Observations of the Diffuse Far-UV Sky with SPEAR

The Spectroscopy of Plasma Evolution from Astrophysical Radiation (SPEAR) instrument, aboard the STSAT-1 satellite mission, is now observing emission from the cosmic far ultraviolet background. The primary science objectives of the mission are to characterize the Galactic interstellar medium and particularly to study interfaces of hot interstellar plasma. The mission plan is to map most of the sky and then to conduct pointed observations of selected targets. We describe the mission and examples of observations of the Vela supernova remnant and the Eridanus Loop superbubble. The SPEAR results provide unprecedented imaging spectra of diffuse emission from highly ionized species including He ii, C iii, C iv, O iii], and Ovi, and from molecular hydrogen. 1. The Instrument, Mission and Data SPEAR is an imaging, dual band spectrograph (S band: λλ 900 – 1150 A , L band: λλ 1350 – 1750 A) optimized for diffuse-radiation measurement (Edelstein, et al. 2003). This bandpass includes many strong atomic emission cooling lines from N, C, O, Si and S, as well as H2 fluorescence lines. The instrument obtains ∼5 ′ imaging resolution along the 7.5o / 4.5o (L/S) field of view and a spectral resolution of λ/∆λ ∼ 500 HEW. SPEAR is the primary payload on the 3-axis stabilized STSAT-1 (S-1) satellite launched into a 700 km low-earth orbit in late 2003. Astrophysical observations are taken for 20 minutes of eclipse during 10 of 15 daily orbits. The S-1 mission is operated from the SaTReC Ground Station in South Korea. Data reduction of each photon includes: pulse height selection, correction of electronics drift and spectral and imaging distortions, and association with time and attitude information. Efficiency was calibrated by measuring known bright stars. In 12 ksec of exposure over its full slit, SPEAR can measure 650 photons cmsrs (hereafter LineUnits or LU) of C iv λ1550 and 5 000 LU of Ovi λ1032 emission. Spectral scale (∼ 1 A error) and resolution were calibrated using aurora and day glow emission lines. Imaging scale and distortion were found by measuring bright stars. The reconstructed attitude resolution is 10 FWHM. 2. SPEAR Observation of the Vela SNR SN explosions are the principal engine producing shock-heated gas in the Galaxy. Our understanding of shock-ISM interactions and the global distribution of ionized gas in galaxies are far from complete. SNRs are among the brightest far ultraviolet (FUV) emitting diffuse objects with emission line intensities that depend on the shock velocity, the ambient medium, and the thermal structure of post-shock cooling regions (e.g.,