THE HIGH-TEMPERATURE BIG BLUE BUMP IN THE SEYFERT-GALAXY REJ1034+396

X-ray, UV and optical spectra are presented for a rare, EUV-bright Sey fert galaxy, RE J1034 + 396 (z = 0.042). When combined, these data des cribe a very-high temperature big blue bump (BBB; kT(eff) similar to 1 00 eV) whose high energy turnover is observed in soft X-rays. The soft X-ray/UV flux ratio is extremely high, L(0.2keV)/L(1200A) similar to 10, compared to AGN in general whose ratio is usually less than 1. The optical/UV continuum betrays no trace of the BBB or the host galaxy. It rises steeply to the red, and is consistent with an underlying far- IR to X-ray power law with an index of similar to 1.3. This may repres ent the first direct identification of an optical power-law component in a non-blazar AGN. We compare the optical to soft X-ray spectrum of RE J1034 + 396 with three mechanisms for the production of the BBB: an optically thick accretion disc (AD); optically thin emission from a h ot plasma; and reprocessing in cool clouds. A thin AD can reproduce th e observed spectrum of RE J1034 + 396, but a free-free spectrum predic ts too much flux in the UV. The reprocessing of a non-thermal continuu m is only possible if there is a steep break between similar to 60 and 1000 Angstrom; a reprocessed free-free spectrum is an alternative. We compare the data with those of the high-redshift quasar E1346 + 266, whose BBB component is similar in shape, but 200 times stronger than t hat of RE J1034 + 396. A simple, thin AD is not appropriate for E1346 + 266 because of its high luminosity, but, when electron scattering is taken into account, an adequate representation is obtained for a blac k hole mass M = 2.3 x 10(8) M. with an accretion rate of 2.7 M. yr(-1) (L/L(Edd) similar to 2). Optically thin (T-brem = 7 x 10(6) K) emissi on and reprocessed free-free are also possible for E1346 + 266.