We obtained several spectra of Aql X-1 in quiescence with the imaging-spectrograph EFOSC-1 at the Cassegrain focus of the ESO 3.6-m telescope. They were taken through the B300 grism and cover the range between 3850 Å and 6850 Å. Two spectra were obtained during the night of 19 May 1988 with 7 Å resolution (using CCD #11) and exposure times of 3600s and 4200s. Two more spectra were obtained one year later on 8 May 1989 with 3.5 Å resolution (using CCD #8) and exposure times of 3600s and 2700s. The second 1989 spectrum was underexposed and will not be discussed further. Weather conditions were good throughout, with seeing of 1.1" and 1.3" respectively, and we used a slit width of 1.5". Spectra of bright stars of known spectral type from mid-GV to early MV (G3: HD 168402, G5: G 724.1, K0: HD 171982, K3: HD 87521, K7: G 747.3, M0: CD -36o6589) were also taken during the 1988 run. All frames were wavelength-calibrated using He-Ar lamp spectra and flat-fielded using internal Tungsten lamp spectra and were reduced using standard MIDAS procedures. The spectra were extracted with the MIDAS long-slit package and corrected for atmospheric extinction using average values for La Silla. The standard star LTT 7987 (Stone and Baldwin 1983) was used for relative flux calibration. The three spectra of Aql X-1, which did not show any significant differences, were then co-added. Since our observed G8 V star (CD -45o12143) turned out to be of a K0 type, we constructed a synthetic G8 V spectrum by averaging our G5 and K0 spectra.
The slit included both stars a and e. As determined using Table 1, the flux coming from star e is 12% of the total flux from stars a and e in the V-band. To derive the spectral type of star a, we constructed a set of template spectra by adding, to each of our observed spectra of known spectral type, normalized to 0.88 at 5500 Å, the observed K7 or M0 spectrum, normalized to 0.12 at 5500 Å (see Table 1 and also the Discussion). We applied different corrections for interstellar reddening to the (a+e) spectrum, corresponding to from 0.3 to 0.6 and normalized the de-reddened (a+e) spectra to unity at 5500 Å. This assumes that both stars are reddened by the same amount (see Discussion). We then compared the set of de-reddened observed (a+e) spectra to the set of template spectra. The template spectra including an M0 component were rejected since they exhibited residual TiO bands which do not appear in the observed (a+e) spectrum and thus are not shown here. The absorption features of the observed (a+e) spectrum, in particular the G-band of CH (4290-4314 Å), are only compatible with a late G or early K type for star a.
Fig. 3 shows four different template spectra (K3+K7, K0+K7, G8+K7 and G5+K7) (grey curves) together with the average (a+e) spectrum de-reddened with = 0.3, 0.45, 0.50 and 0.60, values selected to give a best fit to the template spectra. The depth of the Mgb band at 5175 Å (which was the only late-type feature in the first quiescent spectrum published by Thorstensen et al. 1978) and of the TiO band at 4954 Å increase with later spectral types and are too strong in the K3+K7 combination. The (a+e) de-reddened energy distributions for = 0.60, 0.50 and 0.45 match almost equally well the G5+K7, G8+K7 or the K0+K7 composite templates, respectively, although there are differences at the blue and red ends.
In Fig. 4 we show the result of subtracting the synthetic G8 spectrum, assigned to star a, from the (a+e) spectrum, de-reddened for = 0.50. This should approximate the spectrum of star e. Also shown (grey line) is our observed K7 V spectrum, normalized to 0.12 at 5500 Å. The main features of this difference spectrum are the emission lines at H, H (weaker) and Ca II K and H(+H), which appear strong although the signal-to-noise ratio below 4500 Å in our spectra is low. Emission at H is barely detectable and no emission is detected at HeII 4686 Å. The slight deficit between 4400 and 5000 Å may be of instrumental origin.
Our spectrum is different from that taken by Garcia et al. (1999) when the sum of Aql X-1 plus star a was more than half a magnitude above quiescence (V = 18.68), and which displays a richer emission-line spectrum, including HeII 4686 Å, a consequence of X-ray heating.
© European Southern Observatory (ESO) 1999
Online publication: June 6, 1999