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1. Seven Years of SN 2014C: A Multiwavelength Synthesis of an Extraordinary Supernova

   https://doi.org/10.3847/1538-4357/ac5fa6  

   Thomas, Benjamin P. ; Wheeler, J. Craig ; Dwarkadas, Vikram V. ; Stockdale, Christopher ; Vinkó, Jozsef ; Pooley, David ; Xu, Yerong ; Zeimann, Greg ; MacQueen, Phillip ( May 2022 , The Astrophysical Journal)

   SN 2014C was originally classified as a Type Ib supernova, but at phaseϕ= 127 days, post-explosion strong Hαemission was observed. SN 2014C has since been observed in radio, infrared, optical and X-ray bands. Here we present new optical spectroscopic and photometric data spanningϕ= 947–2494 days post-explosion. We address the evolution of the broadened Hαemission line, as well as broad [Oiii] emission and other lines. We also conduct a parallel analysis of all publicly available multiwavelength data. From our spectra, we find a nearly constant HαFWHM velocity width of ∼2000 km s⁻¹that is significantly lower than that of other broadened atomic transitions (∼3000–7000 km s⁻¹) present in our spectra ([Oi]λ6300; [Oiii]λλ4959, 5007; Heiλ7065; [Caii]λλ7291, 7324). The late radio data demand a fast forward shock (∼10,000 km s⁻¹atϕ= 1700 days) in rarified matter that contrasts with the modest velocity of the Hα. We propose that the infrared flux originates from a toroidal-like structure of hydrogen surrounding the progenitor system, while later emission at other wavelengths (radio, X-ray) likely originates predominantly from the reverse shock in the ejecta and the forward shock in the quasi-spherical progenitor He-wind. We propose that the Hαemission arises in the boundary layer between the ejecta and torus. We also consider the possible roles of a pulsar and a binary companion.

    

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2. Stochastic re-acceleration and magnetic-field damping in Tycho ’s supernova remnant

   https://doi.org/10.1051/0004-6361/201936079  

   Wilhelm, A. ; Telezhinsky, I. ; Dwarkadas, V. V. ; Pohl, M. ( July 2020 , Astronomy & Astrophysics)

   Context.   Tycho ’s supernova remnant (SNR) is associated with the historical supernova (SN) event SN 1572 of Type Ia. The explosion occurred in a relatively clean environment, and was visually observed, providing an age estimate. This SNR therefore represents an ideal astrophysical test-bed for the study of cosmic-ray acceleration and related phenomena. A number of studies suggest that shock acceleration with particle feedback and very efficient magnetic-field amplification combined with Alfvénic drift are needed to explain the rather soft radio spectrum and the narrow rims observed in X-rays. Aims. We show that the broadband spectrum of Tycho ’s SNR can alternatively be well explained when accounting for stochastic acceleration as a secondary process. The re-acceleration of particles in the turbulent region immediately downstream of the shock should be efficient enough to impact particle spectra over several decades in energy. The so-called Alfvénic drift and particle feedback on the shock structure are not required in this scenario. Additionally, we investigate whether synchrotron losses or magnetic-field damping play a more profound role in the formation of the non-thermal filaments. Methods. We solved the full particle transport equation in test-particle mode using hydrodynamic simulations of the SNR plasma flow. The background magnetic field was either computed from the induction equation or follows analytic profiles, depending on the model considered. Fast-mode waves in the downstream region provide the diffusion of particles in momentum space. Results. We show that the broadband spectrum of Tycho can be well explained if magnetic-field damping and stochastic re-acceleration of particles are taken into account. Although not as efficient as standard diffusive shock acceleration, stochastic acceleration leaves its imprint on the particle spectra, which is especially notable in the emission at radio wavelengths. We find a lower limit for the post-shock magnetic-field strength ∼330  μ G, implying efficient amplification even for the magnetic-field damping scenario. Magnetic-field damping is necessary for the formation of the filaments in the radio range, while the X-ray filaments are shaped by both the synchrotron losses and magnetic-field damping. 

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3. The DIVING3D Survey – Deep IFS view of nuclei of galaxies – II. First results: nuclear emission-line properties of the mini-DIVING3D sample

   https://doi.org/10.1093/mnras/stac1235  

   Menezes, R. B. ; Steiner, J. E. ; Ricci, T. V. ; da Silva, Patrícia ( May 2022 , Monthly Notices of the Royal Astronomical Society)

   This work presents the first results of the Deep IFS View of Nuclei of Galaxies (DIVING3D) survey. We analysed the nuclear emission-line spectra of a sub-sample we call mini-DIVING3D, which includes all Southern galaxies with B < 11.2 and |b| > 15°. We verified that $23{{\ \rm per\ cent}} \pm 4{{\ \rm per\ cent}}$ of the galaxies show nuclear emission-line properties characteristic of Low Ionization Nuclear Emission-Line Regions (LINERs). Diagnostic diagram analysis reveals an apparent dichotomy, not detected in previous studies, between objects classified as H ii regions and as LINERs or Seyferts, with very few galaxies classified as transition objects. A possible explanation for this result is that at least part of the transition objects are composite systems, with a central LINER contaminated by the emission from circumnuclear H ii regions. The higher spatial resolution of the DIVING3D survey, in comparison with previous studies, allowed us to isolate the nuclear emission from circumnuclear contaminations, reducing the number of transition objects. We also propose an alternative scenario, in which the emission-line spectra of some transition objects are the result of shock heating by central outflows, together with photoionization by young stars. Clear evidence of active galactic nuclei, in the optical and X-ray spectral bands, were detected in 69 per cent of the LINERs in the mini-DIVING3D sample. Considering the entire mini-DIVING3D sample, evidence of AGNs were detected in 65 per cent of the objects.

    

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4. AT 2022aedm and a New Class of Luminous, Fast-cooling Transients in Elliptical Galaxies

   https://doi.org/10.3847/2041-8213/acf0ba  

   Nicholl, M. ; Srivastav, S. ; Fulton, M. D. ; Gomez, S. ; Huber, M. E. ; Oates, S. R. ; Ramsden, P. ; Rhodes, L. ; Smartt, S. J. ; Smith, K. W. ; et al ( September 2023 , The Astrophysical Journal Letters)

   We present the discovery and extensive follow-up of a remarkable fast-evolving optical transient, AT 2022aedm, detected by the Asteroid Terrestrial impact Last Alert Survey (ATLAS). In the ATLASoband, AT 2022aedm exhibited a rise time of 9 ± 1 days, reaching a luminous peak withM_g≈ −22 mag. It faded by 2 mag in thegband during the next 15 days. These timescales are consistent with other rapidly evolving transients, though the luminosity is extreme. Most surprisingly, the host galaxy is a massive elliptical with negligible current star formation. Radio and X-ray observations rule out a relativistic AT 2018cow–like explosion. A spectrum in the first few days after explosion showed short-lived Heiiemission resembling young core-collapse supernovae, but obvious broad supernova features never developed; later spectra showed only a fast-cooling continuum and narrow, blueshifted absorption lines, possibly arising in a wind withv≈ 2700 km s⁻¹. We identify two further transients in the literature (Dougie in particular, as well as AT 2020bot) that share similarities in their luminosities, timescales, color evolution, and largely featureless spectra and propose that these may constitute a new class of transients: luminous fast coolers. All three events occurred in passive galaxies at offsets of ∼4–10 kpc from the nucleus, posing a challenge for progenitor models involving massive stars or black holes. The light curves and spectra appear to be consistent with shock breakout emission, though this mechanism is usually associated with core-collapse supernovae. The encounter of a star with a stellar-mass black hole may provide a promising alternative explanation.

    

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5. Ejecta, Rings, and Dust in SN 1987A with JWST MIRI/MRS

   https://doi.org/10.3847/1538-4357/ad0036  

   Jones, O. C. ; Kavanagh, P. J. ; Barlow, M. J. ; Temim, T. ; Fransson, C. ; Larsson, J. ; Blommaert, J. A. D. L. ; Meixner, M. ; Lau, R. M. ; Sargent, B. ; et al ( November 2023 , The Astrophysical Journal)

   Supernova (SN) 1987A is the nearest supernova in ∼400 yr. Using the JWST MIRI Medium Resolution Spectrograph, we spatially resolved the ejecta, equatorial ring (ER), and outer rings in the mid-infrared 12,927 days (35.4 yr) after the explosion. The spectra are rich in line and dust continuum emission, both in the ejecta and the ring. The broad emission lines (280–380 km s⁻¹FWHM) that are seen from all singly-ionized species originate from the expanding ER, with properties consistent with dense post-shock cooling gas. Narrower emission lines (100–170 km s⁻¹FWHM) are seen from species originating from a more extended lower-density component whose high ionization may have been produced by shocks progressing through the ER or by the UV radiation pulse associated with the original supernova event. The asymmetric east–west dust emission in the ER has continued to fade, with constant temperature, signifying a reduction in dust mass. Small grains in the ER are preferentially destroyed, with larger grains from the progenitor surviving the transition from SN into SNR. The ER dust is fit with a single set of optical constants, eliminating the need for a secondary featureless hot dust component. We find several broad ejecta emission lines from [Neii], [Arii], [Feii], and [Niii]. With the exception of [Feii] 25.99μm, these all originate from the ejecta close to the ring and are likely to be excited by X-rays from the interaction. The [Feii] 5.34 to 25.99μm line ratio indicates a temperature of only a few hundred K in the inner core, which is consistent with being powered by⁴⁴Ti decay.

    

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