RXTE Monitoring of the Symbiotic Star, CH Cyg

Last updated 2010 August 6


History

The symbiotic star, CH Cygni, is a well-known and highly variable hard X-ray source. The early X-ray observations from HEAO-1 to EXOSAT are summarized by Leahy & Taylor (1987). Ezuka et al. 1998 reported on the ASCA observation, showing a spectacular and complex X-ray spectrum, consisting of a relatively unabsorbed soft component (dominant below 2 keV) and a highly absorbed, hot component (seen above 3 keV), with a 2-10 keV flux of 6.3 x 10-11 ergs cm-2s-1.

CH Cyg is an obvious target for Chandra grating observations, given its brightness and the complex spectrum. Unfortunately, when Pete Wheatley and co. observed it with Chandra/HETG in 2001, it was in a low state (1.4 x 10-12, 3-10 keV). Although the faintness of the central source, however, allowed Galloway & Sokoloski (2004) to detect an X-ray jet in CH Cyg, this observation did not achieve the originally intended objective.

More recently, CH Cyg has been detected in the Swift BAT survey, one of the 4 symbiotic stars to appear in the BAT survey catalog. Over the course of the first 9 month of 2005, the 14-24 keV flux of CH Cyg showed a clear decline (Kennea et al. 2009). During the first 100 days, it probably was even brighter than during the ASCA observations. In 2006, CH Cyg was observed twice with Suzaku, it was in a hard X-ray faint, soft X-ray bright state (Mukai et al. 2007). In the 2-10 keV range, it showed a weak continuum and a strong 6.4 keV Fe line, indicating that the observed flux is now dominated by a scattered component, while something is blocking our direct line of site to the hard X-ray engine of CH Cyg.

Chandra TOO and RXTE monitoring

We secured the rights for a Chandra/HETG observation as a slow TOO, if CH Cyg recovers to at least 70% of the level seen with ASCA, or 4.4 x 10-11 ergs cm-2s-1.

We have also been carrying out an RXTE/PCA monitoring campaign, observing CH Cyg once every 3 weeks, since 2007 July. Through most of this period, CH Cyg stayed in a low state, well below 10-11 ergs cm-2s-1 (2-10 keV). The measured fluxes in many cases are comparable to the estimated systematic uncertainties in the PCA background, and cannot be taken at face value, although they all show the strong 6.4 keV line, suggesting that CH Cyg remained in the same state as during the 2006 Suzaku observations.

In 2009 March (day 900 in the figure below), however, CH Cyg started a gradual recovery towards the high state, with additional fluctuations from pointing to pointing (ATel 2046). Then, in one observation on 2009 July 25 (Day 1038), CH Cyg was seen at 7.4 x 10-11 ergs cm-2s-1 (2-10 keV), as bright as any historical records on this system. However, this appears to have been a short-lived state. In a Swift TOO observation on 2009 August 6, it was down at 1.64 x 10-11 ergs cm-2s-1. Interestingly, this short-lived X-ray bright episode was followed by by an optical flare (ATel 2192). Since then, CH Cyg appears to have returned to the gradual brightening track (ATel 2245).

One RXTE pointing originally scheduled for Sep 26 was not carried out due to a temporary issue with the spacecraft. The two PCA measurements since then (Oct 16 & Nov 7), as well as a series of Swift/XRT observations carried out during Oct 20-29, 2009, indicate that CH Cyg has returned to a high state, comparable to the ASCA level, and the Chandra TOO observations were activated. Contrary to our earlier expectations, this was not a stable X-ray high state: At the time of the Chandra observations, CH Cyg was already beginning to fade, although it (on average at least) satisfied our TOO criterion. During the 7 months since then, CH Cyg has continued its hard X-ray decline. According to Skopal et al., the X-ray behavior is mirrored by the optical light curve (ATel 2394). Moreover, Sokoloski et al. report the cessation of optical flickering (ATel 2707).

RXTE/PCA light curve of CH Cyg from 2007 July 2 to 2010 July 16.

PCA light curve of CH Cyg

The CH Cyg monitoring campaign is part of the RXTE/PCA core program for AO13 and AO14, and is scheduled continue through September, 2010.

References

  • Ezuka, H., Ishida, M. & Makino, F. 1998, ApJ 499, 388
  • Galloway, D.K. & Sokoloski, J.L. 2004, ApJLett 613, L61
  • Kennea, J.A. et al. 2009, ApJ, 701, 1992.
  • Leahy, D.A. & Taylor, A.R. 1987, A&A 176, 262
  • Mukai, K. et al. 2007, PASJ 59, S177

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