Quasi-periodic oscillations in GX 17 + 2

Open Access
Publisher Website
Google Scholar
Abstract
We have made X-ray observations of GX 17 + 2 with the Ginga Observatory and observed the source in three spectral states: the flaring branch, the normal branch and the horizontal branch. Quasi-periodic oscillations (QPO) were observed in all three branches. In the horizontal-branch state the power spectra show low-frequency noise (LFN), with rms strength between 2.8 and 4.5 per cent, and two QPO peaks, with rms strengths between ∼ 1.5 and ∼ 3.5 per cent. The latter are most likely to be a first and second harmonic since the ratio of their frequencies, which both vary with intensity, is 2.04±0.06. The centroid frequency of the first harmonic varies from ∼ 18 to 30 Hz. The FWHM of the second-harmonic QPO peak is substantially higher than that of the first harmonic. In the normal branch, QPO was observed (rms strength between 1.4 and 2.9 per cent), with a frequency of ∼ 7 Hz, independent of the source intensity. When GX 17 + 2 was in the flaring branch state, it showed QPO (rms strength 2.8 to 4.1 per cent) whose frequency, which varied between ∼ 7 and ∼ 18 Hz, was positively correlated with the X-ray intensity. Simultaneously with these 7–18 Hz QPO we also observed a high-frequency peak in the power spectrum at ∼ 120 Hz (rms variation ∼ 2 per cent). No LFN was observed in the normal and flaring branches. Very low-frequency noise (below ∼ 0.03 Hz) was present in all three spectral branches. Our results show that GX 17 + 2 is a clear example of the recently defined group of Z-type sources. The continuous variation of the QPO behaviour as GX 17 + 2 moves from the normal to the flaring branch, similar to what has been observed in Sco X-1 (and perhaps Cir X-1), indicates that the normal-branch QPO and the flaring branch QPO are likely one physical phenomenon.

This publication has 0 references indexed in Scilit: