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1. The long-lived Type IIn SN 2015da: Infrared echoes and strong interaction within an extended massive shell

   https://doi.org/10.1051/0004-6361/201936553  

   Tartaglia, L. ; Pastorello, A. ; Sollerman, J. ; Fransson, C. ; Mattila, S. ; Fraser, M. ; Taddia, F. ; Tomasella, L. ; Turatto, M. ; Morales-Garoffolo, A. ; et al ( March 2020 , Astronomy & Astrophysics)

   In this paper we report the results of the first ~four years of spectroscopic and photometric monitoring of the Type IIn supernova SN 2015da (also known as PSN J13522411+3941286, or iPTF16tu). The supernova exploded in the nearby spiral galaxy NGC 5337 in a relatively highly extinguished environment. The transient showed prominent narrow Balmer lines in emission at all times and a slow rise to maximum in all bands. In addition, early observations performed by amateur astronomers give a very well-constrained explosion epoch. The observables are consistent with continuous interaction between the supernova ejecta and a dense and extended H-rich circumstellarmore »medium. The presence of such an extended and dense medium is difficult to reconcile with standard stellar evolution models, since the metallicity at the position of SN 2015da seems to be slightly subsolar. Interaction is likely the mechanism powering the light curve, as confirmed by the analysis of the pseudo bolometric light curve, which gives a total radiated energy ≳ 10 51 erg. Modeling the light curve in the context of a supernova shock breakout through a dense circumstellar medium allowed us to infer the mass of the prexisting gas to be ≃ 8 M ⊙ , with an extreme mass-loss rate for the progenitor star ≃0.6 M ⊙ yr −1 , suggesting that most of the circumstellar gas was produced during multiple eruptive events. Near- and mid-infrared observations reveal a fluxexcess in these domains, similar to those observed in SN 2010jl and other interacting transients, likely due to preexisting radiatively heated dust surrounding the supernova. By modeling the infrared excess, we infer a mass ≳ 0.4 × 10 −3 M ⊙ for the dust.« less
2. Extreme variability in an active galactic nucleus: Gaia16aax

   https://doi.org/10.1093/mnras/staa186  

   Cannizzaro, G ; Fraser, M ; Jonker, P G ; Pringle, J E ; Mattila, S ; Hewett, P C ; Wevers, T ; Kankare, E ; Kostrzewa-Rutkowska, Z ; Wyrzykowski, Ł ; et al ( March 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present the results of a multiwavelength follow-up campaign for the luminous nuclear transient Gaia16aax, which was first identified in 2016 January. The transient is spatially consistent with the nucleus of an active galaxy at z = 0.25, hosting a black hole of mass ${\sim }6\times 10^8\, \mathrm{M}_\odot$. The nucleus brightened by more than 1 mag in the Gaia G band over a time-scale of less than 1 yr, before fading back to its pre-outburst state over the following 3 yr. The optical spectra of the source show broad Balmer lines similar to the ones present in a pre-outburst spectrum. Duringmore »the outburst, the H α and H β emission lines develop a secondary peak. We also report on the discovery of two transients with similar light-curve evolution and spectra: Gaia16aka and Gaia16ajq. We consider possible scenarios to explain the observed outbursts. We exclude that the transient event could be caused by a microlensing event, variable dust absorption or a tidal encounter between a neutron star and a stellar mass black hole in the accretion disc. We consider variability in the accretion flow in the inner part of the disc, or a tidal disruption event of a star ${\ge } 1 \, \mathrm{M}_{\odot }$ by a rapidly spinning supermassive black hole as the most plausible scenarios. We note that the similarity between the light curves of the three Gaia transients may be a function of the Gaia alerts selection criteria.« less
3. Evidence for a Chandrasekhar-mass explosion in the Ca-strong 1991bg-like type Ia supernova 2016hnk

   https://doi.org/10.1051/0004-6361/201935537  

   Galbany, L. ; Ashall, C. ; Höflich, P. ; González-Gaitán, S. ; Taubenberger, S. ; Stritzinger, M. ; Hsiao, E. Y. ; Mazzali, P. ; Baron, E. ; Blondin, S. ; et al ( October 2019 , Astronomy & Astrophysics)

   Aims . We present a comprehensive dataset of optical and near-infrared photometry and spectroscopy of type Ia supernova (SN) 2016hnk, combined with integral field spectroscopy (IFS) of its host galaxy, MCG -01-06-070, and nearby environment. Our goal with this complete dataset is to understand the nature of this peculiar object. Methods . Properties of the SN local environment are characterized by means of single stellar population synthesis applied to IFS observations taken two years after the SN exploded. We performed detailed analyses of SN photometric data by studying its peculiar light and color curves. SN 2016hnk spectra were compared tomore »other 1991bg-like SNe Ia, 2002es-like SNe Ia, and Ca-rich transients. In addition, we used abundance stratification modeling to identify the various spectral features in the early phase spectral sequence and also compared the dataset to a modified non-LTE model previously produced for the sublumnious SN 1999by. Results . SN 2016hnk is consistent with being a subluminous ( M B  = −16.7 mag, s B V =0.43 ± 0.03), highly reddened object. The IFS of its host galaxy reveals both a significant amount of dust at the SN location, residual star formation, and a high proportion of old stellar populations in the local environment compared to other locations in the galaxy, which favors an old progenitor for SN 2016hnk. Inspection of a nebular spectrum obtained one year after maximum contains two narrow emission lines attributed to the forbidden [Ca  II ] λ λ 7291,7324 doublet with a Doppler shift of 700 km s −1 . Based on various observational diagnostics, we argue that the progenitor of SN 2016hnk was likely a near Chandrasekhar-mass ( M Ch ) carbon-oxygen white dwarf that produced 0.108 M ⊙ of 56 Ni. Our modeling suggests that the narrow [Ca  II ] features observed in the nebular spectrum are associated with 48 Ca from electron capture during the explosion, which is expected to occur only in white dwarfs that explode near or at the M Ch limit.« less
4. An outflow powers the optical rise of the nearby, fast-evolving tidal disruption event AT2019qiz

   https://doi.org/10.1093/mnras/staa2824  

   Nicholl, M ; Wevers, T ; Oates, S R ; Alexander, K D ; Leloudas, G ; Onori, F ; Jerkstrand, A ; Gomez, S ; Campana, S ; Arcavi, I ; et al ( October 2020 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT At 66 Mpc, AT2019qiz is the closest optical tidal disruption event (TDE) to date, with a luminosity intermediate between the bulk of the population and the faint-and-fast event iPTF16fnl. Its proximity allowed a very early detection and triggering of multiwavelength and spectroscopic follow-up well before maximum light. The velocity dispersion of the host galaxy and fits to the TDE light curve indicate a black hole mass ≈106 M⊙, disrupting a star of ≈1 M⊙. By analysing our comprehensive UV, optical, and X-ray data, we show that the early optical emission is dominated by an outflow, with a luminosity evolution L ∝ t2, consistent withmore »a photosphere expanding at constant velocity (≳2000 km s−1), and a line-forming region producing initially blueshifted H and He ii profiles with v = 3000–10 000 km s−1. The fastest optical ejecta approach the velocity inferred from radio detections (modelled in a forthcoming companion paper from K. D. Alexander et al.), thus the same outflow may be responsible for both the fast optical rise and the radio emission – the first time this connection has been observed in a TDE. The light-curve rise begins 29 ± 2 d before maximum light, peaking when the photosphere reaches the radius where optical photons can escape. The photosphere then undergoes a sudden transition, first cooling at constant radius then contracting at constant temperature. At the same time, the blueshifts disappear from the spectrum and Bowen fluorescence lines (N iii) become prominent, implying a source of far-UV photons, while the X-ray light curve peaks at ≈1041 erg s−1. Assuming that these X-rays are from prompt accretion, the size and mass of the outflow are consistent with the reprocessing layer needed to explain the large optical to X-ray ratio in this and other optical TDEs, possibly favouring accretion-powered over collision-powered outflow models.« less
5. The Carnegie Supernova Project II: Observations of the luminous red nova AT 2014ej

   https://doi.org/10.1051/0004-6361/202038019  

   Stritzinger, M. D. ; Taddia, F. ; Fraser, M. ; Tauris, T. M. ; Contreras, C. ; Drybye, S. ; Galbany, L. ; Holmbo, S. ; Morrell, N. ; Pastorello, A. ; et al ( July 2020 , Astronomy & Astrophysics)

   We present optical and near-infrared broadband photometry and optical spectra of AT 2014ej from the Carnegie Supernova Project-II. These observations are complemented with data from the CHilean Automatic Supernova sEarch, the Public ESO Spectroscopic Survey of Transient Objects, and from the Backyard Observatory Supernova Search. Observational signatures of AT 2014ej reveal that it is similar to other members of the gap-transient subclass known as luminous red novae (LRNe), including the ubiquitous double-hump light curve and spectral properties similar to that of LRN SN 2017jfs. A medium-dispersion visual-wavelength spectrum of AT 2014ej taken with the Magellan Clay telescope exhibits a Pmore »Cygni H α feature characterized by a blue velocity at zero intensity of ≈110 km s −1 and a P Cygni minimum velocity of ≈70 km s −1 . We attribute this to emission from a circumstellar wind. Inspection of pre-outbust Hubble Space Telescope images yields no conclusive progenitor detection. In comparison with a sample of LRNe from the literature, AT 2014ej lies at the brighter end of the luminosity distribution. Comparison of the ultra-violet, optical, infrared light curves of well-observed LRNe to common-envelope evolution models from the literature indicates that the models underpredict the luminosity of the comparison sample at all phases and also produce inconsistent timescales of the secondary peak. Future efforts to model LRNe should expand upon the current parameter space we explore here and therefore may consider more massive systems and a wider range of dynamical timescales.« less