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Detection of Photometric Modulation on the Orbital Period in the Eruption Tail of the Recurrent Nova T Pyx

ATel #3782; A. Oksanen (Caisey Harlingten Observatory) and B. E. Schaefer (Louisiana State University)
on 26 Nov 2011; 17:13 UT
Credential Certification: Bradley E. Schaefer (schaefer@lsu.edu)

Subjects: Optical, Nova

Referred to by ATel #: 4452

We report on the detection of photometric modulation with the orbital period for the recurrent nova T Pyx, now in the tail of an eruption (IAUC # 9205 , ATel #3549, ATel #3647). Our V-band observations are made with the 0.5-m telescope at the Caisey Harlingten Observatory near San Pedro de Atacama in Chile with 30-second time resolution (see ATel #3707). We have corrected our photometry for an airmass effect where the strong line emissions visible in the T Pyx spectrum have small differential effects with respect to the comparison stars. We have also subtracted off a smooth trend from the light curve. The result is a flat light curve with RMS equal to 0.010 mag in early September and 0.007 mag in recent weeks. We have 3157 V-band magnitudes from JD 2455815 to 2455885.

We detect a highly significant periodic signal at the orbital period. The sinusoidal modulations started around JD 2455815 (2011 Sep 10). Close examination of the best fit period at ten day intervals shows no significant variation in the period or epoch of minimum. With this, we can fit a sine wave to the entire interval from JD 2488815 to 2455885. Our best fit period is 0.076228 ± 0.000003 days, the time of minimum light is JD 2455850.0251 ± 0.0007, and the best fit peak-to-peak amplitude is 0.0048 ± 0.0003 mag.

Our observed period is almost exactly the pre-eruption orbital period as determined just 40 days before the start of the current eruption (0.07622916 ± 0.00000008 days; Schaefer et al. 2011, arXiv:1109.0065). The epoch of the minimum light is almost exactly that expected from the pre-eruption timing data. The high significance plus the independent recovery of very close to the pre-eruption period and phase both provide strong confidence that the detected modulation is genuine. The observed periodicity is very stable since the 1970's, and it is certainly tied to the orbital period as known from the radial velocity curve (Uthas, Knigge, and Steeghs 2010, MNRAS, 409, 237).

The start time of the periodic modulation (around day 149 after the start of the eruption) is close to the start of the X-ray plateau in the Swift X-ray light curve (ATel #3647, ATel #3707), when the region near the white dwarf has become unveiled. This proximity of the source of the periodic modulations to the white dwarf suggests that the mechanism is associated with a magnetic field on the white dwarf. In this case, the rotation period of the white dwarf must be exactly tied to the orbital period, which is to say that T Pyx is a polar system. The only alternative is that the periodic modulation is somehow tied to the accretion structure in the system, but this seems implausible given that the inclination of T Pyx is 10±2 degrees (Uthas et al. 2010).

T Pyx also shows unstable photometric periodicities away from the stable orbital period (Patterson et al. 1998, PASP, 110, 380). The display of multiple photometric periodicities has always been taken to mean that the white dwarf has a high magnetic field, for examples amongst novae, see RW UMi (Tamburini et al. 2007, A&A, 464, 697), V4633 Sgr (Lipkin & Leibowitz 2008, MNRAS, 387, 289), V4745 Sgr (Dobrotka et al. 2006, MNRAS, 371, 459), V4743 Sgr (Kang et al. 2006, AJ, 132, 608), V697 Sco (Warner & Woudt 2002, PASP, 114, 1222), and V1495 Aql (Retter et al. 1998, MNRAS, 293, 145). The existence of high magnetic fields in the V1500 Cyg stars (including T Pyx, V1500 Cyg, RW UMi, and V4633 Sgr) is required to channel the accretion onto a polar cap such that the nuclear burning can be sustained so as to irradiate the companion star and raise the post-eruption level of accretion (Schaefer & Collazzi 2010, AJ, 139, 1831). In all, the photometric modulations seen in T Pyx are strong evidence that its white dwarf has a very high magnetic field.