The disappearance and reformation of the accretion disc during a low state of FO Aquarii

Abstract

Context. FO Aquarii, an asynchronous magnetic cataclysmic variable (intermediate polar) went into a low state in 2016, from which it slowly and steadily recovered without showing dwarf nova outbursts. This requires explanation since in a low state, the mass-transfer rate is in principle too low for the disc to be fully ionised and the disc should be subject to the standard thermal and viscous instability observed in dwarf novae. Aims. We investigate the conditions under which an accretion disc in an intermediate polar could exhibit a luminosity drop of two magnitudes in the optical band without showing outbursts. Methods. We use our numerical code for the time evolution of accretion discs, including other light sources from the system (primary, secondary, hot spot). Results. We show that although it is marginally possible for the accretion disc in the low state to stay on the hot stable branch, the required mass-transfer rate in the normal state would then have to be extremely high, of the order of 10 19 g s −1 or even larger. This would make the system so intrinsically bright that its distance should be much larger than allowed by all estimates. We show that observations of FO Aqr are well accounted for by the same mechanism that we have suggested as explaining the absence of outbursts during low states of VY Scl stars: during the decay, the magnetospheric radius exceeds the circularisation radius, so that the disc disappears before it enters the instability strip for dwarf nova outbursts. Conclusions. Our results are unaffected, and even reinforced, if accretion proceeds both via the accretion disc and directly via the stream during some intermediate stages; the detailed process through which the disc disappears still requires investigation.

About the dependency of the spin maxima on orbital phase in the intermediate polar MU Cam

“…In that situation the disc-fed accretion should be lower. The disc can completely disappear as was demonstrated by Hameury & Lasota (2017) for the FO Aqr case.…”

The Rise and Fall of the King: The Correlation between FO Aquarii’s Low States and the White Dwarf’s Spin-down

“…Our analysis emphasizes two aspects of the light curve: the relationship between the overall brightness and the power spectrum, and the stability of the eclipse profile throughout the low states. Hameury & Lasota (2017) identified the latter as an important test of the fate of the accretion disk during a low state.…”

“…5 26 1   for a representative period derivative of =´-P 41 0 10  from Figure 10. We do not know where this energy is being deposited, but if it were driving a wind from the system, the resulting decrease in the accretion rate might help to explain why FO Aqr's low states correlate with the WD's spin-down, particularly in light of calculations by Hameury & Lasota (2017) that FO Aqr's high-state mass-transfer rate is only several times above the threshold at which the accretion disk would dissipate (see Section 7.2). Any such wind would likely be detectable as a line-absorption component in ultraviolet spectroscopy.…”

“…Although Murray et al (1999) proposed that tidally induced spiral structure in the inner disk could produce optical modulations at w -W 2( ) in IPs, their mechanism would be most effective in the presence of a viscous accretion disk, a scenario that the calculations of Hameury & Lasota (2017) rule out in FO Aqr's low states. Moreover, it would be difficult to explain why the spiral structure would be present in the low state but not in the high state, when the disk viscosity would be highest.…”

“…These episodes, during which the diminished accretion rate causes the system to fade by several magnitudes, are widely attributed to the passage of star spots across the secondary star near the L1 point (Livio & Pringle 1994). Low states are especially interesting in IPs because of the possibility that the accretion disk will entirely dissipate once the masstransfer rate has dropped below a critical threshold (Hameury & Lasota 2017). However, relatively few low states have been observed in IPs, with Swift J0746.3-1608 (Bernardini et al 2019) and DODra (Breus et al 2017;Andronov & Mishevskiy 2018) being two examples.…”