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\input ibvs2.sty

\begin{document}
\IBVShead{4537}{12 December 1997}

\IBVStitle{OPTICAL PROPERTIES OF HD\,154791 DURING X-RAY OUTBURSTS}

\IBVSauth{L.~TOMASELLA$^1$, U.~MUNARI$^1$, T.~TOMOV$^2$, D.~KOLEV$^2$, M.~MIKO{\L}AJEWSKI$^3$, M.~REJKUBA$^4$}
\vskip 0.3cm plus 0.1cm minus 0.1cm

\IBVSinst{Asiago Astrophysical Observatory, I-36012 Asiago (VI), Italy}
\IBVSinst{NAO Rozhen, P.O. Box 136, 4700 Smolyan, Bulgaria} 
\IBVSinst{Centre for Astronomy, Nicholaus Copernicus University, ul.
Gagarina 11, 87-100 Toru\'n, Poland}
\IBVSinst{Faculty of Physics, University of Zagreb, Bijeni\v{c}ka 32, 10000
          Zagreb, Croatia}
 

\IBVSabs{Photometric and spectroscopic observations of HD 154791 show}
\IBVSabs{that the strong X-ray outburst of October-November 1997} 
\IBVSabs{had no effect on the optical spectroscopic and photometric}
\IBVSabs{properties of the star.}

\SIMBADobj{HD154791}
\IBVStyp{ * }
\IBVSkey{photometry, spectroscopy}


\begintext

Garcia et al. (1983) identified the {\it Ariel V} X-ray source 2A~1704+241
($\equiv$\,4U~1700+24 $\equiv$\,3A~1703+24) with the bright star HD\,154791.
They argued for the object to be a binary in which the M giant is orbited by
a neutron star. According to Garcia et al. the main characteristics of
HD\,154791 are: i) significant variations in the X-rays on time scales from
minutes to years; ii) lack of detectable continuum in the IUE SWP spectra
and presence of C~{\sc iv} 1550\,\AA\ and N~{\sc v} 1238\,\AA\ in emission;
iii) absence of significant variations in optical and infrared photometry;
iv) the M giant optical spectrum is not contaminated by emission lines
tracing the presence of the hot companion.

Simultaneous X-ray, IUE and optical observations in March 1985 were reported
by Dal Fiume et al. (1990). They confirmed the identification of 2A~1704+241
with HD\,154791 and noted some changes in comparison to the observations of
Garcia et al. (1983). Dal Fiume et al.'s X-ray observations are characterized
by a much softer
spectrum and by an overall decrease of the source intensity by a factor of
$\sim$6. The high excitation UV emission lines of C~{\sc iv} and N~{\sc v}
had disappeared.

Recently Remillard (1997, announcement circulated through the VSNET-Alert
electronic service) reported about an X-ray outburst of HD\,154791. During the
first two weeks of November 1997 the flux at 2-10\,keV reached a historical
maximum of about 35\,mCrab.

Being one of the very few binaries supposed to harbor an M giant and a
neutron star, HD\,154791 is in on our long-term observing program. A few days
before and a few ones after Remillard's announcement of the X-ray outburst
we secured high-resolution CCD spectra of HD\,154791 with the
Coud\'e-spectrograph of the 2\,m telescope of the National Astronomical
Observatory Rozhen (resolution $R\sim$\,0.35\,\AA) and with the Echelle
($R\sim$\,0.3\,\AA) spectrograph of Padova \& Asiago Astronomical
Observatory 1.82 m telescope. With the Boller\,\&\,Chivens
($R\sim$\,18\,\AA) spectrograph at the same telescope we secured also low
resolution spectrophotometry. Photometric observations have been obtained
with the UBVRI photoelectric photometer at the 60\,cm Cassegrain telescope
of Torun Observatory (Poland).  SAO\,84893 and $\delta$\,Her have been used
as comparison and check star respectively. The intrinsic errors in the BVRI
bands do not exceed 0\fmm01 and for the U data they are typically between
0\fmm02 and 0\fmm04. The photometric data and spectra closest in time to the
X-ray outburst are presented in Table~1 and Figure~1 respectively. A few
additional spectroscopic data are given in Table~2.

\vskip 5mm
\IBVSfig{22cm}{4537-f1.ps}{
The H$\alpha$ region in the high-resolution
spectra of HD\,154791, obtained on 17 October 1997 (Rozhen) and 15 November
1997 (Asiago), i.e. soon before and near maximum of the X-ray outburst. 
The spectra are normalized to the local continuum level ({\it
two upper panels}). Flux calibrated low-resolution spectrum of HD\,154791
obtained on 20 November 1997 (Asiago) ({\it bottom panel})}
\vskip 5mm

The main result of the present investigation is that the X ray
outburst, still ongoing at the time of our November observations, had no
detectable effect on the optical properties of HD\,154791.

No changes in the high resolution spectra are visible, and no emission line
appeared in the H$\delta$--H$\alpha$ region. The numerous absorption lines
visible in the spectra of Figure~1 are normal in M giants and are produced
by neutral metals. The radial velocities in Table~2 confirm the Garcia et
al. (1983) report of undetectable orbital motion, either caused by low
orbital inclination or by large orbital separation. The H$\alpha$ equivalent
width values presented in Table~2 are constant within the measurement
errors. The overall aspect of the low resolution spectrum in Figure~1 is
still and always that of an M giant.

The photometric data show that there are no variations in the BVRI bands,
even if variability in the U band is present. Our data seem to ascribe the
latter to a sort of flickering activity. Our pre-X-ray-outburst photometry
shows the star about 0\fmm26 fainter in B and $\sim$0\fmm14 brighter in V in
comparison to Garcia et al. (1983).

\vskip 5mm

\centerline{Table 1. UBVRI photometric observations of HD\,154791}
\vskip 3mm
\begin{center}
\begin{tabular}{lcccccc}
\hline
Date & JD\,2400000+ & V & U$-$B & B$-$V & V$-$R & V$-$I \\
\hline
16 Aug 1997 &50677.474 & 7.64 & 2.13 & 1.73 & 1.35 & 2.68 \\
17 Aug 1997 &50678.377 & 7.67 & 1.66 & 1.74 & 1.36 & 2.71 \\
19 Aug 1997 &50680.391 & 7.66 & 1.82 & 1.76 & 1.36 & 2.63 \\
20 Aug 1997 &50681.425 & 7.67 & 1.29 & 1.72 & 1.37 & 2.65 \\
25 Aug 1997 &50686.355 & 7.64 & 1.76 & 1.74 & 1.37 & 2.62 \\
21 Oct 1997 &50743.264 & 7.65 & 1.68 & 1.76 & 1.33 & 2.64 \\
\hline
\end{tabular}
\end{center}

\vskip 5mm

\begin{center}
{Table 2. The radial velocity of the absorption lines and the
equivalent width of H$\alpha$ measured in our high-resolution spectra}
\vskip 3mm
\begin{tabular}{lcccc}
\hline
Date & JD & RV$_{\odot}$ & W$_{\lambda}$ \\
&2400000+ & km\,s$^{\rm -1}$ & \AA \\
\hline
1 Apr 1996 &50175.493 & $-43.3 \pm0.4$ & 0.97  \\
3 May 1996 &50207.402 & $-51.8 \pm0.5$ & 1.00  \\
3 Jun 1996 &50238.395 & $-43.5 \pm0.4$ & 1.01  \\
17 Oct 1997 &50739.178 & $-46.7 \pm0.5$ & 0.98  \\
15 Nov 1997 &50768.216 & $-49.9 \pm0.5$ & 1.04  \\ 
18 Nov 1997 &50771.193 & $-52.8 \pm0.4$ & 1.08  \\
\hline
\end{tabular}
\end{center}

\vskip 5mm

\vskip 5mm
{\it Acknowledgments:\/} This research was supported by the Bulgarian NFSR
Grant No. F\,--\,574/1995 and Polish KBN Grant No.2 P03D 011 13. This
paper was prepared while T.~Tomov was Visiting Professor at the Asiago
Astrophysical Observatory unit of the Italian National Research Council
(CNR).

\references

Dal Fiume D., Poulsen J.M., Frontera F., Manzo G., Re S., Robba N.R.,
Amadori G., Bartolini C., Guarnieri A., Piccioni A., 1990, {\it Nuovo
Cimento}, vol. 13 C, 481

Garcia M., Baliunas S.L., Doxsey R., Elvis M., Fabbiano G., Koenigsberger
G., Patterson J., Schwartz D., Swank J., Watson M.G., 1983, {\it ApJ}, {\bf
267}, 291 

\end{document}

