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Astron. Astrophys. 343, 455-465 (1999)

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2. Observations and data

RXJ0019 has been monitored regularly at the Wendelstein Observatory from 1992 to 1995 (Will & Barwig 1996). Follow-up observations were made in autumn 1996. We were able to observe this object on 15 nights between October and December 1996.

A special approach to investigate RXJ0019 was made in October 1997. We performed simultaneous spectroscopy and spectrophotometry at the 3.5 m and 2.2 m telescope at Calar Alto, respectively. Detailed information about the individual observing campaigns is given in Tables 1 to 3.


[TABLE]

Table 1. Journal of photometric observations with MCCP. HJD = truncated Heliocentric Julian Date -2440000.0, h = hours, IT = integration time



[TABLE]

Table 2. Journal of spectrophotometric observations with MEKASPEK. HJD = truncated Heliocentric Julian Date -2450000.0, h = hours, IT = integration time



[TABLE]

Table 3. Journal of spectroscopic observations. B=blue spectral region, R=red spectral region, UT=universal time, IT=integration time


2.1. Photometry

High-speed photometric observations of RXJ0019 were performed using the multichannel multicolor photometer MCCP (Barwig et al. 1987) at the 80 cm telescope at the Wendelstein Observatory in the Bavarian Alps. With this photometer we can monitor the object, a nearby comparison star and the sky background simultaneously in UBVRI. The MCCP allows a nearly complete elimination of atmospheric transparency variations and extinction effects (Barwig et al. 1987), by subtracting the sky background of each color channel and dividing the object by the comparison star measurement afterwards. Therefore, photometric measurements were possible even under non-photometric conditions. During the 1996 run we were able to add another 88.7 hours of observation time for RXJ0019 to our existing dataset.

As comparison star we used the star at [FORMULA], [FORMULA] (for epoch 2000.0). The intensities of the light-curves presented in Fig. 3 are calculated relative to this star. The error in the relative count rate is [FORMULA]. In order to normalize the individual channels, calibration measurements were performed during photometric conditions once or twice a night. The constancy of the calibration coefficients is indicative of the high stability of our detectors. All observations were made with an integration time of 2 sec. except on the night of Dec 10,1996 where we used an integration time of 1 sec. A detailed journal of the observations is given in Table 1.

2.2. Spectrophotometry

We observed RXJ0019 on October 27 and 30, 1997 with MEKASPEK attached to the 2.2 m telescope at the Calar Alto Observatory. MEKASPEK is a four channel fiber-optic spectrophotometer, developed at the Universitäts-Sternwarte München. With MEKASPEK we can also perform simultaneous measurements of the object, a nearby comparison star and the sky background within the spectral range of 3700...9000Å at a spectral resolution of [FORMULA]. The photon-counting two-dimensional detector (MEPSICRON) has a time resolution of up to 5 ms.

Furthermore, MEKASPEK performs a correct treatment of atmospheric effects and allows an accurate transformation to any broadband photometric system. For more details see Mantel et al. (1993) and Mantel & Barwig (1993). Again, atmospheric effects are eliminated using the standard reduction method (Barwig et al. 1987).

We used an integration time of 2 sec. Each night two calibration measurements were performed to get the calibration coefficients for the normalization of the color channels. We also made flat-field measurements and a measurement of a HgArRb-lamp spectrum for wavelength calibration at the end of each night. Due to unfortunate weather conditions only two of four program nights could be used. Therefore, we could not cover a complete orbital phase of RXJ0019. The details of the MEKASPEK observations are listed in Table 2.

2.3. Spectroscopy

Simultaneously with the spectrophotometric measurements we obtained high-resolution spectroscopic data with the Cassegrain double beam spectrograph (TWIN) attached to the 3.5 m telescope at Calar Alto. The blue and red channel were equipped with the low-noise CCDs (SITe#6a and SITe#4d) with a pixel size of 15 µm and a CCD size of 800[FORMULA]2000 pixel. We used a slit width of [FORMULA]. For the blue channel we chose grating T07 with a spectral resolution of 0.81 Å /pixel and a spectral range of 3300...5000 Å (observations performed in second order) and for the red channel grating T04 with a spectral resolution of 1.08 Å /pixel in a spectral range between 5500...9000 Å. The object was trailed along the slit in order to get phase-resolved, high resolution spectra. Exposure times ranged between 690 and 1800 s and the mean trail velocity was set to [FORMULA]/h. The trailed spectra were binned in the direction of the slit by a factor of 2. Helium-Argon wavelength calibration spectra were taken approximately every hour. During the first observing night no calibration spectra in the blue spectral range could be obtained due to technical problems. Flatfield measurements were taken at the end of the last observing night. Further information about the observation is given in Table 3. The data reduction included bias-subtraction, flatfield-correction, sky-subtraction, cosmic-ray elimination and wavelength-calibration as described by Horne (1986).

After that, the continuum of the spectroscopic data set was calibrated with our simultaneously obtained spectrophotometric data. By applying this method we were able to correct for intensity variations of the emission lines which might have been caused by poor weather conditions (e.g. clouds) or observational difficulties (e.g. variable vignetting of the seeing disk by the slit). Further information is given in imi et al. (1998). In a last step the trailed spectra were phase-folded into 125 phase bins using the updated eclipse ephemeris as given in Sect. 3.1.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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