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Kelley, R.L., S. Rappaport, M.J. Brodheim, L. Cominsky, and R. Stothers, 1981: A search for apsidal motion in 4U0115+63. Astrophys. J., 251, 630-638, doi:10.1086/159509.
We have carred out a pulse arrival-time analysis of the archival Uhuru data from the 1971 transient outburst of the binary X-ray pulsar 4U0115+63. The 3.6 s X-ray pulsations are clearly present in the data, and we show that the average fractional rate of change in pulse period over the 7 yr interval 1971-1978 corresponds to P./P = -2×10-6 / yr. This spin-up rate is consistent with an average source luminosity ~20 times less than that observed during its flare state. The pulse arrival times were tracked for ~7 d, and by combining these data with the 1978 SAS 3 orbital determination, we place a limit on the advance of periastron in the 4U0115+63 system of ώ ≲ 2.1° /yr (95% confidence). The analysis also yields an improved value for the orbital period, Porb = 24.3162°. The constraints on the companion star imposed by the limit on apsidal motion are discussed.
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