Publ. Astron. Soc. Aust, 1995, 12, 66-70 The Magnetic Field of 1H1752+08 Lilia Ferrario, 1 Dayal T. Wickramasinghe, 1 Jeremy Bailey 2 and David Buckley 3 1 Astrophysical Astrophysical Theory Centre, School of Mathematical Sciences, Australian National University, Canberra, ACT 0200, Australia lilia@tardis.anu.edu.au 2 Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia 3 South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa Received 1994 September 26, accepted 1994 November 24 Abstract: We present spectropolarimetric observations of the eclipsing cataclysmic variable 1H1752+08. Modelling of the line intensity and polarisation spectra of 1H1752+08 shows that the magnetic field structure of the white dwarf is off-centre and the mean photospheric field strength is about 7 MG, the lowest measured in a cataclysmic variable (CV). We argue that 1H1752+08 is most probably a low-field AM Herculis system. Keywords: accretion — binaries: close — stars: individual: 1H1752+08 — stars: magnetic fields — cataclysmic variables — white dwarfs 1. Introduction It is now well established that the observations of AM Herculis systems can be explained in terms of high-harmonic cyclotron emission from a hot plasma (T « 10-20 keV) in accretion shocks near the surface of a strongly magnetised (B « 20-60 MG) (1 MG = 10 2 T) white dwarf primary accreting from a low mass M-star. The strength of the field is sufficient to prevent the formation of the usual CV accretion disk and to synchronise the rotation of the two stars with the orbital period (« 80-220 minutes). Accretion occurs directly onto the magnetic white dwarf via an accretion stream, a coupling region, and magnetically confined accretion funnels. The magnetic field strength and structure of AM Herculis systems can be studied by two methods. In a high state of accretion, the spectra are dominated by cyclotron radiation from the accretion shocks, and broad cyclotron lines in emission are often visible in the spectra. Prom the spacing of the cyclotron peaks, it is possible to obtain a tight estimate of the field strength at the accretion region. The radiation from the photosphere of the white dwarf becomes dominant when the mass transfer from the secondary star considerably diminishes or ceases. In this case, the radiation from the photosphere of the white dwarf dominates over the radiation from the accretion shocks, and photospheric Zeeman absorption features can often be detected in the spectra, allowing the properties of the surface- averaged field to be investigated. The X-ray source 1H1742+08 was discovered to be an eclipsing CV with an orbital period of 1 • 88 hours by Silber et al. (1994) as part of the identification program of X-ray sources found during the HE A 0-1 Modulation Collimator survey. Silber et al. (1994) concluded from a study of its eclipse behaviour that 1H1742+08 was an unusual CV with a marginal accretion disc and a hot spot located approximately a quarter of the distance from the primary to the inner Lagrangian point. In this paper, we present spectropolarimetric observations of 1H1742+08 which show Zeeman lines from the photosphere of the white dwarf typical of an AM Herculis system in a low state of accretion. Detailed modelling of the line intensity and polarisation spectra of 1H1752+08 point to a mean photospheric field of about 7 MG. This is the lowest magnetic field so far determined for a magnetic CV. 2. Observations The observations were obtained with the AAO spectropolarimeter on 1993 August 9. Phase- resolved spectropolarimetric observations were ob- tained around the orbit over the wavelength range 4100-7200 A with a spectral resolution of « 10 A. We present in Figure 1 the phase-averaged polarisa- 1323-3580/95/010066S05.00 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1323358000020051 Downloaded from https://www.cambridge.org/core. IP address: 107.173.89.234, on 20 May 2019 at 22:58:20, subject to the Cambridge Core terms of use, available at