In April 1998 the transient source XTE J1808-369 was observed by RXTE, and found by Frank Marshall to be positionally coincident with SAX J1808.4-3658 which had been observed during a previous outburst by SAX in September 1996. Analysis of the public RXTE data by Rudy Wijnands and Michiel van der Klis revealed a sharp peak in the power spectrum at 401 cycles per second. The extreme stability of this frequency over time led to the discovery that this object is a millisecond pulsar, the first such object found to be accreting matter from a companion star.
Additional analysis by Deepto Chakrabarty and Ed Morgan uncovered variations in the times at which the signals from the pulsar arrival at the Earth. From delay times of individual pulses, these workers found that the pulsar lies in a binary with a 2.01 hour orbital period. The finite size of the orbit of the pulsar about its companion means that the pulses sometimes arrive earlier than expected, and sometimes later than expected. A plot of the observed-minus-expected arrival times versus time has a perfectly sinusoidal shape, indicating that the pulsar's orbit is circular. The mass-losing companion star is inferred to have a low mass - about 0.1 solar masses. There are indications that the companion is being cannibalized by X-ray heating from the pulsar. The heating produces a wind of outflowing gas from the star, which may lead to its complete evaporation in another 10 to 100 million years.
Millisecond pulsars are thought to be "spun-up" to high frequencies by the accretion of matter, a process which takes several hundred million to billions of years. Therefore millisecond pulsars are very old, in contrast to pulsars like the one in the Crab nebula which spins 30 times per second and is only about 1000 years old. Millisecond pulsars have been seen throughout the galaxy at radio wavelengths, but always in systems where the mass losing companion has stopped transferring material to the pulsar; the presence of accreting gas in the vicinity of the pulsar tends to block the radio signal from the pulsar. SAX J1808.4-3658 is the first such object for which a persistent millisecond signal has been seen X-rays. Spinning once every 2.5 milliseconds, a point on the equator of this pulsar is moving at about 10 percent the speed of light.
For additional information see http://heasarc.gsfc.nasa.gov/docs/xte/Snazzy/Movies/millisecond.html.
Figure 1: Power Spectrum of RXTE light curve of SAX J1808.4-3658 from 1998 April 11.
Figure 2: Pulse arrival time delays due to the ~2.01 hr orbit of SAX J1808.4-3658.
Figures provided by R. Wijnands and D. Chakrabarty, Captions and supporting text provided by J. Cannizzo
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