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We present spectra of the supernova (SN) impostor AT 2016blu spanning over a decade. This transient exhibits quasi-periodic outbursts with an $$\sim$$113 d period, likely triggered by periastron encounters in an eccentric binary system where the primary star is a luminous blue variable (LBV). The overall spectrum remains fairly consistent during quiescence and eruptions, with subtle changes in line-profile shapes and other details. Some narrow emission features indicate contamination from a nearby H ii region in the host galaxy, NGC 4559. Broader H $$\alpha$$ profiles exhibit Lorentzian shapes with full width at half-maximum intensity (FWHM) values that vary significantly, showing no correlation with photometric outbursts or the 113 d phase. At some epochs, H $$\alpha$$ exhibits asymmetric profiles with a stronger redshifted wing, while broad and sometimes multicomponent P Cygni absorption features occasionally appear, but are again uncorrelated with brightness or phase. These P Cygni absorptions have high velocities compared to the FWHM of the H $$\alpha$$ emission line, perhaps suggesting that the absorption component is not in the LBV’s wind, but is instead associated with a companion. The lack of phase dependence in line-profile changes may point to interaction between a companion and a variable or inhomogeneous primary wind, in an orbit with only mild eccentricity. Recent photometric data indicate that AT 2016blu experienced its 21st outburst around 2023 May/June, as predicted based on its period. This type of quasi-periodic LBV remains poorly understood, but its spectra and erratic light curve resemble some pre-SN outbursts such as those of SN 2009ip. 

Abstract We present complete spectropolarimetric coverage of the Type II supernova (SN) 2023ixf, ranging from 1 to 120 days after explosion. Polarimetry was obtained with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory. As the ejecta interact with circumstellar material (CSM) during the first week, the intrinsic polarization of SN 2023ixf is initially high at ≲1%, dropping steeply within days down to ∼0.4% when the ejecta sweep up the optically thick CSM. The continuum polarization stays low at ∼0.2% thereafter, until it rises again to ∼0.6% as the ejecta transition to the nebular phase. We model this evolution using a combination of archival and newly computed two-dimensional polarized radiative transfer models. In this context, we interpret the early-time polarization as arising from an aspherical CSM with a pole-to-equator density contrast of ≳3. We propose that the surge in polarization at late times originates from an asymmetric distribution of⁵⁶Ni deep in the ejecta. The distinct sources of asymmetries at early and late times are consistent with the temporal evolution of the observed polarization and the polarization angle in SN 2023ixf. 

Abstract Galaxy-cluster gravitational lenses enable the study of faint galaxies even at large lookback times, and, recently, time-delay constraints on the Hubble constant. There have been few tests, however, of lens model predictions adjacent to the critical curve (≲8″) where the magnification is greatest. In a companion paper, we use the GLAFIC lens model to constrain the BalmerL–σrelation for Hiiregions in a galaxy at redshiftz= 1.49 strongly lensed by the MACS J1149 galaxy cluster. Here we perform a detailed comparison between the predictions of 10 cluster lens models that employ multiple modeling assumptions with our measurements of 11 magnified, giant Hiiregions. We find that that the models predict magnifications an average factor of 6.2 smaller, a ∼2σtension, than that inferred from the Hiiregions under the assumption that they follow the low-redshiftL–σrelation. To evaluate the possibility that the lens model magnifications are strongly biased, we next consider the flux ratios among knots in three images of Sp1149, and find that these are consistent with model predictions. Moreover, while the mass-sheet degeneracy could in principle account for a factor of ∼6 discrepancy in magnification, the value ofH₀inferred from SN Refsdal’s time delay would become implausibly small. We conclude that the lens models are not likely to be highly biased, and that instead the Hiiregions in Sp1149 are substantially more luminous than the low-redshift BalmerL–σrelation predicts. 

The death of massive stars is triggered by an infall-induced bounce shock that disrupts the star. How such a shock is launched and propagates through the star is a decade-long puzzle. Some models assume that the shock can be reenergized by absorbing neutrinos, leading to highly aspherical explosions. Other models involve jet-powered shocks that lead to bipolar explosions reflected in the geometry of the shock-breakout emission. We report measurement of the geometry of the shock breakout through unprecedentedly early spectropolarimetry of the nearby type II supernova 2024ggi starting ~1.2 days after the explosion. The measurement indicates a well-defined symmetry axis of the shock breakout, which is also shared by the hydrogen-rich envelope that emerged after the circumstellar matter was engulfed by the ejecta, revealing a persisting and prominent symmetry axis throughout the explosion. These findings suggest that the physical mechanism driving the explosion of massive stars manifests a well-defined axial symmetry and acts on large scales. 

Abstract We present stellar atmosphere modeling of JWST NIRCam photometry of nine highly magnified individual stars in a single galaxy at redshiftz= 0.94 known as the Warhol arc, which is strongly lensed by the galaxy cluster MACS J0416. Seven of these transients were identified by Yan et al. The nine sources are likely red supergiants with temperaturesT_eff ≈ 4000 K. We present new long-slit spectroscopy of the Warhol arc acquired with Keck I telescope and the Large Binocular Telescope, and use these data to help constrain the arc’s oxygen abundance to be $12+\log (\mathrm{O}/\mathrm{H})=8.45\pm 0.08$ . A microlensing simulation is performed on synthetic stellar populations using a range of stellar metallicities and initial mass function (IMF) slopes. The temperature distribution of the simulated detectable stars is sensitive to the choice of stellar metallicity, and setting the stellar metallicity equal to the arc’s nebular metallicity ( $\log (Z_{*}/Z_{\odot })=-0.24$ ) produces a simulated temperature distribution that is consistent with the observations, while lower stellar metallicities ( $\log (Z_{*}/Z_{\odot })<\text{–}0.75$ ) produce simulated temperatures that are inconsistent with the observations. The expected detection rate is strongly anticorrelated with the IMF slope forα > 1.2. For the canonical IMF slopeα = 2.35, the simulation yields expected transient detection rates that agree with the observed detection rates in the Hubble Space Telescope Flashlights filters, but overpredicts the detection rate by a factor of ∼3–12 (<2σtension) in the JWST filters. The simulated detection rate is sensitive to the choice of stellar metallicity, with lower metallicities ( $\log (Z_{*}/Z_{\odot })<\text{–}0.75$ ) yielding a significantly lower simulated detection rate that further reduces the modest tension with the observations. 

Abstract Massive stars at cosmological distances can be individually detected during transient microlensing events, when gravitational lensing magnifications may exceedμ ≈ 1000. Nine such sources were identified in JWST NIRCam imaging of a single galaxy at redshiftz= 0.94 known as the “Warhol arc,” which is mirror imaged by the galaxy cluster MACS J0416.1−2403. Here we present the discovery of two coincident and well-characterized microlensing events at the same location followed by a third event observed in a single filter approximately 18 months later. The events can be explained by microlensing of a binary star system consisting of a red supergiant (T ≈ 4000 K) and a B-type (T ≳ 13,000 K) companion star. The timescale of the coincident microlensing events constrains the estimated projected source-plane size to tens of astronomical units. The most likely binary configurations consistent with the observational constraints on the temperature and luminosity of each star are stars with initial masses $M{1}_{\mathrm{init}}=23.6_{-4.3}^{+5.3}$ M_⊙and an initial mass ratio between the two stars close to unity. A kinematic model that reproduces the observed light curves in all filters gives a relatively small transverse velocity of ∼50 km s⁻¹. This requires the dominant velocity component of several hundreds of kilometers per second to be roughly parallel to the microcaustic. An alternative possibility would be that the three microlensing events correspond to unrelated stars crossing distinct microcaustics, but this would imply a highly elevated rate of events at their common position, even though no underlying knot is present at the location. 

Abstract We present early (≲1 yr) multiwavelength observations and analysis of six luminous fast blue optical transients (LFBOTs). We identified these LFBOTs in Zwicky Transient Facility survey data from their fast light-curve evolution (t_1/2≤ 12 days), blue colors at peak brightness (g − r≤ −0.2 mag), a visible host galaxy, high optical luminosity (M_g < −20 mag), and an X-ray and/or radio detection. With the exception of AT2024aehp (ZTF24abygbss), these transients exhibit peaks in their 10 GHz radio light curves att_rest ≈ 50–100 days, with peak radio luminosities ranging from 10³⁸to 10⁴⁰erg s⁻¹. Modeling the radio emission as synchrotron radiation indicates a fast (v= 0.1–0.3c) shock in a dense (n_e ≈ 10³–10⁴cm⁻³) medium. The X-ray emission varies by ≈2 orders of magnitude in luminosity (10⁴²–10⁴⁴erg s⁻¹) att_rest ∼ 20 days. Analysis of the host-galaxy photometry and spectroscopy for each transient shows that LFBOTs are predominantly nonnuclear (a few kiloparsecs offset) with star-forming host galaxies of stellar masses = 10⁹–10¹¹M_⊙. Unlike all other LFBOTs to date, AT2024aehp exhibited a luminous (M < −19 mag) plateau in the optical light curve and its 6–15 GHz radio emission brightened by over an order of magnitude fromt_rest ≈ 70 to 130 days. The mostly consistent radio behavior between these LFBOTs implies a similar circumburst medium, leading us to prefer a progenitor scenario in which mass is lost in a consistent way shortly prior to the terminal event. 

Abstract We present far- and near-ultraviolet (UV) spectra of the type II supernovae (SNe) SN 2023ixf from days 199 to 722 and SN 2024ggi at days 41 and 232. Both SNe show broad, blueshifted, and asymmetric UV emission lines with an initial maximum velocity of ∼9000 km s⁻¹and narrow unresolved emission in Civλλ1548.9, 1550.8. We compare the optical and UV emission-line profiles, showing that they evolve from two distinct velocity profiles to a single profile tracing the UV emission. We interpret this as shock power from interaction with circumstellar material coming to dominate over the radioactive-decay power from the inner ejecta. Comparing our observations to radiative transfer models with injected shock power, we find SN 2024ggi is best matched byP_shock,abs = 1 × 10⁴¹erg s⁻¹at day 40; SN 2023ixf at day 300 and SN 2024ggi at day 200 are best matched byP_shock,abs = 1 × 10⁴⁰erg s^−1;and SN 2023ixf at day 600 is best matched byP_shock,abs = 5 × 10³⁹erg s⁻¹. From these models, we find that the mass-loss rate of both SNe increased just before the explosion. For SN 2023ixf, our mass-loss rates go from 4 × 10⁻⁵M_⊙yr⁻¹at 600 yr before explosion to 2 × 10⁻²M_⊙yr⁻¹at 15 yr prior to explosion. For SN 2024ggi, we find a mass-loss rate of 9 × 10⁻⁵M_⊙yr⁻¹at 150 yr before explosion and 1 × 10⁻³M_⊙yr⁻¹at 30 yr before explosion. 

Abstract We present photometric and spectroscopic observations of SN 2024abfl, a low-luminosity Type IIP supernova (LLSN) discovered shortly after explosion. The transient reached a peak absolute magnitude ofM_V = −14.9 and exhibited an extended, flat plateau lasting ∼125 days. From the late-time bolometric light curve, we estimate a⁵⁶Ni mass of ∼0.01M_⊙, consistent with other LLSNe. Analytical shock-cooling models fail to reproduce the rapid early rise, indicating that circumstellar matter (CSM) interaction contributed to the initial emission. The spectroscopic evolution is typical of LLSNe, with relatively narrow metal lines and low expansion velocities (≲3000 km s⁻¹) that decline slowly over time. We detect a broad “ledge” feature around 4600 Å within 3 days of explosion, which we interpret as a blend of high-ionization, shock-accelerated CSM lines. Multipeaked Hαprofiles develop during the plateau phase, consistent with complex ejecta–CSM interaction. As one of the best-observed examples of LLSNe, SN 2024abfl exhibits a weak explosion and signatures of nearby CSM, offering new insights into progenitor properties, pre-explosion mass loss, and the diversity of LLSNe. 

Aims.We present optical, ultraviolet, and X-ray observations of supernova (SN) 2024iss, a Type IIb SN that shows a prominent double-peaked light curve. Methods.We modeled the first peak with a semianalytical shock-cooling model and the X-ray emission with a free-free model. We also compared the envelope radius and mass-loss rate with those of other Type IIb SNe to explore the relationships between the progenitor envelope and the circumstellar material. Results.The shock-cooling peak in theV-band light curve reachedM_V = −17.33 ± 0.26 mag, while the⁵⁶Ni-powered second peak attainedM_V = −17.43 ± 0.26 mag. Early spectra show a photospheric velocity of approximately 19 400 km s⁻¹at 3.82 days from the HαP Cygni profile. The Balmer lines persist for at least more than 87 days after the explosion, which is characteristic of hydrogen-rich ejecta. Modeling the first light-curve peak with the shock-cooling model suggests an extended hydrogen envelope with a mass of 0.11 ± 0.04 M_⊙and a radius of 244 ± 43 R_⊙. Fitting the second light-curve peak with an Arnett-like model indicates a typical⁵⁶Ni mass of 0.117 ± 0.013 M_⊙and a relatively low ejecta mass of 1.27 ± 0.34 M_⊙. X-ray observations revealed bright thermal bremsstrahlung emission and indicate a mass-loss rate of 1.6 × 10⁻⁵M_⊙yr⁻¹, which is similar to that of SN 1993J. Conclusions.Supernova 2024iss occupies a transitional position between the two subclasses of extended and compact Type IIb SNe. Its envelope radius and preexplosion mass-loss rate appear to be consistent with the correlation observed in the broader sample. The observational properties of SN 2024iss are compatible with a binary-interaction scenario being the dominant mechanism for envelope stripping.