Neutron stars and stellar-mass black holes are the remnants of massive star explosions¹. Most massive stars reside in close binary systems², and the interplay between the companion star and the newly formed compact object has been theoretically explored³, but signatures for binarity or evidence for the formation of a compact object during a supernova explosion are still lacking. Here we report a stripped-envelope supernova, SN 2022jli, which shows 12.4-day periodic undulations during the declining light curve. Narrow Hα emission is detected in late-time spectra with concordant periodic velocity shifts, probably arising from hydrogen gas stripped from a companion and accreted onto the compact remnant. A new Fermi-LAT γ-ray source is temporally and positionally consistent with SN 2022jli. The observed properties of SN 2022jli, including periodic undulations in the optical light curve, coherent Hα emission shifting and evidence for association with a γ-ray source, point to the explosion of a massive star in a binary system leaving behind a bound compact remnant. Mass accretion from the companion star onto the compact object powers the light curve of the supernova and generates the γ-ray emission.

One of the open questions following the discovery of GW170817 is whether neutron star (NS) mergers are the only astrophysical sites capable of producing r-process elements. Simulations have shown that 0.01–0.1 M_⊙ of r-process material could be generated in the outflows originating from the accretion disk surrounding the rapidly rotating black hole that forms as a remnant to both NS mergers and collapsing massive stars associated with long-duration gamma-ray bursts (collapsars). The hallmark signature of r-process nucleosynthesis in the binary NS merger GW170817 was its long-lasting near-infrared (NIR) emission, thus motivating a systematic photometric study of the light curves of broad-lined stripped-envelope (Ic-BL) supernovae (SNe) associated with collapsars. We present the first systematic study of 25 SNe Ic-BL—including 18 observed with the Zwicky Transient Facility and 7 from the literature—in the optical/NIR bands to determine what quantity of r-process material, if any, is synthesized in these explosions. Using semi-analytic models designed to account for r-process production in SNe Ic-BL, we perform light curve fitting to derive constraints on the r-process mass for these SNe. We also perform independent light curve fits to models without the r-process. We find that the r-process-free models are a better fit to the light curves of the objects in our sample. Thus, we find no compelling evidence of r-process enrichment in any of our objects. Further high-cadence infrared photometric studies and nebular spectroscopic analysis would be sensitive to smaller quantities of r-process ejecta mass or indicate whether all collapsars are completely devoid of r-process nucleosynthesis.

Eruptive mass loss of massive stars prior to supernova (SN) explosion is key to understanding their evolution and end fate. An observational signature of pre-SN mass loss is the detection of an early, short-lived peak prior to the radioactive-powered peak in the lightcurve of the SN. This is usually attributed to the SN shock passing through an extended envelope or circumstellar medium. Such an early peak is common for double-peaked Type IIb SNe with an extended hydrogen envelope but uncommon for normal Type Ibc SNe with very compact progenitors. In this paper, we systematically study a sample of 14 double-peaked Type Ibc SNe out of 475 Type Ibc SNe detected by the Zwicky Transient Facility. The rate of these events is ∼3%-9% of Type Ibc SNe. A strong correlation is seen between the peak brightness of the first and the second peak. We perform a holistic analysis of this sample’s photometric and spectroscopic properties. We find that six SNe have ejecta mass less than 1.5 M ⊙. Based on the nebular spectra and lightcurve properties, we estimate that the progenitor masses for these are less than ∼12 M ⊙. The rest have an ejecta mass >2.4 M ⊙ and a higher progenitor mass. This sample suggests that the SNe with low progenitor masses undergo late-time binary mass transfer. Meanwhile, the SNe with higher progenitor masses are consistent with wave-driven mass loss or pulsation-pair instability-driven mass-loss simulations.

Type Ia supernovae (SNe Ia) are thermonuclear explosions of degenerate white dwarf stars destabilized by mass accretion from a companion star¹, but the nature of their progenitors remains poorly understood. A way to discriminate between progenitor systems is through radio observations; a non-degenerate companion star is expected to lose material through winds² or binary interaction³ before explosion, and the supernova ejecta crashing into this nearby circumstellar material should result in radio synchrotron emission. However, despite extensive efforts, no type Ia supernova (SN Ia) has ever been detected at radio wavelengths, which suggests a clean environment and a companion star that is itself a degenerate white dwarf star^4,5. Here we report on the study of SN 2020eyj, a SN Ia showing helium-rich circumstellar material, as demonstrated by its spectral features, infrared emission and, for the first time in a SN Ia to our knowledge, a radio counterpart. On the basis of our modelling, we conclude that the circumstellar material probably originates from a single-degenerate binary system in which a white dwarf accretes material from a helium donor star, an often proposed formation channel for SNe Ia (refs. ^6,7). We describe how comprehensive radio follow-up of SN 2020eyj-like SNe Ia can improve the constraints on their progenitor systems.

The dividing line between gamma-ray bursts (GRBs) and ordinary stripped-envelope core-collapse supernovae (SNe) is yet to be fully understood. Observationally mapping the variety of ejecta outcomes (ultrarelativistic, mildly relativistic, or nonrelativistic) in SNe of Type Ic with broad lines (Ic-BL) can provide a key test to stellar explosion models. However, this requires large samples of the rare SN Ic-BL events with follow-up observations in the radio, where fast ejecta can be probed largely free of geometry and viewing angle effects. Here, we present the results of a radio (and X-ray) follow-up campaign of 16 SNe Ic-BL detected by the Zwicky Transient Facility (ZTF). Our radio campaign resulted in four counterpart detections and 12 deep upper limits. None of the events in our sample is as relativistic as SN 1998bw and we constrain the fraction of SN 1998bw-like explosions to <19% (3σ Gaussian equivalent), a factor of ≈2 smaller than previously established. We exclude relativistic ejecta with radio luminosity densities in between ≈5 × 10²⁷ erg s⁻¹ Hz⁻¹ and ≈10²⁹ erg s⁻¹ Hz⁻¹ at t ≳ 20 days since explosion for ≈60% of the events in our sample. This shows that SNe Ic-BL similar to the GRB-associated SNe 1998bw, 2003lw, and 2010bh, or to the relativistic SNe 2009bb and iPTF17cw, are rare. Our results also exclude an association of the SNe Ic-BL in our sample with largely off-axis GRBs with energies E ≳ 10⁵⁰ erg. The parameter space of SN 2006aj-like events (faint and fast-peaking radio emission) is, on the other hand, left largely unconstrained, and systematically exploring it represents a promising line of future research.

We report observations of the optical counterpart of the long gamma-ray burst GRB 221009A. Due to the extreme rarity of being both nearby (z = 0.151) and highly energetic (E ( gamma,iso) >= 10(54) erg), GRB 221009A offers a unique opportunity to probe the connection between massive star core collapse and relativistic jet formation across a very broad range of gamma-ray properties. Adopting a phenomenological power-law model for the afterglow and host galaxy estimates from high-resolution Hubble Space Telescope imaging, we use Bayesian model comparison techniques to determine the likelihood of an associated supernova (SN) contributing excess flux to the optical light curve. Though not conclusive, we find moderate evidence (K (Bayes) = 10(1.2)) for the presence of an additional component arising from an associated SN, SN 2022xiw, and find that it must be substantially fainter (<67% as bright at the 99% confidence interval) than SN 1998bw. Given the large and uncertain line-of-sight extinction, we attempt to constrain the SN parameters (M (Ni), M (ej), and E (KE)) under several different assumptions with respect to the host galaxy's extinction. We find properties that are broadly consistent with previous GRB-associated SNe: M (Ni) = 0.05-0.25 M (circle dot), M (ej) = 3.5-11.1 M (circle dot), and E (KE) = (1.6-5.2) x 10(52) erg. We note that these properties are weakly constrained due to the faintness of the SN with respect to the afterglow and host emission, but we do find a robust upper limit on M (Ni) of M (Ni) < 0.36 M (circle dot). Given the tremendous range in isotropic gamma-ray energy release exhibited by GRBs (seven orders of magnitude), the SN emission appears to be decoupled from the central engine in these systems.

The progenitors for many types of supernovae (SNe) are still unknown, and an approach to diagnose their physical origins is to investigate the light-curve brightness and shape of a large set of SNe. However, it is often difficult to compare and contrast the existing sample studies due to differences in their approaches and assumptions, for example, in how to eliminate host galaxy extinction, and this might lead to systematic errors when comparing the results. We therefore introduce the Hybrid Analytic Flux FittEr for Transients (HAFFET), a Python-based software package that can be applied to download photometric and spectroscopic data for transients from open online sources, derive bolometric light curves, and fit them to semianalytical models for estimation of their physical parameters. In a companion study, we have investigated a large collection of SNe Ib and Ic observed with the Zwicky Transient Facility (ZTF) with HAFFET, and here we detail the methodology and the software package to encourage more users. As large-scale surveys such as ZTF and LSST continue to discover increasing numbers of transients, tools such as HAFFET will be critical for enabling rapid comparison of models against data in statistically consistent, comparable, and reproducible ways. Additionally, HAFFET is created with a graphical user interface mode, which we hope will boost the efficiency and make the usage much easier (https://github.com/saberyoung/HAFFET).

Context. The link between the fate of the most massive stars and the resulting supernova (SN) explosion is still a matter of debate, in major part because of the ambiguity among light-curve powering mechanisms. When stars explode as SNe, the light-curve luminosity is typically sustained by a central engine (radioactive decay, magnetar spin-down, or fallback accretion). However, since massive stars eject considerable amounts of material during their evolution, there may be a significant contribution coming from interactions with the previously ejected circumstellar medium (CSM). Reconstructing the progenitor configuration at the time of explosion requires a detailed analysis of the long-term photometric and spectroscopic evolution of the related transient. Aims. In this paper, we present the results of our follow-up campaign of SN 2020faa. Given the high luminosity and peculiar slow light curve, it is purported to have a massive progenitor. We present the spectro-photometric dataset and investigate di fferent options to explain the unusual observed properties that support this assumption. Methods. We computed the bolometric luminosity of the supernova and the evolution of its temperature, radius, and expansion velocity. We also fit the observed light curve with a multi-component model to infer information on the progenitor and the explosion mechanism. Results. Reasonable parameters are inferred for SN 2020faa with a magnetar of energy, Ep = 1.5(-0.2)(+0.5) x 10(50) erg, and spin-down time, t(spin) = 15 +/- 1 d, a shell mass, M-shell = 2.4(-0.4)(+0.5) M-circle dot, and kinetic energy, E-kin(shell) = 0.9(-0.3)(+0.5) x10(51) erg, and a core with M-core = 21.5(-0.7)(+1.4) M-circle dot and E(kin()core) = 3.9(-0.4)(+0.1) x 10(51) erg. In addition, we need an extra source to power the luminosity of the second peak. We find that a hidden interaction with either a CSM disc or several delayed and choked jets is a viable mechanism for supplying the required energy to achieve this e ffect.

Recent observations indicate that hydrogen-poor superluminous supernovae (SLSNe-I) often display bumpy declining light curves. However, the cause of these undulations remains unclear. In this paper, we have improved the magnetar model, which includes flare activities. We present a systematic analysis of a well-observed SLSN-I sample with bumpy light curves in their late phase. These SLSNe-I were identified from multiple transient surveys, such as the Pan-STARRS1 Medium Deep Survey and the Zwicky Transient Facility. Our study provides a set of magnetar-powered model light curve fits for five SLSNe-I, which accurately reproduce the observed light curves using reasonable physical parameters. By extracting the essential characteristics of both the explosions and central engines, these fits provide valuable insights into investigating their potential association with gamma-ray burst engines. We found that the SLSN flares tend to be the dim and long extension of GRB flares in the peak luminosity versus peak time plane. Conducting large-scale, high-cadence surveys in the near future could enhance our comprehension of both SLSN undulating light curve properties and their potential relationship with GRBs by modeling their light curve characteristics.

The fate of stars in the zero-age main-sequence (ZAMS) range ≈8–12 M_⊙ is unclear. They could evolve to form white dwarfs or explode as electron-capture supernovae (SNe) or iron core-collapse SNe (CCSNe). Even though the initial mass function indicates that this mass range should account for over 40% of all CCSN progenitors, few have been observationally confirmed, likely due to the faintness and rapid evolution of some of these transients. In this paper, we present a sample of nine Ca-rich/O-poor Type IIb SNe detected by the Zwicky Transient Facility with progenitors likely in this mass range. These sources have a [Ca ii] λλ7291, 7324/[O i] λλ6300, 6364 flux ratio of ≳2 in their nebular spectra. Comparing the measured [O i] luminosity (≲10³⁹ erg s⁻¹) and derived oxygen mass (≈0.01 M_⊙) with theoretical models, we infer that the progenitor ZAMS mass for these explosions is less than 12 M_⊙. The ejecta properties (M_ej ≲ 1 M_⊙ and E_kin ∼ 10⁵⁰ erg) are also consistent. The low ejecta mass of these sources indicates a class of strongly-stripped SNe that is a transition between the regular stripped-envelope SNe and ultra-stripped SNe. The progenitor could be stripped by a main-sequence companion and result in the formation of a neutron star−main sequence binary. Such binaries have been suggested to be progenitors of neutron star−white dwarf systems that could merge within a Hubble time and be detectable with LISA.