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We present upgraded infrastructure for Searches After Gravitational waves Using ARizona Observatories (SAGUARO) during LIGO, Virgo, and KAGRA’s fourth gravitational-wave (GW) observing run (O4). These upgrades implement many of the lessons we learned after a comprehensive analysis of potential electromagnetic counterparts to the GWs discovered during the previous observing run. We have developed a new web-based target and observation manager (TOM) that allows us to coordinate sky surveys, vet potential counterparts, and trigger follow-up observations from one centralized portal. The TOM includes software that aggregates all publicly available information on the light curves and possible host galaxies of targets, allowing us to rule out potential contaminants like active galactic nuclei, variable stars, solar system objects, and preexisting supernovae, as well as to assess the viability of any plausible counterparts. We have also upgraded our image-subtraction pipeline by assembling deeper reference images and training a new neural-network-based real–bogus classifier. These infrastructure upgrades will aid coordination by enabling the prompt reporting of observations, discoveries, and analysis to the GW follow-up community, and put SAGUARO in an advantageous position to discover kilonovae in the remainder of O4 and beyond. Many elements of our open-source software stack have broad utility beyond multimessenger astronomy, and will be particularly relevant in the “big data” era of transient discoveries by the Vera C. Rubin Observatory.

We perform a comprehensive search for optical precursor emission at the position of SN 2023ixf using data from the DLT40, ZTF, and ATLAS surveys. By comparing the current data set with precursor outburst hydrodynamical model light curves, we find that the probability of a significant outburst within 5 yr of explosion is low, and the circumstellar material (CSM) ejected during any possible precursor outburst is likely smaller than ∼0.015M_⊙. By comparing to a set of toy models, we find that, if there was a precursor outburst, the duration must have been shorter than ∼100 days for a typical brightness ofM_r≃ −9 mag or shorter than 200 days forM_r≃ −8 mag; brighter, longer outbursts would have been discovered. Precursor activity like that observed in the normal Type II SN 2020tlf (M_r≃ −11.5) can be excluded in SN 2023ixf. If the dense CSM inferred by early flash spectroscopy and other studies is related to one or more precursor outbursts, then our observations indicate that any such outburst would have to be faint and only last for days to months, or it occurred more than 5 yr prior to the explosion. Alternatively, any dense, confined CSM may not be due to eruptive mass loss from a single red supergiant progenitor. Taken together, the results of SN 2023ixf and SN 2020tlf indicate that there may be more than one physical mechanism behind the dense CSM inferred around some normal Type II supernovae.

We present high-cadence photometric and spectroscopic observations of SN 2023axu, a classical Type II supernova with an absoluteV-band peak magnitude of –17.2 ± 0.1 mag. SN 2023axu was discovered by the Distance Less Than 40 Mpc (DLT40) survey within 1 day of the last nondetection in the nearby galaxy NGC 2283 at 13.7 Mpc. We modeled the early light curve using a recently updated shock cooling model that includes the effects of line blanketing and found the explosion epoch to be MJD 59971.48 ± 0.03 and the probable progenitor to be a red supergiant. The shock cooling model underpredicts the overall UV data, which point to a possible interaction with circumstellar material. This interpretation is further supported by spectral behavior. We see a ledge feature around 4600 Å in the very early spectra (+1.1 and +1.5 days after the explosion), which can be a sign of circumstellar interaction. The signs of circumstellar material are further bolstered by the presence of absorption features blueward of Hαand Hβat day >40, which is also generally attributed to circumstellar interaction. Our analysis shows the need for high-cadence early photometric and spectroscopic data to decipher the mass-loss history of the progenitor.

We report on analysis using the JWST to identify a candidate progenitor star of the Type II-plateau (II-P) supernova SN 2022acko in the nearby, barred spiral galaxy NGC 1300. To our knowledge, our discovery represents the first time JWST has been used to localize a progenitor system in pre-explosion archival Hubble Space Telescope (HST) images. We astrometrically registered a JWST NIRCam image from 2023 January, in which the SN was serendipitously captured, to pre-SN HST F160W and F814W images from 2017 and 2004, respectively. An object corresponding precisely to the SN position has been isolated with reasonable confidence. That object has a spectral energy distribution (SED) and overall luminosity consistent with a single-star model having an initial mass possibly somewhat less than the canonical 8 M⊙ theoretical threshold for core collapse (although masses as high as 9 M⊙ for the star are also possible); however, the star’s SED and luminosity are inconsistent with that of a super-asymptotic giant branch star that might be a forerunner of an electron-capture SN. The properties of the progenitor alone imply that SN 2022acko is a relatively normal SN II-P, albeit most likely a low-luminosity one. The progenitor candidate should be confirmed with follow-up HST imaging at late times, when the SN has sufficiently faded. This potential use of JWST opens a new era of identifying SN progenitor candidates at high spatial resolution.

We present the optical spectroscopic evolution of SN 2023ixf seen in subnight cadence spectra from 1.18 to 15 days after explosion. We identify high-ionization emission features, signatures of interaction with material surrounding the progenitor star, that fade over the first 7 days, with rapid evolution between spectra observed within the same night. We compare the emission lines present and their relative strength to those of other supernovae with early interaction, finding a close match to SN 2020pni and SN 2017ahn in the first spectrum and SN 2014G at later epochs. To physically interpret our observations, we compare them to CMFGEN models with confined, dense circumstellar material around a red supergiant (RSG) progenitor from the literature. We find that very few models reproduce the blended Niii(λλ4634.0,4640.6)/Ciii(λλ4647.5,4650.0) emission lines observed in the first few spectra and their rapid disappearance thereafter, making this a unique diagnostic. From the best models, we find a mass-loss rate of 10⁻³–10⁻²M_⊙yr⁻¹, which far exceeds the mass-loss rate for any steady wind, especially for an RSG in the initial mass range of the detected progenitor. These mass-loss rates are, however, similar to rates inferred for other supernovae with early circumstellar interaction. Using the phase when the narrow emission features disappear, we calculate an outer dense radius of circumstellar materialR_CSM,out≈ 5 × 10¹⁴cm, and a mean circumstellar material density ofρ= 5.6 × 10⁻¹⁴g cm⁻³. This is consistent with the lower limit on the outer radius of the circumstellar material we calculate from the peak Hαemission flux,R_CSM,out≳ 9 × 10¹³cm.

We analyze pre-explosion near- and mid-infrared (IR) imaging of the site of SN 2023ixf in the nearby spiral galaxy M101 and characterize the candidate progenitor star. The star displays compelling evidence of variability with a possible period of ≈1000 days and an amplitude of Δm≈ 0.6 mag in extensive monitoring with the Spitzer Space Telescope since 2004, likely indicative of radial pulsations. Variability consistent with this period is also seen in the near-IRJandK_sbands between 2010 and 2023, up to just 10 days before the explosion. Beyond the periodic variability, we do not find evidence for any IR-bright pre-supernova outbursts in this time period. The IR brightness ($M_{K_s}=-10.7$mag) and color (J−K_s= 1.6 mag) of the star suggest a luminous and dusty red supergiant. Modeling of the phase-averaged spectral energy distribution (SED) yields constraints on the stellar temperature ($T_{\mathrm{eff}}={3500}_{-1400}^{+800}$K) and luminosity ($\log L/L_{\odot }=5.1\pm 0.2$). This places the candidate among the most luminous Type II supernova progenitors with direct imaging constraints, with the caveat that many of these rely only on optical measurements. Comparison with stellar evolution models gives an initial mass ofM_init= 17 ± 4M_⊙. We estimate the pre-supernova mass-loss rate of the star between 3 and 19 yr before explosion from the SED modeling at$\dot{M}\approx 3\times {10}^{-5}$to 3 × 10⁻⁴M_⊙yr⁻¹for an assumed wind velocity ofv_w= 10 km s⁻¹, perhaps pointing to enhanced mass loss in a pulsation-driven wind.

We present the densely sampled early light curve of the Type II supernova (SN) 2023ixf, first observed within hours of explosion in the nearby Pinwheel Galaxy (Messier 101; 6.7 Mpc). Comparing these data to recently updated models of shock-cooling emission, we find that the progenitor likely had a radius of 410 ± 10R_⊙. Our estimate is model dependent but consistent with a red supergiant. These models provide a good fit to the data starting about 1 day after the explosion, despite the fact that the classification spectrum shows signatures of circumstellar material around SN 2023ixf during that time. Photometry during the first day after the explosion, provided almost entirely by amateur astronomers, does not agree with the shock-cooling models or a simple power-law rise fit to data after 1 day. We consider the possible causes of this discrepancy, including precursor activity from the progenitor star, circumstellar interaction, and emission from the shock before or after it breaks out of the stellar surface. The very low luminosity (−11 mag >M> −14 mag) and short duration of the initial excess lead us to prefer a scenario related to prolonged emission from the SN shock traveling through the progenitor system.

We present very early photometric and spectroscopic observations of the Type Ia supernova (SN Ia) 2023bee, starting about 8 hr after the explosion, which reveal a strong excess in the optical and nearest UV (UandUVW1) bands during the first several days of explosion. This data set allows us to probe the nature of the binary companion of the exploding white dwarf and the conditions leading to its ignition. We find a good match to the Kasen model in which a main-sequence companion star stings the ejecta with a shock as they buzz past. Models of double detonations, shells of radioactive nickel near the surface, interaction with circumstellar material, and pulsational delayed detonations do not provide good matches to our light curves. We also observe signatures of unburned material, in the form of carbon absorption, in our earliest spectra. Our radio nondetections place a limit on the mass-loss rate from the putative companion that rules out a red giant but allows a main-sequence star. We discuss our results in the context of other similar SNe Ia in the literature.

We present five far- and near-ultraviolet spectra of the Type II plateau supernova, SN 2022acko, obtained 5, 6, 7, 19, and 21 days after explosion, all observed with the Hubble Space Telescope/Space Telescope Imaging Spectrograph. The first three epochs are earlier than any Type II plateau supernova has been observed in the far-ultraviolet revealing unprecedented characteristics. These three spectra are dominated by strong lines, primarily from metals, which contrasts with the featureless early optical spectra. The flux decreases over the initial time series as the ejecta cool and line blanketing takes effect. We model this unique data set with the non–local thermodynamic equilibrium radiation transport codeCMFGEN, finding a good match to the explosion of a low-mass red supergiant with energyE_kin= 6 × 10⁵⁰erg. With these models we identify, for the first time, the ions that dominate the early ultraviolet spectra. We present optical photometry and spectroscopy, showing that SN 2022acko has a peak absolute magnitude ofV= − 15.4 mag and plateau length of ∼115 days. The spectra closely resemble those of SN 2005cs and SN 2012A. Using the combined optical and ultraviolet spectra, we report the fraction of flux as a function of bluest wavelength on days 5, 7, and 19. We create a spectral time-series of Type II supernovae in the ultraviolet, demonstrating the rapid decline of flux over the first few weeks of evolution. Future observations of Type II supernovae are required to map out the landscape of exploding red supergiants, with and without circumstellar material, which is best revealed in high-quality ultraviolet spectra.