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\IBVShead{6030}{10 July 2012}

\IBVStitle{Photometric Behavior of Eight B\lowercase{e}/X-ray Binaries in the SMC}

\IBVSauth{Sarraj,~I.$^1$; Sanders,~R.J.$^1$; Schmidtke,~P.C.$^1$}

\IBVSinst{School of Earth and Space Exploration, Arizona State University, Box 871404,
Tempe, Arizona 85287 USA}

\IBVSinsto{E-mail: Ibrahim.Sarraj@asu.edu, Raymond.J.Sanders@asu.edu, Paul.Schmidtke@asu.edu}

\IBVSkey{Be stars, X-ray binaries}

\SIMBADobj{XMMU J004834.5-730230}
\SIMBADobj{CXOU J005047.9-731817}
\SIMBADobj{CXOU J005057.2-731008}
\SIMBADobj{CXOU J005245.0-722844}
\SIMBADobj{CXOU J005252.2-724830}
\SIMBADobj{CXOU J005446.2-722523}
\SIMBADobj{XMMU J010030.2-722035}
\SIMBADobj{XMMU J010435.7-722143}


\IBVSabs{We have analyzed OGLE-II and MACHO data for eight Be/X-ray binaires in the SMC with no}
\IBVSabs{previous photometric analysis.  Our goal was to detect periodic variations on several possible}
\IBVSabs{timescales: orbital outbursts and/or non-radial pulsations of the primary star.  One system}
\IBVSabs{shows optical outbursts that reoccur every 17.5 days.  This is likely to be an orbital period.}
\IBVSabs{Three other systems show low-amplitude sinusoidal pulsations with periods near one day.}

\begintext

We have examined the photometric behavior of eight Be/X-ray binaries in the SMC.  Systems
like these contain a mass-accreting neutron star in orbit with a Be star and show a variety of
optical signatures (see Schmidtke \& Cowley 2006; Coe et al.\ 2005).  For binaries in which the
neutron star pierces the Be star's envelope, recurring outbursts are observed that mark the orbital
period.  Non-radial pulsations of the Be star itself are apparent in some systems (see Balona
2010).  Our sample is listed in Table 1 and consists of  eight systems from the surveys of Antoniou
et al.\ (2009b), Haberl \& Pietsch (2004), and Laycock et al.\ (2010).  None of the X-ray sources
is known to be a pulsar.  The magnitudes of all optical counterparts are consistent
with B-type stars in the SMC.  Except for object 2 (CXOU~J005047.9$-$731817), the spectrum of
each source was discussed by Antoniou et al.\ (2009a).  They classified the stars as type B1.5e
or earlier, with strong H$\alpha$ emission.  Hence, the observed $B$$-$$V$ colors (between $-0.10$
and 0.00) imply large contributions to the light from cooler circumstellar disks surrounding the
primary stars.

\vspace{2mm}

\begin{table}[h]
\centerline{{\bf Table~1.} Source Identifications, Magnitudes, Colors, and Spectral Types}
\footnotesize
\begin{center}
\begin{tabular}{cccccccc}
Object & X-ray Source & OGLE-II & MACHO & [M2002]$^a$ & $V^a$ & $B$$-$$V^a$ & Sp.Typ.$^b$\\
\hline
\hline
1  & XMMU~J004834.5$-$730230 & smc\_sc5\_43566$^c$ & 212.15851.9 & 11182 & 14.84 & $-$0.05 & B1.5e\\
2  & CXOU~J005047.9$-$731817 & smc\_sc5\_180008 & ... & 17703 & 14.50 & $-$0.03 & ...\\
3  & CXOU~J005057.2$-$731008 & smc\_sc5\_271074 & ... & 18200 & 14.33 & 0.00 & B0.5e\\
4  & CXOU~J005245.0$-$722844 & ... & 207.16145.12$^d$ &  24501 & 14.65 & $-$0.10 & O9-B0e\\
5  & CXOU~J005252.2$-$724830 & smc\_sc6\_147662 & 208.16140.14$^e$ & 24914 & 14.19 & 0.00 & O2((f))+OBe\\
6  & CXOU~J005446.2$-$722523 & smc\_sc7\_70843 & 207.16259.37 & 31155 & 15.25 & $-$0.10 & B1e\\
7  & XMMU~J010030.2$-$722035 & smc\_sc8\_204456$^f$ & 207.16603.13 & 49014 & 14.60 & $-$0.07& B1.5e\\
8  & XMMU~J010435.7$-$722143 & smc\_sc10\_61612 & 206.16887.12 & 59680 & 15.15 & $-$0.03 & B1.5e\\
\hline
\end{tabular}
\end{center}
$^a$From Massey\ (2002)
$^b$From Antoniou et al.\ (2009a)\\
$^c$Also smc\_sc4\_178950
$^d$Also 208.16145.10
$^e$Also 207.16140.17
$^f$Also smc\_sc9\_35989
\end{table}

Photometric data for all objects were obtained from the MACHO and OGLE web sites.  Long-term
trends in the observations were removed by subtracting low-order polynomial fits from segments of
data. The flattened values were then searched for periodicities in the frequency range 0\,--\,3 day$^{-1}$,
using the Period04 analysis package (Lenz \& Breger 2005).


For sources in this study, significant
one-day aliasing is present in the Fourier spectra due to the character of the data sampling.
This problem can be further complicated for short-period signals if the fundamental frequency is itself
changing, as discussed by Schmidtke \& Cowley (2012).  They find that small variations in a pulsation
period have the effect of reducing the significance of a short-period signal while artificially increasing
the apparent significance of a long-period alias.  Hence, the highest peak in a Fourier plot might not
be the proper choice.  In selecting the most likely period for each object studied here, we considered
not only the relative strengths of peaks in the Fourier spectrum of the entire data set but also the
Fourier spectrum from each individual season, the shape of the resulting (folded) light curve, and the
agreement of data taken in different bandpasses.

Periodic signals were found in four systems.  We did not identify a meaningful signal for objects
3, 5, 7, and 8.  The maximum amplitude in each of these stars was $<$0.01 mag.  The results are
summarized in Table 2, and individual systems are discussed below.

\begin{table}[h]
\centerline{{\bf Table~2.} Photometric Periods and Amplitudes}
\small
\begin{center}
\begin{tabular}{ccccccc}
 &  \multicolumn{2}{c}{OGLE-II $I$} & \multicolumn{2}{c}{MACHO Blue} & \multicolumn{2}{c}{MACHO Red}\\
Object & Period & Amplitude & Period & Amplitude & Period & Amplitude\\
 & (days) & (mag) & (days) & (mag) & (days) & (mag)\\
\hline
\hline
1  & 1.053 & 0.037 & 1.025 & 0.022 & ... & ...\\
2  & 1.050 & 0.017 & ... & ... & ... & ...\\
4  & ... & ... & ... & ... & 17.541 & 0.042\\
6  & 0.731 & 0.015 & 0.730 & 0.008 & 0.730 & 0.010 \\
\hline
\end{tabular}
\end{center}
\end{table}

{\bf Object 1 (XMMU~J004834.5$-$730230):}
A periodic signal with P=1.053 days was found in season 1 of the OGLE-II $I$ data.  The folded
light curve, shown in Figure 1, is sinusoidal in shape with a full amplitude of 0.037 mag.  This
behavior is consistent with non-radial pulsations of the Be star.  Similar variations, with a period
of 1.025 days and an amplitude of 0.022 mag, are present in MACHO blue-bandpass data.
\newline

{\bf Object 2 (CXOU~J005047.9$-$731817):}
The high-precision position of the {\it Chandra} X-ray source is coincident with OGLE-II star
smc\_sc5\_180008, but not with emission-line object [MA93]396 (Meyssonnier \& Azzopardi 1993),
which lies $\sim$5\arcs\ to the north.  Although the X-ray spectral parameters are consistent with
a Be/X-ray binary (Antoniou et al.\ 2009b), H$\alpha$ emission has not yet been confirmed in
smc\_sc5\_180008.  A weak optical periodicity, however, is present in season 4 of the OGLE-II $I$
data for this star, as shown in Figure 2.  These variations, with P=1.050 days and an amplitude of
0.017 mag, are approximately sinusoidal and might be due to non-radial pulsations of the primary star. 
\newline

{\bf Object 4 (CXOU~J005245.0$-$722844):}
A signal with a period of 17.541 days and an amplitude of 0.042 mag is clearly present in all seasons
of the MACHO red-bandpass data.  The folded light curve, shown in Figure 3, has a non-sinusoidal
shape.  The distinct brightening might represent an orbital outburst lasting $\sim$0.4P.
\newline

{\bf Object 6 (CXOU~J005446.2$-$722523):}
Periodic variations, with P=0.731 days and an amplitude of 0.015 mag, are present in season 4 of the
OGLE-II $I$ data.  Although the amplitude is small, see Figure 4, the folded light curve is roughly
sinusoidal and might be caused by non-radial pulsations.  Similar signals were found in both
red and blue MACHO data, reinforcing this interpretation.


\IBVSfig{4.5cm}{6030-f1.eps}{Light curve of detrended OGLE-II data and Fourier spectrum for
object 1.  The data are folded with P=1.053 days, which is the period marked by $\ast$ in the
Fourier spectrum.}
\IBVSfigKey{6030-f1.eps}{XMMU J004834.5-730230}{light curve}

\IBVSfig{4.5cm}{6030-f2.eps}{Light curve of detrended OGLE-II data and Fourier spectrum for
object 2.  The data are folded with P=1.050 days, which is the period marked by $\ast$ in the
Fourier spectrum.}
\IBVSfigKey{6030-f2.eps}{CXOU J005047.9-731817}{light curve}

\IBVSfig{4.5cm}{6030-f3.eps}{Light curve of detrended MACHO data and Fourier spectrum for
object 4.  The data are folded with P=17.541 days, which is the period marked by $\ast$ in the
Fourier spectrum.}
\IBVSfigKey{6030-f3.eps}{CXOU J005245.0-722844}{light curve}

\IBVSfig{4.5cm}{6030-f4.eps}{Light curve of detrended OGLE-II data and Fourier spectrum for
object 6.  The data are folded with P=0.731 days, which is the period marked by $\ast$ in the
Fourier spectrum.}
\IBVSfigKey{6030-f4.eps}{CXOU~J005446.2-722523}{light curve}

\newpage
Acknowledgments: This paper utilizes public domain data obtained
by the MACHO Project, jointly funded by the US Department of Energy
through the University of California, Lawrence Livermore National
Laboratory under contract No.\ W-7405-Eng-48, by the National Science
Foundation through the Center for Particle Astrophysics of the University
of California under cooperative agreement AST-8809616, and by the Mount
Stromlo and Siding Spring Observatory, part of the Australian National
University.

The OGLE-II database, as described by Udalski et al.\ (1997), Zebrun et al.\ (2001),
and Szymanski (2005), was also used in this investigation.

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\end{document}

