On the X-ray fast-time variability of Sco X-2 (GX 349+2)

¹ School of Physics, Australian Defence Force Academy, University of New South Wales, Canberra ACT 2600, Australia e-mail: r.sood@adfa.edu.au
² Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: erikk@sron.nl
³ Astronomical Institute, Utrecht University, PO Box 80000, 3507 TA Utrecht, The Netherlands
⁴ Institute of Space and Astronautical Science, 1-1 Yoshinodai 3-chome, Sagamihara-shi, Kanagawa 229-8510, Japan e-mail: dotani@astro.isas.ac.jp

Corresponding author: P. M. O'Neill, p.m.oneill@adfa.edu.au

Received: 11 October 2000
Accepted: 2 February 2001

Abstract

We have analysed archived Ginga data on the Z source Sco X-2 (GX 349+2). We present the first detailed investigation of its X-ray fast-time variability, as a function of position in the Z track. During the two-day observation over the period 5-7 March 1989, the source was in the so-called flaring branch, and the lower part of the so-called normal branch. We found broad peaked noise with a centroid frequency and width of ~4-7 Hz and ~6-12 Hz respectively. The peaked noise was strongest in the lower flaring branch, with a maximum fractional rms amplitude of ~3% . We conclude that it is not a manifestation of atoll source high frequency noise, as had been suggested, and compare it with the power spectral features seen in other Z sources. We find that the peaked noise is markedly different to the quasi-periodic oscillations found in the normal and flaring branches of Sco X-1; however it bears some resemblance to that seen in the flaring branch of Cyg X-2 at low overall intensities.