RX J1712.6-2414: a polarized intermediate polar from the ROSAT Galactic Plane Survey

We report the discovery of a new intermediate polar, RX J1712.6-2414, discovered during the ROSAT Galactic Plane Survey optical identification programme. Photometric and polarimetric observations establish that the system exhibits two periodicities at 1003 and 927s (or aliases thereof) in the two data sets respectively. This makes RX J1712.6-2414 only the third IP to have a positive detection of circular polarization, and the second for which periodic variations have been detected. Furthermore, the degree of polarization is the largest yet observed in an IP: up to -5 per cent in the I band, and with a semi-amplitude of ~1 per cent in the V, R and I bands. The extensive photometric observations, spanning 3yr, show a convincing 1003-s or 1027-s period, although the amplitude of this signal varies markedly from non-detection to a ~15 per cent semi-amplitude. Detectability of the photometric signal in RX J1712.6-2414 appears to be anticorrelated with its brightness. Low-amplitude (K~10 km s^-1) periodic radial velocity variations at the 1003-s photometric period are observed in at least two nights of data. These variations are particularly pronounced for the HeII lambda4686-A line, where the amplitude is as much as 20 km s^-1. Interpretation of the 927-s polarimetric and 1003-s photometric periods as the spin and synodic (beat) periods, respectively, implies an orbital period of 3.41h, or an alias of this. However, no photometric, and more significantly no radial velocity variations are seen at these or any nearby periods, with an upper limit of ~10 km s^-1. This may be evidence that RX J1712.6-2414 is a low-inclination system, which is supported by the negative sign of the circular polarization over the entire spin cycle, implying that only one accreting pole is observable. We interpret the synodic photometric and radial velocity variations as evidence for discless accretion in RX J1712.6-2414, or at least direct accretion on to the white dwarf's magnetosphere, either directly by the accretion stream, or via disc overflow. Analysis of the ROSAT sky survey data for RX J1712.6-2414 shows that the source had a mean PSPC count rate of 0.52 count s^-1, although the total exposure time was only 302s. The X-ray spectrum is consistent with moderately absorbed (N_H~10^21 cm^-2) and reasonably hard (kT>~1 keV) thermal bremsstrahlung, which implies an L_x/L_opt ratio of ~0.1. These properties are consistent with an intermediate polar model for RX J1712.6-2414, probably involving discless accretion.