High-speed energy-resolved STJ photometry of the eclipsing binary UZ For

We present high-time-resolution optical photometry of the eclipsing binary UZ For using a superconducting tunnel junction (STJ) device, a photon-count ing array detector with intrinsic energy resolution. Three eclipses of the similar to 18-mag 126.5-min orbital binary were observed using a 6x6 array of tantalum STJs at the 4.2-m William Herschel Telescope on La Palma. The d etector presently provides individual photon arrival-time accuracy to about 5 mus, and a wavelength resolution of about 60 nm at 500 nm, with each arr ay element capable of counting up to similar to 5000 photon s(-1). The data allow us to place accurate constraints on the accretion geometry from our time- and spectrally resolved monitoring, especially of the eclipse ingress and egress. We find that there are two small accretion regions, located cl ose to the poles of the white dwarf. The positions of these are accurately constrained, and show little movement from eclipse to eclipse, even over a number of years. The colour of the emission from the two regions appears si milar, although their X-ray properties are known to be significantly differ ent: we argue that the usual accretion shock may be absent at the non-X-ray -emitting region, and instead the flow here interacts directly with the whi te dwarf surface; alternatively, a special grazing occultation of this regi on is required. There is no evidence for any quasi-periodic oscillations on time-scales of the order of seconds, consistent with relatively stable cyc lotron cooling in each accretion region.