Diffuse Background Measurement with EUVE's Deep-Survey Telescope First-Light Images

The Extreme Ultraviolet Explorer's (EUVE's) deep-survey telescope image system views the sky through two wavelength band filters, 50--190 Angstroms and 160--360 Angstroms, spanning the short-wavelength portion of the EUV. A part of the spacecraft's commissioning, a deep first-light sky exposure in the direction of (l=5.6, b=+1.3) was taken. Analysis has permitted a precise assessment of the observing background for this image, from which we obtain two immediate results: (1) in the 50--190 Angstroms band we see an effective continuum flux of 1.71 +/- 0.17 photons cm(-2) s(-1) sr(-1) Angstroms(-1) , consistent with the Wisconsin B band survey in this direction and the ROSAT WFC S1 flux; (2) in the 160--360 band we establish a 3 sigma upper flux limit of 3.63 photons cm(-2) s(-1) sr(-1) Angstroms(-1) , far lower than previous efforts. Finally, we apply these fluxes to constrain isothermal plasma emission models for the local interstellar medium; we find that no simple isothermal model satisfactorily fits all the data, but that areas of closest agreement occur for log T of 5.85 and 6.20. This work has been supported by NASA contract NAS5-30180.