A 421-d activity cycle in the BeX recurrent transient A0538-66 from MACHO monitoring
/ Authors
C. Alcock, R. Allsman, D. Alves, T. Axelrod, A. Becker, D. Bennett, P. Charles, K. Cook, A. Drake, K. Freeman
and 16 more authors
M. Geha, K. Griest, M. Lehner, S. Marshall, K. McGowan, D. Minniti, C. Nelson, B. Peterson, P. Popowski, M. Pratt, P. Quinn, C. Stubbs, W. Sutherland, A. Tomaney, T. Vandehei, D. Welch
/ Abstract
ABSTRACT We present a ∼5-yr optical light curve of the recurrent Be/X-ray transient A0538–66obtained as a by-product of the MACHO Project. These data reveal both a long-termmodulation at P = 420.8±0.8 d and a short-term modulation at 16.6510±0.0022 dwhich, within errors, confirms the previously found orbital period. Furthermore, theorbital activity is only seen at certain phases of the 421 d cycle suggesting that thelong-term modulation is related to variations in the Be star envelope.Key words: binaries: close - stars: individual: A0538–66 - X-rays: stars 1 INTRODUCTIONThe recurrent X-ray transient A0538–66 was discovered withAriel V when two outbursts, separated by ∼17 d, were ob-served (White and Carpenter 1978). Further outbursts wereobserved with HEAO-1 which, when the source was active,was found to have a periodicity of 16.668 d (Johnston etal. 1979; Johnston et al. 1980; Skinner et al. 1980; Skinner ⋆ email: kem@astro.ox.ac.uk 1980), the precision of which led to its interpretation as be-ing orbital. Skinner (1981) used archival plates taken over∼50 years in order to obtain an improved value of 16.6515 dfor this periodicity, based on the recurrence of the outbursts.A0538–66 has been optically identified with a B star(V∼15) (Johnston et al. 1980), and an X-ray pulse periodof 69 ms indicates that the compact object is a neutronstar (Skinner et al. 1982). In quiescence (X-ray ’off’) thecolour, magnitude and radial velocity are consistent with aB2 III-IV star in the LMC (Charles et al. 1983), but during
Journal: Monthly Notices of the Royal Astronomical Society