On VI Emission Line Detection Limit of Far-Ultraviolet Imaging Spectrograph

Abstract

There have been a few attempts to measure diffuse line emission between 900 //A and 1200 //A, and only in a limited number of sight lines has it been detected. The main contributions to the equilibrium radiative cooling curve between 10^4.5 K to 10^6 K are from the doublet of O VI 1032 and 1038 in the FUV spectral region. There are several bright airglow lines which could interfere with attempts to observe the O VI lines. The nearest lines H I 1025 //A, O I 1027 //A have a combined intensity of about 10^5.5 photons/s/cm^2/sr. In the present study, the detectability simulation of O VI doublet is performed using a Monte-Carlo technique and chi-square statistics. The analysis results are compared with the previous observations and with the predictions of several interstellar medium models, and are used to limit manufacturing and alignment errors of FIMS optical system.