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The peculiarities of TW Pictoris

Andy Norton (OU)

We present the results of a long ROSAT HRI observation of the candidate intermediate polar TW Pic. The power spectrum of the X-ray lightcurve shows no sign of either the previously proposed white dwarf spin period or the proposed binary orbital period (1.996hr and 6.06hr respectively). In the absence of a coherent X-ray pulsation, there is no evidence to support the suggestion that TW Pic is an intermediate polar.

We also present UBVRI photometry of TW Pic which show that on one occasion it faded by ca. 4 magnitudes, when compared with its `normal' brightness. This measurement was simultaneous with the ROSAT All-Sky Survey observation of TW Pic, during which the X-ray flux was also reduced in comparison with our later X-ray observation.

TW Pic lies close to the critical dividing line in the mass-loss/orbital-period plane which separates thermally stable systems (permanent superhumpers and nova-like variables) from those that are thermally unstable (SU UMa and U Gem stars). Using this evidence and re-interpreting previously published analyses, we suggest that TW Pic might display permanent negative superhumps with a period of about 6.06 hr and that its orbital period is around 6.36 hr. Under this interpretation, we show that TW Pic follows the same relationship between superhump period excess and orbital period as the rest of the negative superhump systems. The observed period of 1.996hr is then the second harmonic of the superhump period, shifted by about 1%, as is seen in several other superhump systems. We also suggest that TW Pic may be a VY Scl star (or `anti-dwarf nova'). If these suggestions are confirmed, TW Pic would be the longest period cataclysmic variable to display superhumps, and also the longest period VY Scl star.

Soon to be re-submitted to MNRAS; preprints from anonymous ftp site:
physics.open.ac.uk/pub/astro/ajnorton/twpic.ps

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