SS Cygni as a hard X-ray source identified with the scanning modulation collimator on HEAO 1

Abstract

X-RAY emission from the dwarf nova SS Cygni was first detected at soft (∼ ¼ keV) X-ray wavelengths when the source was in an optical high state1–3. More recently, ANS satellite observers4 have reported the detection of soft X-ray emission when SS Cygni was in an optical low state. These latter observations show the flux of the soft X-ray source to be about an order of magnitude less than the values when in the optical high state. The soft X-ray positional uncertainty is ∼ 0.2deg2. The correlation between optical and soft X-ray behaviour convincingly identifies SS Cygni as a soft X-ray source. Hard ( > 1 keV) X-ray emission from a 15′ × 1.5° area containing SS Cygni was also reported by the ANS observers during the optical low state and identified with SS Cygni4. A more accurate position (0.03 deg2) has been reported by Ariel 5 observers5. We report here a 0.4 arc min2 measurement of the position of the X-ray source with the HEAO 1 modulation collimator experiment which leads to an unequivocal identification of SS Cygni as an X-ray emitter in the 2–10 keV energy range. We have also searched the Uhuru data to investigate the historical light curve in hard X rays.