(Fig. 1). The first BB temperature estimate was obtained via careful extrapolation of the UVOT UV M2 and P60 g + i light curves back to the first P48 (detection) point (see inset of Fig. 1 and text for details). The earlytime BB temperature estimates, within the first half day after explosion, are also in agreement with our temperature estimates from the modeling of the early Keck spectra (Fig. 4), showing the highly ionised emission lines at temperatures > ∼ 50 kK. The shaded region and the solid black line (a running mean of the region) denote bolometric luminosity estimates based on the multiband photometry ( Fig. 1) according to three methods used to calculate the total flux from the SED: interpolation, order-4 polynomial fit, and BB fits. The top end of the shaded region can be regarded as our best lower limit on the real bolometric luminosity, based on the photometric observations. The red triangles denote a (more conservative) lower limit on the bolometric luminosity obtained from our spectra (Fig. 2, Fig. 3), beginning with the early set of 4 Keck spectra at < ∼ 10 hr after explosion and ending with our latest spectrum at 57.2days. The blue triangles show the luminosity as obtained by our best BB temperature and radius estimates (Fig. 4), L = 4πR 2 σT 4 ; the luminosity in the first point, at ∼ 3.8 hr after explosion, exceeds 10 44 erg s −1 .

The early part of a supernova (SN) light-curve is dominated by radiation escaping from the expanding shockheated progenitor envelope. For polytropic Hydrogen envelopes, the properties of the emitted radiation are described by simple analytic expressions and are nearly independent of the polytropic index, n. This analytic description holds at early time, t < few days, during which radiation escapes from shells initially lying near the stellar surface. We use numerical solutions to address two issues. First, we show that the analytic description holds at early time also for non-polytropic density profiles. Second, we extend the solutions to later times, when the emission emerges from deep within the envelope and depends on the progenitor's density profile. Examining the late time behavior of polytropic envelopes with a wide range of core to envelope mass and radius ratios, 0.1 ≤ M c /M env ≤ 10 and 10 −3 ≤ R c /R ≤ 10 −1 , we find that the effective temperature is well described by the analytic solution also at late time, while the luminosity L is suppressed by a factor, which may be approximated to better than 20[30]% accuracy up to t = t tr /a by A exp[−(at/t tr ) α ] with. This description holds as long as the opacity is approximately that of a fully ionized gas, i.e. for T > 0.7 eV, t < 14(R/10 13.5 cm) 0.55 d. The suppression of L at t tr /a obtained for standard polytropic envelopes may account for the first optical peak of double-peaked SN light curves, with first peak at a few days for M env < 1M ⊙ . Subject headings: radiation hydrodynamics -shock waves -supernovae: general 1. INTRODUCTION During a supernova (SN) explosion, a strong radiation mediated shock wave propagates through and ejects the stellar envelope. As the shock expands outwards, the optical depth of the material lying ahead of it decreases. When the optical depth drops below ≈ c/v sh , where v sh is the shock velocity, radiation escapes ahead of the shock and the shock dissolves. In the absence of an optically thick circum-stellar material, this breakout takes place once the shock reaches the edge of the star, producing an X-ray/UV flash on a time scale of R/c (seconds to a fraction of an hour), where R is the stellar radius. The relatively short breakout is followed by UV/optical emission from the expanding cooling envelope on a day timescale. As the envelope expands its optical depth decreases, and radiation escapes from deeper shells. The properties of the breakout and post-breakout cooling emission carry unique information on the structure of the progenitor star (e.g. its radius and surface composition) and on its pre-explosion evolution, which cannot be directly inferred from observations at later time. The detection of SNe on a time scale of a day following the explosion, which was enabled recently by the progress of wide-field optical transient surveys, yielded important constraints on the progenitors of SNe of type Ia, Ib/c and II. For a recent comprehensive review of the subject see Waxman & Katz (2016).At radii r close to ...

The spectrum of radiation emitted following shock breakout from a star's surface with a power-law density profile ρ ∝ x n is investigated. Assuming planar geometry, local Compton equilibrium and bremsstrahlung emission as the dominant photon production mechanism, numerical solutions are obtained for the photon number density and temperature profiles as a function of time, for hydrogen-helium envelopes. The temperature solutions are determined by the breakout shock velocity v 0 and the pre-shock breakout density ρ 0 , and depend weakly on the value of n. Fitting formulas for the peak surface temperature at breakout as a function of v 0 and ρ 0 are provided, with T peak ≈ 9.44 exp[12.63(v 0 /c) 1/2 ] eV, and the time dependence of the surface temperature is tabulated. The time integrated emitted spectrum is a robust prediction of the model, determined by T peak and v 0 alone and insensitive to details of light travel time or slight deviations from spherical symmetry. Adopting commonly assumed progenitor parameters, breakout luminosities of ≈ 10 45 erg s −1 and ≈ 10 44 erg s −1 in the 0.3-10 keV band are expected for BSG and RSG/He-WR progenitors respectively (T peak is well below the band for RSGs, unless their radius is ∼ 10 13 cm). > 30 detections of SN1987A-like (BSG) breakouts are expected over the lifetime of ROSAT and XMM-Newton. An absence of such detections would imply that either the typical parameters assumed for BSG progenitors are grossly incorrect or that their envelopes are not hydrostatic. The observed spectrum and duration of XRF 080109/SN2008D are in tension with a non-relativistic breakout from a stellar surface interpretation.