PULSAR FLUX STABILITY AND REFRACTIVE INTERSTELLAR SCINTILLATION

We have measured accurate flux densities of 25 pulsars at 310, 416, and 750 MHz for 43 consecutive days with the NRAO 91 m telescope. We chose pulsars covering a wide range of dispersion measures (DM) to distinguish intrinsic luminosity variations from diffractive and refractive interstellar scintillations (RISS). The pulsars PSR 1818-04, 1933+16, and 2111+46 have such high DMs and/or low space velocities that their predicted RISS time scales at low frequencies are ≫43 days. These pulsars were observed to have quite constant flux densities, implying that their luminosities must be intrinsically stable on time scales from several minutes (the duration of a single flux-density measurement) to several weeks (the duration of the experiment). Since the DM is primarily a property of the interstellar medium along the line of sight to a pulsar, rather than of the pulsar itself or its environment, we conclude that most pulsars have intrinsically stable luminosities. A number of pulsars with intermediate dispersion measures (e.g., PSR 0329 + 54) showed moderate (ΔS/〈S〉 ≈ 0.3) flux-density variations on time scales from days to weeks. We ascribe these variations to refraction in the interstellar medium. It follows that an intrinsically nonvariable extragalactic source whose angular diameter is not much larger than the scattering angle θ should also scintillate. The observed values θ ≈ 10-4 to 10-3 arcsec approach the angular sizes of the most compact extragalactic sources at these frequencies, so our pulsar observations confirm the suggestion of Rickett, Coles, and Bourgois that low-frequency (v ≤ 1 GHz) variability of extragalactic sources can be caused by RISS.