Abstract
The 2006 outburst of GK Perseii differed significantly at optical and ultraviolet wavelengths from typical outbursts of this object. We present multi-wavelength (X-ray, UV and optical) Swift and AAVSO data, giving unprecedented broad-band coverage of the outburst, allowing us to follow the evolution of the longer-than-normal 2006 outburst across these wavelengths. In the optical and UV we see a triple-peaked morphology with maximum brightness ~1.5 magnitudes lower than in previous years. In contrast, the peak hard X-ray flux is the same as in previous outbursts. We resolve this dichotomy by demonstrating that the hard X-ray flux only accounts for a small fraction of the total energy liberated during accretion, and interpret the optical/UV outburst profile as arising from a series of heating and cooling waves traversing the disc, caused by its variable density profile.
Highlights
The magnetic cataclysmic variable (CV) star GK Persei (GK Per) underwent an unusual dwarf-nova-like outburst in 2006–2007
The outbursts are still believed to be analogous to normal DN outbursts, i.e. they are thought to be caused by enhanced mass transfer through the accretion disc due to thermal instability therein (e.g. Bianchini, Sabbadin & Hamzaoglu 1982; Simon 2002; Bianchini et al 2003)
In intermediate polar (IP) it is often assumed that most of the accretion luminosity is emitted as hard X-rays (e.g. Evans & Hellier 2007 showed the bolometric luminosity of the soft component to be 0.1 of the bolometric luminosity of the hard component); since the 0.3–10 keV bandpass of the X-ray telescope (XRT) covers only the hard tail of the blackbody component, the details of the soft emission are not well constrained
Summary
The magnetic cataclysmic variable (CV) star GK Persei (GK Per) underwent an unusual dwarf-nova-like outburst in 2006–2007. In outburst the modulation is strong and single peaked, whereas in quiescence it is weak and double peaked (Watson et al 1985; Norton, Watson & King 1988; Hellier, Harmer & Beardmore 2004) Modulation at this period has been seen in optical spectroscopy (Morales-Rueda, Still & Roche 1999) and photometry (Patterson 1991). Observations with Ginga (Ishida et al 1992) and EXOSAT (Watson et al 1985; Norton et al 1988) show that in hard X-rays (∼2–10 keV) the typical outburst flux is ∼10 times the quiescent flux These and RXTE observations (Hellier et al 2004) show the typical outburst 2–10 keV flux to be ∼ 2.5 × 10−10 erg cm−2 s−1. We obtained Target of Opportunity observations with Swift (Gehrels et al 2004) which began on 2006 December 20 and were repeated regularly throughout the outburst
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