X-RAY AND GAMMA-RAY FLASHES FROM TYPE Ia SUPERNOVAE?

We investigate two potential mechanisms that will produce X-ray and gamma-ray flashes from Type Ia supernovae (SN-Ia). The first mechanism is the breakout of the thermonuclear burning front as it reaches the surface of the white dwarf (WD). The second mechanism is the interaction of the rapidly expanding envelope with material within an accretion disk in the progenitor system. Our study is based on the delayed detonation scenario because this can account for the majority of light curves, spectra, and statistical properties of Branch-normal SN-Ia. Based on detailed radiation-hydro calculations which include nuclear networks, we find that both mechanisms produce brief flashes of high-energy radiation with peak luminosities of 10(48)-10(50) erg s(-1). The breakout from the WD surface produces flashes with a rapid exponential decay by 3-4 orders of magnitude on timescales of a few tenths of a second and with most of the radiation in the X-ray and soft gamma-ray range. The shocks produced in gases in and around the binary will produce flashes with a characteristic duration of a few seconds with most of the radiation coming out as X-rays and gamma-rays. In both mechanisms, we expect a fast rise and slow decline and, after the peak, an evolution from hard to softer radiation due to adiabatic expansion. In many cases, flashes from both mechanisms will be superposed. The X-and gamma-ray visibility of an SN-Ia will depend strongly on self-absorption within the progenitor system, specifically on the properties of the accretion disk and its orientation toward the observer. Such X-ray and gamma-ray flashes could be detected as triggered events by gamma-ray burst (GRB) detectors on satellites, with events in current GRB catalogs. We have searched through the GRB catalogs (for the BATSE, HETE, and Swift experiments) for GRBs that occur at the extrapolated time of explosion and in the correct direction for known Type Ia supernovae with radial velocity of less than 3000 km s(-1). For the Burst and Transient Source Experiment (BATSE) about 12.9 +/- 3.6 nearby SNe Ia should have been detected, but only 0.8 +/- 0.7 non-coincidental matches have been found. With the High Energy Transient Explorer (HETE) and Swift satellites, we expect to see 5.6 +/- 1.3 SN-Ia flashes from known nearby SNe Ia but, yet, no SN-Ia flashes were detected. With the trigger thresholds for these experiments and the upper limits on the SN-Ia distances, we show that the bolometric peak luminosity of SN-Ia flashes must be less similar to 10(46) erg s(-1). Our observational limit is several orders-of-magnitude smaller than the peak luminosities predicted for both the early flashes. We attribute this difference to the absorption of the X-and.-rays by the accretion disk of large-scale height or common envelope that would be smothering the WD.