Astron. Astrophys. 338, 933-946 (1998)

2. HST/GHRS observations

HST/GHRS observations of AM Her were carried out in January 1997 during a rare opportunity when the system was located in a continuous viewing zone of HST (Table 1). The total exposure time was slightly longer than the binary orbital period, min. At the time of the observations, AAVSO observations showed AM Her to be in a high state at . The GHRS spectra were taken in the ACCUM mode through the 2" Large Science Aperture (LSA). In order to cover both wings of the central wavelength was set to 1292 Å resulting in a spectral coverage of 1150-1435 Å with a nominal FWHM resolution of Å ( km s^-1). The spectra were acquired with a time resolution of 31.4 sec, corresponding to an orbital phase resolution of , with a net exposure time of 27.2 sec per spectrum. The observation was interrupted for min for a SPYBAL calibration, causing a gap in the phase coverage of . A total of 341 spectra were obtained.

Table 1. HST/GHRS observations of AM Her on Jan 4, 1997.

Regular monitoring observations with the GHRS/G140L grating have revealed a slowly decreasing sensitivity below 1200 Å (Sherbert et al. 1997). The response at 1150 Å was reduced in late 1997 by 15% with respect to the sensitivity just after the Service Mission 1 in December 1993. We therefore recalibrated our GHRS data of AM Her with the calhrs routine of stsdas using the time-dependent flux calibration data given by Sherbert & Hulbert (1997). We caution, however, that the absolute fluxes at the very blue end of the spectra ( Å) may be still somewhat on the low side (Sect. 3.4).

The mid-exposure times of the individual GHRS spectra were converted into magnetic orbital phases using the ephemeris determined by Tapia (see Heise & Verbunt 1988). The magnetic phases were subsequently converted into conventional binary orbital phases via the relation

The offset was obtained by comparison to the zero point in the optical ephemeris determined by Martin (1988; see also Southwell et al. 1995): corresponds to the inferior conjunction of the secondary. Note that the period given by Heise & Verbunt (1988) has a smaller error than that given by Southwell et al. (1995) and the former should preferably be used. Even though is historically used in the analysis of UV and X-ray light curves of AM Her, we adopt as the natural choice for the discussion of the geometry within the binary system. For convenience, light curves and radial velocity curves will be also labelled with .

© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998
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