57-second oscillations in Nova Centauri 1986 (V842 Cen)

Abstract

High-speed photometry in 2008 shows that the light curve of V842 Cen possesses a coherent modulation at 56.825 s, with sidebands at 56.598 and 57.054 s. These have appeared since this nova remnant was observed in 2000 and 2002. We deduce that the dominant signal is the rotation period of the white dwarf primary and the sidebands are caused by reprocessing from a surface moving with an orbital period of 3.94 h. Thus, V842 Cen is an intermediate polar (IP) of the DQ Herculis subclass, is the fastest rotating white dwarf among the IPs and is the third fastest known in a cataclysmic variable. As in other IPs, we see no dwarf nova oscillations, but there are often quasi-periodic oscillations in the range 350-1500 s. There is a strong brightness modulation with a period of 3.78 h, which we attribute to negative superhumps, and there is an even stronger signal at 2.886 h which is of unknown origin but is probably a further example of that seen in GW Lib and some other systems. We used the Swift satellite to observe V842 Cen in the ultraviolet and in X-rays, although no periodic modulation was detected in the short observations. The X-ray luminosity of this object appears to be much lower than that of other IPs in which the accretion region is directly visible.

XMM–Newton EPIC and OM observation of Nova Centauri 1986 (V842 Cen)

“…We detect a period at 3.64 ± 0.46 h which is within the errors of the ∼3.94 h orbital period (Fig. 1) estimated by Woudt et al (2009). We are not able to detect the ∼57 s white dwarf spin period in the X‐ray data.…”

“…The pulsed fraction of the 3.51‐h modulation decreases with energy as observed in the X‐ray light curves of magnetic cataclysmic variables, possibly due either to occultation of the accretion column by the white dwarf body or phase‐dependent to absorption. We do not find the 57‐s white dwarf spin period, with a pulse amplitude of 4 mmag, reported by Woudt et al in either the OM data, which are sensitive to pulse amplitudes ≳0.03 mag, or the EPIC data, sensitive to pulse fractions p ≳ 14 ± 2 per cent.…”

SPIN-UP/SPIN-DOWN MODELS FOR TYPE Ia SUPERNOVAE

“…Neutron stars (NSs), for example, can be spun up to periods of a few milliseconds (Lorimer 2008). Similarly a number of fast-spinning WDs that must have been spun up by accretion are known, for example, WZ Sge, 27.87 s (Patterson 1980); AE Aqr, 33.06 s (Patterson 1979); V842 Cen, 56.82 s (Woudt et al 2009); and V455 And, 67.2 s (Araujo-Betancor et al 2005). These periods are much longer than for NSs, due to the much higher moment of inertia of WDs, but similar to the millisecond pulsars, the surface velocity is only a factor of a few lower than the escape speed.…”

SALT observations of southern post-novae

“…Schmidtobreick et al (2005c) show the spectrum of the remnant obtained in 2003. Woudt et al (2009b) estimated the orbital period of V842 Cen to 3.…”