On the multi-periodicities in the X-ray dipper XB1916-053

Abstract

Using the Rossi X-ray Timing Explorer and the Nordic Optical Telescope, we have obtained the highest ever quality X-ray/white-light high-speed photometry of XB 1916-053. We refine the X-ray period (PX) to 3000.6 ± 0.2 s via a restricted cycle counting approach. Using our complete optical light curve, we have extended the optical period (Popt) ephemeris by another 4 yr, providing further evidence for its stability, although a slightly longer period of 3027.555 ± 0.002 s now provides a marginally better fit. Moreover, modulations at both Px and Popt are present in the optical data, with the former dominating the nightly light curves (i.e. a few cycles of data). We have also attempted to determine the 'beat' period, as seen in the repeating evolution of the X-ray dip structure, and the variation in primary dip phase. We find that a quasi-period of 4.74 ± 0.05 d provides the best fit to the data, even then requiring phase shifts between cycles, with the expected 3.90-d 'beat' of PX and Popt appearing to be less likely. Finally, considering the nature of each of these temporal phenomena, we outline possible models, which could explain all of the observed behaviour of this enigmatic source, focusing on which of PX or Popt is the binary period.