A VLA search for the geminga pulsar: A Bayesian limit on a scintillating source

We derive an upper limit of 3 mJy (95% confidence) for the flux density at 317 MHz of the Geminga pulsar (J0633 + 1746). Our results are based on 7 ho urs of fast-sampled VLA data, which we averaged synchronously with the puls e 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 interstel lar scintillations on a timescale much shorter than our observing span. Our Bayesian method is quite general and can be applied to calculate the fluxe s of other scintillated sources. We also present a Bayesian technique for c alculating the flux in a pulsed signal of unknown width and phase. Comparin g our upper limit of 3 mJy with the quoted flux density of Geminga at 102 M Hz, we calculate a lower limit to its spectral index of alpha approximate t o 2.7 [F(nu) proportional to nu(-alpha)]. We discuss some possible reasons for Geminga's weakness at radio wavelengths, and the likelihood that many o f the unidentified EGRET sources are also radio-quiet or radio-weak Geminga -like pulsars.