2 Observations

2.1 Spectroscopic observations

On May 23, 2000, three spectra (10 min exposure each) were obtained with the ByuFOSC-2 focal reducer (Movsessian et al. 2000) attached to the prime focus of the Byurakan Observatory 2.6-m telescope; the "green" grism (4200-6900 Å) was used giving a dispersion of 2.7 Å pix^-1. On May 25, another spectrum (15 min exposure) was obtained with the "red'' grism (5400-7600 Å) giving a dispersion of 2.1 Å pix^-1. The detector was a $1060\times514$, $24\times24~\mu$m pixel Thomson CCD. The spectra were flux calibrated using the standard star BD+28$^\circ$4211 (Stone 1977). The continuum is very blue; several emission lines are present, the strongest being H$\alpha$, H$\beta$, He II $\lambda$4686, He I $\lambda$6678 and the "Bowen blend" emission feature at $\sim$4645 Å.

During the period 22-28 September 2000 more detailed spectroscopic observations were carried out with the CARELEC spectrograph (Lemaître et al. 1989) attached to the Cassegrain focus of the OHP 1.93-m telescope. We used a 600 lmm^-1 grating resulting in a dispersion of 66 Å mm^-1. The detector was a $1024\times2048$, $13.5\times13.5~\mu$m pixel EEV 42-20 CCD. Exposure times were generally 10 min. Seven columns of the CCD ($\sim$3 $^{\prime\prime}$) were extracted. The slit width was 2 $.\!\!^{\prime\prime}$1, corresponding to a projected slit width on the detector of 52 $\mu$m i.e. 3.8 pixels. The resolution, as measured on the night sky lines, was $\sim$3.4 Å FWHM. The wavelength range was 4280-6100 Å. The spectra were wavelength calibrated using an Ar lamp and flux calibrated using the standard stars Feige15, BD+28$^\circ$4211 and EG247, taken from Oke (1974) and Stone (1977). To obtain accurate velocity measurements, we applied wavelength corrections by measuring the [O I] $\lambda$5577 night sky line on each spectrum (the corrections were in the range -1.3, -0.4 Å).

The profiles of all emission lines are complex and variable. Some H$\beta$ profiles have a flat top, others display a shoulder while a few show a clear central absorption; we derived radial velocities using our own software, as described in Gonçalves et al. (1999), by fitting each profile with a main emission component and a minor unresolved Gaussian absorption (see Sect. 5). This procedure derives velocity information essentially from the line flanks. The radial velocities for the main emission component vary between -205 and +125 kms^-1.

Figure 1: Finding chart for RX J1643.7+3402. This is a 30-s $R_{\rm c}$ exposure taken on September 23, 2000 with the OHP 1.2-m telescope. The field of view is $6\times 6$ arcmin. The variable is indicated by two tick marks. Magnitudes and colors for numbered field stars are given in Table 2. Stars 2 and 3 were used as comparisons. The bright star (V=6) at left is HD 151087.

Figure 2: Mean spectrum of RX J1643.7+3402, obtained at the OHP 1.93-m telescope with the CARELEC spectrograph, showing the blue continuum, approximately $F_{\lambda } \propto \lambda ^{-2.7}$, and the main emission lines discussed in the text (the total exposure time is 387 min).

 

 

Table 1: Equivalent widths of the main emission lines as measured on the integrated spectrum.

Line	EW (Å)
H$\gamma$	1.6
Bowen Blend	2.0
He II $\lambda$4686	2.5
He I $\lambda$4713	0.12
H$\beta$	2.8
He I $\lambda$4921	0.33
He I $\lambda$5016	0.32
C IV $\lambda$5804	0.62

We coadded all our spectra (see Fig. 2). The resulting spectrum shows, in addition to the lines seen on the low resolution BAO spectra, several He I lines (4388, 4471, 4713, 4921, 5016 and 5875 Å) and an additional He II line (5411 Å); there is also a weak, broad ($\sim$33 Å FWHM) C IV $\lambda$5804 emission line (this is in fact a doublet at 5801.5 and 5812.1 Å). Table 1 gives the equivalent widths (EW) of the main emission lines measured on this spectrum. Blends of atmospheric water vapor absorption lines are present between 5680 and 5800 Å and between 5870 and 6000 Å.

2.2 Photometric observations

We obtained photometric observations of the RX J1643.7+3402 field on nine nights (18-27 September 2000) using the OHP 1.2-m telescope and a CCD $\rm TK1024\times1024$ camera. The pixel size is 24 $\mu$m which projects to 0 $.\!\!^{\prime\prime}$7 on the sky. Most of the frames used for monitoring the brightness were 30 s V-band exposures. The effective time resolution (exposure time + read-out time) was 113 s on the first four nights, but on subsequent nights we used a single $512\times512$ detector quadrant ($6\times 6$ arcmin field of view), which resulted in a read-out time of 30 s and an effective time resolution of 60 s. The longest runs possible at this time of year were about 2.5 to 3 h long. Total or partial simultaneous coverage with the spectroscopic observations at the 1.93-m was achieved on six nights (22-27 September). Dome flat-fields were obtained using controlled daylight illumination. All necessary data reduction was done using the ESO-MIDAS software package.

Figure 3: CCD photometry of RX J1643.7+3402 obtained with the 1.2-m OHP telescope in the V band on the dates shown. The magnitude scale is the same for all panels but is displayed only once. Note the strong (0.1 to 0.2 mag) rapid variations with a 15 min time scale which are present on most nights. The $\sim$0.1 mag periodic modulation at 9.25 c/d (2 $.\!\!^{\rm h}$595) reported in the text can be discerned best on the nights of September 18, 22, 24 and 26.

We derived differential magnitudes for the variable star and two nearby comparison stars using custom optimal-aperture photometry software. Sky values were derived for each star from concentric annuli designed to correct essentially for scattered light from the nearby bright star HD 151087. Results for our photometry runs are shown in Fig. 3 where the magnitudes, derived from 655 images, have been transformed into the standard V-band system. These light curves show that the object varied between V magnitude 12.5 and 12.7 during our September 2000 observing run. Superimposed on a $\sim$0.1 mag slow modulation, there are rapid variations of 0.1 to 0.2 magnitude amplitude repeating with a time scale of $\sim$15 min.

Observations using the entire $UBVR_{\rm c}I_{\rm c}$ system filter set were secured on September 23, together with frames of standards in M 13 (Forbes & Dawson 1986; Arp & Johnson 1955) for photometric calibration.

 

 

Table 2: Magnitudes and colors of field stars shown in the finding chart for RX J1643.7+3402.

Name	V	U-B	B-V	$V-R_{\rm c}$	$R_{\rm c}-I_{\rm c}$
Star 2	12.97	-0.03	+0.54	+0.32	+0.26
Star 3	12.16	+0.94	+1.04	+0.61	+0.44
Star 4	14.74	+0.89	+1.08	+0.59	+0.48
Star 5	16.30	-0.21	+0.43	+0.34	+0.29
Star 6	16.68	+0.04	+0.66	+0.39	+0.31
Star 7	16.43	+1.18	+1.56	+1.02	+1.01
Star 8	16.14	+0.55	+0.87	+0.46	+0.38

Table 2 gives magnitudes and colors for several stars in the field including the two comparisons (stars 2 and 3, see Fig. 1). The calibration errors lie within $\pm$0.02 and $\pm$0.05 mag, depending on the magnitude. Two different measurements of RX J1643.7+3402 were obtained for each color index, except U-B. The results are given in Table 3. The rapid variability seen in the data of Fig. 3 may have slightly affected the derived colors since measurements in any pair of filters were taken one minute apart (5 min in the case of the U filter).