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

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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, [FORMULA] min. At the time of the observations, AAVSO observations showed AM Her to be in a high state at [FORMULA]. The GHRS spectra were taken in the ACCUM mode through the 2" Large Science Aperture (LSA). In order to cover both wings of [FORMULA] the central wavelength was set to 1292 Å resulting in a spectral coverage of 1150-1435 Å with a nominal FWHM resolution of [FORMULA] Å ([FORMULA] km s-1). The spectra were acquired with a time resolution of 31.4 sec, corresponding to an orbital phase resolution of [FORMULA], with a net exposure time of 27.2 sec per spectrum. The observation was interrupted for [FORMULA] min for a SPYBAL calibration, causing a gap in the phase coverage of [FORMULA]. 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 ([FORMULA] 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 [FORMULA] using the ephemeris determined by Tapia (see Heise & Verbunt 1988). The magnetic phases were subsequently converted into conventional binary orbital phases [FORMULA] 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): [FORMULA] 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 [FORMULA] is historically used in the analysis of UV and X-ray light curves of AM Her, we adopt [FORMULA] 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 [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998