J. V. Paradijs, University of Amsterdam; H. Pedersen, European Southern Observatory; and W. H. G. Lewin, Massachusetts Institute of Technology, report the detection of ~ 4-hr modulation in the broad-band optical flux of the optical counterpart V801 Ara of the x-ray burst source 4U/MXB 1636-53. The observations were made in 1980 July and 1981 Aug. with the Danish 1.5-m telescope at La Silla and cover a total of ~ 23 hr of observing. If the modulation is strictly periodic, only three periods are possible: 3.28, 3.78 or 4.42 hr, the uncertainty in each case being +/- 0.05 hr. The full amplitude of the modulation was ~ 25 percent in the 1980 data and 40 percent in the 1981 data. An orbital period of ~ 4 hr is consistent with an M0V star (~ 0.4 Ms) that fills its Roche lobe.
International Astronomical Union Circular
- Pub Date:
- August 1981