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Abstract:

Context. At late stages, massive stars experience strong mass-loss rates, losing their external layers and thus producing a dense H-rich circumstellar medium (CSM). After the explosion of a massive star, the collision and continued interaction of the supernova (SN) ejecta with the CSM power the SN light curve through the conversion of kinetic energy into radiation. When the interaction is strong, the light curve shows a broad peak and high luminosity that lasts for several months. For these SNe, the spectral evolution is also slower compared to non-interacting SNe. Notably, energetic shocks between the ejecta and the CSM create the ideal conditions for particle acceleration and the production of high-energy (HE) neutrinos above 1 TeV. Aims. We study four strongly interacting Type IIn SNe, 2021acya, 2021adxl, 2022qml, and 2022wed, in order to highlight their peculiar characteristics, derive the kinetic energy of their explosion and the characteristics of the CSM, infer clues on the possible progenitors and their environment, and relate them to the production of HE neutrinos. Methods. We analysed spectro-photometric data of a sample of interacting SNe to determine their common characteristics and derive the physical properties (radii and masses) of the CSM and the ejecta kinetic energies and compare them to HE neutrino production models. Results. The SNe analysed in this sample exploded in dwarf star-forming galaxies, and they are consistent with energetic explosions and strong interaction with the surrounding CSM. For SNe 2021acya and 2022wed, we find high CSM masses and mass-loss rates, linking them to very massive progenitors. For SN 2021adxl, the spectral analysis and less extreme CSM mass suggest a stripped-envelope massive star as a possible progenitor. SN 2022qml is marginally consistent with being a Type Ia thermonuclear explosion embedded in a dense CSM. The mass-loss rates for all the SNe are consistent with the expulsion of several solar masses of material during eruptive episodes in the last few decades before the explosion. Finally, we find that the SNe in our sample are marginally consistent with HE neutrino production

Abstract:

Most stripped-envelope supernova progenitors are thought to be formed through binary interaction, losing hydrogen and/or helium from their outer layers. Ultrastripped supernovae are an emerging class of transient that are expected to be produced through envelope stripping by a neutron star companion. However, relatively few examples are known, and the outcomes of such systems can be diverse and are poorly understood at present. Here we present spectroscopic observations and high-cadence, multiband photometry of SN 2023zaw, a rapidly evolving supernova with a low ejecta mass. SN 2023zaw was discovered in a nearby spiral galaxy at D = 39.7 Mpc. It has significant Milky Way extinction, E(B − V)MW = 0.21, and significant (but uncertain) host extinction. Bayesian evidence comparison reveals that nickel is not the only power source and that an additional energy source is required to explain our observations. Our models suggest that an ejecta mass of Mej ∼ 0.07 M⊙ and a synthesised nickel mass of MNi ∼ 0.007 M⊙ are required to explain the observations. We find that additional heating from a central engine, or interaction with circumstellar material, can power the early light curve.

Abstract:

Fast X-ray transients (FXTs) are extragalactic bursts of X-rays first identified in archival X-ray data and are now routinely discovered in real time by the Einstein Probe, which is continuously surveying the night sky in the soft (0.5–4 keV) X-ray regime. In this Letter, we report the discovery of the second optical counterpart (AT 2024gsa) to an FXT (EP 240414a). EP 240414a is located at a projected radial separation of 27 kpc from its likely host galaxy at z = 0.4018 ± 0.0010. The optical light curve of AT 2024gsa displays three distinct components. The initial decay from our first observation is followed by a rebrightening episode, displaying a rapid rise in luminosity to an absolute magnitude Mr ∼ −21 after two rest-frame days. While the early optical luminosity and decline rate are similar to those of luminous fast blue optical transients, the color temperature of AT 2024gsa is distinctly red and we show that the peak flux is inconsistent with a thermal origin. The third component peaks at Mi ∼ −19 at ≳16 rest-frame days post-FXT, and is compatible with an emerging supernova. We fit the riz-band data with a series of power laws and find that the decaying components are in agreement with gamma-ray burst afterglow models, and that the rebrightening may originate from refreshed shocks. By considering EP 240414a in context with all previously reported known-redshift FXT events, we propose that Einstein Probe FXT discoveries may predominantly result from (high-redshift) gamma-ray bursts, and thus appear to be distinct from the previously discovered lower-redshift, lower-luminosity population of FXTs.

Abstract:

We present multiwavelength data of SN 2020acct, a double-peaked stripped-envelope supernova (SN) in NGC 2981 at ∼150 Mpc. The two peaks are temporally distinct, with maxima separated by 58 rest-frame days and a factor of 20 reduction in flux between. The first is luminous (Mr = −18.00 ± 0.02 mag) and blue (g − r = 0.27 ± 0.03 mag) and displays spectroscopic signatures of interaction with hydrogen-free circumstellar material. The second peak is fainter (Mr = −17.29 ± 0.03 mag) and has some spectroscopic similarities to an evolved stripped-envelope SN, with strong forbidden [Ca ii] and [O ii] features. No other known double-peaked SN exhibits a light curve similar to that of SN 2020acct. We find the likelihood of two individual SNe occurring in the same star-forming region within that time to be highly improbable, while an implausibly fine-tuned configuration would be required to produce two SNe from a single binary system. We find that the peculiar properties of SN 2020acct match models of pulsational pair instability (PPI), in which the initial peak is produced by collisions of shells of ejected material, shortly followed by core collapse. Pulsations from a star with a 72 M⊙ helium core provide an excellent match to the double-peaked light curve. The local galactic environment has a metallicity of 0.4 Z⊙, a level where massive single stars are not expected to retain enough mass to encounter the PPI. However, late binary mergers or a low-metallicity pocket may allow the required core mass. We measure the rate of SN 2020acct–like events to be <3.3 × 10−8 Mpc−3 yr−1 at z = 0.07, or <0.1% of the total core-collapse SN rate.