ICARUS 133, 184–191 (1998) ARTICLE NO. IS985936 Observations of Jupiter at 3.5 cm and 6.0 cm Associated with the Impact of Comet P/Shoemaker–Levy 9 Robyn Millan Astronomy Department, University of California, Berkeley, California 94720 E-mail: rmillan@ssl.berkeley.edu Derck Smits Hartebeesthoek Radio Astronomy Observatory, South Africa and Imke de Pater Astronomy Department, University of California, Berkeley, California 94720 Received May 22, 1997; revised January 2, 1998 poorer quality than those obtained from other stations (Klein et al. 1995, Bolton et al. 1995) and are not included Jupiter was observed before, during, and after the July 1994 impacts of Comet P/Shoemaker–Levy 9. Observations were in this report. There are two other data sets at 6 cm; one made with the HartRAO 26-m antenna at 3.5 and 6.0 cm. In obtained with the Effelsberg radio telescope in Germany agreement with observations at other wavelengths, an increase (Bird et al. 1996) and the other with the Medicina and in the total flux density was seen at both wavelengths during Noto telescopes (Venturi et al. 1996). We compare our the impact week, followed by a slow decay. At 6.0 cm the 6-cm data with the 6-cm Effelsberg data. nonthermal flux density was found to increase by 1.1  0.1 Jy The 3.5-cm data reported here are particularly interest- (40%) and at 3.5 cm by 1.5  0.1 Jy (75%). The relaxation ing because they are the highest quality data at short wave- of the flux density to preimpact levels was found to decay with lengths taken during the impact period. Two other data an exponential lifetime of 215  10 days at 6.0 cm and at 300 sets at short wavelengths have been reported (Bird et al.  15 days at 3.5 cm. The exponential decay time is considerably shorter, however, during the first few months after the impacts. 1996, Venturi et al. 1996). Both of these data sets are In addition, pronounced variations in the flux density at both limited in that they either have poor statistics or do not wavelengths were seen during the first two months after the have good coverage of the event. The data presented in impacts, with a significant increase in the flux density about this paper cover the entire event, before, during, and after 85 days after the last impact.  1998 Academic Press the impacts, with high signal-to-noise. 1. INTRODUCTION 2. OBSERVATIONS Continuum measurements of Jupiter’s brightness were Between July 16 and July 22, 1994, fragments of Comet P/Shoemaker-Levy 9 (SL9) crashed into Jupiter, offering made at 3.5 and 6.0 cm using a dual-feed system operated in a beam-switched Dicke mode (see Table I for summary a unique opportunity for astronomers around the world to observe the collision of two bodies in the solar system. of observational and calibration information). Both sets of observations cover the period from June 1994 through To witness impacts involving millions of megatons of en- ergy, a worldwide campaign was organized to monitor October 1995. Observations of a single polarization were made at both wavelengths; at 6.0 cm linear east–west (LIN changes in Jupiter’s microwave emission (de Pater et al. 1995). In addition, the 26-m radio telescope of the Harte- EW) polarization was used, and right circular polarization (RCP) was used at 3.5 cm. The flux density at each wave- beesthoek Radio Astronomy Observatory (HartRAO) in South Africa was used to observe Jupiter’s brightness be- length was measured using a five-point stepping sequence in declination to correct for pointing errors in this coordi- fore, during, and after the impacts at wavelenths of 3.5, 6.0, 13, and 18 cm. The 13- and 18-cm data sets are of nate; the stepping positions in terms of beam parameters 184 0019-1035/98 $25.00 Copyright  1998 by Academic Press All rights of reproduction in any form reserved.