GX 9+9: Variability of the X-Ray Orbital Modulation

TL;DR

This study examines the variability of the 4.19-hour X-ray orbital modulation in GX 9+9, revealing periods of strong and weak modulation over several years and analyzing its characteristics across multiple observations.

Contribution

It provides the first detailed analysis of the temporal variability of the X-ray orbital modulation in GX 9+9 over multiple years using ASM, PCA, Chandra, and optical data.

Findings

The 4.19-hour modulation was weak or absent before late 2004 and then became strong for about 2 years.

The modulation appears as a nonsinusoidal intensity dip covering less than 30% of the cycle.

Optical observations did not show increased modulation during the X-ray strong modulation period.

Abstract

Results of observations of the Galactic bulge X-ray source GX 9+9 by the All-Sky Monitor (ASM) and Proportional Counter Array (PCA) onboard the Rossi X-ray Timing Explorer are presented. The ASM results show that the 4.19 hour X-ray periodicity first reported by Hertz and Wood in 1987 was weak or not detected for most of the mission prior to late 2004, but then became strong and remained strong for approximately 2 years after which it weakened considerably. When the modulation at the 4.19 hour period is strong, it appears in folded light curves as an intensity dip over slightly less than 30% of a cycle and is distinctly nonsinusoidal. A number of PCA observations of GX 9+9 were performed before the appearance of strong modulation; two were performed in 2006 during the epoch of strong modulation. Data obtained from the earlier PCA observations yield at best limited evidence of the presence of phase-dependent intensity changes, while the data from the later observations confirm the presence of flux minima with depths and phases compatible with those apparent in folded ASM light curves. Light curves from a Chandra observation of GX 9+9 performed in the year 2000 prior to the start of strong modulation show the possible presence of shallow dips at the predicted times. Optical observations performed in 2006 while the X-ray modulation was strong do not show an increase in the degree of modulation at the 4.19 hour period. Implications of the changes in modulation strength in X-rays and other observational results are considered.