BVRI time series observations of the 7.1 hour period in Nova Mon 2012

ATel #4803; F.-J. Hambsch (Andromeda Observatory, Mol, Belgium), T. Krajci (Astrokolkhoz observatory, Mayhill, New Mexico) & D. P. K. Banerjee (Physical Research Laboratory, Ahmedabad, India)
on 10 Feb 2013; 12:55 UT
Credential Certification: Dipankar P.K. Banerjee (orion@prl.res.in)

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We report differential time series optical photometry in the BVRI-bands of Nova Mon 2012 on 2013 February 4, 5 and 7 UT with two 35 cm telescopes (Celestron 14) at the private Astrokolkhoz Observatory in Mayhill, New Mexico using SBIG ST8 and ST9 CCD cameras with fields of view of 31 x 21 and 16 x 16 sq. arcmin, respectively. On each of these nights more than 520 images were taken in I and V band covering nearly 7h of observing time per night. In the B and R bands, so far observations have been conducted only during one night (Feb. 7). Exposure times varied from 30 seconds (V-band), 45 sec (I-band), 20 sec (R-band) and 40 sec (B-band). The AAVSO sequence for Nova Mon 2012 was used to determine absolute magnitudes using AUID 000-BKQ-000 as reference star and AUID 000-BKQ-001 as check star. The brightness of the comparison stars was comparable to the brightness of the nova.

We have confirmed the reported 7.1 hour periodicity in the light curve of the Nova Mon 2012 (ATEL #4727, #4737, #4764 and references therein). Temporal analysis of our time series data using the PDM method shows a strong and significant optical period in the I-band of P = 0.2965+/- 0.0072 days with a full amplitude of about 0.13 magnitudes. The most prominent part of the light curve is the presence of a principal minimum which is deep and sharply peaked and which repeats at the period P. There is the possibility of a second weaker minimum with a depth of just 0.03 to 0.04 magnitudes, separated from the primary minimum. The weakness of this feature necessitates further observations for its proper confirmation but the possible presence of two minima within one period is reminiscent of ellipsoidal variability. The epoch of minimum brightness deduced from our data is t0 = JD 2456305.6821, slightly different from ATel # 4737. There was no obvious decline in the overall night-to-night brightness of the nova for Feb. 4 and 5 but the magnitude dropped on Feb. 7 by about 0.07 mag. A long term light curve obtained by one of the authors (FJH) in V and I-band covering more than 140 days since the outburst of Nova Mon 2012 can be found in the AAVSO database.

An interesting new result we find is that the amplitude of the light curve variation increases with wavelength having values of about 0.06, 0.08, 0.1 and 0.13 magnitudes in the B, V, R and I bands, respectively. This could be explained by a combination of two factors. First, the companion star to the white dwarf is a cool, late type star which contributes most of its light at longer wavelengths (I-band in our case). And second, the tidally distorted Roche lobe of this component is exhibiting ellipsoidal variability. If this is indeed the case, then it would require that the orbital period be twice of the observed 7.1 hr light curve period. Further observations and analyses of the data are underway to confirm these aspects.