Early-time light curves of Type Ia Supernovae (SNe Ia) encode critical information about their progenitor systems. We characterize the rise of normal SNe Ia using a volume-complete sample of 972 events from the Zwicky Transient Facility Data Release 2, an order of magnitude larger than any previous dataset for similar analyses. Fitting light curves up to 30% of peak flux with a power-law model under a hierarchical Bayesian framework, we provide robust population-level constraints on the rise time (t_rise; μ=18.55±0.08 days, σ=1.42±0.07 days), rise index (α; μ=2.10±0.04, σ=0.48±0.03 in ZTF r), and g-r color evolution (α_g - α_r; μ=0.20±0.02, σ=0.17±0.02). These power-law fits are sensitive to the chosen truncation epoch if data beyond ∼40% of peak flux are included, but generally converge when restricted to earlier epochs. The relation between rise morphology and light-curve width ( x_1 stretch) bifurcates into two distinct regimes: high-stretch SNe Ia show clear trends where a higher x_1 correlates with shallower rises and more persistent blue colors, whereas low-stretch SNe Ia lack such trends. While rise times correlate positively with x_1 overall, this relation flattens significantly within the high-stretch population. Searching for anomalies, we identify several normal SNe Ia with unusually long rise times, which potentially exhibit short-duration (≲2 days) flux excesses over a smooth rise. Long-duration (∼5 days) flux excesses appear common within the high-stretch population and are tied to the shallow rises and early blue colors, pointing to widespread outward ^56Ni mixing. Multi-dimensional explosion models with more realistic progenitor setups are needed to fully reproduce the observed dichotomy in rise morphology and stretch.

Modern wide-field time-domain surveys facilitate the study of transient, variable and moving phenomena by conducting image differencing and relaying alerts to their communities. Machine learning tools have been used on data from these surveys and their precursors for more than a decade, and convolutional neural networks (CNNs), which make predictions directly from input images, saw particularly broad adoption through the 2010s. Since then, continually rapid advances in computer vision have transformed the standard practices around using such models. It is now commonplace to use standardized architectures pre-trained on large corpora of everyday images (e.g., ImageNet). In contrast, time-domain astronomy studies still typically design custom CNN architectures and train them from scratch. Here, we explore the effects of adopting various pre-training regimens and standardized model architectures on the performance of alert classification. We find that the resulting models match or outperform a custom, specialized CNN like what is typically used for filtering alerts. Moreover, our results show that pre-training on galaxy images from Galaxy Zoo tends to yield better performance than pre-training on ImageNet or training from scratch. We observe that the design of standardized architectures are much better optimized than the custom CNN baseline, requiring significantly less time and memory for inference despite having more trainable parameters. On the eve of the Legacy Survey of Space and Time and other image-differencing surveys, these findings advocate for a paradigm shift in the creation of vision models for alerts, demonstrating that greater performance and efficiency, in time and in data, can be achieved by adopting the latest practices from the computer vision field.

Modern time-domain surveys like the Zwicky Transient Facility (ZTF) and the Legacy Survey of Space and Time (LSST) generate hundreds of thousands to millions of alerts, demanding automatic, unified classification of transients and variable stars for efficient follow-up. We present Applying multimodal learning to Classify transient Detections Early (AppleCiDEr), a novel framework that integrates four key data modalities (photometry, image cutouts, metadata, and spectra) to overcome limitations of single-modality classification approaches. Our architecture introduces (i) two transformer encoders for photometry, (ii) a multimodal convolutional neural network (CNN) with domain-specialized metadata towers and Mixture-of-Experts fusion for combining metadata and images, and (iii) a CNN for spectra classification. Training on similar to 30,000 real ZTF alerts, AppleCiDEr achieves high accuracy, allowing early identification and suggesting follow-up for rare transient spectra. The system provides the first unified framework for transients classification using real observational data, with seamless integration into brokering pipelines, demonstrating readiness for the LSST era.

Supernovae (SNe) come in various flavors and are classified into different types based on emission and absorption lines in their spectra. SN candidates are now abundant with the advent of large systematic sky surveys like the Zwicky Transient Facility (ZTF), however, the identification bottleneck lies in their spectroscopic confirmation and classification. Fully robotic telescopes with dedicated spectrographs optimized for SN follow-up have eased the burden of data acquisition. However, the task of classifying the spectra still largely rests with the astronomers. Automating this classification step reduces human effort and can make the SN type available sooner to the public. For this purpose, we have developed a deep-learning based program for classifying core-collapse supernovae (CCSNe) with ultra-low resolution spectra from the SED-machine spectrograph on the Palomar 60 inch telescope. The program consists of hierarchical classification task layers, with each layer composed of multiple binary classifiers running in parallel to produce a reliable classification. The binary classifiers utilize recurrent neural networks and convolutional neural networks architecture and are designed to take multiple inputs to supplement spectra with g - and r -band photometry from ZTF. On non-host-contaminated and good quality SEDM spectra (“gold” test set) of CCSNe, CCSNscore is ∼94% accurate in distinguishing between hydrogen-rich (Type II) and hydrogen-poor (Type Ibc) CCSNe. With light curve input, CCSNscore classifies ∼83% of the gold set with high confidence (score ≥0.8 and score-error < 0.05), with ∼98% accuracy. Based on SNIascore’s and CCSNscore’s real-time performance on bright transients ( m pk ≤ 18.5) and our reporting criteria, we expect ∼0.5% (∼4) true SNe Ia to be misclassified as SNe Ibc and ∼6% (∼17) of true CCSNe to be misclassified between Type II and Type Ibc annually on the Transient Name Server.

The nature of the progenitor systems and explosion mechanisms that give rise to Type Ia supernovae (SNe Ia) are still debated. The interaction signature of circumstellar material (CSM) being swept up by the expanding ejecta can constrain the type of system from which it was ejected. However, most previous studies have focussed on finding CSM ejected shortly before the SN Ia explosion, which still resides close to the explosion site resulting in short delay times until the interaction starts. We used a sample of 3628 SNe Ia from the Zwicky Transient Facility (ZTF) that were discovered between 2018 and 2020 and searched for interaction signatures greater than 100 days after peak brightness. By binning the late-time light curve data to push the detection limit as deep as possible, we identified potential late-time rebrightening in three SNe Ia (SN 2018grt, SN 2019dlf, and SN 2020tfc). The late-time optical detections occur between 550 and 1450 d after peak brightness, have mean absolute r-band magnitudes of -16.4 to -16.8 mag, and last up to a few hundred days, which is significantly brighter than the late-time CSM interaction discovered in the prototype, SN 2015cp. The late-time detections in the three objects all occur within 0.8 kpc of the host nucleus and are not easily explained by nuclear activity, another transient at a similar sky position, or data quality issues. This is suggestive of environment or specific progenitor characteristics playing a role in the production of potential CSM signatures in these SNe Ia. Through simulating the ZTF survey, we estimate that < 0.5% of normal SNe Ia display a late-time (> 100 d post peak) strong H alpha-dominated CSM interaction. This is equivalent to an absolute rate of 8(-4)(+20) to 54(-26)(+91) Gpc(-3) yr(-1) assuming a constant SN Ia rate of 2.4 x 10(-5) Mpc(-3) yr(-1) for z <= 0.1. Weaker interaction signatures of H alpha emission, more similar to the strength seen in SN 2015cp, could be more common but are difficult to constrain with our survey depth.

We are searching for hot, constant-color, offset optical flares in the Zwicky Transient Facility data stream that are >10 '' from any galaxy in public imaging data from the PanSTARRS survey. Here, we present the first discovery from this search: AT 2024puz, a luminous multiwavelength transient offset by 5 kpc from a similar to 10(8) M-circle dot galaxy at z = 0.356 with a low-moderate star formation rate (0.01 +/- 0.003 M-circle dot yr(-1)). It produced luminous 10(44.79 +/- 0.04) erg s(-1) optical/UV emission that evolved on a similar to 20 day timescale, as well as 10(44.12 +/- 0.03) erg s(-1) X-ray emission with a photon-index . No associated radio or millimeter emission was detected. We show that the early time optical emission is likely powered by reprocessing of high-energy, accretion-powered radiation, with a possible contribution from a shock in a dense circumtransient medium. If the shock is dominant at early times, the circumtransient medium has a mass similar to 0.1-1 M-circle dot, a radius 10(15) cm, and a density profile shallower than similar to r(-1). A near-infrared excess appears at late-times and is suggestive of reprocessing within a wind or other circumtransient medium. The X-rays are most consistent with a central engine. We suggest that AT 2024puz may be associated with an accretion event onto a 50-10(5) M-circle dot black hole, where the lower masses are preferred based on the large projected offset from the host galaxy. AT2024puz exhibits properties similar to both luminous, fast, blue optical transients and tidal disruption events, but is intermediate between them in its energetics and evolution timescale. This highlights the need for broader exploration of the landscape of hot optical transients to trace their origins.

Multipeaked supernovae with precursors, dramatic light-curve rebrightenings, and spectral transformation are rare, but are being discovered in increasing numbers by modern night-sky transient surveys like the Zwicky Transient Facility. Here, we present the observations and analysis of SN 2023aew, which showed a dramatic increase in brightness following an initial luminous (−17.4 mag) and long (∼100 days) unusual first peak (possibly precursor). SN 2023aew was classified as a Type IIb supernova during the first peak but changed its type to resemble a stripped-envelope supernova (SESN) after the marked rebrightening. We present comparisons of SN 2023aew’s spectral evolution with SESN subtypes and argue that it is similar to SNe Ibc during its main peak. P-Cygni Balmer lines are present during the first peak, but vanish during the second peak’s photospheric phase, before H α resurfaces again during the nebular phase. The nebular lines ([O i ], [Ca ii ], Mg i ], H α ) exhibit a double-peaked structure that hints toward a clumpy or nonspherical ejecta. We analyze the second peak in the light curve of SN 2023aew and find it to be broader than that of normal SESNe as well as requiring a very high ^56 Ni mass to power the peak luminosity. We discuss the possible origins of SN 2023aew including an eruption scenario where a part of the envelope is ejected during the first peak and also powers the second peak of the light curve through interaction of the SN with the circumstellar medium.

We present a sample of 34 normal SNe II detected with the Zwicky Transient Facility, with multi-band UV light-curves starting at $t \leq 4$ days after explosion, as well as X-ray detections and upper limits. We characterize the early UV-optical colors and provide prescriptions for empirical host-extinction corrections. We show that the $t > 2\,$days UV-optical colors and the blackbody evolution of the sample are consistent with the predictions of spherical phase shock-cooling (SC), independently of the presence of `flash ionization" features. We present a framework for fitting SC models which can reproduce the parameters of a set of multi-group simulations without a significant bias up to 20% in radius and velocity. Observations of about half of the SNe II in the sample are well-fit by models with breakout radii $<10^{14}\,$cm. The other half are typically more luminous, with observations from day 1 onward that are better fit by a model with a large $>10^{14}\,$cm breakout radius. However, these fits predict an early rise during the first day that is too slow. We suggest these large-breakout events are explosions of stars with an inflated envelope or a confined CSM with a steep density profile, at which breakout occurs. Using the X-ray data, we derive constraints on the extended ($\sim10^{15}$ cm) CSM density independent of spectral modeling, and find most SNe II progenitors lose $<10^{-4} M_{\odot}\, \rm yr^{-1}$ a few years before explosion. This provides independent evidence the CSM around many SNe II progenitors is confined. We show that the overall observed breakout radius distribution is skewed to higher radii due to a luminosity bias. We argue that the $66^{+11}_{-22}\%$ of red supergiants (RSG) explode as SNe II with breakout radii consistent with the observed distribution of field RSG, with a tail extending to large radii, likely due to the presence of CSM.

The Keck Planet Finder (KPF) is a fiber-fed, high-resolution, echelle spectrometer that specializes in the discovery and characterization of exoplanets using Doppler spectroscopy. In designing KPF, the guiding principles were high throughput to promote survey speed and access to faint targets, and high stability to keep uncalibrated systematic Doppler measurement errors below 30 cm s(-1). KPF achieves optical illumination stability with a tip-tilt injection system, octagonal cross-section optical fibers, a double scrambler, and active fiber agitation. The optical bench and optics with integral mounts are made of Zerodur to provide thermo-mechanical stability. The spectrometer includes a slicer to reformat the optical input, green and red channels (445-600 nm and 600-870 nm), and achieves a resolving power of similar to 97,000. Additional subsystems include a separate, medium-resolution UV spectrometer (383-402 nm) to record the Ca II H & K lines, an exposure meter for real-time flux monitoring, a solar feed for sunlight injection, and a calibration system with a laser frequency comb and etalon for wavelength calibration. KPF was installed and commissioned at the W. M. Keck Observatory in late 2022 and early 2023 and is now in regular use for scientific observations. This paper presents an overview of the as-built KPF instrument and its subsystems, design considerations, and initial on-sky performance.

Dome-C in the Antarctic Plateau is a privileged site for Astronomy, with one of the lowest concentrations of water vapor in the world, providing a pristine atmospheric window for IR observations. Together with the long winter nights, this allows for extended continuous observational campaigns. At the Concordia Station, ASTEP has taken advantage of the weather and long nights to observe long-period transiting exoplanets for over a decade. With the Cryoscope Pathfinder we now plan to take advantage of the dark IR window between 2.35 and 2.55𝜇m. The unique design of Cryoscope Pathfinder is optimized for a very wide field of view and very thermal background. It is a cryogenic 0.26 m telescope designed for observations in K-dark with a field of view of 16 deg^2. This is the first step for a much more ambitious project, the full scale 1-meter class Cryoscope telescope, with a field of view of 50 deg^2. The initial science drivers are the study of exoplanets and of the infrared transient sky, where it will play a major role in the localization of gravitational wave sources. Furthermore, many other science topics will be enabled by Cryoscope and through synergies with other surveys.

The detonation of a thin (≲0.03 M ⊙) helium shell (He-shell) atop a ∼1 M ⊙ white dwarf (WD) is a promising mechanism to explain normal Type Ia supernovae (SNe Ia), while thicker He-shells and less massive WDs may explain some recently observed peculiar SNe Ia. We present observations of SN 2020jgb, a peculiar SN Ia discovered by the Zwicky Transient Facility (ZTF). Near maximum brightness, SN 2020jgb is slightly subluminous (ZTF g-band absolute magnitude −18.7 mag ≲ M g ≲ −18.2 mag depending on the amount of host-galaxy extinction) and shows an unusually red color (0.2 mag ≲ g ZTF − r ZTF ≲ 0.4 mag) due to strong line-blanketing blueward of ∼5000 Å. These properties resemble those of SN 2018byg, a peculiar SN Ia consistent with an He-shell double detonation (DDet) SN. Using detailed radiative transfer models, we show that the optical spectroscopic and photometric evolution of SN 2020jgb is broadly consistent with a ∼0.95–1.00 M ⊙ (C/O core + He-shell) progenitor ignited by a ≳0.1 M ⊙ He-shell. However, one-dimensional radiative transfer models without non-local-thermodynamic-equilibrium treatment cannot accurately characterize the line-blanketing features, making the actual shell mass uncertain. We detect a prominent absorption feature at ∼1 μm in the near-infrared (NIR) spectrum of SN 2020jgb, which might originate from unburnt helium in the outermost ejecta. While the sample size is limited, we find similar 1 μm features in all the peculiar He-shell DDet candidates with NIR spectra obtained to date. SN 2020jgb is also the first peculiar He-shell DDet SN discovered in a star-forming dwarf galaxy, indisputably showing that He-shell DDet SNe occur in both star-forming and passive galaxies, consistent with the normal SN Ia population.

We report on our plans to upgrade the detector systems in the 2022–2024 time frame for three of the workhorse instruments (NIRC2, DEIMOS, and NIRES) operated by the W. M. Keck Observatory. The upgrades are done in collaboration with Observatory partner institutions and other Maunakea observatories. The main motivating factors behind these upgrades are to tackle obsolescence of hardware and software components, to boost observing efficiency, to enhance the instrument throughput, and to add new observing functionality.

ABSTRACT Supernova (SN) siblings – two or more SNe in the same parent galaxy – are useful tools for exploring progenitor stellar populations as well as properties of the host galaxies such as distance, star-formation rate, dust extinction, and metallicity. Since the average SN rate for a Milky Way-type galaxy is just one per century, a large imaging survey is required to discover an appreciable sample of SN siblings. From the wide-field Zwicky Transient Facility (ZTF) Bright Transient Survey (which aims for spectroscopic completeness for all transients which peak brighter than r < 18.5 mag) we present 10 SN siblings in five parent galaxies. For each of these families, we analyse the SN’s location within the host and its underlying stellar population, finding agreement with expectations that SNe from more massive progenitors are found nearer to their host core and in regions of more active star formation. We also present an analysis of the relative rates of core collapse and thermonuclear SN siblings, finding a significantly lower ratio than past SN sibling samples due to the unbiased nature of the ZTF.

We report second-timescale infrared photometry of the nova V1674 Her using Palomar Gattini-IR. These observations constitute the first infrared and highest temporal resolution data (resolution of 0.84 s) of the nova reported to date. PGIR observed in this fast readout mode for more than an hour on three nights between 3 and 6 days after discovery. We searched for periodic variability using a Lomb-Scargle periodogram and did not detect anything down to a three sigma upper limit of 0.093 mag. This suggests that the periodic variability detected in the nova by Patterson et al. 2021 was lower by at least a factor of about 1.65 in the first week of the eruption.

We present optical and infrared (IR) light curves of the enshrouded massive binary NaSt1 (WR 122) with observations from Palomar Gattini-IR (PGIR), the Zwicky Transient Facility (ZTF), the Katzman Automatic Imaging Telescope (KAIT), and the All-Sky Automated Survey for Supernovae (ASAS-SN). The optical and IR light curves span between 2014 July and 2020 Oct., revealing periodic, sinusoidal variability from NaSt1 with a $P=305.2\pm1.0$ d period. We also present historical IR light curves taken between 1983 July and 1989 May that also indicate NaSt1 exhibits long-term IR variability on timescales of $\sim$decades. Fixed-period sinusoidal fits to the recent optical and IR light curves show that amplitude of NaSt1's variability is different at different wavelengths and also reveal significant phase offsets of $\sim18$ d between the ZTF $r$ and PGIR $J$ light curves.We interpret the $\sim300$ d period of the observed variability as the orbital period of a binary system in NaSt1. Assuming a circular orbit and adopting a range of combined stellar mass values in the range 20-100 M$_\odot$ in NaSt1, we estimate orbital separations of $\sim2$-4 au. We suggest that the sinusoidal photometric variability of NaSt1 may arise from variations in the line-of-sight optical depth toward circumstellar optical/IR emitting regions throughout its orbit due to colliding-wind dust formation. We provide an interpretation on the nature of NaSt1 and speculate that the mass-transfer process may have been triggered by Roche-lobe overflow (RLOF) during an eruptive phase of a Ofpe/WN9 star. Lastly, we claim that NaSt1 ceased RLOF mass transfer $\lesssim3400$ yr ago.