X-ray and optical observations of BL Hydri

We acquired hard X-ray and unfiltered optical photometric data of the AM He rculis system BL Hydri. The X-ray data were obtained using the Proportional Counter Array detector of the Rossi X-Ray Timing Explorer satellite on 199 7 October 17. The optical data were obtained using the 1.5 m telescope of t he Cerro Tololo Inter-American Observatory on 1997 September 24. At the tim e of our observations, BL Hyi was in an unusual high-luminosity state with average visual magnitude m(v) approximate to 14.4 and orbital phase-average d 2-10 keV X-ray flux approximate to 1.5 x 10(-11) ergs cm(-2) s(-1), rough ly 50% larger than had been seen previously. The X-ray light curves suggest ed that two accretion poles were active. The primary X-ray emission region was extended, covering similar to 45 degrees in longitude on the white dwar f, while the secondary X-ray emission region was consistent with a point so urce with flux similar to 33% that of the primary region. The X-ray spectra were well fitted by absorbed bremsstrahlung with Gaussian emission-line mo dels; and absorbed power law with Gaussian emission-line models. For the th ermal models, the X-ray spectra were consistent with kT(X) approximate to 1 1 keV and an emission line at E approximate to 6.7-6.8 keV with equivalent width EW = 0.86-1.2 keV. The absorbed power-law models had slope alpha appr oximate to 2.1 and an emission line at E = 6.7-6.8 keV with EW = 1.1-1.5 ke V. The optical data showed, 2.7% +/- 0.2% rms quasi-periodic oscillations ( QPOs) over the frequency range 0.2-0.8 Hz modulated strongly on the orbital period suggesting that the QPOs were connected with the dominant accretion hot spot. We did not did corresponding QPOs in the X-ray emission for an u pper limit of 20% rms. There were no detections of other short-period QPOs or coherent features in either the optical or the X-ray data. The X-ray and optical data were consistent with a radiative shock model. Based primarily on the X-ray continuum and line spectrum, we infer that the mass of the wh ite dwarf in the BL Hyi system is 0.3-0.7 M., depending on the relative cyc lotron and hard X-ray luminosities.