Annu. Rev. Astron. Astrophys. 1991. 29: 59-88 Copyright © 1991 by Annual Reviews. All rights reserved

5. THE COSMIC BACKGROUND FROM 912 TO 1200 Å

The 912-1200 Å band has many transitions which, if present as spectral features, will be useful as diagnostics of astronomical phenomena, including CIII 977, NIII 990, H Lyman 1027, OVI 1032/1038, SiIV 1020, NII 1085, and the H₂ Lyman and Werner bands. Despite the obvious promise of studies of diffuse radiation in this bandpass, very few measurements have been made. The primary reason for this is technical: the (comparatively) intense geocoronal/interplanetary hydrogen Lyman line at 1216 Å is a ubiquitous source of instrumentally scattered radiation since it is ~ 10⁶ times more intense than other sources of astronomical flux. Scattered H Lyman radiation can be eliminated in instruments carrying out measurements longward of 1250 Å through the use of filters with appropriate short wavelength cutoffs, but no simple mechanisms exist for eliminating instrumentally scattered H Lyman radiation at shorter wavelengths.

Bixler, Bowyer, and Grewing (1984) measured the cosmic background from 1040 to 1080 Å with a photometer with a very small field of view (0.23 square degrees), which was used in combination with a lithium fluoride/indium filter placed at the focal plane of a 1 m rocket-borne telescope. Although the effects of starlight were easily accessed and the filter combination was successful in reducing the H Lyman flux to a negligible level, the overall sensitivity of the measurement was low and yielded only an upper limit of ~ 10⁴ CU for this band. Opal and Weller (1984) employed an indium filter to define a bandpass from 700 to 1100 Å, which was used with a photometer with a field of view of ~ 50 square degrees. The instrument was flown on a satellite that provided substantial integration time, but, because of stellar contamination, only an upper limit of ~ 10⁴ CU was obtained for the diffuse background.

A number of studies of the cosmic background from 912 to 1200 Å have been carried out with the Voyager 2 spectrometer (Broadfoot et al. 1981). This instrument had ~ 30 Å resolution for diffuse radiation and suffered from several sources of background, including an onboard radioactive power generator and substantial instrumental scattering. A variety of approaches was employed to correct for these effects. The astronomical flux derived (or the upper limits to an astronomical flux) is typically 40 times smaller than the raw counting rate. While the corrections applied appear to be well thought-out, a reduction of this scale does admit to some uncertainty.

Shemansky, Sandel, and Broadfoot (1979) obtained data from three regions in the Cygnus loop with the Voyager instrument. They detected emission centered at 980 and 1035 Å, but the 30 Å resolution complicated their analysis. These authors concluded that the 980 Å feature was CIII 977 and the 1035 Å feature was CII 1037. This interpretation of the 1035 Å feature is not consistent with any reasonable theoretical picture, however, and is most assuredly wrong since it is contradicted by data at longer wavelengths obtained with IUE.

Blair et al. (1991) have carried out a detailed analysis of substantial regions of the Cygnus loop using extensive Voyager data. Two features were detected, one at 980 Å and one at 1035 Å. These authors conclude that the 980 Å feature is CIII 977 (but NIII 990 is also expected to be present in the Voyager spectrum and would be blended with the CIII line, given the instrument resolution) and that the 1035 Å feature is primarily OVI 1032, 1038. They provide evidence that these lines produce the majority of the cooling in the Cygnus loop. They conclude that the OIV emission emanates from directly behind the main blast wave as defined by the X-ray emission, while the CIII 977 is associated with cooler material that also produces optical emission.

Chris Martin (1990, private communication) has recently observed the Cygnus loop in a sounding rocket flight. He detected and resolved the OVI 1032/1038 emission at almost 10 .

Holberg (1986) used the Voyager 2 spectrometer with a very long integration time (~ 1.5 x 10⁶ sec) in an attempt to detect the cosmic far ultraviolet background in a view direction near the Galactic pole. He obtained upper limits on an astronomical flux of ~ 4 x 10² to 10⁴ CU over the 912-1200 Å band.

Edelstein and Bowyer (1990, private communication) flew a double dispersion nebular spectrometer on a sounding rocket to search for emission in the 970-1100 Å band from the Galactic corona. The theoretical work of Edgar and Chevalier (1986) indicates that, given the intensity of the CIV 1550 line reported by Martin and Bowyer (1990), the OVI 1032 Å line will be predominant in this band. A preliminary analysis of the data obtained by Edelstein and Bowyer indicates that this line is at least a factor of 2 to 4 less than that predicted by Edgar and Chevalier.