A Vla Search for the Geminga Pulsar: a Bayesian Limit on a Scintillating Source

We derive an upper limit of 3 mJy (95% conÐdence) for the Ñux density at 317 MHz of the Geminga pulsar (J0633]1746). Our results are based on 7 hours of fast-sampled VLA data, which we averaged synchronously with the pulse period using a period model based on CGRO/EGRET gamma-ray data. Our limit accounts for the fact that this pulsar is most likely subject to interstellar scintillations on a timescale much shorter than our observing span. Our Bayesian method is quite general and can be applied to calculate the Ñuxes of other scintillated sources. We also present a Bayesian technique for calculating the Ñux in a pulsed signal of unknown width and phase. Comparing our upper limit of 3 mJy with the quoted Ñux density of Geminga at 102 MHz, we calculate a lower limit to its spectral index of a B 2.7 [F(l) P l~a]. We discuss some possible reasons for GemingaÏs weakness at radio wavelengths, and the likelihood that many of the unidentiÐed EGRET sources are also radio-quiet or radio-weak Geminga-like pulsars. Subject headings : methods : statistical È pulsars : individual (Geminga) È radio continuum: stars