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\IBVShead{5776}{1 June 2007}

\IBVStitle{H\boldmath{$\alpha$} Observations of the Galactic Microquasar LSI+61\boldmath{$\deg$}303}

\IBVSauth{Zamanov, R.K.; Stoyanov, K.A.; Tomov, N.A.}

\IBVSinsto{Institute of Astronomy, Bulgarian Academy of Sciences, Tsarigradsko shosse Blvd.\
72, 1784 Sofia, Bulgaria\\ e-mail: rkz@astro.bas.bg; kstoyanov@astro.bas.bg}

\SIMBADobjAlias{V615 Cas}{LSI+61303}

\IBVSkey{variable stars, spectral variability}

\IBVSabs{We present results of spectroscopic observations of the Be/X-ray}
\IBVSabs{binary LSI+61303 during the period January 2000-April 2007.}
\IBVSabs{The signs of 4.5 years modulation are visible in H-alpha,}
\IBVSabs{even during the time of lower equivalent width of H-alpha).}

\begintext

LSI+61\deg303 (V615 Cas, GT0236+610) is a Be/X-ray binary star at a distance of 2.3 kpc
(Steele et al., 1998) and with radio outbursts every 26.496 d (Gregory, 2002, and references
therein) which is assumed to be the orbital period.  The variable radio counterpart of the
system was resolved at milliarcsecond scales as a rapidly processing relativistic compact jet
(Massi et al., 2004), so LSI+61\deg303 joined the group of Galactic microquasars.  It is also
a variable $\gamma$-ray source (Albert et al., 2006). The compact object is probably a black
hole orbiting around a Be star in a highly eccentric orbit (Casares et al., 2005).  Spectral
observations show that the H$\alpha$ emission is variable on time scales days-years (see
Grundstrom et al., 2007, and references therein). Here we present the results of our H$\alpha$
spectroscopy during the period January 2000--April 2007.

We have secured 53 spectra with the Coud\'e spectrograph of the 2-m RCC telescope at the
Bulgarian National Astronomical Observatory Rozhen and Photometrics AT200 CCD{}.  The
wavelength coverage is from 6500 \AA\ to about 6700 \AA\ at resolution 0.2 \AA/pixel.  For
each spectrum, we have measured the equivalent width (EW)  of the H$\alpha$ emission line and
the separation between the blue and red humps ($\Delta V$). The measured quantities are given
in Table 1. The typical error of our measurements is $\pm$10\% in EW, and $\pm 10$ km~s$^{-1}$
in $\Delta V$.

In Fig.~1 (left panel) we show a few examples of the H$\alpha$ line. From up to down are
plotted our spectra 20000127, 20000820, and 20000623. In all our spectra the H$\alpha$ line is
in emission with two peaks and EW(H$\alpha$) is always $>8$~\AA.  We have not observed a third
peak in the emission, as visible in the September 2001 observations of Liu \& Yan (2005), nor
very weak emission in H$\alpha$ as detected by Grundstrom et al.\ (2007) at JD2451468.

In Fig. 1 (right panels)  we plot the long-term variability of EW(H$\alpha$) and $\Delta V$.  
We also use data from Paredes et al.\ (1991), Zamanov et al.\ (1999, 2001), Liu \& Yan (2005),
and Grundstrom et al.\ (2007). EW(H$\alpha$) achieved values $\approx18$~\AA\ during the two
prominent maxima at JD2448800 and at JD2450000. It seems that there are three minima of
EW(H$\alpha$) at about JD2449200, JD2451200, and JD2453270, when EW(H$\alpha$) was $\sim
7$~\AA.  During the last 2000 days, there is not a prominent maximum. After JD2451000, the
EW(H$\alpha$) is always $<14$~\AA. We see a clear minimum in $\Delta V$ at JD2451900, when
$\Delta V$ dropped to $\Delta V \le 280$~km~s$^{-1}$, values similar to those observed during
the maximum of EW(H$\alpha$) at JD2450000.

The distance between the blue and red peak ($\Delta V$) is connected with the outer size of
the H$\alpha$ emitting disk: $\Delta V /( 2 v \sin i)=(R_{\rm out}/R_{*})^{-1/2}$ for a
Keplerian disk (Huang, 1972).  Adopting for a typical B0 star radius $R_*=10~R_\odot$, and $v
\sin i=360$~km~s$^{-1}$ (Hutchings \& Crampton, 1981), we obtain that $R_{\rm out}$ varies
from 3.7~$R_*$ (37~$R_{\odot}$) to 7.7~$R_*$ (77~$R_{\odot}$).  These values are in the range
1.2--100~$R_*$ as derived by Hanuschik et al.\ (1988) in other Be stars.
 
It deserves noting that the sudden drop on 1 week scale of the EW(H$\alpha$) observed by
Grundstrom et al.\ (2007) at JD2451468 is not accompanied with dramatic changes in $\Delta V$,
indicating that the disk size does not change on such time scale.

Using the PDM method (Stellingwerf, 1978), we did not detect a clear periodicity in H$\alpha$
line parameters in the interval 200--3000 days.  However, when we plot the data folded with
the radio period $P=1667$~days (Gregory, 2002) we see that the modulation is clearly visible
(Fig.~2).  All of the data (and the subsets of data when EW${}<12$~\AA\ and EW${}\ge12$~\AA)  
show signs of the 1667 day modulation in EW(H$\alpha$) and $\Delta V$.  The maximum of
EW(H$\alpha$)  and the minimum of $\Delta V$ are at phase $0.25\pm0.10$. At the minimum
$\Delta V \approx 260$~km~s$^{-1}$, and at phase 0.75 it achieves $\sim370$~km~s$^{-1}$.

{\small
\begin{center}
Table 1. Parameters of the H$\alpha$ line in the spectrum of LSI+61\deg303 
\begin{tabular}{ccrc|ccrc}
\noalign{\medskip}
\hline
date      & JD & EW(H$\alpha$)& $\Delta V$ & date & JD & EW(H$\alpha$) & $\Delta V$ \\ 
yyyymmdd  & 2400000+  &  [\AA] & [km~s$^{-1}$] & yyyymmdd  &2400000+ &  [\AA] & [km~s$^{-1}$] \\
\hline       
20000127 & 51571.29 & 13.1  & 351 & 20010206 & 51947.40 &  8.8  & 307 \\
20000621 & 51717.48 &  8.6  & 325 & 20010207 & 51948.29 &  9.7  & 307 \\
20000621 & 51717.51 &  8.3  & 338 & 20010208 & 51949.29 & 10.3  & 299 \\
20000621 & 51717.52 & 10.0  & 313 & 20010317 & 51986.23 &  9.7  & 281 \\
20000623 & 51718.50 &  7.8  & 325 & 20010317 & 51986.24 & 10.2  & 294 \\
20000623 & 51718.51 &  8.8  & 401 & 20010407 & 52007.24 & 10.5  & 319 \\
20000623 & 51718.52 &  9.6  & 338 & 20010709 & 52100.57 & 11.8  & 345 \\
20000623 & 51719.48 &  8.1  & 313 & 20010727 & 52118.53 & 10.8  & 256 \\
20000623 & 51719.50 &  8.5  & 363 & 20010903 & 52156.44 & 13.4  & 332 \\
20000623 & 51719.51 &  9.5  & 338 & 20010904 & 52157.36 & 12.4  & 332 \\
20000817 & 51774.38 &  9.4  & 313 & 20011003 & 52186.55 & 12.0  & 331 \\
20000817 & 51774.39 &  9.0  & 300 & 20020123 & 52298.34 &  9.2  & 280 \\
20000818 & 51775.39 &  9.4  & 275 & 20020622 & 52448.54 &  8.6  & 332 \\
20000818 & 51775.40 &  9.5  & 288 & 20020624 & 52450.55 &  9.7  & 357 \\
20000819 & 51776.39 & 10.4  & 325 & 20021020 & 52568.46 & 11.0  & 332 \\
20000820 & 51777.39 & 10.3  & 325 & 20021112 & 52591.45 & 10.3  & 306 \\
20000820 & 51777.40 & 12.3  & 300 & 20030717 & 52838.58 &  9.3  & 390 \\
20000821 & 51778.35 &  9.8  & 300 & 20030718 & 52838.58 & 11.8  & 362 \\
20000821 & 51778.36 & 10.2  & 325 & 20031205 & 52979.46 & 11.4  & 312 \\
20000822 & 51779.35 & 10.3  & 338 & 20031208 & 52982.39 & 12.4  & 349 \\
20000822 & 51779.36 & 11.0  & 338 & 20031208 & 52982.40 & 12.8  & 375 \\
20000917 & 51805.50 & 10.1  & 338 & 20041001 & 53280.50 & 10.6  & 300 \\
20000917 & 51805.52 & 10.5  & 351 & 20060116 & 53752.40 & 12.4  & 361 \\
20001205 & 51884.43 & 11.4  & 332 & 20061202 & 54072.46 & 10.0  & 300 \\
20001206 & 51885.48 & 11.8  & 299 & 20070401 & 54192.25 & 10.6  & 337 \\
20001208 & 51887.43 & 10.2  & 281 & 20070402 & 54493.24 &  9.5  & 340 \\
20010204 & 51945.32 & 12.0  & 332 & \\
\noalign{\medskip}
\hline
\end{tabular}
\end{center}
}

\IBVSfig{10cm}{5776-f1.eps}{Profiles of the H$\alpha$ emission line in the spectrum of
LSI+61\deg303 (left panel). Long-term variability of the EW(H$\alpha$) and $\Delta V$ (right
panels).  Squares indicate the previous data, and crosses indicate our new observations}
 \IBVSfigKey{5776-f1.eps}{LSI+61303}{spectrum}

\IBVSfig{9cm}{5776-f2.eps}{Variability of the EW(H$\alpha$) and $\Delta V$ folded with period
$P=1667$ days. Filled squares indicate EW(H$\alpha$)${}\ge12$~\AA, open squares indicate
EW(H$\alpha$)${}<12$~\AA}
 \IBVSfigKey{5776-f2.eps}{LSI+61303}{}


Possible origins of 4.5 year modulation are:
 \begin{itemize}
 \item(1) precessing relativistic jet (Gregory et al., 1989);
 \item(2) quasi-cyclic Be star envelope variations (Gregory et al., 1989);
 \item(3) precession of the Be star (Lipunov \& Nazin, 1994);
 \item(4) outward-moving density enhancement in the equatorial disk (Gregory \& Neish, 2002);
 \item(5) variability of the Be star mass loss;
 \item(6) variability of the size of the circumstellar disk.
 \end{itemize}

The circumstellar disks in Be/X-ray binaries are truncated by the gravitational influence of
the compact object (Okazaki \& Negueruela, 2001). Very likely in LSI+61\deg303 a precession of
the Be star leads to variations of the truncation radius, which combined with variable
mass-loss rate of the Be star, creates the 4.5 year modulation in H$\alpha$ and radio
emission.


To conclude, the main results of our spectral observations of the Be/X-ray binary and galactic
microquasar LSI+61\deg303 are:
 \begin{itemize}
 \item{\bf (i)} In our observations the equivalent width of the H$\alpha$ emission line varied
from 8 \AA\ to 14 \AA.
 \item{\bf (ii)} The separation of the H$\alpha$ peaks varied from 250 to 400 km~s$^{-1}$.
 \item{\bf (iii)} The signs of 1667~day modulation are visible in the H$\alpha$ parameters,
even during the time of lower EW(H$\alpha$).
 \end{itemize}

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