Astron. Astrophys. 347, L51-L54 (1999)

2. Photometry

2.1. Quiescence

Three dithered 5-min V-band exposures and 3 dithered 5-min I-band exposures (using actually a Gunn i filter) were obtained on 1999 April 9 with the imaging spectrograph EFOSC-2 at the Cassegrain focus of the ESO 3.6-m telescope at La Silla, using a Loral/Lesser CCD (#40) with 15 µ pixels, yielding a projected pixel size of 0.157". Conditions were clear, the seeing was 1" for the V frames and 0.8" for the I frames. After standard MIDAS reduction procedures, the medianed V and I frames were measured using the DAOPHOT NSTAR routine of Stetson (1987). A clean point spread function (PSF) profile was derived from three nearby isolated stars after three neighbour-removing iterations. Two comparison stars, C1 and C2, situated (12.7" N, 8.7" S) and (14" N, 25" S) respectively from Aql X-1, were used to derive the V and I magnitudes. Their Johnson-Cousins magnitudes, measured at Observatoire de Haute-Provence with the 1.2-m telescope, are V = 17.48, I = 16.12 for C1 and V = 17.42, I = 16.29 for C2 (Chevalier and Ilovaisky 1999c). We estimate the accuracy of these magnitudes to be 0.05 mag in each band.

A close-up view (10"10") in the I-band of the stars around Aql X-1 (star a) is shown in Fig. 1a while Fig. 1b shows the same image after subtraction of the PSF profile at the location of stars a, b and c, revealing the interloper star e, in addition to star d. The NSTAR fitting process, which treats the positions as input parameters to be optimized, was incomplete at this point as the subtracted image shows strong residuals, including star e. An estimate of the goodness of fit can be derived from the parameters CHI and SHARP produced by the NSTAR routine. CHI is the ratio of the observed pixel-to-pixel scatter in the fitting residuals to the expected scatter based on the values of read-out noise and gain, and should not exceed unity if the fit is good. SHARP measures the difference between the half-width at half-maximum for a star and that for the PSF and is close to zero for isolated stars. When only stars a, b and c were included in the fit (Fig. 1b), star a yielded CHI = 4.62 and SHARP = 0.027, indicating a poor fit.

Fig. 1. a  Left panel: A close-up view of our medianed I-band frame, taken with EFOSC-2 at the ESO 3.6-m telescope on 1999 April 9, showing a 10"10" field around Aql X-1 (0.157"/pixel). The FWHM for the image profiles is 0.8". Star a was previously believed to be the optical counterpart of this recurrent transient. The image is reversed with North at the bottom and East to the left. b  Second panel: The resulting image after subtraction of the cleaned third-generation PSF profile fitted to stars a, b and c. A new object, star e, is now clearly visible but the residuals indicate a poor over-all fit due to the presence of star e. c  Third panel: Here star e has been included in the fitting process and subtracted out with an immediate improvement in the residuals. Only star d remains. d  Right panel: All stars have now been removed and the residuals are very small.

Fig. 1c shows star d which is left after subtraction of stars a, e, b, c and Fig. 1d shows the cleaned image after removing the profiles fitted to a, b, c, d, and e. Inclusion of star e (Fig. 1d) in the fit yielded CHI = 0.85 and SHARP = 0.007 for both stars a and e, showing a good fit. The same procedure was applied to the medianed V frame. The final results of the NSTAR photometry are given in Table 1. The main sources of the errors given in Table 1 are the uncertainty on the absolute calibration (zero-point) of the comparison stars C1 and C2 (color terms are relatively small) and the error on the NSTAR fitting for stars e and d. These determinations are of much better quality than our previous estimates (Chevalier and Ilovaisky 1999a) based on the 1989 V-band frames which were obtained with a seeing of 1.2-1.3" and a projected pixel size of 0.33". On these frames star d was undetected and the magnitude of star e, which appeared as a faint residual after NSTAR fitting to the a, b, c group of stars, was underestimated. Under these conditions, the measured magnitude for star a, V = 19.26, is an overestimate by 0.1-0.15 magnitude due to the contamination by star e and by the surrounding objects. Measurements obtained with smaller telescopes are affected in a similar fashion, depending on the projected pixel size and seeing.

Table 1. Photometric results from PSF fitting using DAOPHOT NSTAR

2.2. Outburst

During early 1999 May, the source started a new outburst (Jain et al. 1999, Chevalier and Ilovaisky 1999b) and on May 21, near outburst maximum, we secured 1-min CCD frames in V and I with EFOSC-2. The FWHM of the image profiles was 1.1" in I. Inspection of the frames showed that the barycenter of the variable object image in outburst did not coincide with the position of star a. We analyzed the outburst frames with DAOPHOT using as a starting point the table of star positions derived by NSTAR from the quiescent frames. From the positions of eight near-by objects we find that the variable object is located -0.05 0.03" in right-ascension and -0.03 0.06" in declination from the position of star e, which is located 0.48" West of star a (Table 1). The lower signal-to-noise ratio in these short exposures did not allow accurate photometry of star a but subtraction of a PSF profile fitted to star e shows a residual compatible with star a. On these images, star e has V = 17.03 and = 1.03, with a slight contamination from a. Fig. 2 shows the same field of view as Fig. 1 with the outburst frame shown as a positive print and the quiescent frame overlaid as a negative print. The image in activity appears shifted by 0.42 0.03" to the West from the position of star a, as measured in quiescence.

Fig. 2. The 1-min I-band CCD image obtained near outburst maximum on 1999 May 21 with EFOSC-2 is shown as a positive print and the image corresponding to quiescence (Fig. 1a) has been aligned and overlayed as a negative print (field size and orientation are the same as in Fig. 1a). The seeing for the two frames was different. Note how the image of the object in outburst is clearly shifted to the West relative to the image in quiescence, showing that star e of Fig. 1b is the true optical counterpart of Aql X-1.

© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999
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