AAS 199th meeting, Washington, DC, January 2002
Session 6. Binary Stars
Display, Monday, January 7, 2002, 9:20am-6:30pm, Exhibit Hall

## [6.14] Eclipsing Binary Millisecond Pulsars: Observations of Orbit and Pulse Phase Variability

D.J. Nice (Princeton University), Z. Arzoumanian (NASA/GSFC), S.E. Thorsett (UC-Santa Cruz)

Eclipsing binary pulsar systems are remnants of accreting low mass X-ray binaries. They consist of millisecond pulsars in tight orbits (orbital periods of a few hours) with low mass companions (0.02 to 0.2 M\odot). The low mass stars fill their Roche lobes and lose mass through a combination of stellar wind (induced by heating from pulsar irradiation) and/or Roche lobe overflow.

Timing measurements of these millisecond pulsars can be used to map the orbits with exquisite precision, allowing detection of small changes in orbital elements.

We have monitored two eclipsing binary pulsars, B1744-24A and B1957+20, over ten years at Arecibo, Green Bank, and the VLA. PSR B1957+20 exhibits increases and decreases in its orbital period of order \Delta Pb/Pb~10-7 on a time scale of a few years; this may be due to angular momentum transfer between the orbit and the low mass star. In contrast, the orbital period of PSR B1744-24A has continuously decreased, with a time scale |Pb/\dot{Pb}|~200 Myr; this evolution is five times faster than expected from gravitational radiation, indicating that angular momentum is being carried off by some other mechanism, perhaps magnetic braking of the secondary.

Both of these pulsars show unusual irregularities in pulse arrival times, with pulse phase residuals of order 30 and 500 \mus, respectively, for B1957+20 and B1744-24A, and variability on a time scale of years for both sources. Since such large timing noise'' is not seen in other spun-up pulsars, we conclude it is a consequence of mass flow in the system. Small variations in dispersion measure are also seen for these systems but are not correlated with the pulse phase noise.

The author(s) of this abstract have provided an email address for comments about the abstract: dnice@princeton.edu