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\IBVShead{6160}{11 February 2016}

\IBVStitle{Two new Blazhko stars: XZ UM\lowercase{i} and VX S\lowercase{cl}}


\IBVSauth{Skarka, M.$^{1,2}$; Dolinsk\'{y}, J.$^3$; Jury\v{s}ek, J.$^{2,4,5}$; Ho\v{n}kov\'{a}$^{2}$, K.; Ma\v{s}ek, M.$^{2,4}$; \\Li\v{s}ka, J.$^{2,3}$; Zejda, M.$^{2,3}$}

\IBVSinst{Konkoly Observatory, Research Centre for Astronomy and
Earth Sciences, Hungarian Academy of Sciences, H-1121 Budapest, Konkoly Thege Mikl\'{o}s \'{u}t 15-17, Hungary, e-mail: marek.skarka@csfk.mta.hu}
\IBVSinst{Variable Star and Exoplanet Section of the Czech Astronomical Society, Vala\v{s}sk\'{e} Mezi\v{r}\'{i}\v{c}\'{i}, Vset\'{i}nsk\'{a} 78, Vala\v{s}sk\'{e} Mezi\v{r}\'{i}\v{c}\'{i}, 757 01, Czech Republic}
\IBVSinst{Department of Theoretical Physics and Astrophysics, Masaryk University, Kotl\'{a}\v{r}sk\'{a} 2, 611 37 Brno, Czech Republic}
\IBVSinst{Institute of Physics, Czech Academy of Sciences, Na Slovance 1999/2, 182 21 Praha, Czech Republic}
\IBVSinst{Astronomical Institute, Faculty of Mathematics and Physics, Charles University in Prague, V Hole\v{s}ovi\v{c}k\'{a}ch~2, Praha 8, 180 00 Praha, Czech Republic}

\SIMBADobjAliasJ2000{XZ UMi}{GSC 04416-00214}{14 39 51.612 +74 45 02.20}
\SIMBADobjAliasJ2000{VX Scl}{GSC 07005-00642}{01 35 23.658 -35 07 42.57}
\GCVSobj{XZ UMi}
\GCVSobj{VX Scl}
\IBVStyp{RRL}
\IBVSkey{photometry}{RR Lyrae}{Blazhko effect}

\IBVSabs{A brief report about a discovery of modulation in two RRab Lyrae stars XZ~UMi and VX Scl is}
\IBVSabs{presented. The suspicion of modulation comes from our observations, but the modulation periods}
\IBVSabs{(41.1\,d for XZ UMi and 67.3\,d for VX Scl) were estimated based on the analysis of NSVS}
\IBVSabs{(XZ UMi) and SuperWASP data (VX Scl). Both stars show indications of period change. The peaks}
\IBVSabs{close to the basic pulsation frequency of VX Scl could be the signs of possible double modulation.}

\begintext

\section{Introduction}
Amplitude and phase modulation of the light curves of pulsating stars is quite common. In RR Lyrae stars pulsating in the fundamental mode almost every other star is considered to show some kind of modulation (Jurcsik et al. 2009; Benk\H{o} et al. 2010). The nature of this phenomenon, called the Blazhko effect (Bla\v{z}ko 1907), and its observed characteristics are still poorly understood (see e.g. Kov\'{a}cs 2009, 2015, and Szab\'{o} 2014). Also the reasons why some of the stars are modulated, and some not, are not clear (e.g. Skarka 2014b).  

We give a brief note about a discovery of modulation in two RRab stars XZ~UMi and VX Scl. The targets were observed as a part of the Czech RR Lyrae Observational Project (Skarka et al. 2013; Skarka et al. 2015). In both stars we observed scattering around maximum light which is typical of Blazhko stars. Because our data were insufficient for reliable confirmation of the Blazhko effect and determination of the modulation period, publicly available data from sky surveys archives were utilized.

\section{XZ UMi}
Variability of XZ UMi was firstly noted by Wils et al. (2006) in NSVS data (Wozniak et al. 2004, 148 data points, time span of one year). We observed this star in seven nights from December 2014 to April 2015 at Masaryk University Observatory (MUO), Brno, Czech Republic, with a 24inch Newtonian telescope equipped with MII G2-4000 camera. We obtained about 450 data points in Johnson-Cousins {\it BVR$_{\rm{c}}$I$_{\rm{c}}$} filters. For the sake of clarity we give only data for $B$-filter, because the modulation is most apparent in this filter. Identification of comparison stars can be found in Table 1 and in the left panel of Fig. 1.

From the top panel of Fig. 2 it is apparent that XZ UMi is a Blazhko star with very distinct amplitude modulation. From our measurements we estimate the pulsation period as 0.58521(2)\,d (we used Period04 software, Lenz\,\&\,Breger 2005). This value is slightly different from the NSVS-based pulsation period (0.58506(2)\,d). Although both values are determined on the basis of not optimal data samples, the difference of 1.5$\cdot 10^{-4}$\,d could indicate period change. 


\begin{table}
\centering
\caption{Cross-identification of XZ UMi and comparison stars. Coordinates and magnitudes come from the UCAC4 (APASS photometry) catalogue (Zacharias et al. 2013).}
\begin{tabular}{ccccc}\hline	
	ID UCAC4    & RA(2000)		& DEC(2000)		& $V$ [mag] & $B-V$\,[mag] \\ \hline 
	\multicolumn{5}{l}{XZ UMi}\\
	824-016693 & 14 39 51.623 & +74 45 02.08 & 11.884 & 0.732 \\
	\multicolumn{5}{l}{comp star}\\
	824-016709 & 14 41 10.264 & +74 36 29.03 & 11.772 & 0.895 \\
	\multicolumn{5}{l}{check star}\\
	824-016696 & 14 40 18.613 & +74 40 37.14 & 12.032 & 1.055 \\ 
	\hline
	\end{tabular}
\end{table}
	
\IBVSfig{7cm}{6160-f1.eps}{
Identification of stars used as comparison and check stars.}
\IBVSfigKey{6160-f1.eps}{XZ UMi+VX Scl}{finding chart}


Our data are insufficient for the determination of modulation period, but in the NSVS survey data a side peak just next to the basic pulsation frequency was identified after removing the only two detectable pulsation components (Fig. 3, top panel). The separation between the basic pulsation peak (in this figure highlighted by the dashed line) and the side peak is 0.0243(1)\,c/d. Blazhko period is then 41.1(2)\,d. The detected frequencies in NSVS data set are in Table 2.

\IBVSfig{7.3cm}{6160-f2.eps}{
XZ UMi data phased with period of 0.58521(2)\,d and zero epoch 2457092.4632(9).}
\IBVSfigKey{6160-f2.eps}{XZ UMi}{light curve}

\IBVSfig{7.3cm}{6160-f3.eps}{
The top panel shows NSVS frequency spectra of XZ UMi after extraction of detectable pulsation components ($f_{0}$ and $2f_{0}$), while the bottom panels show the data phased with the Blazhko period. The location of the basic pulsation frequency $f_{0}$ is marked by the dashed line in the top panel.}
\IBVSfigKey{6160-f3.eps}{XZ UMi}{periodogram}

\begin{table}
\centering
\caption{Frequencies detected in the NSVS data of XZ UMi.}
	\begin{tabular}{llll}\hline
	\multicolumn{1}{c}{ID} & \multicolumn{1}{c}{$f$\,[c/d]} & \multicolumn{1}{c}{$A$\,[mag]} & \multicolumn{1}{c}{$\phi$\,[rad]} \\ \hline	
	$f_{0}$	&	1.70915(5)	&	0.196(7)	&	2.06(5)\\
	$f_{0}+f_{\rm b}$	&	1.7334(2)	&	0.062(7)	&	0.36(11)	\\
	$2f_{0}$	&	3.4185(1)	&	0.086(7)	&	4.92(2)	\\	  
	\hline
	\end{tabular}
\end{table}


\section{VX Scl}

This star was suspected of modulation by Skarka (2014a). We observed it in 9 nights (2014 - 2015) using FRAM (F(/Ph)otometric Robotic Atmospheric Monitor, Ebr et al. 2014) - a 300/3000~mm Schmidt-Cassegrain telescope placed near Malarg\"{u}e at the Pierre Auger Observatory, Argentina. This telescope is equipped with MII G2-1600 camera with {\it BVRI} filters. We obtained more than 300 data points in each of the filters, but we show only data in {\it V}-filter because in this filter we got the largest number of measurements around maxima. 

The scatter around maximum light suggests modulation (Fig. 4). However, our data were insufficient to certainly decide about the Blazhko effect and for determination of the modulation period. Fortunately, VX Scl was observed by the SuperWASP survey in 2006 and 2007 (Pollacco et al. 2006).

\begin{table}
\centering
\caption{VX Scl and comparison stars.}
	\begin{tabular}{ccccc}\hline	
	ID UCAC4    & RA(2000)		& DEC(2000)		& $V$ [mag] & $B-V$\,[mag] \\ \hline 
	\multicolumn{5}{l}{VX Scl}\\
	275-001600 & 01 35 23.658 & $-$35 07 42.57 & 11.928 & 0.311 \\
	\multicolumn{5}{l}{comp star}\\
	275-001595 & 01 35 01.127 & $-$35 11 01.03 & 11.587 & 0.470 \\
	\multicolumn{5}{l}{check star}\\
	274-001597 	 & 01 35 37.542 & $-$35 17 10.53 & 11.021 & 0.565 \\	  
 \hline
	\end{tabular}
\end{table}

\IBVSfig{7.3cm}{6160-f4.eps}{
VX Scl data phased with the basic pulsation period of $1/f_{0}=0.637061(1)$\,d and zero epoch 2456975.6521(5). Only data from four nights are shown in the top panel for better clarity.}
\IBVSfigKey{6160-f4.eps}{VX Scl}{light curve}

Skarka (2014a) did not analyse SuperWASP data because they did not fulfil high demands for his analysis. We removed all data points with the uncertainty higher than 0.02\,mag. Only data from camera 221 were used for frequency analysis (7712 points, time span of two years). After removing detectable pulsation components ($kf_{0}=k \cdot 1.569708(2)$\,c/d, where $k=\langle 1,7\rangle$) side peaks near $f_{0}$, $2f_{0}$\footnote{When analysing seasons separately, in the first part of the data also $3f_{0}+f_{\rm b}$ was detected.} were revealed (see the top panel of Fig. 5 where the frequency spectrum around $f_{0}$ is shown). The spacing is 0.01486(4)\,c/d which is equivalent to modulation period of 67.3(2)\,d. SuperWASP data phased with the modulation period are shown in the bottom panel of Fig. 5. The modulation envelope created from maximum magnitudes with amplitude of about 0.15\,mag is well apparent. 

\IBVSfig{7.3cm}{6160-f5.eps}{
Frequency spectrum of VX Scl based on SuperWASP data after subtraction of seven detectable main pulsation components with position of $f_{0}$ highlighted by the dashed line (top panel). The bottom panel shows SuperWASP data phased with the modulation period of 67.3\,d.} 
\IBVSfigKey{6160-f5.eps}{VX Scl}{periodogram}

The full light-curve solution for SuperWASP data is given in Table 4. Beside these peaks, additional peaks appeared near 0.001, 4, 5, and 7\,c/d when both seasons were analysed together. The peaks were not detected in the first part of the data. Therefore we suspect them from the artificial nature arising from the shifts between seasons and nights in the second part of the data  (see the bottom part of Fig. 4 from where it is apparent that the data from season 2007 have significantly larger scatter than from the season 2006).

The basic pulsation period of VX Scl is probably slowly changing which could be deduced from the bottom panel of Fig. 4 where data from both seasons are shifted in phase. In addition, bunch of peaks close to $f_{0}$ supports this hypothesis. Alternatively, we cannot exclude that VX Scl could be multi-modulated Blazhko star -- the closest nearly-equidistant peaks to $f_{0}$ indicate the modulation period of about 274\,d\footnote{No corresponding peaks were detectable near $kf_{0}$.} (peak marked as $f_{0}-f_{\rm m?}$ in Table 4). However, for more reliable analysis larger data sets would be needed, because the separation of the peaks from $f_{0}$ is only twice the Rayleigh criterion.


\begin{table}[]
\centering
\caption{Frequencies identified in the SuperWASP data of VX Scl.}
	\begin{tabular}{llll}\hline
	\multicolumn{1}{c}{ID} & \multicolumn{1}{c}{$f$\,[c/d]} & \multicolumn{1}{c}{$A$\,[mag]} & \multicolumn{1}{c}{$\phi$\,[rad]} \\ \hline	
$f_{0}$	&	1.569708(2)	&	0.3148(11)	&	4.758(1)	\\
$f_{0}-f_{\rm m?}$	&	1.56606(3)	&	0.0246(10)	&	2.55(1)	\\
$f_{0}+f_{\rm b}$	&	1.58457(4)	&	0.0161(9)	&	1.56(2)	\\
$2f_{0}$	&	3.139315(5)	&	0.1381(8)	&	6.001(1)	\\
$2f_{0}+f_{\rm b}$	&	3.15401(7)	&	0.0093(8)	&	1.17(2)	\\
$3f_{0}$	&	4.709001(5)	&	0.1074(8)	&	2.412(1)	\\
$4f_{0}$	&	6.278645(8)	&	0.0742(8)	&	3.462(2)	\\
$5f_{0}$&	7.84835(1)	&	0.0489(8)	&	0.674(2)	\\
$6f_{0}$	&	9.41799(3)	&	0.0239(8)	&	1.182(7)	\\
$7f_{0}$	&	10.98767(6)	&	0.0113(8)	&	5.57(1)	\\
	  
 \hline
	\end{tabular}
	\end{table}


\section{Conclusions}

Presented analysis showed that RRab stars XZ UMi and VX Scl are modulated with periods of 41.1\,d and 67.3\,d, respectively. The attention to these targets was brought on the basis of our original measurements. The results for both stars should be considered as preliminary estimates because both targets probably show more complex behaviour. From our figures it is apparent that the pulsation period of both stars could be variable. In VX~Scl we have a suspicion of either double modulation with additional Blazhko component with a period of about 274\,d, or of secular period change apparent in SuperWASP data. In both targets better data sets are needed for more reliable analysis.
\vskip 0.2cm
\noindent{\bf Acknowledgement:} MS acknowledges the support of the postdoctoral fellowship programme of the Hungarian Academy of Sciences at the Konkoly Observatory as a host institution. This research made use of the International Variable Star Index (VSX) database, operated at AAVSO, Cambridge, Massachusetts, USA. Support of MUNI/A/1110/2014 and LH14300 is acknowledged (JL, MZ). The operation of the robotic telescope FRAM has been supported by the grants of the Ministry of Education of the Czech Republic (MSMT-CR LG13007 and LG15014) and by the grant No. 14-17501S of the Czech Science Foundation. This paper makes use of data from the DR1 of the WASP data as provided by the WASP consortium, and the computing and storage facilities at the CERIT Scientific Cloud, reg. no. CZ.1.05/3.2.00/08.0144 which is operated by Masaryk University, Czech Republic.

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\endreferences


\IBVSedata{6160-t5.txt}
\IBVSdataKey{6160-t5.txt}{XZ UMi}{photometry}

\IBVSedata{6160-t6.txt}
\IBVSdataKey{6160-t6.txt}{VX Scl}{photometry}


\end{document}
