THE ULTRAVIOLET-SPECTRUM OF THE NARROW-LINE SEYFERT-1 GALAXY RE J1034+396

REJ1034+396 has one of the hottest big blue bumps of any Seyfert 1 (kT (BB) similar to 120 eV) and thus provides a valuable insight into the physics in the nuclei of active galaxies. In this paper, we present UV spectroscopy of REJ1034+396, taken with the Faint Object Spectrograph on the Hubble Space Telescope. With a spectral resolution of similar to 1-2 Angstrom FWHM and a typical signal-to-noise ratio of similar to 15 per diode, this is one of the first detailed UV spectra of an obje ct in the narrow-line Seyfert 1 (NLS1) class, The spectrum probes the physics and kinematics of the high-ionization and coronal line gas, an d the strength and form of the big blue bump component in the UV. We d etect many emission lines, including Ly alpha: CIV lambda 1549, He II lambda 1640, CIII] lambda 1909 and Mg II lambda 2798. We also identify a feature at 2647 Angstrom (in the rest frame) with highly ionized ir on ([Fe XI] lambda 2649); a line of the same species ([Fe XI] lambda 7 892) has also been seen in the optical spectrum, The velocity widths o f the UV lines are relatively narrow (FWHM<2000 km s(-1)), although CI V lambda 1549 appears to have a broad underlying component with a FWHM typical of quasars (similar to 5500 km s(-1)). The FWHM are similar t o those of the optical lines, which suggests that all line emission in REJ1034+396, i.e. including that of high-and low-ionization species a nd the forbidden lines, may be dominated by an intermediate-velocity ( FWHM similar to 1000 km s(-1)), intermediate-density (log N-e similar to 7.5 cm(-3)) region of gas. The slope of the UV continuum (alpha(uv) similar to 0.9) is soft (i.e. red) relative to quasars and the UV to soft X-ray flux ratio is unusually low (the 0.2 keV/1200 Angstrom flux ratio is 1/200), implying that the big blue bump component is very we ak in the UV. The present-epoch UV to soft X-ray continuum is consiste nt with earlier data, demonstrating that this extreme big blue bump co mponent is also very stable, unlike many other NLS1s which show extrem e patterns of variability.