OSSE Observations of Supernova 1998bu

M.D. Leising, L.S. The

J.D. Kurfess

P. Höflich

Abstract

Supernova 1998bu was discovered on May 9.9 in NGC 3368 (M 96), and was already present on a pre-discovery plate on May 3.1. SN 1998bu shows spectral and light curve properties characteristic of a rather typical Type Ia supernova, reaching a peak visual magnitude of V @ 11.9. This is probably the nearest SN Ia in the Compton Observatory era, offering the best chance yet for CGRO to detect ⁵⁶Co lines, as well as early ⁵⁶Ni lines for some models. Very early observations turned out to be impractical; the first data were accumulated on days 18-21 post-explosion. NASA committed a great deal of CGRO time to later observations, including OSSE observations during the intervals 26-53, 60-110, 117-137, 144-158, and 165-186 days. Together these data provide a line sensitivity at 847 keV of roughly 3 10^-5 cm^-2 s^-1 averaged over all these times. There is no convincing evidence for ⁵⁶Co line emission in the OSSE data. The upper limits rule out g-ray bright models, such as some delayed-detonations. Reaching the ultimate sensitivity of these data will require additional effort, as orbital conditions at the current CGRO altitude are responsible for unusually large and variable backgrounds.

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On 16 Jul 1999, 09:19.