Super-Orbital Variability in Hard X-rays

TL;DR

This study analyzes long-term hard X-ray variability in five low mass X-ray binaries, detecting some super-orbital periods and phase shifts that suggest possible transient jet formation linked to accretion states.

Contribution

It provides new detections of super-orbital periods in hard X-ray bands and explores their phase relationships, offering insights into accretion and jet phenomena in these systems.

Findings

Detection of the 170-day period in 4U 1820-303 up to 24 keV.

Detection of the 46-day period in 4U 1636-536 up to 100 keV with phase shift.

No significant periodic modulation in 4U 1916-053, Cyg X-2, or Sco X-1.

Abstract

We present the results of a study with the \emph{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 $\sim$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 $\sim$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 $\sim180^\circ$ with respect to each other. This phase shift, when taken together with the near-coincident onset of the $\sim$46 d modulation and the low/hard X-ray state, leads us to speculate that the modulation could herald transient jet formation.