COMPTEL upper limits for the Co γ-ray emission from SN1998bu

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
² FRM-II, Technische Universität München, Lichtenbergstr. 1, 85747 Garching, Germany
³ SRON Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁴ Space Science Center, University of New Hampshire, Durham, NH 03824, USA
⁵ Science Division, ESTEC, ESA, 2200 AG Noordwijk, The Netherlands

Corresponding author: R. Georgii, rgeorgii@frm2.tum.de

Received: 20 February 2002
Accepted: 1 August 2002

Abstract

Supernova 1998bu in the galaxy M 96 was observed by COMPTEL for a total of 88 days starting 17 days after the explosion. We searched for a signal in the 847 keV and 1238 keV lines of radioactive ⁵⁶Co from this type Ia supernova. Using several different analysis methods, we did not detect SN1998bu. Our measurements should have been sensitive enough to detect ⁶⁰Co gamma-rays as predicted from supernova models. Our flux limit is  photons cm^-2 s^-1; this would correspond to 0.35  of ejected ⁵⁶Ni, if SN1998bu were at a distance of 11.3 Mpc and transparent to MeV gamma rays for the period of our measurements. We discuss our measurements in the context of common supernova models, and conclude disfavoring a supernova event with large mixing and major parts of the freshly-generated radioactivity in outer layers.