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Astron. Astrophys. 338, L13-L16 (1998)

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2. Observations

RX J0925.7-4758 was observed during two consecutive nights on 1997 June 7 and 8 UT with the ESO-Danish 1.54 m telescope and DFOSC equipped with the LORAL-Lesser 2K3EB-C1W7 chip. All spectra have 15 mn exposure times and on all occasions we used grism #7 with a slit width of [FORMULA] in order to accommodate the rather bad seeing ([FORMULA]) prevailing during the entire run. In addition to these spectroscopic data, several 2 mn long V band exposures were accumulated over the two nights. During the first night, 3 spectra were obtained in about 1 hour from JD start times 2,450,606.5499 to 2,450,606.5821. The 5 spectra collected during the second night span 2.8 hours from JD 2,450,607.4666 to 2,450,607.5823.

All data were reduced using standard MIDAS procedures. Two dimensional spectra were calibrated in wavelength using He + Ne arc lamps exposures closest in time to the science frame. A S/N optimized procedure extracted one dimensional spectra. The spectral range extends from 3,800 to 6,900 Å with a pixel size of 1.47 Å. Spectral resolution is degraded by the large slit width and bad seeing. FWHM resolution estimated from the minimum observed width of H[FORMULA] is about 7.3 Å.

In Fig. 1 we plot the average of the three June 7 spectra exhibiting the jet feature together with the average of the five June 8 spectra which are typical of the normal state of the source. The redshifted H[FORMULA] component unfortunately coincides in wavelength with the He i [FORMULA]6678.15 and He ii [FORMULA]6683.2 emission lines. No jet feature is seen at He ii [FORMULA]4686 which is the second brightest emission line in the observed wavelength range. Using H[FORMULA] jet lines as template, we can put an upper limit of 2.4 Å to the equivalent width (EW) of the blue He ii component. Average spectra of both nights show evidence for P Cyg profile in the central H[FORMULA] emission.

[FIGURE] Fig. 1. Mean spectra obtained on June 7 showing the jet feature plotted together with that obtained on June 8 representing the normal state of the source. The June 7 spectrum is shifted up by 3 flux units for clarity. Upper panel: H[FORMULA] region. Lower panel: No jet feature is detected around the He ii [FORMULA]4686 emission line. The N iii-C iii [FORMULA]4640-60 complex emission is also visible whereas H[FORMULA] is not detected

Fig. 2 shows that the jet velocity does not change on a time scale of one hour. Within the statistical uncertainties, the jet profile remains constant, showing a clear asymmetric extension towards low absolute velocities. During the first night, the central H[FORMULA] line exhibits shoulders moving in velocity from one spectrum to the other ([FORMULA] 20 mn). Such variations are not seen during the following night.

[FIGURE] Fig. 2. Individual 15 mn long spectra obtained on June 7. Spectra are shifted in flux for clarity. Time goes from bottom to top

We tried to correct the red jet component profile for contaminating He i and He ii lines by subtracting the mean June 8 spectrum shifted by the 44 km s-1 velocity difference. In the wavelength intervals void of sharp line features (i.e. excluding the H[FORMULA] and He i/He ii lines), we smoothed the June 8 continuum with a Gaussian of 7.3 Å FWHM, equal to the spectral resolution. This procedure preserves the best statistics and allows correction for weak water vapour absorption bands which are abundant bluewards of H[FORMULA]. The resulting `pure' jet spectrum is shown on Fig. 3. Peak intensities of both components compare remarkably well. After correction, the slight asymmetry of the raw red component profile (see Fig. 2) seems to have vanished. This could be due to changing He i/He ii line emission strength between the first and the second night. Independently of any correction error due to He i/He ii lines, the red and blue component profiles appear to have different widths, the red profile being about 8 Å (360 km s-1) less extended towards low absolute velocities than the blue one. The EW of the blue component is 4.1Å, slightly larger than those of RX J0019.8+2156 (3 Å, Becker et al. 1998) and RX J0513.9-6951 (2.6 Å, Southwell et al. 1996). In contrast, the EW of the central H[FORMULA] (4.9 Å) and He ii [FORMULA]4686 (7.8 Å) lines in RX J0925.7-4758 are significantly smaller than those of other supersoft sources exhibiting jets. The upper limit on the He ii [FORMULA]4686 to H[FORMULA] EW ratio for blueshifted components ([FORMULA] 0.6) is compatible with those observed in other sources and does not suggest a lower excitation level in the jet of RX J0925.7-4758.

[FIGURE] Fig. 3. Jet spectrum obtained by subtracting the average June 8 spectrum shifted in velocity from the average June 7 one.

The V magnitude of RX J0925.7-4758 was 17.213[FORMULA]0.016 and 17.133[FORMULA]0.026 on June 7 and 8 UT respectively. These magnitudes are within the range of those reported for the source since 1992 (Motch et al. 1994; Motch 1996) and there is therefore no evidence that the appearance of the jet is accompanied by any large change in optical continuum emission.

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© European Southern Observatory (ESO) 1998

Online publication: September 8, 1998