Astron. Astrophys. 354, L29-L32 (2000)

3. Discussion

The blue spectrum of the optical counterpart to 1WGA J1958.2+3232 is displayed in Fig. 1. The spectrum is typical of a cataclysmic variable with no obvious absorption stellar features and strong emission in all Balmer lines (down to H12). The absence of photospheric features rules out the possibility that 1WGA J1958.2+3232 is a Be/X-ray binary - see, for example, Steele et al. (1999), where it is shown that even for the Be stars with the strongest emission veiling, the photospheric features allow spectral classification to the spectral subtype.

In the spectrum of 1WGA J1958.2+3232, on the other hand, as is typical in intermediate polars, HeII  4686Å and the Bowen complex are strongly in emission. Many other HeI and HeII transitions are also in emission. The Balmer lines are all double-peaked and asymmetric with a stronger blue peak (note that the profile of H is modified by the interstellar CaII  3968Å line). The asymmetry is still stronger in the HeII lines and can be seen in the weaker HeI lines. The centroids of emission lines (determined by fitting a single Gaussian to the profile) show no displacement from the rest wavelength within the resolution achieved. The blue peaks of the HI and HeII lines are displaced by .

Fig. 2 displays H and HeI 6678Å at higher resolution. The double-peaked shape can be seen in greater detail in the H line. This is evidence for the presence of an accretion disc surrounding the white dwarf. The exact shape of the lines must depend on the orbital phase at which the observation was taken. Given that the X-ray flux is strongly pulsed and an accretion disc is present, the object must be an intermediate polar. Therefore the observed X-ray variation should represent the spin period of the cataclysmic variable or the beat period between the spin and orbital periods, since it should be an asynchronous system. The sharpness of the peaks indicates that the 25-min exposure does not represent a significative portion of the orbit (otherwise the peaks would be blurred). This is consistent with expected orbital periods of a few hours.

In the lower resolution spectrum taken two weeks later (Fig. 3), H and the HeI are single-peaked and red-dominated, indicating that the source was observed at a different orbital phase. Even though the resolution is rather lower than in the WHT spectrum, a peak separation similar to that measured in the first spectrum () should have been resolved. The interstellar NaI lines are not detectable above the noise level. Due to their weakness and the irregularity of the continuum, no diffuse interstellar bands (DIB) can be measured even in the higher resolution spectra. We set upper limits for the Equivalent Width (EW) of the DIBs at 4430Å and 6613Å as EWÅ and Å, both of which are consistent with (Herbig 1975). This is in accordance with the measurements of interstellar absorption in this direction () by Neckel & Klare (1980), who find and at 1 kpc for the two fields between which 1WGA J1958.2+3232 approximately lies.

In the WHT observations, we set the slit in such a way as to also observe the nearby star dubbed "Candidate A" by Israel et al. (1999), which is about away from the optical counterpart to 1WGA J1958.2+3232, and therefore could provide some information on the reddening in that direction. Even though Israel et al. (1999) claim that this object is an early-type star, comparison with the spectra of several stars taken from the electronic database of Leitherer et al. (1996) shows that its spectral type is F8V (see Fig. 4). We cannot see the 4430Å DIB down to the level of the many weak features in the spectrum, which gives an upper limit of Å. From the measured and the intrinsic for an F8V star (Popper 1980) we obtain the interstellar reddening . Using the relation of Crawford & Mandwewala (1976) , this implies , significantly higher than the upper limit that could be derived from the interstellar 4430Å DIB, which implies , according to the relation by Herbig (1975). Assuming for a main-sequence F8 star (Deutschman et al. 1976) and the standard reddening , this star is situated at a distance .

Fig. 4. Spectrum of the star called Candidate A in Israel et al. (1999), which is only from 1WGA J1958.2+3232. The comparison spectra correspond to HD 5015 (top, F8V) and HD 22879 (bottom, F9V) and are taken from the database of Leitherer et al. (1996).

Given its brightness, 1WGA J1958.2+3232 should be located at a distance (see Israel et al. 1998), i.e., farther away than the F8V star and therefore would have a higher reddening. If the reddening is , the soft X-ray flux could be absorbed, which would explain the relatively low of the source when compared to less distant intermediate polars (see Israel et al. 1998). We note that the interstellar lines indicate a lower reddening, but in the F8V star this estimate is also rather lower than the photometric determination of the reddening.

With a pulse period of , this system falls in between the two groups of short and long period intermediate polars defined by Norton et al. (1999), and characterised by different X-ray pulse shapes. Clearly further X-ray observations of the source are needed and either RXTE or Chandra could provide more detailed timing observations. Also, future time-resolved photometric and spectroscopic observations are needed in order to determine the orbital period and whether the observed X-ray pulsations correspond to the spin period.

© European Southern Observatory (ESO) 2000

Online publication: January 31, 2000
helpdesk.link@springer.de