QPO

In the analysis of GK Per, a strange phenomenon can be observed¡Xquasi-periodic oscillations (QPO). ¡§QPOs are defined as low-coherence brightness oscillations thought to be associated with material within the inner accretion flows of CVs (Morales-Rueda, Still & Roche, hereafter MSR, 1999). Though the source can be considered periodic, its period has a certain variance. The state of the GK Per system affects the observed QPO period (PQPO) (Yi & Kenyon 1997). Various people have analyzed the optical periodicity, but their results vary greatly. For example, Patterson (1981) observed the source during quiescence in 1978, finding PQPO to be about 380 seconds. During the decline phase of GK Per in 1983, Mazeh et al. (1985) found PQPO ~360 sec, and also determined PQPO ~400 s during two nights of GK Per¡¦s outburst phase (Yi & Kenyon 1997). Watson, King & Osborne (1985) also observed a ~5000 s QPO through the analysis of X-ray data.

No consensus has been reached as to a single model for the QPO phenomenon. Theoretical models suggest dense blobs in orbit within the inner disk (Bath 1973), pulsations on the surface of the white dwarf (Papaloizou & Pringle 1978), or oscillating a

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More data is required to come to a conclusion in regard to the structure of the accretion disk. Analysis

The spectroscopy of GK Per obtained by MSR exhibit broad Balmer and HeI lines, as well as high excitation lines of HeII, NIII and CIII. With this model, both phenomena are taken into account, and no new models of QPO formation are required. The asymmetry appears periodic in the blue-shifted material, and seem to be caused by some type of absorption. coustic waves in the inner regions of the accretion disk (Okuda et al. Observations

To support their hypotheses, MSR observed the source between February 26 and 28 in 1996. By subtracting this fit from the spectra, they correlated the absorption spectra with their templates. Fitting their data with that of previous researchers, MSR calculated ƒ× = 30 „b 1 km/s, K = 119 „b 2 km/s, and ƒÖ0 = 0. Kraft (1964), Gallagher & Oinas (1974), Crampton, Cowley & Fisher (1994) and Reinsch (1994) all identify these as characteristics of a K-type secondary star. 5m Isaac Newton Telescope (INT) at La Palma, they were able to get visual spectrophotometry of GK Per. As a result, the signal will be biased toward the blue (MSR 1999). However, these models can only account for QPOs of a few hundred seconds. Since GK Per has a relatively weak magnetic field, the accretion disk will come very close to the star, completely surrounding the magnetic field. Again using the Lomb-Scargle algorithm, power spectra were produced for the three nights of observations.

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