Curtis McCully

We present the first X-ray detection of the supernova (SN) impostor AT 2016blu in NGC 4559. AT 2016blu exhibited 27 detected quasiperiodic (∼113 day) outbursts between 2012 and 2026 June, presumably triggered by periastron passages in an eccentric binary system in which the primary is a ≳33 M _⊙ luminous blue variable (LBV). AT 2016blu was serendipitously observed by Chandra in 2001–2002, prior to its first documented outburst in 2012, but the stacked archival data show no X-ray detection. We monitored the source’s visual-wavelength variability around its predicted outburst time and triggered a Chandra Target of Opportunity (ToO) program in 2026 March, obtaining observations with detections over five closely spaced epochs. These data indicate an X-ray luminosity of log L X ≈ 38.64 ± 0.11 erg s ⁻¹ and an accretion rate of M ̇ ≳ 8 × 1 0 − 8 M ⊙ yr − 1 , consistent with the presence of a compact companion. We therefore conclude that AT 2016blu is the first known case of an LBV SN impostor where X-ray observations clearly favor intermittent accretion onto a compact object as the cause of the outbursts. Given that the system consists of a massive star and a compact companion, AT 2016blu is a high-mass X-ray binary (HMXB), similar to SN 2010da, although the donor star in SN 2010da is a red supergiant (RSG).

We present a comprehensive analysis of the long-term photometric and spectroscopic monitoring campaign of the Type IIn supernova (SN) 2024kgi. The SN reaches a peak r -band absolute magnitude of −19.81 ± 0.06 mag and a bolometric luminosity of ∼2 × 10 ⁴³ erg s ⁻¹ . We observe a break in the lightcurve at day ∼312, after which the luminosity decline changes from t ^−1.0 to t ^−4.5 , likely due to the shock sweeping the dense circumstellar matter (CSM). We introduce a semi-analytical lightcurve modeling approach for CSM interaction in transients that accounts for variable diffusion time. We hence estimate a CSM mass of 2.3 4 − 1.6 + 11.8 M ⊙ , with a density profile of ρ _csm ∝ r ^−2.74 , indicating progressively increasing mass loss toward the SN explosion. The ejecta signatures emerge as broad H, He, and Ca ii triplet lines at day ∼80, much earlier than expected from a spherically symmetric CSM, suggesting asymmetry in the CSM. After the lightcurve break, the H lines exhibit a wavelength-dependent deficit in the red-wing flux, indicating new dust formation in the ejecta and/or in the postshock gas. We observe a near-IR (NIR) excess from day ∼40 onward. We attribute the early NIR excess before the lightcurve break solely to the NIR echo from preexisting dust, as there are no other indications for new dust formation. The late-time NIR excess likely has contributions from both the preexisting and newly formed dust. The steady increase in mass loss, slow CSM velocity, and asymmetric CSM favor a binary interaction-induced mass loss.

We report on multiband photometric and spectroscopic observations of SN 2023rve, a nearby Type II supernova (SN II) discovered in galaxy NGC 1097 ( D = 15.4 ± 3.2 Mpc). Nearby SNe II provide constraints on late-stage evolution and progenitor mass loss, in particular the role of circumstellar material (CSM) in shaping SN II observables. SN 2023rve peaks with an absolute V -band magnitude of −17.1 and declines at a rate of 0.90 ± 0.02 mag/50 days during the plateau. The bolometric light curve implies a ⁵⁶ Ni mass of 0.0064 M _⊙ . Using hydrodynamic light-curve modeling, we infer an intermediate-mass progenitor (∼14–18 M _⊙ ), a low explosion energy of 0.27 × 10 ⁵¹ erg, and a dense CSM component with radial extent of 2900 R _⊙ and density parameter of 10 ¹⁸ g cm ⁻¹ . This supports growing evidence that enhanced pre-SN mass loss influences the diversity of SNe II. The nebular spectra of SN 2023rve show narrow He i lines and an absence of [O i ] lines unprecedented among Type II SNe. Comparison with other SNe II shows that only two other known objects, both with higher velocities, lack oxygen signatures at similar epochs, <10% of the sample. The lack of oxygen emission combined with low explosion energy, a long plateau, and a small synthesized nickel mass may be consistent with partial fallback of material onto the compact remnant. We also discuss alternative explanations for the suppressed oxygen emission, including dust formation, oxygen–calcium mixing, and ongoing CSM interaction.

SN 2024abup is a nearby broad-lined Type Ic supernova (SN Ic-bl) in NGC 0681 at a distance of 23.3 ± 1.6 Mpc. As energetic explosions of massive stars, SNe Ic-bl are considered a plausible site for rapid-neutron capture nucleosynthesis ( r -process) and chemical enrichment from short-lived progenitors. They may also contribute to dust production in the early Universe. We present JWST near- to mid-infrared (NIR+MIR) observations (1–14 μ m) of SN Ic-bl 2024abup at +41 days after the V -band maximum (+54 days after explosion), the first-ever JWST+MIR observation of an SN Ic-bl along with radio and optical data. Using the spectral synthesis code SUMO , we identify the observed broad IR line features in SN 2024abup and find significant contributions from C, O, Mg, and carbon monoxide (CO)—the earliest detection of molecules in a core-collapse SN so far. The spectrum shows continuum emission at wavelengths greater than 1.5 μ m, which could be explained by dust—preexisting, newly formed, or a combination—heated by the SN. We do not find unambiguous evidence for infrared signatures of r -process elements, though our search is hampered by the presence of many broad and blended features from the non- r -process elements such as [Te iii ] + Mg ii at 2.1 μ m. These new observations indicate that SNe Ic-bl could be a contributor to early-universe dust production and suggest that if r -process elements are produced, revealing their presence from spectra requires very high-quality data and models to disentangle blends.

Hydrogen-rich superluminous supernovae without narrow emission lines (SLSNe-II) are rare transients whose powering mechanisms remain debated, particularly the role of circumstellar medium (CSM) interaction. We present photometric and spectroscopic observations of SN 2019cqc to investigate its power source and progenitor mass loss. We model the lightcurve using MOSFiT with the csm and csmni models to constrain the explosion and CSM parameters. SN 2019cqc peaked at M _g = −20.21 ± 0.07 and emitted ∼1.7 × 10 ⁵⁰ erg of energy in the form of radiation. Photometric and spectroscopic modeling indicates that CSM interaction dominates in both scenarios, with best-fit CSM and ejecta masses of ∼4.5 M _⊙ and ∼32 M _⊙ , respectively. The inferred CSM properties imply an extreme eruptive mass loss at a rate of ∼0.2–0.3 M _⊙ , yr ⁻¹ in the years preceding the core collapse, consistent with a variable progenitor of luminous blue variable progenitor. We observe a persistent blueshifted asymmetry in the H α emission line. At early times (≲+110 days), this is attributed to electron scattering with the bulk velocity of ejecta, while the profile at late epochs (≳ +402 days onward) suggests the subsequent formation of dust in the ejecta. Additionally, we identify a distinct, short-lived feature at ∼4600 Å, likely a blend of ionized C iii /N iii lines powered by the interaction of the SN-shock with the extended atmosphere.

Mapping how the explosion properties of Type II supernovae (SNe II) relate to the properties of their progenitors can provide strong constraints for understanding the final evolutionary stages of massive stars. Type IIP SNe, linked to the explosions of single red supergiant (RSG) stars, have recently been found to require some form of interaction with circumstellar material (CSM) to reproduce the rapid rise to the plateau often seen in their light curves. In this work, we present observations and analysis of the Type IIP SN 2020bij, characterized by a slow rise to its plateau as well as high expansion velocities. We identify four other SNe IIP from the literature (ASASSN-14 kg, SN 2018fif, SN 2021yja, and SN 2023axu) with similarly slowly rising light curves and find that they also show high expansion velocities. Using both analytical and numerical models, all five events can be explained with weak to no CSM interaction. We therefore propose that these events constitute a new subclass of Type IIP SNe that could be associated with relatively confined CSM. Early and dense photometric coverage of future SNe IIP together with early spectroscopic observations will further map this subclass and its physical properties. Understanding such rare events could be key to constraining the diversity of late-stage mass loss in RSGs.

In this paper, we first present observations of SN 2024acyl, a normal Type Ibn supernova (SN), with a large projected offset (∼35 kpc) from its host galaxy. The low star formation rate measured at the explosion site raises the possibility that the progenitor of SN 2024acyl may not have been a massive star. We then examine, more broadly, the spectral diversity of Type Ibn supernovae (SNe Ibn) around 20–35 days after peak brightness and identify two distinct groups: Group I, which shows bluer rest-frame optical color and narrower He I emission lines; and Group II, which shows redder rest-frame optical color and broader He I lines. Group I also tends to show higher peak luminosities. The diversity we identify appears to be closely connected to the diversity observed around the peak and to persist into late phases (> 80 days after peak). Given its redder color and broader He I lines, we classify SN 2024acyl as belonging to Group II. Based on the current dataset, we find no clear connection between this spectral diversity and either the host environments of SNe Ibn or their pre-explosion activity. The observed diversity in SNe Ibn likely reflects differences in circumstellar material properties and/or explosion energetics. These differences could result from a range of progenitor properties, such as different helium star mass, orbital period, and companion type if they are in binary systems, and may indicate fundamentally diverse progenitors. Whether a continuous distribution exists between the two groups remains to be determined and will require further data to explore.

We present spectroscopic and photometric observations of SN 2021ukt, a peculiar short-plateau object that was originally identified as a Type IIn supernovae (SN) and later underwent an unprecedented transition to a Type Ib (possibly Type IIb) SN. The early time light curves of SN 2021ukt exhibit an ∼25 day plateau. Such a short phase of hydrogen recombination suggests a rather thin H-rich outer envelope of the progenitor star. The narrow Balmer emission lines in the spectra of SN 2021ukt during the first week indicate the interaction between the expanding ejecta and the immediate circumstellar material (CSM). This H α line is observed throughout its helium-rich ejecta-dominated phase and nebular phase, suggesting persistent interaction with a radially extended CSM profile. We explore synthetic light curves among grids of parameters generated by MESA + STELLA . We also compare the spectrophotometric evolution of SN 2021ukt with several SNe that exhibit a short plateau and persistent ejecta–CSM interaction. An estimate of the progenitor mass of SN 2021ukt is made based on the flux ratio between [Ca ii ] λλ 7291, 7324 and [O i ] λλ 6300, 6364 during its nebular phase. Our analysis suggests that the progenitor star of SN 2021ukt has a zero-age main-sequence mass of M _ZAMS ≈ 12 M _⊙ , a mass of radioactive ⁵⁶ Ni synthesized in the SN ejecta of M _Ni ≈ 0.05 M _⊙ , and a mass of the H-rich envelope of M _H,env ≲ 0.5 M _⊙ . This study adds to the growing sample of transitional SNe, reinforcing evidence for a continuum of underrepresented progenitors whose evolutionary pathways lie between those of standard SN models.

Interacting supernovae probe the twilight years of massive stars, exhibiting signatures of interaction between the supernova ejecta and surrounding material expelled from the progenitor. We present the peculiar interacting supernova, SN 2025ngs in NGC 5961 (37.8 Mpc). This transient toes the line between strongly interacting supernovae (Type IIn) and Type IIP supernovae. SN 2025ngs presents photometrically as a short-plateau supernova, with a plateau duration t PT ≈ 70 days. Interaction features subside within a week postexplosion, consistent with the growing number of flash supernovae, giving way to a short period where a typical IIP spectrum is exhibited. Towards the drop off the plateau, interaction features re-emerge, exhibiting complex H α profiles throughout the rest of the transient evolution. We compare with models of early spectra, finding the abundances generally consistent with a supergiant progenitor with a high mass-loss rate (10 ⁻³ M _⊙ yr ⁻¹ ). Early, high-resolution spectra reveal a double-horned H α profile, providing strong evidence for shock interaction with a proximate disk-like circumstellar medium. Spectroscopically, SN 2025ngs closely resembles the luminous SN 1998S, despite photometric differences, with SN 2025ngs having a relatively modest peak magnitude of M _V = −17.9 mag, adding another member to the surprisingly diverse 98S-like group.

Although Type Ia supernovae (SNe Ia) are found in all galaxies types, recent local Hubble constant measurements have disfavored using SNe Ia in early-type/quiescent galaxies, aiming instead for better consistency with SNe Ia in star-forming/late-type host galaxies calibrated by Cepheid distances. Here, we investigate the feasibility of a parallel distance ladder using SNe Ia exclusively in quiescent, massive ( log M * / M ⊙ ≥ 10 ) host galaxies, calibrated by tip of the red giant branch (TRGB) distances. We present TRGB measurements to four galaxies: three measured from the Hubble Space Telescope with the Advanced Camera for Surveys F814W filter and one measured from the JWST NIRCam F090W filter. Combined with literature measurements, we define a TRGB-calibrator sample of five high-mass, early-type galaxies that hosted well-measured SNe Ia: NGC 1316 (SN 2006dd), NGC 1380 (SN 1992A), NGC 1404 (SN 2007on, SN 2011iv), NGC 4457 (SN 2020nvb), and NGC 4636 (SN 2020ue). We jointly standardize these calibrators with a fiducial sample of 124 Hubble-flow SNe Ia from the Zwicky Transient Facility that are matched in host-galaxy and light-curve properties. Results with this homogenized subsample show a Hubble residual scatter under 0.11 mag, lower than usually observed in cosmological samples of the full SN Ia distribution. We obtain a measurement of the Hubble constant, H ₀ = 75.3 ± 2.9 km s ⁻¹ Mpc ⁻¹ , including statistical and estimated systematic uncertainties, and discuss the potential to further improve the precision of this approach. As calibrator and SN samples grow, we advocate that future cosmological applications of SNe Ia use subsamples matched in host-galaxy and SN properties across redshifts.