Optical properties of the new polar RXJ1940.2–1025

Abstract

We present detailed optical spectroscopic and photometric observations of the recently discovered X-ray source, RXJ1940.2–1025. This object, whose periodic X-ray emission had previously been confused with the X-ray signal from the nearby active galaxy, NGC 6814, is almost certainly an AM Her binary. Its 3.4-h orbital light curve is strongly modulated, and possesses a prominent dip feature lasting about 850s. The dip ingress and egress are well resolved (lasting about 200s each) at optical wavelengths. Blueward of 5500 Å the profile is flat-bottomed and near total. In contrast, we find that a reduced but significant residual flux is present in the Balmer and HeII emission lines throughout the dip. The line fluxes, however, recover much more slowly than the continuum at the end of the dip. At wavelengths beyond 5500 Å, the clear signature of an M4 dwarf secondary star becomes visible during dip minimum, yielding an estimate of 230 pc for the distance to RXJ1940.2–1025. Using our broad-band photometry, we demonstrate that the dip feature recurs with a period of 12116.2±0.4s. The accuracy with which we can determine this period is sufficient to extrapolate back to the epoch of the ROSAT PSPC observations. We can thus demonstrate that the optical dip occurs in the middle of the X-ray ‘low state’, and is not associated with the prominent broader minimum evident in the soft X-ray light curves. We find, however, that there is evidence for an X-ray dip feature corresponding to the optical dip both in the ROSAT data, and in earlier Ginga hard X-ray data. Under the assumption that the X-ray and optical dips are the same phenomenon, we derive a refined ephemeris for the dips, yielding a dip period of 12116.30±0.01s. Whilst, on balance, the X-ray and optical properties of the dip suggest absorption in the accretion flow rather than an eclipse by the companion star as the cause, the inferred wavelength dependence of the optical opacity of the absorber does not follow that expected for free-free absorption in a homogeneous absorber. This may be circumvented by invoking an inhomogeneous accretion flow.