The radius and surface composition of an exploding massive star, as well as the explosion energy per unit mass, can be measured using early UV observations of core-collapse supernovae (SNe). We present the first results from a simultaneous GALEX/PTF search for early ultraviolet (UV) emission from SNe. Six SNe II and one Type II superluminous SN (SLSN-II) are clearly detected in the GALEX near-UV (NUV) data. We compare our detection rate with theoretical estimates based on early, shock-cooling UV light curves calculated from models that fit existing Swift and GALEX observations well, combined with volumetric SN rates. We find that our observations are in good agreement with calculated rates assuming that red supergiants (RSGs) explode with fiducial radii of 500 R e , explosion energies of 10 51 erg, and ejecta masses of 10 M e . Exploding blue supergiants and Wolf-Rayet stars are poorly constrained. We describe how such observations can be used to derive the progenitor radius, surface composition, and explosion energy per unit mass of such SN events, and we demonstrate why UV observations are critical for such measurements. We use the fiducial RSG parameters to estimate the detection rate of SNe during the shock-cooling phase (<1 day after explosion) for several ground-based surveys (PTF, ZTF, and LSST). We show that the proposed wide-field UV explorer ULTRASAT mission is expected to find >85 SNe per year (∼0.5 SN per deg 2 ), independent of host galaxy extinction, down to an NUV detection limit of 21.5 mag AB. Our pilot GALEX/PTF project thus convincingly demonstrates that a dedicated, systematic SN survey at the NUV band is a compelling method to study how massive stars end their life.

High cadence transient surveys are able to capture supernovae closer to their first light than before. Applying analytical models to such early emission, we can constrain the progenitor stars properties. In this paper, we present observations of SN 2018 fif (ZTF18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018 fif was surrounded by relatively small amounts of circumstellar material (CSM) compared to all previous cases. This particularity, coupled with the high cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman (2017). Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shockcooling model. We find that the progenitor of SN 2018 fif was a large red supergiant, with a radius of R = 1174 +208 −81 R and an ejected mass of M ej = 5.6 +9.1 −1.0 M. Our model also gives information on the explosion epoch, the progenitor inner structure, the shock velocity and the extinction. The large radius differs from previously modeled objects, and the difference could be either intrinsic or due to the relatively small amount of CSM around SN 2018 fif, perhaps making it a "cleaner" candidate for applying shock-cooling analytical models.

We present visible-light and ultraviolet (U V ) observations of the supernova PTF 12glz. The SN was discovered and monitored in near-U V and R bands as part of a joint GALEX and Palomar Transient Factory campaign. It is among the most energetic Type IIn supernovae observed to date (≈ 10 51 erg). If the radiated energy mainly came from the thermalization of the shock kinetic energy, we show that PTF 12glz was surrounded by ∼ 1 M of circumstellar material (CSM) prior to its explosive death. PTF 12glz shows a puzzling peculiarity: at early times, while the freely expanding ejecta are presumably masked by the optically thick CSM, the radius of the blackbody that best fits the observations grows at ≈ 8000 km s −1 . Such a velocity is characteristic of fast moving ejecta rather than optically thick CSM. This phase of radial expansion takes place before any spectroscopic signature of expanding ejecta appears in the spectrum and while both the spectroscopic data and the bolometric luminosity seem to indicate that the CSM is optically thick. We propose a geometrical solution to this puzzle, involving an aspherical structure of the CSM around PTF 12glz. By modeling radiative diffusion through a slab of CSM, we show that an aspherical geometry of the CSM can result in a growing effective radius. This simple model also allows us to recover the decreasing blackbody temperature of PTF 12glz. SLAB-Diffusion, the code we wrote to model the radiative diffusion of photons through a slab of CSM and evaluate the observed radius and temperature, is made available on-line.

The radius and surface composition of an exploding massive star, as well as the explosion energy per unit mass, can be measured using early ultraviolet (UV) observations of core-collapse supernovae (CC SNe). We present the results from a simultaneous GALEX and Palomar Transient Factory (PTF) search for early UV emission from SNe. We analyze five CC SNe for which we obtained N U V measurements before the first ground-based R-band detection. We introduce SOPRANOS, a new maximum likelihood fitting tool for models with variable temporal validity windows, and use it to fit the Sapir & Waxman (2017) shock cooling model to the data. We report four Type II SNe with progenitor radii in the range of R * ≈ 600 − 1100R ⊙ and a shock velocity parameter in the range of v s * ≈ 2700 − 6000 km s −1 (E/M ≈ 2 − 8 × 10 50 erg/M ⊙ ) and one type IIb SN with R * ≈ 210R ⊙ and v s * ≈ 11000 km s −1 (E/M ≈ 1.8 × 10 51 erg/M ⊙ ). Our pilot GALEX/PTF project thus suggests that a dedicated, systematic SN survey in the N U V band, such as the wide-field UV explorer ULTRASAT mission, is a compelling method to study the properties of SN progenitors and SN energetics.

High cadence transient surveys are able to capture supernovae closer to their first light than before. Applying analytical models to such early emission, we can constrain the progenitor stars properties. In this paper, we present observations of SN 2018 fif (ZTF18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018 fif was surrounded by relatively small amounts of circumstellar material (CSM) compared to all previous cases. This particularity, coupled with the high cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman (2017). Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shockcooling model. We find that the progenitor of SN 2018 fif was a large red supergiant, with a radius of R = 1174 +208 −81 R and an ejected mass of M ej = 5.6 +9.1 −1.0 M . Our model also gives information on the explosion epoch, the progenitor inner structure, the shock velocity and the extinction. The large radius differs from previously modeled objects, and the difference could be either intrinsic or due to the relatively small amount of CSM around SN 2018 fif, perhaps making it a "cleaner" candidate for applying shock-cooling analytical models.