The Orbital Period of the Classical Nova V458 Vul

V.P. Goranskij, N.V. Metlova (SAI, Moscow Univ.); E.A. Barsukova, A.N. Burenkov (Special Astrophysical Observatory, Russia) and V.Ya. Soloviev (Kazan' Univ., Russia)

Published in Astronomer's Telegram No.1631.

Classical nova V458 Vul (N Vul 2007 No.1) was detected as a supersoft X-ray source (SSS) by the Swift XRT several times in the time range between 2007 October 18 and 2008 June 18 (J. Drake et al., ATel #1246 and #1603). Our V photometry shows the plateau in the light curve continued since January till June 2008. This feature accompanies usually the SSS phases in some classical novae. The fragmentary monitoring during plateau shows night-to-night variability with the amplitudes between 1.2 and 0.4 mag and rapid variability by 0.1 mag in the time scale of an hour.

We performed special CCD V monitoring of this nova in 12 nights between 2008 June 25 and July 6 UT using 38-cm telescope of the Crimean Astrophysical Observatory (Ukraine) to establish the cause of this variability. A total number of observations is 612. The duration of observing set used to be of about 6 hours each night. The light decay by 0.05 mag/day suggests that plateau has finished in the middle of June. Additionally, slow light variations in the scale of few hours superimposed. The total range of variations in this period was between 14.6 and 15.5 in V band. When the light-decay trend eliminated, Fourier analysis reveals pronounced periodicity with one of possible periods, 3.34, 1.42, 0.769 and 0.587 day, which are connected each other as 1 day aliases. The last period value is chosen as a preferential one because the intra-night trends do not contradict to the general trend of the mean phase light curve. The full amplitude of periodic variations is equal to 0.4 mag, and the light curve has a saw-tooth shape. Similar light curves were observed in other SSS novae, e.g. CI Aql or V723 Cas. The chosen period was corrected using previously observed local maxima. Its refined value is 0.58946(+/-0.00018) day, and Epoch Max = JD hel 2454461.479 (+/-0.030).

Fig. 1. Amplitude spectrum of V458 Vul. The confidence levels of peaks in the spectrum are shown by horizontal lines and per cents on the right edge of the Figure.

Fig. 2. The light curve of V458 Vul folded with the period of 0.58946 day (top). The light-decay trend is eliminated to reduce data to plateau level. The light curve of a check star folded with the same period to demonstrate the accuracy of observations (middle). The averaged orbital light curve of V458 Vul (bottom).

We analysed spectra of V458 Vul taken with SAO RAS 1-m reflector on 2007 August 18.90 and October 18.68 UT (spectral range 3800-7530A, resolution 8A). The date of the last spectrum coincides with the date of the first X-ray detection by Swift. Our spectra confirm the description of spectral evolution by T.N.Tarasova (IBVS #5807): the features of FeII class nova were completely changed by ones of He/N nova in the SSS phase. No signs of high excitation Fe ion emission are seen in spectrum taken on October 18. HeII 4686A emission is blended with CIII/NIII and HeI lines, but its intensity is less than a half of Hbeta what gives the upper limit of the central source temperature of 190000K. [Fe X] 6374A emission is very faint, its detection is marginal.

Fig. 3. The light curve of V458 Vul drawn versus time. Horizontal line marks plateu, SSS - marks the first and the last detection of X-ray SuperSoft Source, digits 1-4 show the local maxima used to correct the orbital period.

Fig. 4. The spectra of V458 Vul taken on 2007 August 18 and October 18. The line identifications are given.


Dr. Albert Zijlstra (JBCA, Univ. of Manchester) reported about new time-resolved optical spectroscopic observations of V458 Vul which reveal radial velocity variations in the He II 5412 and 4686 A emission lines with the period 0.06812255 (0.00000017) day (Rodriguez-Gil et al. ArXiv:1006.1075). It is identified with the orbital period of the binary system. T(0) = 2454652.52694 (8 x 10^-5). Period 0.58946 day and its aliases determined by Goranskij et al. (ATel 1631) do not become apparent. V458 Vul is therefore the planetary nebula central binary star with the shortest period known. We try to identify this period in our photometric observations. We have two sets of extensive monitoring in the V band: JD 2454644 - 2454655 (612 obs.) and JD 2455060 - 2455073 (415 obs.). These data were cleaned for slow trends and low frequency noise. Period 0.06812255 day appears in the first set with the amplitude 0.03 mag and roughly sinusoidal light curve, and in the second set with the amplitude of 0.05 mag with a small eclipse-like dip near the phase of T(0). The averaged light curves are shown in the following Figure.

Figure. Mean light curves of V458 Vul calculated with the period of 0.06812255 day.


J.J.Drake, K.Page, J.Osborne, M.Bode, R.Corradi, P.Rodriguez-Gil, J.Drew, P.Gaensicke, D.Steeghs, C.Knigge, J.Sokoloski, ATel 1246, 2007.
J.J.Drake, K.Page, J.Osborne, A.Beardmore, J.-U.Ness, S.Starrfield, G.Schwarz, M.Tsujimoto, M.Barlow, R.Wesson, M.Bode, R.Corradi, P.Rodriguez-Gil, P.Gaensicke, D.Steeghs, C.Knigge, D.Takai, A.Zijlstra, ATel 1603, 2007.
T.N.Tarasova, IBVS No.5807, 2007.
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Download photometric data collection (in ASCII codes; JD hel., V,B,U,R in the columns of the table).