2 Observations

   

Table 1: Observation log and inferred longitudinal magnetic field of $\eta$ Aql obtained from LSD profiles. HJD is the heliocentric Julian Date, S/N is the peak signal-to-noise ratio in the reduced spectrum, $N_{\rm LSD}$ is the mean noise level in the associated LSD profile, and $\ensuremath {B_{\rm z}}\pm \sigma _{\rm B}$ is the inferred longitudinal magnetic field and its standard error. Phases are according to the pulsation ephemeris ${\rm JD} = 2450100.861 + 7.176726 \cdot E$ of Kiss & Vinkó (2000).

HJD	Pulsation	S/N	$N_{\rm LSD}$	$\ensuremath {B_{\rm z}}\pm \sigma _{\rm B}$
-2 450 000	phase		(%)	(G)
2101.436	0.759	760	$3.6\times 10^{-3}$	$6 \pm 4$
2101.480	0.765	760	$3.6\times 10^{-3}$	$-7 \pm 4$
2101.518	0.770	680	$4.0\times 10^{-3}$	$0 \pm 5$
2101.548	0.774	650	$4.3\times 10^{-3}$	$3 \pm 5$
2101.578	0.779	620	$4.6\times 10^{-3}$	$4 \pm 5$
2101.608	0.783	660	$4.2\times 10^{-3}$	$-8 \pm 5$
2101.638	0.787	620	$4.5\times 10^{-3}$	$-8 \pm 5$
2102.558	0.915	860	$3.3\times 10^{-3}$	$-9 \pm 5$
2102.603	0.921	790	$4.5\times 10^{-3}$	$-13 \pm 4$
2102.647	0.928	710	$4.1\times 10^{-3}$	$-14 \pm 5$
2103.405	0.033	1050	$2.3\times 10^{-3}$	$-7 \pm 3$
2103.445	0.039	1040	$2.3\times 10^{-3}$	$-6 \pm 3$
2103.554	0.054	1060	$2.3\times 10^{-3}$	$-6 \pm 3$
2103.655	0.068	750	$3.6\times 10^{-3}$	$-1 \pm 4$

Figure 1: Averaged observed (crosses and error bars) and synthesised (smooth line) Stokes I (lower) and V (upper) spectra of $\eta$ Aql, obtained on 10-11 July 2001 UT (pulsation phase $\sim$0.8). The calculated spectrum corresponds to a longitudinal magnetic field of -70 G, as expected at this phase based on Plachinda's results. Various predicted Stokes V Zeeman signatures which should be easily detectable in individual spectral lines in this region are clearly absent in the observed spectrum. Asymmetries in the observed Stokes I spectrum are due to the pulsation velocity field, which is accounted for kinematically in the model (as per Donati et al. 2001). We have employed $v\sin i=13.3$ km s$^{\rm -1}$, $\xi =4$ km s$^{\rm -1}$, a solar abundance table and 14.92 km s$^{\rm -1}$ and 7.29 km s$^{\rm -1}$ for the first- and second-order Fourier coefficients of the pulsational radial velocity variation. Note that Stokes V has been magnified $10\times$ and shifted up by +1.05 for display purposes.

During the nights of 2001 July 10-13 UT we obtained 14 new Stokes V spectra of $\eta$ Aql using the MuSiCoS spectropolarimeter at Pic du Midi observatory. To check the instrument performance, we also obtained 3 observations (1 per night) of the magnetic Ap standard $\alpha ^2$ CVn. The spectra span a total range of 450-660 nm, with a typical resolving power of 35 000. Details of the instrument, and of the observing and reduction procedures are provided by Wade et al. (2000a). The observation log is reported in Table 1.