Number 124, April 16, 1993 by Phillip F. Schewe and Ben Stein SUPERNOVA 1993J , first spotted on March 28, is the brightest supernova in the northern hemisphere in several decades and the brightest supernova of any kind since SN 1987A. Alert astronomers and a fleet of versatile satellites contributed to what is the most thorough observation (at diverse wavelengths) of a supernova in its earliest stages. At the American Physical Society meeting in April in Washington, DC this week, Stephen Holt of NASA/Goddard---representing the Japanese satellite ASCA---and Walter Lewin of MIT---representing the German satellite Rosat---reported the detection of prompt x rays from SN 1993J. The distance of the supernova, 12 million light years away in galaxy M81, and the early arrival and intensity of the x rays (at the highest x-ray energies, the supernova was brighter than the galactic core) allowed scientists to estimate a supernova temperature of 100 million K and to deduce that the progenitor star had been a red supergiant star. According to this hypothesis, prompt x rays would arise from the supernova shock wave plowing into the high-density, low-velocity stellar wind (given off in the centuries before the blast) surrounding the star. By contrast, in the case of supernova 1987A, whose progenitor was a blue supergiant, the shock had to catch up to a low-density, high-velocity stellar wind, thus delaying the arrival of x rays for several months. George Sonneborn of NASA/Goddard reported on ultraviolet measurements of SN 1993J with the IUE satellite. From these he expects to learn about the red giant's stellar wind and the way in which it approached its violent death. Other telescopes such as the Hubble Space Telescope and the Gamma Ray Observatory will be looking at SN 1993J. However, because the new supernova is some 70 times further from Earth than SN 1987A, detected emissions will be about 5000 times weaker, virtually eliminating the probability of seeing neutrinos. THE TOP QUARK has not been discovered yet in high energy proton-antiproton collisions at Fermilab, and researchers there have pushed up their estimate of the minimum mass the top must have, if indeed it exists. Speaking at the APS meeting, Tony Liss of the University of Illinois reported that data from the CDF detector preclude a top quark mass less than 108 GeV. David Buchholz of Northwestern reported a comparable value, 103 GeV, for the D0 detector. Both of the mammoth detectors (each employing more than 400 physicists) have recorded a small number of events (officially 2 for CDF and 1 for D0) suggestive of top-quark production, events in which a high energy muon or electron, along with jets of other particles, emerges from the proton-antiproton collision. The Tevatron scientists admit, however, that such events might also be ascribed to a variety of non-top background reactions. Fermilab director John Peoples said that an inventory of at least 10 events in each detector would be necessary before one could even consider declaring that the top had been produced unambiguously. This underscores the statistical nature of the search for the top; the Tevatron has more than enough energy to create quarks with masses of 200 GeV or more, but the basic probability (the cross section) of this happening in any one interaction is extremely small. What is needed is much more data, and an important way of doing that is to increase the luminosity, the rate at which beam particles can be brought to bear at the interaction point. After a scheduled shutdown (June-Oct) the Tevatron is expected to operate with a larger luminosity and at a higher energy, 2 TeV.