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Astron. Astrophys. 350, 517-528 (1999)

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2. Observations and data reduction

The UV and optical campaign on FO Aqr was carried out between September and October 1995 with HST, IUE and at the South African Astronomical Observatory. The journal of the observations is reported in Table 1.


Table 1. Journal of observations.

2.1. The HST data

HST Faint Object Spectrograph observations of FO Aqr were performed on September 10, 1995. The observations were carried out in the rapid mode during 7 consecutive HST orbits. Due to target acquisition procedures the total effective on source exposure time was 4.07 h, yielding six continuous exposure slots as detailed in Table 1. The orbital period was unevenly sampled since it is commensurable with the HST orbit. The G160L grating was operated with the blue digicon covering the range 1154-2508 Å at a resolution of 6.8 Å diode-1 and with the 0.86" upper square aperture, supposed to be free from the 1500-1560 Å photocathode blemish which is known to affect the circular apertures (HST Data Handbook, 1997). A total of 797 spectra were collected, each with an effective exposure time of [FORMULA] 18 s.

The standard routine processing ST ScI pipeline applied to the data at the time of the observations revealed the presence of anomalous features when comparing the reduced spectra with the IUE data, and in particular in the regions 1500-1590 Å and 1950-2010 Å . The data were then re-processed using the STSDAS/CALFOS routine within IRAF using the latest reference files for sensitivity correction and appropriate aperture flatfield provided by the ST ScI Spectrograph Group in summer 1998. The calibrated spectra appear then free from the above features although the IUE fluxes are on average [FORMULA] times larger than the FOS ones (see Fig. 1). A check against systematic effects using spectra of standard stars observed with both IUE and FOS only confirms the known lower flux (on average of [FORMULA]) of IUE, with respect to that of FOS (Gonzalez-Riestra 1998). The residual flux difference may be due to the fact that the IUE data were acquired a month later than the FOS spectra.

[FIGURE] Fig. 1. Upper panel: Grand average FOS (solid line) and IUE (dotted line) spectra of FO Aqr in the range 1150-2500 Å together with line identification. Bottom panel: Enlargement of the [FORMULA] feature in the average FOS spectrum. The solid line represents the observed spectrum, the dotted line a damped Lorentzian profile convolved with a 7 Å FWHM gaussian corresponding to [FORMULA] and the dashed line represents the ratio between the two, enhancing the emission feature at 1215 Å .

The G160L grating provides zero order light covering the full range between 1150 and 5500 Å, with an effective wavelength at 3400 Å, which provides useful simultaneous broad-band photometry. The flux calibration of the zero-order signal (Eracleous & Horne 1994) updated for errors and post-COSTAR sensitivity and aperture throughput (HST Data Handbook 1997) has been applied to the signal extracted in the zero-order feature of each of the 797 exposures.

2.2. The IUE data

On October 15, 1995 ten IUE SWP (1150-1980 Å) and ten LWP (1950-3200 Å) low resolution ([FORMULA] 6 Å) spectra were acquired during consecutive 16 h with exposure times equal or twice the [FORMULA] to smear out effects of the rotational pulsation (Table 1). The SWP and LWP exposures sample the orbital period.

The spectra have been re-processed at VILSPA using the IUE NEWSIPS pipeline used for the IUE Final Archive which applies the SWET extraction method as well as the latest flux calibrations and close-out camera sensitivity corrections (Garhart et al. 1997). Line-by-line images have been inspected for spurious features which have been identified and removed.

2.3. The optical photometry

Optical photometry was conducted in the period October 18-23 1995 at the SAAO 0.75 m telescope and UCT Photometer employing a Hamamatsu R93402 GaAs photomultiplier. BVRI (Cousins) photometry was carried out on the first three nights performing symmetric modules with integration times of 30 s or 20 s respectively for the B and I, or V and R filters. The typical time resolution for the sequence of all four filters was [FORMULA]120 s, with interruption every [FORMULA] 10 min for sky measurements. The times and durations of individual runs are reported in Table 1. The orbital period has not been fully sampled.

Additional fast photometry in white light has been carried out on October 21 and 23, 1995 using the same photometer, but employing a second channel photomultiplier for monitoring of a nearby comparison star. Continuous 5 s integrations were obtained, with occasional interruptions (every 15-20 min) for sky measurements (lasting [FORMULA] 20-30 s). During the two nights, the observations were carried out for 4.3 h and 2.5 h respectively.

The photometric data have been reduced in a standard manner with sky subtraction, extinction correction and transformation to the standard Cousins system using observations of E-region standards obtained on the same night.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999