ULTRASOFT X-RAY COMPONENTS IN THE HIGH-TEMPERATURE EXTREME

A search for high-redshift (z > 0.5) ultrasoft X-ray AGN in pointed RO SAT PSPC observations is made, using the WGACAT catalogue. Evidence fo r a further three such objects is found, adding to three identified pr eviously (E1346+266, EXO 1346.4+2637 and RX J0947.0+4721). The flux of one new object [1WGA J1342.4+2720; L(0.1-2keV)similar to 2 x 10(45) e rg s(-1)] was found to be variable (a factor of 1.7 over 2 d), and thi s object has relatively narrow permitted line emission. These properti es, also seen in E1346+266 and EXO 1346.4+2637, are typical of ultraso ft AGN at all redshifts, in particular the narrow-line Seyfert 1s (NLS 1s). The rarity of high-z ultrasoft AGN places limits on the 'temperat ure' of the soft X-ray component in the rest frame of AGN in general. It also provides an opportunity to investigate models for the soft com ponent in the high-temperature extreme, e.g., for accretion disc model s, testing the physics at the inner edge of the disc, and the effects of Comptonization in a hot, optically thin corona.