Science from the GB 85-3 pulsar monitoring projects.

April 11, 2000

Telescope 85-3, a 26 meter (85-foot diameter) dish telescope, is run on behalf of pulsar groups at Berkeley and Princeton. About 35 pulsars have been monitored daily since 1989 at 327 MHz and 610 MHz. In 1995, the "Green Bank-Berkeley Pulsar Processor" (GBPP) was installed as the main data acquisition instrument. Designed by D. Backer and colleagues at U. Cal.- Berkeley, it does coherent de-dispersion and averaging of pulse data. Pulsars are monitored for possible timing "glitches", to improve accuracy of timing models, and to study the interstellar medium.

Monitoring of pulsar flux densities has shown that many pulsars have long term variations due to interstellar refractive scintillation, although some, such as the Vela pulsar, have variations not consistent with scintillation theory (Kaspi and Stinebring, Ap.J. 392, 530, 1992).

Timing glitches have been detected in the Crab and Vela pulsars, and in PSR 1737-30 (Nice, in "Isolated Pulsars", vanRiper, ed., Cambridge 1992). The largest glitch ever seen in the Vela pulsar (or any pulsar) occurred on January 16, 2000, and was reported in IAU circular 7347 by a group at the U. of Tasmania. This glitch was also observed with 85-3, and the data are being analyzed by the Berkeley group and compared with Tasmanian and Chandra observations. Preliminary results indicate df/f ~ 4E-6. The Crab pulsar has had a cluster of six glitches during 1995-1999 (Wong & Backer, 2000, submitted to ApJ).

Recent measurements with 85-3 have supported a Crab pulsar timing experiment using NRL's USA satellite at X-ray energies and Palomar in the IR band. Also, the 85-3 observing has been modified recently to include pulsars in the VLBA pulsar astrometry program of Chatterjee and Cordes.

A dual-frequency study of giant pulses from the Crab pulsar was done in 1996 using the VLA at 1.4 GHz and the GB 85-3 telescope at 0.61 GHz. The detection of a giant pulse at the VLA was used as a trigger, sent via internet, to a computer at Green Bank to trigger the start of data acquisition of the pulse at 0.61 GHz. This was possible because the dispersion measure (56.8 pc cm-3) causes a delay of about 0.5 second in the arrival time of the pulse at 0.61 GHz with respect to the pulse at 1.4 GHz. The trigger message arrived over the internet in about 0.2 seconds, making it possible for the GBPP to record the same pulse as recorded across the country at the VLA. The results, published recently (Sallmen, Backer, et. al. 1999 ApJ 517, 460), show that the emission mechanism for the giant pulses must be very broad band in nature, and that the splitting of the pulses into multiple components is likely intrinsic to the emission from the pulsar, i.e., not due to propagation effects.

During 1995-1999 the propagation parameters of the Crab pulsar have been violently variable. The parameters -- refractive gain, scattering, dispersion, multipathing -- are excellently studied with the two bands at 85-3 (327 MHz and 610 MHz) along with the dense sampling in time. The interpretation is that the line of sight passed through a particularly disturbed region of the nebular interface between the expanding synchrotron emitting region and the supernova ejecta. The first paper on the wild events during 1997.5-1998.0 has been submitted (Backer, Wong & Valanju 2000 ApJ). A second paper is in preparation that will cover the entire period up to present and incorporate history of propagation activity going back to 1969.

Note that both 85-3 and the GBI are potentially usable for other types of observing programs. The recent consolidation of all telescope operations at Green Bank into the same building means that signals from receivers on any telescope can be linked to any of the available back ends: continuum, spectral processor, spectrometer, or VLBI. Offers are welcome. At the moment A. Roshi is exploring use of 85-3 for studies of low frequency recombination lines. The 610-MHz IF will be linked to the NRAO Spectral Processor.