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

The pulsating X-ray source 4U 1626-67 is an accreting neutron star in a binary system with a very low mass companion. The authors observed the source with EXOSAT continuously for 23 hr on 1986 March 30 - 31 UT. These observations allow them to set a stringent upper limit on the projected semimajor axis of the orbit of the neutron star. The corresponding upper limit on the mass function is 1.3×10-6M_sun;. The authors conclude that if the orbital inclination angle, i, equals 90°, then the optical companion star has a mass of ⪉0.02 M_sun;. However, they find that a companion star mass ≥0.06 M_sun; is required if gravitational radiation is responsible for driving the mass transfer in this system. Only for i ⪉ 16° can a companion star mass this large be accomodated by the limit on the orbital amplitude. The authors also present in this work their results on the flaring activity in 4U 1626-67 on time scales of ≡1000 s, the energy-dependent pulse profiles, and the pulse period history over the past decade.