BVRI CCD Photometry of Two New Variable Stars in the Field of Seyfert 1 Galaxies Mrk 478 and Mrk 1513
V.T. Doroshenko
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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
Crimea, Ukraine
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Article in PDF |
photometry is presented
for two new variable stars in the fields of the Seyfert galaxies
Mrk 478 and Mrk 1513. The new variables change their brightness
periodically, with the periods
1. Introduction
In the course of studying comparison stars in the fields of several Seyfert galaxies, we found two variable stars. One of them is in the field of Mrk 478, and the other one is located in the field of Mrk 1513. For accurate photometry of galaxies, it is important to search for possible variable stars among the reference-star candidates selected in the AGN field in order to reject them as comparison stars. That is why we perform a study of stars in the fields of those AGNe which we have been observing at the Crimean Astrophysical Observatory from 2002 to the present time.
2. Observations and reductions
The CCD monitoring of Seyfert galaxies was performed in the prime focus of the 70-cm telescope of the Crimean Astrophysical Observatory using the Ap7p CCD camera. This camera has a
-pixel chip. The field of view of our images is
. The scale of images is 1
755 per pixel. We
used the 
, 
, 
, 
, 
filters. The transformation of
our instrumental magnitudes to the standard Johnson-Cousins
system was made using Stetson's Standard fields where star
magnitudes are given in the 
system (Doroshenko et al.
2005a). It should be noted that the filter is more similar to
the filter in the Cousins system. As a rule, for each object
we made 4-6 series of exposures per night with each filter, while
before and after that we obtained a single image to determine the
bias and two images to find the dark current, with the largest
exposure for the filter. At least 4-6 flat-field (FF) images
for each filter were usually obtained in twilight at sunset or
sunrise for each change in the CCD setup at the telescope. The
images for bias, dark current, and FF were cleared from spikes and
then used in the standard procedure of CCD-image processing. All
our photometric measurements were made by aperture photometry with
respect to the star selected as a standard in the field of the
galaxy. We used a circular aperture of 15
diameter to
measure the signal from an object. The sky background was usually
determined around each object in an annulus aperture. We took into
consideration errors associated with photon statistics, with
readout noise, with correlated changes in the bias, and errors
associated with the distance between the measured object and the
comparison star. The stars in the field of the two galaxies in
question were calibrated earlier by Doroshenko et al. (2013).
3. Mrk 478, star 4
In the field of the Seyfert 1 galaxy Mrk 478 (14
42
07
47,
+35
26
22
9, J2000, 2MASS), we found a variable star
No. 4 (USNO-A2.0 1200-07369079). It is circled in the finding
chart (Fig. 1). Its coordinates are
,
(J2000.0, UCAC4). The galaxy Mrk 478
is marked by two bars; the comparison and check stars are
numbered.
 |
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 |
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 |
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
4. Mrk 1513=II Zw 136, star 9
Mrk 1513 is a well-known Seyfert 1 galaxy (21322782,
+100819
3, J2000, 2MASS). The finding chart for the
star No. 9 and other stars selected for photometry is shown in
Fig. 5. The coordinates of the star No. 9 (GSC 1123-00250) are
,
(J2000.0, UCAC4).
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.
5. Conclusion
We found two new variable stars that change their brightness periodically. The star No. 4 in the field of Mrk 478 varies with the period, and the star No. 9 in the field of
Mrk 1513 varies with the period
. It is possible
that these stars belong to the class of close binary eclipsing
systems.
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
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Fig. 5.
The finding chart for the selected stars in
the field of Mrk 1513. The size of the chart is |
 |
Fig. 7.
The phased light curves for the star
No. 9 in the field of Mrk 1513, plotted with the period
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.