SUPERNOVA 1999em IN NGC 1637

D.W. Fox and W.H.G. Lewin, Massachusetts Institute of Technology, report on behalf of a large collaboration: “SN1999em has been imaged twice with the Chandra X-ray Observatory (ACIS-S). On November 1-2, 112 photons were detected during a 32 ksec observation; preliminary spectral fits indicate a soft source spectrum (photon index ~1.8) with a 0.1-8.0 keV flux of 2e-14 ergs/cm^2/s, corresponding to an unabsorbed source luminosity of 2e38 ergs/s at 7.8 Mpc (Y.-J. Sohn & T.J. Davidge 1998, AJ 115, 130). On November 11-12, 60 photons were detected during a 32 ksec observation; assuming a (relatively) constant spectrum, this corresponds to a source luminosity of 1e38 ergs/s.”

C.K. Lacey, Naval Research Laboratory, R.A. Sramek, National Radio Astronomy Observatory, S.D. Van Dyk, IPAC/Caltech, and K.W. Weiler, Naval Research Laboratory report: “Radio observations with the NRAO Very Large Array on 1999 November 1, 3, 4, 5, 9, 11, 13, and 16 yield non-detections of the Type IIP SN 1999em. The best upper limits (3 sigma) from all of the radio observations for SN 1999em are: <0.86 mJy at 43.3 GHz (7 mm), <0.77 mJy at 22.5 GHz (1.3 cm), <0.83 mJy at 15.0 GHz(2 cm), and <0.26 mJy at 8.4 GHz (3.6 cm), which correspond to upper limits on the radio spectral luminosity of <6.2, <5.6, <6.0, and <1.9 x 10^25 erg/s/Hz, respectively, for an assumed distance of 7.8 Mpc (Y.-J. Sohn & T.J. Davidge 1998, AJ 115, 130). The full set of measured upper limits is available on http://rsd-www.nrl.navy.mil/7214/weiler/sne-home.html. Whether our upper limits imply such a low CSM density that the radio emission is undetectable, or such a high CSM density that it is optically thick to radio emission at this early epoch, remains to be determined through further monitoring.”

“The combination of X-ray detection and radio non-detection is unusual. The only other known example is the Type IIP SN 1994W in NGC 4027 which has never been detected in the radio (3 sigma upper limit <1.1 x 10^26 erg/s/Hz at 8.4 GHz on 1998 February 10) but has a reported ROSAT HRI detection of ~8 x 10^39 erg/s on 1997 October 22 (E.M. Schlegel, astroph/9910425). This may imply a significant difference in the physical properties of the presupernova systems between the more readily radio and X-ray detectable Type IIL SNe, such as SN 1979C and SN 1980K, and the Type IIP progenitor systems. We encourage acquisition of additional optical spectra of SN 1999em while its X-ray emission is still strong.”

(Submitted to IAUC on 24 November 1999)