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Optical Afterglow of the Hard X-ray Flare in the Symbiotic Star CH Cygni

ATel #2192; A. Skopal, S. Shugarov, D. Chochol (Astronomical Institute, Slovak Academy of Sciences, Tatranska Lomnica), T. Pribulla (Astrophysikalisches Institut und Universitaets-Sternwarte, Jena), M. Wolf (Astronomical Institute, Charles University, Prague)
on 9 Sep 2009; 08:41 UT
Credential Certification: Augustin Skopal (skopal@ta3.sk)

Subjects: Optical, X-ray, Binary, Variables

Referred to by ATel #: 2245, 2394

We report on the recent optical photometric and spectroscopic signatures of the hard X-ray recovery of the symbiotic system CH Cyg (see ATEL #2046). Our UBVR_C photometry taken at the observatories of the Astronomical Institute of the Slovak Academy of Sciences showed that CH Cyg started a gradual brightening in the U photometric passband from 2008 October (U ~ 11.9) to the present. During our last observation from 2009 September 7 it was seen at U = 10.82. On 2009 July 25, Mukai et al. (ATEL #2046) detected a very bright hard X-ray (2-10 keV) flare, whereas our simultaneous observations showed only a small brightening from U = 11.29 on July 24.817 to U = 11.05 on July 25.925. The optical afterglow of the hard X-ray flare was detected later on July 28.099 UT, when CH Cyg peaked at U = 10.13. The ~ 2 days delay of the optical afterglow suggests particle density of a few times 10^7 cm^-3 in the emitting region ionized by the X-ray photons. The UBVR_C light curves are available at http://www.ta3.sk/~astrskop/symbphot/ch_cyg/chcyg_flare.png .

On 2008 March 17.415 and on 2009 August 27.852 UT, we obtained high-resolution spectra of CH Cyg, in the range of 6400-6700A, at the David Dunlap Observatory, University of Toronto, and the Ondrejov Observatory, Academy of Sciences of the Czech Republic, respectively. Spectra showed pronounced signatures of the red giant continuum. The hydrogen H-alpha emission was cut with a strong absorption shifted to about of -30 km/s relative to the systemic velocity, with the red peak width of only 0.8 and 1.4A at the half of its maximum that was as high as double of the local continuum. Nebular emissions of [NII] 6548 and 6584A were well pronounced on the 2008 spectrum with a maximum of the half continuum level for the 6584A line, while on the 2009 spectrum they were seen as faint features. On both spectra, their profiles were broad and flat at their tops with the width of about 3A. This evolution suggests an increase of particle density of their emitting region. The recent photometric and spectroscopic observations of CH Cyg thus indicate its transition to a new active phase. Therefore, further multifrequency observations from the ground-based and space observatories are required to understand better this intriguing symbiotic system.