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Session 111 - Pulsars.
Display session, Saturday, January 10
Exhibit Hall,
Highly accurate timing of radio millisecond pulsars has numerous applications, including experimental tests of general relativity and cosmology. Great improvements over existing timing measurements can be achieved by removing the dispersive effects of the interstellar medium in a phase-coherent manner. Only recently have computer speeds approached levels allowing the routine use of this method over bandwidths of several MHz or more.
We have designed the Princeton Mark IV system to implement coherent dedispersion in software. A fast analog-to-digital converter samples quadrature components of the received voltages in two orthogonal polarization channels, allowing 10\,MHz bandwidth with 2-bit sampling or 5\,MHz bandwidth with 4-bit sampling, and producing a continuous throughput of 10\,MB/s. The resulting data stream passes through a SPARC 20 computer and onto DLT 7000 tape drives and/or a 108\,GB disk array. Data analysis is performed off-line by one of several fast (1.25\,Gflop) parallel processors. The effects of dispersion are removed by convolving the data time-series with the inverse of the interstellar medium ``chirp function.'' Cross-products are formed from the dedispersed signals, and then can be folded modulo the pulse period. In this fashion we obtain full Stokes parameters for every observation. Narrow-band radio-frequency interference can be effectively excised as part of the coherent signal-processing task, while time segments contaminated with broad-band noise are dropped from the analysis.
We present the results of several months of observations at the 76\,m Lovell telescope at Jodrell Bank, U. K. \,\, Pulse profiles with full polarization information and flux calibration are given for a number of millisecond pulsars. A concentrated campaign to observe the double-neutron-star binary PSR B1534+12 has resulted in a greatly improved measurement of the orbital period derivative, and hence a strong test of the predictions of general relativity.
