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ATEL # 1631; 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)
on 25 Jul 2008; 10:57 UT
Password Certification: Vitaly Goranskij (firstname.lastname@example.org)
Subjects: Optical, Binaries, Nova, Variables
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 = JDhel 2454461.479 (+/-0.030).
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.
Light Curves and Spectra