A Third X-Ray Spectral Mode in Scorpius X-1: Similarities to the Other Bright Bulge Sources

Abstract

Sco X-1 normally shows two spectral/quasi-periodic oscillation (QPO) modes. In this paper, new observations of Sco X-1 are reported which show it in a third mode which is similar to the 'horizontal branch' displayed by Cyg X-2 and suggests that the Galactic bulge sources follow a common, Z-shaped spectral pattern. In this mode, the spectral hardness remains constant, the intensity decreases, and the power spectrum shows a broad noise component at about 2 Hz. In all branches, Sco X-1 shows a previously unreported noise continuum which cuts off at about 60 Hz.