Short Wavelength Time Dependence

SWP MXHI Time Dependence

This figure shows the time dependence of SWP NEWSIPS large aperture high dispersion extractions. It shows the short wavelength region of all the available well exposed high dispersion spectra (71 in all) of the B0 V star tau Sco obtained over the lifetime of IUE. The spectra are normalized by their mean spectrum. The temporal ordinate is simply the accumulated number of spectra, and is not linear in time. No smoothing has been done, so individual spectra can be distinguished in the temporal direction.

Notice the following:

  1. The regions of the strong lines (e.g., the C+2 1175 Å multiplet, the interstellar Ly alpha line, and the N +4 1238-42 Å resonance doublet) all grow stronger over the lifetime of the mission.

  2. The regular structure which appears throughout the image. This corresponds to the echelle ripple pattern, with the low flux portion of the pattern becoming darker over the life time of the mission.

  3. The flux in the core of the saturated interstellar Ly alpha line is not zero in the mean spectrum.

  4. At the shortest wavelengths, the extracted fluxes change by more than 40% at low flux levels over the lifetime of the mission.
Together, the first two points suggest a time dependent systematic effect which depends upon the flux level of the unextracted spectrum, with low flux levels being strongly affected and higher flux levels less so. The third point indicates that this trend results from low flux levels being overestimated toward the beginning of the mission.

The time dependent effects are strongest in this region. They are weakest at intermediate (1300-1600 Å) wavelengths and somewhat weaker at the longest (1700-1950 Å) wavelengths.