Supernova 1987A in the Large Magellanic Cloud

P. Bouchet, I. J. Danziger, and L. B. Lucy, European Southern Observatory, report: "Continuing infrared photometry in the J, H, K, L, M, N1, N2, N3, Q bands at La Silla, combined with UBVRI photometry reported from Cerro Tololo (cf. IAUC 4881, 4910) shows that the bolometric light curve on 1989 Nov. 10 (day 991) lies 1 x 10E38 erg sE-1 above a linear extrapolation from earlier epochs (Suntzeff and Bouchet 1989, A.J., in press). This levelling off was already apparent from the previous observation (Aug. 14; day 903), though at a lower level of significance, and is confirmed by observations in less-than-ideal conditions on Dec. 20 (day 1030), when blackbody fitting gives T = 160 K and log L = 38.30 +/- 0.05. Because more than 80 percent of the flux is now emitted redward of the M band, the levelling-off is almost completely due to the near constancy of the flux integrated over the M, N, Q bands for days 903, 991, and 1030. This implies that a hitherto-undetected energy source is now contributing significantly to the total energy output. If it were due to 57Co, the original amount would have to be 20-25 times the anticipated 0.0017 solar mass (Woosley and Pinto 1988, Workshop on Gamma-ray Spectroscopy), but this is contradicted by the observed [Co II] 10.52-micron line strength on day 526 (Danziger et al. 1989, Proceedings of Santa Cruz Workshop). A thermal echo from external dust seems unlikely since it would coincidentally need to have a color temperature (150-180 K) similar to that of the supernova's emission. Moreover, the corresponding scattering echo (cf. IAUC 4746) is not evident in the smooth UBVR light curves (IAUC 4881, 4910). Nevertheless, CCD frames should be inspected for new echoes within 5" of the SN. (Uncertainties in luminosities derived by fitting blackbody curves to the far-infrared data have been checked using emission curves for isothermal dust clouds of astronomical silicate. With a dust mass of 0.05 solar mass, inefficient dust emission at wavelengths greater than 30 microns lowers the luminosity by only 10E0.1.) Pulsar emission, absorbed and reradiated by dust in the ejecta (IAUC 4746) seems the most likely explanation of our observations. Other signatures of such input should be sought."