Ma, C. P. | Search NASA

NASA NTRS1988

4U 1626-67 - The binary with the smallest known mass function

The pulsing X-ray source 4U 1626-67 is an accreting neutron star in a binary system with a very low mass companion. The source was observed by Exosat continuously for 23 hr on March 30-31, 1986 UT. It is found that, if the orbital inclination angle equals 90 deg, the optical companion star has a mass of less than about 0.002 solar mass; however, it is found that a companion star mass of greater than about 0.06 solar mass is required if gravitational radiation is responsible for driving the mass transfer in this system. Also presented are results on flaring activity in the system on time scales of about 1000 s, the energy-dependent pulse profiles, and the pulse period history over the past decade.

Levine, A.↗

NASA NTRS1987

The evolutionary status of 4U 1820-30

The evolution of an ultracompact binary wherein mass transfer is driven by the emission of gravitational radiation is calculated, emphasizing the systematic study of how the rate of change in orbital period depends on the various system parameters. Analytic scaling laws are derived describing how the mass of the secondary, the mass transfer rate, and the rate of change of period depend on the binary system parameters, and these simple relations are applied to the 4U 1820-30 system. Detailed numerical evolutionary calculations are performed utilizing Zapolsky-Salpeter (1969) models for cold stars, but allowing for the possibility that the secondary has not yet reached a completely degenerate configuration. The results are used to set significant constraints on the deviation from a completely degenerate configuration of the secondary, the systemic mass-loss parameters, and the mass of the neutron star in the 4U 1820-30 system.

Rappaport, S.↗