Solving the Kilo-Second QPO Problem of the Intermediate Polar GK Persei
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Mon. Not. R. Astron. Soc. 000, 1–14 (1999) Solving the kilo-second QPO problem of the intermediate polar GK Persei L. Morales-Rueda1, M. D. Still2 and P. Roche1 1Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QJ ([email protected], [email protected]) 2Physics and Astronomy, University of St. Andrews , North Haugh, St. Andrews, Fife KY16 9SS ([email protected]) Accepted 1999 January 20. Received 1998 December 16; in original form 1998 July 29. ABSTRACT We detect the likely optical counterpart to previously reported X-ray QPOs in spec- trophotometry of the intermediate polar GK Per during the 1996 dwarf nova outburst. The characteristic timescales range between 4000–6000s. Although the QPOs are an order of magnitude longer than those detected in the other dwarf novae we show that a new QPO model is not required to explain the long timescale observed. We demon- strate that the observations are consistent with oscillations being the result of normal- timescale QPOs beating with the spin period of the white dwarf. We determine the spectral class of the companion to be consistent with its quiescent classification and find no significant evidence for irradiation over its inner face. We detect the white dwarf spin period in line fluxes, V/R ratios and Doppler-broadened emission profiles. Key words: accretion, accretion discs – binaries: close – line profiles – stars: cataclysmic variables – stars: individual: GK Per – X-rays: stars. 1INTRODUCTION stability within an accretion disc (Osaki 1974). GK Per out- bursts have been modelled as such by Cannizzo & Kenyon GK Per (Nova Per 1901; Campbell 1903), belongs to a sub- (1986) and Kim, Wheeler & Mineshige (1992). group of cataclysmic variables (CVs) called Intermediate Polars (IPs). In these systems an asynchronously-rotating, This paper is a continuation of paper i (Morales-Rueda, magnetic white dwarf accretes material from a less-massive, Still & Roche 1996), in which we presented spectrophoto- late-type companion filling its Roche lobe. Gas leaving the metric observations of GK Per taken on the rise to its 1996 companion star attempts to form an accretion disc around outburst (Mattei et al. 1996). We reported the detection the primary star but its magnetic field either prevents the of quasi-periodic oscillations (QPOs) within the Doppler- formation of the disc or truncates it near the white dwarf. broadened emission lines of H i and He ii. This provides GK Per was identified with the X-ray source A0327+43 an opportunity to map the velocity structure of the oscilla- by King, Ricketts & Warwick (1979) and confirmed as an IP tions. QPOs are defined as low-coherence brightness oscil- by the detection of a 351 s X-ray spin pulse by Watson, King lations thought to be associated with material within the & Osborne (1985; hereafter WKO) and Norton, Watson & inner accretion flows of CVs. Theoretical models developed King (1988). The same period was subsequently found in to explain QPOs consider the presence of dense blobs of ma- optical photometry by Patterson (1991). GK Per has the terial orbiting in the inner regions of the accretion disc (Bath longest orbital period from the sample of known CVs, Porb 1973), or non-radial pulsations over the surface of the white = 2 d, (Crampton, Cowley & Fisher 1986; hereafter CCF). dwarf (Papaloizou & Pringle 1978), or radially-oscillating The wide binary separation combined with a relatively weak acoustic waves in the inner disc (Okuda et al. 1992; Godon magnetic field ( 1 MG) means that a truncated accretion 1995). In these models, the QPO timescales match observa- disc must be present∼ if current theories of disc formation tions of dwarf novae and are of the order of a few hundred are correct (Hameury, King & Lasota 1986). The presence seconds. However the QPO periods detected in GK Per are of a disc has yet to be confirmed by direct observation, al- an order of magnitude longer than this. Previous to this though the system does undergo dwarf nova outbursts every paper they have only been detected in X-ray data taken 2–3 years where its optical brightness increases from 13th to during outbursts; WKO discovered them in 1.5–8.5 keV EX- 10th magnitude (Sabbadin & Bianchini 1983). The most- OSAT data at the peak of the 1983 outburst, while Ishida likely mechanism for dwarf nova outbursts is a thermal in- et al. (1996) report a second detection at 0.7–10 keV with 2 L. Morales-Rueda, M. D. Still, P. Roche Table 1. Journal of observations. E is the cycle number plus binary phase with respect to the ephemeris given by Crampton, Cowley & Fisher (1986). Phases have been adjusted by π/2so that phase 0 corresponds to superior conjunction of the white dwarf. Date Start End Start End No. of (1996 Feb) (UT) (E 2 000) spectra − 26 20.06 0.12 617.119 617.204 109 27 20.04 0.07 617.620 617.704 116 28 20.06 0.04 618.121 618.204 117 sure times and resolution of the data were already described in paper i. After debiasing and flat-fielding the frames by tungsten lamp exposures, spectral extraction proceeded according to the optimal algorithm of Horne (1986). The data were wave- length calibrated using a CuAr arc lamp and corrected for instrumental response and extinction using the flux stan- Figure 1. The visual light curve of GK Per during the dard HZ 15 (Stone 1977). The spectrograph slit orientation 1996 outburst obtained from the Variable Star Network of PA 249.1◦ allowed a 15th magnitude nearby star approx- (http://www.kusastro.kyoto-u.ac.jp/vsnet/). The arrows indi- cate the times of our spectrophotometric observations. imately 0.5 arcsec ENE of GK Per to be employed as cali- bration for light losses on the slit. We also have available to us spectroscopy of various K- ASCA during the rise to the 1996 outburst discussed in this type stars from 1995 October 11 to 13 obtained from the paper. INT and from 1995 October 30 to November 2 with the To explain the long timescales WKO suggested the 2.1 m telescope in the McDonald Observatory in Texas. The QPO mechanism is caused by beating between the 351 s INT instrumental setup was identical to the one used for the 1996 observations described above. For the McDonald white dwarf spin period and inhomogeneous gas orbiting at the inner edge of the accretion disc. Hellier & Livio (1994; data, the low-to-moderate resolution spectrometer ES2 was hereafter HL) noted that the X-ray hardness ratio varies employed in conjunction with the TI1 CCD and a grating ruled at 1200 lines mm 1 covering the wavelength region over the QPO cycle as expected from photoelectric absorp- − ˚ ˚ 1 tion by cool gas and that a period of a few thousand sec- λ4196 A–λ4894 A giving a resolution of 200 km s− at Hβ. onds is consistent with the orbital frequency of gas if it is The spectra were flat-fielded, optimally extracted and deposited onto the disc by a gas stream which has partially wavelength calibrated also in the standard manner. Flux avoided impacting the outer disc rim and follows a ballistic calibrations were applied using observations of the standards trajectory. They propose that the QPO mechanism is X- HD19445 (Oke & Gunn 1983) and Feige 110 (Stone 1977) for ray absorption by vertically-extended blobs of gas orbiting the October and November data respectively. Table 2 gives a list of the K-type templates observed over both runs. at this preferred inner impact radius. In paper i we deter- mined that the characteristic velocity structure of the optical counterpart to the QPOs observed by Ishida et al. (1996) is consistent with blobs in the inner disc. In the current paper 3RESULTS we present further analysis which indicates that the optical QPO is also driven by absorption, but favours strongly a 3.1 Average spectra beat model over the disc-overflow interpretation. Fig. 2 presents the average of all the data collected on 1996 Feb 28. It is characterised by a flat continuum, broad Balmer and He i lines in emission, high excitation lines of He ii,Niii and C iii and numerous faint, narrow absorption features of 2 OBSERVATIONS Fe i,Cai,Tiii and Sr ii that had been identified as signa- Between 1996 February 26 and 28, 6–8 days before the tures of the K-type secondary star by Kraft (1964), Gal- ASCA pointings of Ishida et al. (1996), we obtained spec- lagher & Oinas (1974), CCF and Reinsch (1994). trophotometry of GK Per using the Intermediate Dispersion We employed K star spectral templates to determine Spectrograph mounted on the 2.5 m Isaac Newton Telescope which luminosity class best matched the secondary star in (INT) on La Palma. Table 1 gives a journal of observations. this system during outburst and search for signatures of in- In Fig. 1 we show a visual light curve obtained by the Vari- creased X-ray irradiation. Using CCF’s fit to the orbital ra- able Star Network during the 1996 outburst, with arrows dial velocity of the secondary star we shifted out the orbital indicating the days on which we made observations. The motion of the absorption lines with a quadratic rebinning quick readout mode was used in conjunction with a Tek- algorithm. We binned in velocity the spectra of GK Per and tronix CCD windowed to 1024 150 pixels to reduce dead the K-type templates to ensure that they all had identical time and obtain good sampling× of the spin cycle.