits on the luminosity of SN 2015G's progenitor, while nondetections of any luminous radio afterglow and optical nondetections of outbursts over the past two decades provide constraints upon its mass-loss history.

Nature 502, 346–349 (2013); doi:10.1038/nature12569 In this Letter, we have identified an important error affecting Fig. 4 and Extended Data Fig. 6, as well as the values of some parameters derived from our model fits. We stress that this error in no way affects the discussion or the conclusions. Inbuilding the bolometric light curve of the superluminous supernova PTF 12dam, our code assumed that photometry from the Swift satellite was calibrated in the Vega magnitude system.

We discuss 76 large amplitude transients ( m > 1.5) occurring in the nuclei of galaxies, nearly all with no previously known active galactic nucleus (AGN). They have been discovered as part of the Pan-STARRS1 (PS1) 3π survey, by comparison with Sloan Digital Sky Survey (SDSS) photometry a decade earlier, and then monitored with the Liverpool Telescope, and studied spectroscopically with the William Herschel Telescope (WHT). Based on colours, light-curve shape, and spectra, these transients fall into four groups. A few are misclassified stars or objects of unknown type. Some are red/fast transients and are known or likely nuclear supernovae. A few are either radio sources or erratic variables and so likely blazars. However the majority (∼66 per cent) are blue and evolve slowly, on a time-scale of years. Spectroscopy shows them to be AGN at z ∼ 0.3 − 1.4, which must have brightened since the SDSS photometry by around an order of magnitude. It is likely that these objects were in fact AGN a decade ago, but too weak to be recognized by SDSS; they could then be classed as ‘hypervariable’ AGN. By searching the SDSS Stripe 82 quasar database, we find 15 similar objects. We discuss several possible explanations for these slow-blue hypervariables – (i) unusually luminous tidal disruption events; (ii) extinction events; (iii) changes in accretion state; and (iv) large amplitude microlensing by stars in foreground galaxies. A mixture of explanations (iii) and (iv) seems most likely. Both hold promise of considerable new insight into the AGN phenomenon.

SNe Ia, with their remarkably homogeneous light curves and spectra, have been used as standardizable candles to measure the accelerating expansion of the universe. Yet, their progenitors remain elusive. Common explanations invoke a degenerate star (white dwarf) that explodes upon almost reaching the Chandrasekhar limit, by either steadily accreting mass from a companion star or violently merging with another degenerate star. We show that circumstellar interaction in young Galactic supernova remnants can be used to distinguish between these single and double degenerate (DD) progenitor scenarios. Here we propose a new diagnostic, the surface brightness index, which can be computed from theory and compared with Chandra and Very Large Array (VLA) observations. We use this method to demonstrate that a DD progenitor can explain the decades-long flux rise and size increase of the youngest known galactic supernova remnant (SNR), G1.9+0.3. We disfavor a single degenerate scenario for SNR G1.9+0.3. We attribute the observed properties to the interaction between a steep ejecta profile and a constant density environment. We suggest using the upgraded VLA, ASKAP, and MeerKAT to detect circumstellar interaction in the remnants of historical SNe Ia in the Local Group of galaxies. This may settle the long-standing debate over their progenitors.

ABSTRACT Here we revisit line identifications of type I supernovae (SNe I) and highlight trace amounts of unburned hydrogen as an important free parameter for the composition of the progenitor. Most one-dimensional stripped-envelope models of supernovae indicate that observed features near 6000–6400 Å in type I spectra are due to more than Si ii λ 6355. However, while an interpretation of conspicuous Si ii λ 6355 can approximate 6150 Å absorption features for all SNe Ia during the first month of free expansion, similar identifications applied to 6250 Å features of SNe Ib and Ic have not been as successful. When the corresponding synthetic spectra are compared with high-quality timeseries observations, the computed spectra are frequently too blue in wavelength. Some improvement can be achieved with Fe ii lines that contribute redward of 6150 Å; however, the computed spectra either remain too blue or the spectrum only reaches a fair agreement when the rise-time to peak brightness of the model conflicts with observations by a factor of two. This degree of disagreement brings into question the proposed explosion scenario. Similarly, a detection of strong Si ii λ 6355 in the spectra of broadlined Ic and super-luminous events of type I/R is less convincing despite numerous model spectra used to show otherwise. Alternatively, we suggest 6000–6400 Å features are possibly influenced by either trace amounts of hydrogen or blueshifted absorption and emission in H α , the latter being an effect which is frequently observed in the spectra of hydrogen-rich, SNe II.

We report VLA radio observations of the Type Ib SN 2016bau (ATel #8818, #8820) in NGC 3631 (distance of 13 Mpc, according to NED). We observed the position of the supernova with VLA at a mid-time of 2016 March 30.42 UT utilizing two basebands of ~1 GHz bandwidth each, centered at 8.6 & 11.0 GHz. We detect a radio source with a flux density of 1.0 +/- 0.03 mJy in the lower sideband (8.6 GHz) at a position consistent with the optical supernova emission.

We present a novel method for the light-curve characterization of Pan-STARRS1 Medium Deep Survey (PS1 MDS) extragalactic sources into stochastic variables (SVs) and burst-like (BL) transients, using multi-band image-differencing time-series data. We select detections in difference images associated with galaxy hosts using a star/galaxy catalog extracted from the deep PS1 MDS stacked images, and adopt a maximum a posteriori formulation to model their difference-flux time-series in four Pan-STARRS1 photometric bands gP1, rP1, iP1, and zP1. We use three deterministic light-curve models to fit BL transients; a Gaussian, a Gamma distribution, and an analytic supernova (SN) model, and one stochastic light-curve model, the Ornstein–Uhlenbeck process, in order to fit variability that is characteristic of active galactic nuclei (AGNs). We assess the quality of fit of the models band-wise and source-wise, using their estimated leave-out-one cross-validation likelihoods and corrected Akaike information criteria. We then apply a K-means clustering algorithm on these statistics, to determine the source classification in each band. The final source classification is derived as a combination of the individual filter classifications, resulting in two measures of classification quality, from the averages across the photometric filters of (1) the classifications determined from the closest K-means cluster centers, and (2) the square distances from the clustering centers in the K-means clustering spaces. For a verification set of AGNs and SNe, we show that SV and BL occupy distinct regions in the plane constituted by these measures. We use our clustering method to characterize 4361 extragalactic image difference detected sources, in the first 2.5 yr of the PS1 MDS, into 1529 BL, and 2262 SV, with a purity of 95.00% for AGNs, and 90.97% for SN based on our verification sets. We combine our light-curve classifications with their nuclear or off-nuclear host galaxy offsets, to define a robust photometric sample of 1233 AGNs and 812 SNe. With these two samples, we characterize their variability and host galaxy properties, and identify simple photometric priors that would enable their real-time identification in future wide-field synoptic surveys.

We report the late-time evolution of Type IIb Supernova (SN IIb) 2013df. SN 2013df showed a dramatic change in its spectral features at 1 year after the explosion. Early on it showed typical characteristics shared by SNe IIb/Ib/Ic dominated by metal emission lines, while later on it was dominated by broad and flat-topped Halpha and He I emissions. The late-time spectra are strikingly similar to SN IIb 1993J, which is the only previous example clearly showing the same transition. This late-time evolution is fully explained by a change in the energy input from the ^56Co decay to the interaction between the SN ejecta and dense circumstellar matter (CSM). The mass loss rate is derived to be (5.4 +- 3.2) x 10^-5 Msun/yr (for the wind velocity of 20 km/s), similar to SN 1993J but larger than SN IIb 2011dh by an order of magnitude. The striking similarity between SNe 2013df and 1993J in the (candidate) progenitors and the CSM environments, and the contrast in these natures to SN 2011dh, infer that there is a link between the natures of the progenitor and the mass loss: SNe IIb with a more extended progenitor have experienced a much stronger mass loss in the final centuries toward the explosion. It might indicate that SNe IIb from a more extended progenitor are the explosions during a strong binary interaction phase, while those from a less extended progenitor have a delay between the strong binary interaction and the explosion.

Gamma-ray bursts (GRBs) are characterized by ultra-relativistic outflows, while supernovae are generally characterized by non-relativistic ejecta. GRB afterglows decelerate rapidly, usually within days, because their low-mass ejecta rapidly sweep up a comparatively larger mass of circumstellar material. However, supernovae with heavy ejecta can be in nearly free expansion for centuries. Supernovae were thought to have non-relativistic outflows except for a few relativistic ones accompanied by GRBs. This clear division was blurred by SN 2009bb, the first supernova with a relativistic outflow without an observed GRB. However, the ejecta from SN 2009bb was baryon loaded and in nearly free expansion for a year, unlike GRBs. We report the first supernova discovered without a GRB but with rapidly decelerating mildly relativistic ejecta, SN 2012ap. We discovered a bright and rapidly evolving radio counterpart driven by the circumstellar interaction of the relativistic ejecta. However, we did not find any coincident GRB with an isotropic fluence of more than one-sixth of the fluence from GRB 980425. This shows for the first time that central engines in SNe Ic, even without an observed GRB, can produce both relativistic and rapidly decelerating outflows like GRBs.

In the last three decades, about 50 radio supernovae have been detected as a result of targeted searches of optically discovered supernovae in the local universe. Despite this relatively small number some diversity among them has already been identified which is an indication of the underlying richness of radio supernovae waiting to be discovered. For example, comparison of star formation and supernova discovery rate imply that as many as half of the supernovae remain undetected in the traditional optical searches, either because of intrinsic dimness or due to dust obscuration. This has far reaching consequences to the models of stellar and galaxy evolution. A radio sky survey would be ideal to uncover larger supernova population. Transient radio sky would benefit significantly from such a survey. With the advent of advanced gravitational wave detectors a new window is set to open on the local Universe. Localization of these gravitational detectors is poor to identify electromagnetic counterparts of the gravitational wave sources. However, the longer lasting radio emission accompanied in these sources could be effectively identified in a radio sky survey. We advocate a medium area ($\approx$ a few thousand $\rm deg^{2}$) radio sky survey at C-band. Alternatively, a survey at S-band has advantage of larger sky coverage without serious loss of the science case as presented here. Understanding the background in radio sky will be of paramount importance for the upcoming sensitive radio facilities including the Square Kilometer Array.

Observations spanning a large wavelength range, from X-ray to radio, of the Type IIb supernova (SN) 2011hs are presented, covering its evolution during the first year after explosion. The optical light curve presents a narrower shape and a fainter luminosity at peak than previously observed for Type IIb SNe. High expansion velocities are measured from the broad absorption H i and He i lines. From the comparison of the bolometric light curve and the time evolution of the photospheric velocities with hydrodynamical models, we found that SN 2011hs is consistent with the explosion of a 3-4 M-circle dot He-core progenitor star, corresponding to a main-sequence mass of 12-15 M-circle dot, that ejected a mass of Ni-56 of about 0.04 M-circle dot, with an energy of E = 8.5 x 10(50) ERG. Such a low-mass progenitor scenario is in full agreement with the modelling of the nebular spectrum taken at similar to 215 d from maximum. From the modelling of the adiabatic cooling phase, we infer a progenitor radius of approximate to 500-600 R-circle dot, clearly pointing to an extended progenitor star. The radio light curve of SN 2011hs yields a peak luminosity similar to that of SN 1993J, but with a higher mass-loss rate and a wind density possibly more similar to that of SN 2001ig. Although no significant deviations from a smooth decline have been found in the radio light curves, we cannot rule out the presence of a binary companion star.