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An improved limit on the flux of magnetic monopoles in the vicinity of the solar system is obtained, assuming that monopoles strongly catalyze nucleon decay (the Rubakov-Callan effect). Flux limits are presented for monopole velocities from ${10}^{\ensuremath{-}5}c$ to ${10}^{\ensuremath{-}1}c$ and for monopole-nucleon cross sections between ${10}^{\ensuremath{-}27}$ ${\mathrm{cm}}^{2}$ and ${10}^{\ensuremath{-}21}$ ${\mathrm{cm}}^{2}$. For a representative velocity $\ensuremath{\beta}\ensuremath{\approx}{10}^{\ensuremath{-}3}$, and cross section $\ensuremath{\sigma}\ensuremath{\approx}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{2}$, we obtain a limit ${F}_{m}l2.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{\ensuremath{-}2}$${\mathrm{sr}}^{\ensuremath{-}1}$${\mathrm{sec}}^{\ensuremath{-}1}$ and for $\ensuremath{\sigma}\ensuremath{\approx}{10}^{\ensuremath{-}25}$ ${\mathrm{cm}}^{2}$, ${F}_{m}l1.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{\ensuremath{-}2}$${\mathrm{sr}}^{\ensuremath{-}1}$${\mathrm{sec}}^{\ensuremath{-}1}$ at 90% C.L.

Muons recorded by the IMB proton decay detector during the radio outburst from Cygnus X-3 in January 1991 are studied. Data are examined for both aperiodic excesses and those phase modulated at the x-ray period of Cygnus X-3. No correlation between the muon data and Cygnus X-3 is found. Further, this observation provides flux limits of ${\ensuremath{\Phi}}_{90% \mathrm{C}.\mathrm{L}.}\ensuremath{\le}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ \ensuremath{\mu}${\mathrm{cm}}^{\ensuremath{-}2}$${\mathrm{s}}^{\ensuremath{-}1}$ at 1570 meters of water equivalent on the 20th and 23rd, in contrast with other reported signals.

A basic tenet of special relativity is that all massless particles travel at a constant, energy-independent velocity. Astrophysical data, including observation of the Crab pulsar at {similar to}100 MeV and the recent detection of the pulsar in Hercules X-1 at energies {ge}100 TeV, are used to place new experimental constraints on energy-dependent deviations from constant velocity for massless particles. Previous experiments reached energies {similar to}10 GeV; this analysis improves the previous constraints by 7 orders of magnitude.

We have examined muon data collected with the IMB (Irvine-Michigan-Brookhaven) proton-decay detector in search of a muon flux associated with the x-ray binary Cygnus X-3. A sample of 3.6 x 10/sup 4/ muons from the direction of Cygnus X-3 during the period September 1982 until April 1984 shows no evidence of its characteristic 4.8-h periodicity. In the phase interval 0.65 to 0.90 where a signal has been reported by other workers at a similar depth we obtain a 90%-confidence-level limit to the flux of 6.9 x 10/sup -11/ cm/sup -2/sec/sup -1/ for a depth of 1800 m water equivalent. This limit does not conflict with the reported signal. Three subsets of the data taken during Cygnus X-3 radio bursts have been separately analyzed. We find some evidence for an excess in the phase interval 0.50--0.60 at the time of the October 1983 outburst. We examine this excess in detail.