Super-Orbital Variability in Hard X-rays
Abstract
We present the results of a study with the Swift Burst Alert Telescope in the 14 -- 195 keV range of the long-term variability of 5 low mass X-ray binaries with reported or suspected super-orbital periods -- 4U 1636-536, 4U 1820-303, 4U 1916-053, Cyg X-2 and Sco X-1. No significant persistent periodic modulation was detected around the previously reported periods in the 4U 1916-053, Cyg X-2 or Sco X-1 light curves. The 170 d period of 4U 1820-303 was detected up to 24 keV, consistent with variable accretion due to the previously proposed triple system model. The 46 d period in 4U 1636-536 was detected up to 100 keV, with the modulation in the low and high energy bands found to be phase shifted by 180 with respect to each other. This phase shift, when taken together with the near-coincident onset of the 46 d modulation and the low/hard X-ray state, leads us to speculate that the modulation could herald transient jet formation.