Positions of galactic X-ray sources Cir X-1, TrA X-1 and 3U1626—67

Abstract

THE positions of three celestial X-ray sources were measured with the rotating modulation collimators (RMC) on SAS-3 during a survey of the galactic plane1 and are precise to ≤ 25″ (ref. 2). One of these sources, Cir X-1 (ref. 3), is a highly variable X-ray source often compared with the black-hole candidate, Cyg X-1. Another, TrA X-1 = A1524–61, is a 1974 X-ray nova4, and the third, 3U1626–67 (ref. 5), is a 7.68-s X-ray pulsar6. The results reported here support recently proposed optical and radio identifications7–9 of these three sources. In two cases (TrA X-1 and 3U1626–67), they have been instrumental in bringing about the proposed identifications. Cir X-1 exhibits a binary periodicity10 of 16.6 d, extreme aperiodic variability on timescales of > 0.1s (refs 11, 12) and 1–3 s (ref. 13), and flaring14,15 with time constants of a few ms. This variability and the absence of a shorter (spin) period13,15 have led to suggestions11,13–15 that this object may be similar to Cyg X-1. The absence of a compelling optical or radio counterpart, however, has prevented the further exploration of this hypothesis.