Welcome to the EGRET Pulsar Search Homepage

The EGRET Pulsar Search Collaboration consists of:

Background Information

The gamma-ray source Geminga was first discovered in 1972 by the SAS-2 satellite. Despite being a very bright source at energies above 100 MeV, it has no counterparts at any other wavelengths, except for the X-ray band. Consequently, its true identity was largely a mystery for the 20 years following its discovery. A breakthrough came in 1992 when observations by the ROSAT satellite discovered soft X-ray pulsations from Geminga, thus revealing it to be a radio-quiet gamma-ray pulsar . This raises the tantalizing possibility that many of the unidentified sources detected by the EGRET instrument on-board the Comption Gamma Ray Observatory could be Geminga-like pulsars. We have therefore embarked on a systematic search for pulsations from these unidentified EGRET sources.

Pulsation Search Technique and Strategy

Due to the extremely low event rate even from bright gamma-ray sources (about 150 photons per day), long observation intervals of several weeks are typically required to attain a sufficiently high signal-to-noise ratio for detection. In 1987, Buccheri, Ozel and Sacco wrote that although the Fast Fourier Transform (FFT) could reduce computation time by about orders of magnitude over epoch folding in the search for high-energy periodicity, ``the enormous problems of computer memory to face (arrays of 10^10 locations) make the search unpracticable'. Fortunately, the exponential rate at which computational power evolved over the past decade has now made this search strategy highly feasible. The Center for Advanced Computing Research (CACR) at Caltech owns the 512-node Intel Touchstone Delta which has a memory of 8 Gigabytes and an attained operating speed of 10 GFLOPS.

The parallel FFT and the associated pulsation detection software which we have implemented on the Delta are capable of processing a billion-point time series in less than 5 minutes. In view of the spin-down expected of most rotation-powered pulsars, a wide range of frequency derivatives must be searched. For each trial frequency derivative, the search code performs the following sequence of procedures:

Candidate events are then scrutinized using more sensitive techniques like epoch-folding. The following is the power spectrum from an FFT of 2^28 points of data for the EGRET viewing period 1.0 observation of Geminga. The double-peaked nature of the Geminga pulse profile results in most of the power appearing at 8.4 Hz, which is the 2nd harmonic of the pulsar rotational frequency. Unfortunately, our Geminga search results are being described here instead of Nature because of the Rossi Prize work done by Jules Halpern and Steve Holt with the ROSAT data in the interval between the EGRET data acquisition and its public availability.

Project Status

We have begun extensive searches on archival data for GRO J1744-28, GRO J2019+40 and GRO J0617+22. No Geminga-like pulsars are present in these data to the limits of our sensitivity, and we can also rule out strong Vela-like pulsars. To date, we have consumed more than 80,000 CPU node hours, corresponding to about 561 teraflops, on the Delta. We are carrying out searches for pulsations at higher frequency derivatives, making use of all available supercomputer time. In addition, searches on archival data for other promising candidates are in progress.