RX-J1712.6-2414 - A POLARIZED INTERMEDIATE POLAR FROM THE ROSAT GALACTIC PLANE SURVEY

We report the discovery of a new intermediate polar, RXJ1712.6 - 2414, discovered during the ROSAT Galactic Plane Survey optical identificat ion programme. Photometric and polarimetric observations establish tha t the system exhibits two periodicities at 1003 and 997 s (or aliases thereof) in the two data sets respectively. This makes RX J1712.6 - 24 14 only the third LP to have a positive detection of circular polariza tion, and the second for which periodic variations have been detected. Furthermore, the degree of polarization is the largest yet observed i n an IP: up to -5 per cent in the I band, and with a semi-amplitude of similar to 1 per cent in the V, R and I bands. The extensive photomet ric observations, spanning 3 yr, show a convincing 1003-s or 1027-s pe riod, although the amplitude of this signal varies markedly from non-d etection to a similar to 15 per cent semi-amplitude. Detectability of the photometric signal in RX J1712.6 - 2414 appears to be anticorrelat ed with its brightness. Low-amplitude (K similar to 10 km s(-1)) perio dic radial velocity variations at the 1003-s photometric period are ob served in at least two nights of data. These variations are particular ly pronounced for the He u lambda 4686-Angstrom line, where the amplit ude is as much as 20 km s(-1). Interpretation of the 927-s polarimetri c and 1003-s photometric periods as the spin and synodic (beat) period s, respectively, implies an orbital period of 3.41 h, or an alias of t his. However, no photometric, and more significantly no radial velocit y variations are seen at these or any nearby periods, with an upper li mit of similar to 10 km s(-1). This may be evidence that RX J1712.6 - 2414 is a low-inclination system, which is supported by the negative s ign of the circular polarization over the entire spin cycle, implying that only one accreting pole is observable. We interpret the synodic p hotometric and radial velocity variations as evidence for discless acc retion in RX J1712.6 - 2414, or at least direct accretion on to the wh ite 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 cou nt s(-1), although the total exposure time was only 302 s. The X-ray s pectrum is consistent with moderately absorbed (N-H similar to 10(21) cm(-2)) and reasonably hard (kT greater than or similar to keV) therma l bremsstrahlung, which implies an L(x)/L(opt) ratio of similar to 0.1 . These properties are consistent with an intermediate polar model for RX J1712.6 - 2414, probably involving discless accretion.