Search for: All records

Abstract SN 2018ivc is an unusual Type II supernova (SN II). It is a variant of SNe IIL, which might represent a transitional case between SNe IIP with a massive H-rich envelope and SNe IIb with only a small amount of the H-rich envelope. However, SN 2018ivc shows an optical light-curve evolution more complicated than that of canonical SNe IIL. In this paper, we present the results of prompt follow-up observations of SN 2018ivc with the Atacama Large Millimeter/submillimeter Array. Its synchrotron emission is similar to that of SN IIb 1993J, suggesting that it is intrinsically an SN IIb–like explosion of an He star with a modest (∼0.5–1 M ⊙ ) extended H-rich envelope. Its radio, optical, and X-ray light curves are explained primarily by the interaction between the SN ejecta and the circumstellar material (CSM); we thus suggest that it is a rare example (and the first involving the “canonical” SN IIb ejecta) for which the multiwavelength emission is powered mainly by the SN–CSM interaction. The inner CSM density, reflecting the progenitor activity in the final decade, is comparable to that of SN IIb 2013cu, which shows a flash spectral feature. The outer CSM density, and therefore the mass-loss rate in the final ∼200 yr, is higher than that of SN 1993J by a factor of ∼5. We suggest that SN 2018ivc represents a missing link between SNe IIP and SNe IIb/Ib/Ic in the binary evolution scenario. 

We present VLT–FORS spectropolarimetric observations of the type II supernova (SN) 2012aw taken at seven epochs during the photospheric phase, from 16 to 120 d after explosion. We corrected for interstellar polarization by postulating that the SN polarization is naught near the rest wavelength of the strongest lines – this is later confirmed by our modeling. SN 2012aw exhibits intrinsic polarization, with strong variations across lines, and with a magnitude that grows in the 7000 Å line-free region from 0.1% at 16 d up to 1.2% at 120 d. This behavior is qualitatively similar to observations gathered for other type II SNe. A suitable rotation of Stokes vectors places the bulk of the polarization in q , suggesting the ejecta of SN 2012aw is predominantly axisymmetric. Using an upgraded version of our 2D polarized radiative transfer code, we modeled the wavelength- and time-dependent polarization of SN 2012aw. The key observables may be explained by the presence of a confined region of enhanced 56 Ni at ~4000 km s −1 , which boosts the electron density in a cone having an opening angle of ~50 deg and an observer’s inclination of ~70 deg to the axis of symmetry. With this fixed asymmetry in time, the observed evolution of the SN 2012aw polarization arises from the evolution of the ejecta optical depth, ionization, and the relative importance of multiple versus single scattering. However, the polarization signatures exhibit numerous degeneracies. Cancellation effects at early times imply that low polarization may even occur for ejecta with a large asymmetry. An axisymmetric ejecta with a latitudinal-dependent explosion energy can also yield similar polarization signatures as asymmetry in the 56 Ni distribution. In spite of these uncertainties, SN 2012aw provides additional evidence for the generic asymmetry of type II SN ejecta, of which VLT–FORS spectropolarimetric observations are a decisive and exquisite probe. 

We present deep, nebular-phase spectropolarimetry of the Type II-P/L SN 2013ej, obtained 167 days after explosion with the European Southern Observatory’s Very Large Telescope. The polarized flux spectrum appears as a nearly perfect (92% correlation), redshifted (by ∼4000 km s⁻¹) replica of the total flux spectrum. Such a striking correspondence has never been observed before in nebular-phase supernova spectropolarimetry, although data capable of revealing it have heretofore been only rarely obtained. Through comparison with 2D polarized radiative transfer simulations of stellar explosions, we demonstrate that localized ionization produced by the decay of a high-velocity, spatially confined clump of radioactive⁵⁶Ni—synthesized by and launched as part of the explosion with final radial velocity exceeding 4500 km s⁻¹—can reproduce the observations through enhanced electron scattering. Additional data taken earlier in the nebular phase (day 134) yield a similarly strong correlation (84%) and redshift, whereas photospheric-phase epochs that sample days 8 through 97 do not. This suggests that the primary polarization signatures of the high-velocity scattering source only come to dominate once the thick, initially opaque hydrogen envelope has turned sufficiently transparent. This detection in an otherwise fairly typical core-collapse supernova adds to the growing body of evidence supporting strong asymmetries across nature’s most common types of stellar explosions, and establishes the power of polarized flux—and the specific information encoded by it in line photons at nebular epochs—as a vital tool in such investigations going forward.

 

ABSTRACT We present optical spectroscopy together with ultraviolet, optical, and near-infrared photometry of SN 2019hcc, which resides in a host galaxy at redshift 0.044, displaying a sub-solar metallicity. The supernova spectrum near peak epoch shows a ‘w’ shape at around 4000 Å which is usually associated with O ii lines and is typical of Type I superluminous supernovae. SN 2019hcc post-peak spectra show a well-developed H α P-Cygni profile from 19 d past maximum and its light curve, in terms of its absolute peak luminosity and evolution, resembles that of a fast-declining Hydrogen-rich supernova (SN IIL). The object does not show any unambiguous sign of interaction as there is no evidence of narrow lines in the spectra or undulations in the light curve. Our tardis spectral modelling of the first spectrum shows that carbon, nitrogen, and oxygen (CNO) at 19 000 K reproduce the ‘w’ shape and suggests that a combination of non-thermally excited CNO and metal lines at 8000 K could reproduce the feature seen at 4000 Å. The Bolometric light-curve modelling reveals that SN 2019hcc could be fit with a magnetar model, showing a relatively strong magnetic field (B > 3 × 1014 G), which matches the peak luminosity and rise time without powering up the light curve to superluminous luminosities. The high-energy photons produced by the magnetar would then be responsible for the detected O ii lines. As a consequence, SN 2019hcc shows that a ‘w’ shape profile at around 4000 Å, usually attributed to O ii, is not only shown in superluminous supernovae and hence it should not be treated as the sole evidence of the belonging to such a supernova type. 

We present DELIGHT, or Deep Learning Identification of Galaxy Hosts of Transients, a new algorithm designed to automatically and in real time identify the host galaxies of extragalactic transients. The proposed algorithm receives as input compact, multiresolution images centered at the position of a transient candidate and outputs two-dimensional offset vectors that connect the transient with the center of its predicted host. The multiresolution input consists of a set of images with the same number of pixels, but with progressively larger pixel sizes and fields of view. A sample of 16,791 galaxies visually identified by the Automatic Learning for the Rapid Classification of Events broker team was used to train a convolutional neural network regression model. We show that this method is able to correctly identify both relatively large (10″ <r< 60″) and small (r≤ 10″) apparent size host galaxies using much less information (32 kB) than with a large, single-resolution image (920 kB). The proposed method has fewer catastrophic errors in recovering the position and is more complete and has less contamination (<0.86%) recovering the crossmatched redshift than other state-of-the-art methods. The more efficient representation provided by multiresolution input images could allow for the identification of transient host galaxies in real time, if adopted in alert streams from new generation of large -etendue telescopes such as the Vera C. Rubin Observatory.

 

SN 2018aoz is a Type Ia SN with aB-band plateau and excess emission in infant-phase light curves ≲1 day after the first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN 2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show that the SN is intermediate between two subtypes of normal Type Ia: core normal and broad line. The excess emission may be attributable to the radioactive decay of surface iron-peak elements as well as the interaction of ejecta with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on Hαand Heifavor a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [Oi] and Heidisfavors a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN 2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Feii] and [Niii]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1D models are incompatible with the infant-phase excess emission,$B_{\max }–V_{\max }$color, and weak strength of nebular-phase [Caii]. Although the explosion processes of SN 2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear to be normal after ∼1 day.