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P. Szkody, K. Nishikida, D.K. Erb (U of WA), M. Uemura, T. Kato (Kyoto U)
YY Dra is one of the few Intermediate Polars that have dwarf nova like outbursts. Normally, this 4.0 hr orbital period close binary is at an optical brightness of 16th magnitude. Previous Ariel, HEAO and ROSAT X-ray observations showed it to be a strong 2-10 keV source, with pulses evident at 265 sec while optical light shows periodicities at 265, 275 and 550 sec. These periodicities are interpreted as due to two accreting poles on a magnetic white dwarf that is rotating with a 530s period while the 275/550 pulses are reprocessed light from structures fixed in the binary frame. The two accreting poles would have to be equally bright in X-rays to account for the observations.
In September, 1999, YY Dra underwent an outburst and rose to 10.7 mag. We obtained TOO observations with RXTE at 2, 3 and 4 days past the peak of outburst and followed the outburst with optical photometry. Comparison to quiescent observations in the RXTE archive shows that the X-ray pulse amplitude is smaller during the outburst than quiescence. This is in marked contrast to the RXTE observations of the outbursts of the IPs XY Ari and EX Hya, where the pulse amplitude dramatically increased during outburst. We discuss these results, along with the spectral changes and optical pulses, in the context of models suggested for the outbursts in IPs and the different accretion patterns in these 3 systems.
This research was partially funded by NASA grant 62-1246.