Astron. Astrophys. 338, 947-956 (1998)

4. Results from spectroscopy

Some preliminary results of our spectroscopy have been presented by Van Teeseling et al. (1996b). Although our spectra have a much lower spectral resolution than those of C97, we will discuss them again in some more detail in the context of our photometric results, and because they appear to be significantly different, with more prominent Balmer absorption, than those presented by C97.

4.1. Average spectrum

The average spectrum is shown in Fig. 7. Using only the spectra obtained during the photometric first two nights, we derive an average magnitude of and a colour of , which are consistent with the values derived from our photometry.

Fig. 7. Average flux-calibrated spectrum of 1E 0035

The average spectrum shows clear He II emission with an equivalent width of Å and a radial velocity of km s^-1. It is possible that there is also very weak He II and O VI emission. H is in absorption with an average equivalent width of Å and a radial velocity of km s^-1. In the average spectrum there is no clear evidence for Balmer absorption lines other than H. The He II emission and the H absorption are consistent with the spectra obtained by S96 and C97, except for the much higher mean radial velocities.

The FWHM of the normalized average He II is Å, which is comparable to the spectral resolution. The FWHM of the normalized average H is Å. The phase-resolved spectra and the results of C97 show that this non-zero intrinsic width of H is the result of a variable radial velocity and of a significant intrinsic broadening in some spectra.

We do not find any evidence for H emission as observed by C97. Since the equivalent width of the H+He II blend in the spectra of C97 is significantly larger than that of He II , we would have detected similar H emission, even with our much lower spectral resolution. Clearly, the phase-averaged optical spectrum of 1E 0035 is variable, which may be the result of a variable amount of irradiation. Indeed, C97 mention that 1E 0035 was somewhat brighter during their spectroscopic run, than during earlier observations.

4.2. Phase-resolved spectra

The average flux level of the 4 spectra in the phase interval from -0.12 to 0.13 () is lower by about than that of the 5 spectra in the phase interval from 0.25 to 0.43 (), which is consistent with the photometric orbital modulation. For both averaged spectra we find .

Although H is the only clear absorption feature in the average spectrum, some of the spectra, as shown in Fig. 8, also show other Balmer lines (including H) in absorption. The Balmer absorption lines seem to be more prominent near optical orbital minimum, i.e. when the companion star is closest to us. Also C97 found that H absorption appeared to be strongest near minimum light.

Fig. 8. Flux-calibrated spectra of 1E 0035 near optical minimum

To increase the signal-to-noise ratio, we have combined the three spectra taken in the first night (with phases 0.05, 0.29, and 0.46) with the first three spectra taken in the second night (with phases 0.10, 0.28, and 0.46). We have normalized each spectrum by fitting a spline function to the continuum and dividing the spectrum by this spline. Fig. 9 shows all normalized spectra and the normalized average spectrum in the wavelength region containing He II and H: both the He II emission and the H absorption are variable in strength and wavelength.

Fig. 9. Normalized spectra at different photometric phases. The top panel shows the normalized average spectrum. The dotted lines give the average wavelengths of the He II emission and the H absorption

We have measured the radial velocity of He II and H in each of the 9 normalized spectra by cross-correlating the spectra with the normalized average spectrum and by fitting Gaussian profiles to the lines. The He II radial velocity shows a large variability of km s^-1, but we could not correlate this variability with the photometric orbital phase. The H radial velocity, however, shows a 99% significant correlation with the photometric orbital phase, and has the blue-to-red zero crossing close to minimum optical light (Van Teeseling et al. 1996b). Better spectra are required to confirm the reality of this correlation between the H radial velocity and the orbital phase. The semi-amplitude of a sine fit to the H radial velocity is km s^-1. The average radial velocity of H, corrected for the earth's motion, is km s^-1. This is significantly higher than the mean heliocentric radial velocity of the SMC of km s^-1 (with a spread of km s^-1; see Westerlund 1997 and references therein) and the mean H (emission) velocity of -88 km s^-1 measured by C97.

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
helpdesk@link.springer.de