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Type IIb supernovae (SNe IIb) are core-collapse events whose optical spectra show strong hydrogen features that disappear over time, implying that their progenitors were nearly, but not completely, stripped of their hydrogen envelopes prior to core collapse. Thus, compared to hydrogen-rich SNe II, SNe IIb can provide a closer examination of the underlying structure of the progenitor system, particularly during early photospheric phases (less than +70 days relative to max. light). I will present early-time multi-epoch optical spectropolarimetry of several SNe IIb, obtained using the SPOL instrument at the University of Arizona. Using polarization diagnostics provides a way to track structural changes in the depleted hydrogen envelopes of these SNe as deeper layers of helium and other elements emerge and evolve. I find significant temporal polarization increases in the absorption wings of their H and He lines. Some of these line features make "loops" in Stokes Q-U diagrams, suggesting non-axisymmetic structure in the ejecta, perhaps arising from a transient absorbing clump. Furthermore, the majority of these SNe show polarimetric evidence for aspherical explosions along a preferred, or dominant, axis. I discuss the implications these findings have on the 3D geometry of the explosions by comparing the observed polarization to published synthetic spectropolarimetry that models axial symmetry and clump structures in stripped-envelope, core-collapse SNe. This comparative study naturally facilitates a broader discussion around the unresolved question as to what extent this SNe subclass shows common polarization characteristics. 

Type IIb supernovae (SNe IIb) are core-collapse events whose optical spectra show strong hydrogen features that disappear over time, implying that their progenitors were nearly, but not completely, stripped of their hydrogen envelopes prior to core collapse. Thus, compared to hydrogen-rich SNe II, SNe IIb can provide a closer examination of the underlying structure of the progenitor system, particularly during early photospheric phases (less than +70 days relative to max. light). I will present early-time multi-epoch optical spectropolarimetry of several SNe IIb, obtained using the SPOL instrument at the University of Arizona. Using polarization diagnostics provides a way to track structural changes in the depleted hydrogen envelopes of these SNe as deeper layers of helium and other elements emerge and evolve. I find significant temporal polarization increases in the absorption wings of their H and He lines. Some of these line features make "loops" in Stokes Q-U diagrams, suggesting non-axisymmetic structure in the ejecta, perhaps arising from a transient absorbing clump. Furthermore, the majority of these SNe show polarimetric evidence for aspherical explosions along a preferred, or dominant, axis. I discuss the implications these findings have on the 3D geometry of the explosions by comparing the observed polarization to published synthetic spectropolarimetry that models axial symmetry and clump structures in stripped-envelope, core-collapse SNe. This comparative study naturally facilitates a broader discussion around the unresolved question as to what extent this SNe subclass shows common polarization characteristics. 

ABSTRACT V906 Carinae was one of the best observed novae of recent times. It was a prolific dust producer and harboured shocks in the early evolving ejecta outflow. Here, we take a close look at the consequences of these early interactions through study of high-resolution Ultraviolet and Visual Echelle spectrograph spectroscopy of the nebular stage and extrapolate backwards to investigate how the final structure may have formed. A study of ejecta geometry and shaping history of the structure of the shell is undertaken following a spectral line $$\rm {\small SHAPE}$$ model fit. A search for spectral tracers of shocks in the nova ejecta is undertaken and an analysis of the ionized environment. Temperature, density, and abundance analyses of the evolving nova shell are presented. 

Abstract The contemporaneous detection of gravitational waves and gamma rays from GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron star mergers as progenitors of short-duration gamma-ray bursts (GRBs) and cosmic sources of heavy r -process nuclei. However, the nature (and life span) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with short GRB 180618A with extended gamma-ray emission—with ultraviolet and optical multicolor observations starting as soon as 1.4 minutes post-burst. The spectrum is consistent with a fast-fading afterglow and emerging thermal optical emission 15 minutes post-burst, which fades abruptly and chromatically (flux density F ν ∝ t − α , α = 4.6 ± 0.3) just 35 minutes after the GRB. Our observations from gamma rays to optical wavelengths are consistent with a hot nebula expanding at relativistic speeds, powered by the plasma winds from a newborn, rapidly spinning and highly magnetized neutron star (i.e., a millisecond magnetar), whose rotational energy is released at a rate L th ∝ t −(2.22±0.14) to reheat the unbound merger-remnant material. These results suggest that such neutron stars can survive the collapse to a black hole on timescales much larger than a few hundred milliseconds after the merger and power the GRB itself through accretion. Bright thermal optical counterparts to binary merger gravitational wave sources may be common in future wide-field fast-cadence sky surveys. 

Context. There has been significant technological and scientific progress in our ability to detect, monitor, and model the physics of γ -ray bursts (GRBs) over the 50 years since their first discovery. However, the dissipation process thought to be responsible for their defining prompt emission is still unknown. Recent efforts have focused on investigating how the ultrarelativistic jet of the GRB propagates through the progenitor’s stellar envelope for different initial composition shapes, jet structures, magnetisation, and, consequently, possible energy dissipation processes. Study of the temporal variability – in particular the shortest duration of an independent emission episode within a GRB – may provide a unique way to distinguish the imprint of the inner engine activity from geometry and propagation related effects. The advent of new high-energy detectors with exquisite time resolution now makes this possible. Aims. We aim to characterise the minimum variability timescale (MVT) defined as the shortest duration of individual pulses that shape a light curve for a sample of GRBs in the keV–MeV energy range and test correlations with other key observables such as the peak luminosity, the Lorentz factor, and the jet opening angle. We compare these correlations with predictions from recent numerical simulations for a relativistic structured – possibly wobbling – jet and assess the value of temporal variability studies as probes of prompt-emission dissipation physics. Methods. We used the peak detection algorithm MEPSA to identify the shortest pulse within a GRB time history and preliminarily calibrated MEPSA to estimate the full width at half maximum duration. We then applied this framework to two sets of GRBs: Swift GRBs (from 2005 to July 2022) and Insight Hard Modulation X-ray Telescope (Insight-HXMT) GRBs (from June 2017 to July 2021, including the exceptional 221009A). We then selected 401 GRBs with measured redshift to test for correlations. Results. We confirm that, on average, short GRBs have significantly shorter MVTs than long GRBs. The MVT distribution of short GRBs with extended emission such as 060614 and 211211A is compatible only with that of short GRBs. This is important because it provides a new clue concerning the progenitor’s nature. The MVT for long GRBs with measured redshift anti-correlates with peak luminosity; our analysis includes careful evaluation of selection effects. We confirm the anti-correlation with the Lorentz factor and find a correlation with the jet opening angle as estimated from the afterglow light curve, along with an inverse correlation with the number of pulses. Conclusions. The MVT can identify the emerging putative new class of long GRBs that are suggested to be produced by compact binary mergers. For otherwise typical long GRBs, the different correlations between MVT and peak luminosity, Lorentz factor, jet opening angle, and number of pulses can be explained within the context of structured, possibly wobbling, weakly magnetised relativistic jets. 

Abstract We present photometric and spectroscopic data for SN 2022joj, a nearby peculiar Type Ia supernova (SN Ia) with a fast decline rate (Δm_15,B= 1.4 mag). SN 2022joj shows exceedingly red colors, with a value of approximatelyB−V≈ 1.1 mag during its initial stages, beginning from 11 days before maximum brightness. As it evolves, the flux shifts toward the blue end of the spectrum, approachingB−V≈ 0 mag around maximum light. Furthermore, at maximum light and beyond, the photometry is consistent with that of typical SNe Ia. This unusual behavior extends to its spectral characteristics, which initially displayed a red spectrum and later evolved to exhibit greater consistency with typical SNe Ia. Spectroscopically, we find strong agreement between SN 2022joj and double detonation models with white dwarf masses of around 1M_⊙and a thin He shell between 0.01 and 0.05M_⊙. Moreover, the early red colors are explained by line-blanketing absorption from iron peak elements created by the double detonation scenario in similar mass ranges. The nebular spectra in SN 2022joj deviate from expectations for double detonation, as we observe strong [Feiii] emission instead of [Caii] lines as anticipated, though this is not as robust a prediction as early red colors and spectra. The fact that as He shells get thinner these SNe start to look more like normal SNe Ia raises the possibility that this is the triggering mechanism for the majority of SNe Ia, though evidence would be missed if the SNe are not observed early enough. 

Abstract We present a JWST mid-infrared (MIR) spectrum of the underluminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) ∼130 days post-explosion. We identify the first MIR lines beyond 14μm in SN Ia observations. We find features unique to underluminous SNe Ia, including the following: isolated emission of stable Ni, strong blends of [Tiii], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Coiii] 11.888μm feature and the SN light-curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements, we constrain the mass of the exploding WD. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD ( $M_{\mathrm{WD}}$≈1.37M_⊙) of high central density (ρ_c≥ 2.0 × 10⁹g cm⁻³) seen equator-on, which producedM(⁵⁶Ni) =0.324M_⊙andM(⁵⁸Ni) ≥0.06M_⊙. The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of subsonic carbon burning followed by an off-center deflagration-to-detonation transition beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.