Peremennye Zvezdy (Variable Stars) 33, No. 5, 2013
Received 09 July; accepted 17 July.
Southern station of the Moscow
M.V. Lomonosov State University, Moscow, Russia, Nauchny, 98409
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Fig. 1. The finding chart for the comparison stars in the field of Mrk 478. The size of the chart is , north is at the top, east is to the left. The variable star No. 4 is circled, the galaxy is marked by two bars.
The magnitudes of the selected stars are given in Doroshenko et al. (2013). Our photometry of stars in the Mrk 478 field was made relative to the star No. 1 with the aperture , in the course of 32 nights from January 04/05, 2012 to July 19/20, 2012. As a rule, 4 images in each filter were obtained on each observing night, the time resolution for this galaxy being about 4.35 minutes. We used the criterion to find variable stars. The value per one degree of freedom (dof) was calculated from the light curves of the selected star. If the star under consideration is not variable, then is close to 1. For variable stars, should be significantly larger than 1. We concluded that the star No. 4 was a variable star with a high confidence level because per dof was 13.1, 16.4, 21.7, 11.6, and 10.1 in the bands, respectively, whereas the stars Nos. 1-3, Nos. 5-8 had , and they can be used as check stars as well. The star No. 1 was calibrated using 4 nights in 2012 against the main reference stars in the fields of 14 different AGNe. Next, we calculated the unweighted mean values for these different calibrations. The mean magnitudes for the star No. 1 from 29 measurements are the following: , , , , and . Table 1, available electronically in the html version of this paper, presents the photometry of the star No. 4 in the Johnson-Cousins system. The mean magnitude and color indices of the star No. 4 are the following: , , , , , , and . This color index is characteristic of the F8-G0 spectral type if the star belongs to the main sequence and if the extinction in the direction of the star is rather small. The star can have an earlier spectral type if the extinction is appreciable. The amplitude of variability, derived from the averaged phased light curves, slightly decreases from to : Amp , Amp , Amp , and Amp . Figure 3 shows the phased light curves for the star No. 4, plotted for the double period, .
In addition to the observations made in Crimea, data for this star are also available in the 2013 release of the Catalina Sky Survey (Drake et al. 2009). The light curve in the CSS is obtained from observation without filter. However, to obtain more accurate values in the Johnson band from , the authors of the survey give transformation formulae depending on the color indices and ; I applied them. Photometry of the star No. 4 in the field of Mrk 478 from the Catalina Sky Survey is given in Table 2, available electronically in the html version, where Modified Julian Dates were transformed to Julian Dates and , to standard magnitudes using the the formula , according to CSS prescripts.
The combined light curve (CSS + Crimean data) in the band is shown in Fig. 2, where the CSS data are plotted in red color and the Crimean data, in black color. The agreement between these data at close times is very good.
Fig. 2. The -band light curve of the star No. 4, based on the Catalina Sky Survey (open circles with red error bars) and Crimean (open circles with black error bars) observations.
The Fourier analysis of the light curve of this star revealed a peak at the frequency cycles/day ( ). Figure 3 displays the Fourier spectrum for the -band photometry of the star No. 4.
Fig. 3. The -band power spectrum for the combined (CSS+Crimean) photometry of the star No. 4 in the field of Mrk 478.
The particular type of variability is difficult to determine from the photometric data only, without any spectroscopic information. However, taking into account the variability period and amplitude and the shape of the phased light curves for the period , we can suppose that the star No. 4 may belong to eclipsing variable stars, namely, to the W UMa type of variable stars. In this case, the phased light curves plotted with the double period can give us a more definite information about the variable star type. Such curves for the star No. 4, with the period day, are presented in Fig. 4. The phased light curves correspond to the elements: . The phased light curve in the band shows a slight difference in the depths of brightness minima and resembles light curves of close binary eclipsing systems.
Fig. 4. The phased light curves for the star No. 4 plotted with the period . In the band, the CSS and Crimean data were used. Only Crimean data are presented in the other bands.
The stars selected as the comparison star candidates are numbered. Their magnitudes are presented in Doroshenko et al. (2013). The photometry of stars in the Mrk 1513 field was made relative to the star No. 3 with the aperture , in the course of 82 nights in 2010-2012. The comparison star No. 3 was calibrated during 5 good (in the photometrical sense) nights in 2010. Thirty-six estimates of its brightness were made with respect to the main stars in 12 different galaxies, which were observed on the same nights as Mrk 1513. The magnitudes of the comparison star No. 3 are the following: , , , and . The magnitudes of other stars were also computed relative to the star No. 3 using all observational nights. The mean magnitudes of the star No. 9 are , , , , ; the mean color indices are , , . The color indices of this star are in general agreement with the spectral type F2-G0, if this star belongs to the main sequence and if the extinction in the direction of the star is small enough.
The Crimean photometry of the star No. 9 is presented in Table 3, available electronically in the html version of this paper.
The values calculated for light curves of the star No. 9 are significantly higher than 1: , , , and , compared to other selected stars, which have . This fact indicates the variability of the star No. 9. The amplitudes of variability of the star No. 9 are the following: Amp , Amp , Amp , and Amp . The uncertainties were computed using a Monte-Carlo simulation.
The light curves of the star No. 9 tend to fainter brightness with time. This is clearer seen in the band. I have found that the star No. 9 changes its brightness periodically. The Fourier analysis of the light curves of this star revealed the maximum peak at the frequency cycles/day ( ); Fig. 6 demonstrates this power spectrum. It should be noted that the points in 2012 correspond to the minimum brightness on the phased light curve. It is quite possible that the depth of minimum varies with time.
Just as in the case of the star No. 4 in the field of the galaxy Mrk 478, discussed above, it is also difficult to determine accurately the variability type of the star No. 9. However, if we suppose that the star is in a close binary system, then the phased light curve plotted for the doubled period, (Fig. 7), shows that the depth of the first and second minima are different, but the heights of the maxima are approximately the same. This fact may indicate that the star belongs to the W UMa type with the elements . Further studies are required to investigate the stationarity of the orbit or to find out whether there are any systematic changes of the minima depths in time.
Acknowledgements. I thank the anonymous referee for helpful remarks, Drs. V.P. Goranskij and A.A. Slyapnikov for useful discussion. This work was supported by the grant 12-02-01237-a from the Russian Foundation for Basic Research.
Drake, A.J., Djorgovski, S.G., Mahabal, A., et al. 2009, Astrophys. J., 696, 870
Doroshenko, V.T., Sergeev, S.G., Merkulova, N.I., et al. 2005a, Astrophysics, 48, 156
Doroshenko, V.T., Sergeev, S.G., Merkulova, N.I., et al. 2005b, Astrophysics, 48, 304
Doroshenko, V.T., Sergeev, S.G., Efimov, Yu.S., et al. 2013, Astrophysics, in press
Fig. 5. The finding chart for the selected stars in the field of Mrk 1513. The size of the chart is , north is at the top, east is to the left. The variable star No. 9 is circled; the galaxy is marked by two bars.
Fig. 7. The phased light curves for the star No. 9 in the field of Mrk 1513, plotted with the period .