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The radius and surface composition of an exploding massive star,as well asthe explosion energy per unit mass, can be measured using early UV observationsof core collapse supernovae (SNe). We present the first results from asimultaneous GALEX/PTF search for early UV emission from SNe. Six Type II SNeand one Type II superluminous SN (SLSN-II) are clearly detected in the GALEXNUV data. We compare our detection rate with theoretical estimates based onearly, shock-cooling UV light curves calculated from models that fit existingSwift and GALEX observations well, combined with volumetric SN rates. We findthat our observations are in good agreement with calculated rates assuming thatred supergiants (RSGs) explode with fiducial radii of 500 solar, explosionenergies of 10^51 erg, and ejecta masses of 10 solar masses. Exploding bluesupergiants and Wolf-Rayet stars are poorly constrained. We describe how suchobservations can be used to derive the progenitor radius, surface compositionand explosion energy per unit mass of such SN events, and we demonstrate why UVobservations are critical for such measurements. We use the fiducial RSGparameters to estimate the detection rate of SNe during the shock-cooling phase(<1d after explosion) for several ground-based surveys (PTF, ZTF, and LSST). Weshow that the proposed wide-field UV explorer ULTRASAT mission, is expected tofind >100 SNe per year (~0.5 SN per deg^2), independent of host galaxyextinction, down to an NUV detection limit of 21.5 mag AB. Our pilot GALEX/PTFproject thus convincingly demonstrates that a dedicated, systematic SN surveyat the NUV band is a compelling method to study how massive stars end theirlife.

The detection rate of early UV emission from supernovae: A dedicated GALEX/PTF survey and calibrated theoretical estimates