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\input ibvs2.sty 
 
\begin{document} 
\IBVShead{5851}{26 September 2008}
  
\IBVStitletl{The longitudinal magnetic field of the roAp star HD\,99563} 
 
\IBVSauth{Elkin, V.G.$^1$; Freyhammer L.M.$^1$; Kudryavtsev D.O.$^2$; Romanyuk, I.I.$^2$} 
 
\IBVSinst{Centre for Astrophysics, University of Central Lancashire, 
Preston PR1 2HE, United Kingdom} 
\IBVSinst{Special Astrophysical Observatory, Nizhniy Arkhyz, KChR, 
Russia} 
 
 
\SIMBADobj{HD 99563} 
\IBVStyp{  roAp  } 
\IBVSkey{spectropolarimetry} 
\IBVSabs{Based on spectropolarimetric observations, we report the first}
\IBVSabs{detection of a periodic angular variation of the longitudinal magnetic}
\IBVSabs{field for the roAp star HD 99563.} 
 
\begintext 
 
%\vspace*{-0.8cm} 
\noindent 
The chemically peculiar star HD\,99563 is an important member of the class of 
rapidly oscillating Ap (roAp) stars and its monoperiodic rapid oscillation shows 
the highest known radial velocity amplitude among these (Elkin et al. 2005). The 
rapid pulsation was discovered photometrically by Dorokhova \& Dorokhov (1998) and 
was comprehensively studied by Handler et al. (2006) in a multisite photometric 
campaign. The longitudinal magnetic field in HD\,99563 was first detected by 
Hubrig et al. (2004) and later confirmed by Hubrig et al. (2006).
 
The roAp stars as a class are in general well-described by the oblique pulsator 
model, which assumes a dipole magnetic field axis that is aligned with the stellar 
pulsation axis. It is important to observe the longitudinal magnetic field over 
the stellar rotation period for HD\,99563 to provide a geometrical model of the 
magnetic field structure. 
 
 We obtained observations of the magnetic field with the 6-m telescope BTA (Big 
Telescope Alt-azimuthal) of the Special Astrophysical Observatory in Russia. The 
observations and the data reduction were performed similarly to the procedures 
described by Kudryavtsev et al. (2006). The results of our magnetic field 
measurements together with those obtained by Hubrig et al. (2004, 2006) are given 
in Table 1.


% \vskip 1cm

 \begin{table}[ht] 
 \caption[]{The longitudinal magnetic field measurements for HD\,99563. The 
columns give:  HJD of the middle of the exposure, rotational phase using the 
ephemeris of Handler et al. (2006), the longitudinal magnetic field ${B_l}$, the 
standard deviation of $B_l$ and the observing place. The first three measurements 
were taken from  Hubrig et al. (2004, 2006) who used ESO VLT and the other six are 
our observations with 6-m BTA.   } 
\small 
\begin{center} 
\begin{tabular}{crccc} 
\hline 
   HJD (2450000+) & Phase  & $B_l$ [G] &  $\sigma$ [G] & Telescope  \\ 
\hline 
2494.483 & 0.0728 & -688  &  145   & VLT  \\
3012.749 & 0.0617 & -235  &   73   & VLT  \\
3015.727 & 0.0845 & -670  &   84   & VLT  \\
3395.550 & 0.5273 & +580  &  100   & BTA  \\
3718.580 & 0.4660 & +680  &  100   & BTA  \\
3719.553 & 0.8002 & -320  &  280   & BTA  \\
3784.569 & 0.1289 & -110  &  120   & BTA  \\
3786.526 & 0.8010 & -750  &  160   & BTA  \\
3812.523 & 0.7290 & +270  &  120   & BTA  \\
 \hline 
 \end{tabular} 
 \end{center} 
 \end{table} 


\IBVSfig{10cm}{5851-f1.ps}{Variation of the longitudinal magnetic field in 
HD\,99563 (filled circles - new observations from the 6-m telescope, triangles -
results by Hubrig et al. 2004, 2006). A best-fit sine curve is shown with the full 
line, while the dotted line is for a sine curve including the first harmonic. Both 
curves fit the observations well, but several measurements with similar phases do 
show a scatter. The differences in the two points at the phase near 0.8 may be 
explained by relatively large errors. Although the differences in the measurements 
from VLT, obtained at nearly equal rotational phases, are still not clear.} 
\IBVSfigKey{5851-f1.ps}{HD 99563}{}

We have a total of nine points spread over the rotation period. Fig.\,1 shows the 
variation of the longitudinal field with the stellar rotation period according to 
the ephemeris given by Handler et al. (2006): 
\begin{center} 
\begin{tabular}{l} 
 ${\rm HJD} = 2452031.29627 + 2.91179 E.$ 
\end{tabular} 
\end{center} 

 This rotation period is quite reliable and additionally supported by  Doppler 
imaging and line profile variations (Freyhammer et al 2008). The number of 
magnetic field measurements is not sufficient to determine the rotation period 
independently. Least-squares sine fitting was performed with the program Period04 
(Lenz \& Breger 2004), which for a pure sine curve uses: 
\begin{center} 
\IBVSeqfig{
\begin{tabular}{l} 
$B_l = B_{0} + B_{1}\sin(2\pi(\omega_{1}\,t + \phi_{1}))$. 
\end{tabular} 
}{\IBVSalteqfig{5851-e1}}
\end{center} 

The observations are illustrated in Fig.\,1 with the fitted curve shown as a solid 
line for the determined parameters: mean magnetic field $B_{0} = 21 \pm 58$\,G and 
amplitude \hbox{$B_{1}$ = 701 $\pm$ 114\,G}. Alternatively, for a least-squares sine fit 
that also includes the first harmonic, i.e., 
\begin{center} 
\IBVSeqfig{
\begin{tabular}{l} 
%\begin{equation} 
$B_l = B_0 + B_1\sin(2\pi(\omega_{1}\,t + \phi_1)) + 
 B_{2}\sin(2\pi(2\omega_{1}\,t + \phi_{2})),$ 
%\end{equation} 
\end{tabular} 
}{\IBVSalteqfig{5851-e2}}
\end{center} 
we get the curve shown with a dotted line in the figure. This fit has the magnetic 
field parameters: $B_{0}$ = 6 $\pm$ 77\,G, B$_{1}$ = 846 $\pm$ 149\,G, B$_{2}$ = 
311 $\pm$ 186\,G. The harmonic amplitude is significant only at the 1.7$\sigma$ 
level, indicating that a purely sinusoidal fit is sufficient for the present 
observations.  
 
The photometry by Handler et al. (2006) shows a double wave light variation with 
the rotation period, and maximum brightness in the $U$ and $B$ filters at phases 
0.25 and 0.75 and minimum brightness at phases 0.0 and 0.50. The variations in the 
$V$, $R$, $I$ filters are in antiphase to $U$ and $B$ and have lower amplitudes. 
This behaviour is typical for a dipole rotator where two opposing spots come into 
view over the rotation period. 
 
The minima of the $U$ and $B$ filters thus coincide with the times when one of the 
magnetic poles is closest to the line-of-sight, i.e., the times of magnetic maxima 
and minima in Fig.\,1. The fit to the magnetic measurements possibly suggests a 
minor phase offset of $\Delta \varphi_{\rm rot}=-0.046\pm0.027$ (units of 
fractional phase) as the magnetic minimum in Fig.\,1 occurs near phase 0.95, while 
the positive extremum coincides with phase 0.45. This phase difference has, 
however, only a 1.7$\sigma$ significance, and considering the uncertainties in the 
magnetic curve, more observations are needed at a higher precision. 

 
Handler et al. (2006) calculated the radius of HD\,99563 to be 2.38\,${\rm 
R}_{\odot}$. Taking into account the longitudinal field variations and the 
projected rotation velocity of $v \sin i = 28.5 \pm 1.1$\,km\,s$^{-1}$ (Elkin et 
al. 2005), the geometrical parameters for an oblique rotator model are: 
inclination angle between rotation axis and line of sight $i = 43.^\circ5$ and 
magnetic obliquity (angle between rotation axis and the magnetic dipole axis) 
$\beta = 88.^\circ4$. These are similar to those determined by Handler et al. 
(2006). 
 
 
%\vspace*{1.cm} 
 
 
\references 
 
Dorokhova T. N., Dorokhov N. I., 1998, {\it Contributions of the Astronomical 
Observatory Skalnat\'{e} Pleso}, {\bf 27}, 338 

Elkin, V. G., Kurtz, D. W., Mathys, G., 2005, {\it MNRAS}, {\bf 364}, 864 

Freyhammer, L.M. et al., 2008, in preparation 

Handler, G., Weiss, W. W., Shobbrook, R. R., Paunzen, E., Hempel, A., Anguma, S. 
K., Kalebwe, P. C., Kilkenny, D., Martinez, P., Moalusi, M. B., Garrido, R., 
Medupe, R., 2006, {\it MNRAS}, {\bf 366}, 257 

Hubrig, S., Szeifert, T., Sch{\"o}ller, M., Mathys, G., Kurtz D.~W., 2004, {\it 
A\&A}, {\bf 415}, 685 

Hubrig, S., North, P., Sch{\"o}ller, M., Mathys, G., 2006, { \it AN}, {\bf 327}, 289 

Kudryavtsev, D.O., Romanyuk, I.I., Elkin, V.G., Paunzen E., 2006, {\it MNRAS}, 
{\bf 372}, 1804 

Lenz, P., Breger, M., 2005, {\it Comm.\ in Asteroseismology}, {\bf 146}, 53 

\endreferences 
 
\end{document} 
