spacer link to MAST page spacer logo image spacer

Processing of the FUSE Spectra

Return to the Main page

All the FUSE spectra were obtained through the 30"x30" LWRS Large Square Aperture in TIME TAG mode. The data were processed with the latest and final version of CalFUSE (v3.2.3; for more details see Dixon, W.V., et al. 2007, PASP, 119, 527). The FUSE data comes in the form of eight spectral segments (SiC1a, SiC1b, SiC2a, SiC2b, LiF1a, LiF1b, LiF2a, and LiF2b) which are combined together to give the final FUSE spectrum. The spectral regions covered by the spectral channels overlap, and these overlap regions are then used to renormalize the spectra in the SiC1, LiF2, and SiC2 channels to the flux in the LiF1 channel. We then produce a final spectrum that covers the full FUSE wavelength range 905-1187A. The low sensitivity portions of each channel were discarded. In most channels there exists a narrow dark stripe of decreased flux in the spectra running in the dispersion direction, known as the "worm", which can attenuate as much as 50percent of the incident light in the affected portions of the spectrum; - this is due to shadows thrown by the wires on the grid above the detector. Because of the temporal changes in the strength and position of the "worm", CALFUSE cannot correct target fluxes for its presence. Therefore, we carried out a visual inspection of the FUSE channels to locate the worm and we manually discarded that portion of the spectrum affected by the worm. We combined the individual exposures and channels to create a time-averaged spectrum weighting the flux by the exposure time and sensitivity of the input exposure and channel of origin.

For some of the systems, we had to discard the two SiC channels completely. For other systems, large portions of the SiC channels had to be removed and, as a consequence, some spectra exhibit a gap around 1080A. The channels that were the most reliable were the 1aLiF, 1bLiF and 2aSiC. The 2aLiF channel was moderately reliable and was used especially when the worm was affecting the 1bLiF channel. The 1aSiC and 1bSiC channels were the most discarded, together with the 2bLiF channel. The 2bSiC channel was discarded only when it was found to be of much lower quality than the LiF channels.

Details of the data processing for each individual system is given in the reference(s) given in the link below the figure(s) "These data and models were published in the paper".

The FUSE Spectral Lines

The main emitting components contributing to the FUSE spectra of CVs are the accretion disk and the WD, and usually the disk dominates during outburst and the WD dominates during quiescence. The main feature of the spectra is the broad Lyman beta absorption feature, which can easily be used to assess the gravity and temperature of the emitting gas. At higher temperatures, as the continuum rises in the shorter wavelengths, the higher orders of the Lyman series also become visible.

Additional broad absorption lines of metals (C, S, Si, ..) are detected and help determine the chemical abundances and projected rotational velocity of the emitting gas. In the present spectra, the main absorption features observed are:

  • C II (1010A),
  • C III (1175A),
  • Si III (1108-1114A and 1140-1144A),
  • Si IV (1067A and 1120-1130A),
  • S IV (1073A),
  • N II (1085A when not contaminated by air glow).

On top of the spectrum, broad emission lines are also found in some of the systems, mainly the O VI doublet and C III (977A and 1175A).

Most of the FUSE spectra show some ISM molecular hydrogen absorption. The most affected targets reveal a spectrum "sliced" at almost equal intervals (12A); starting at wavelengths around 1110A and continuing towards shorter wavelengths all the way down to the hydrogen cut-off around 915A. In the affected FUSE spectra, we identified the most prominent molecular hydrogen absorption lines by their band (Werner or Lyman), upper vibrational level (1-16), and rotational transition (R, P, or Q) with lower rotational state (J=1,2,3).

In addition, the targets that are weak FUSE sources exhibit sharp emission lines from air glow (geo- and helio-coronal in origin; some of which is due to sunlight reflected inside the telescope), such as

  • H I series,
  • S VI (934A and 944A),
  • O VI doublet,
  • C III (977A), and
  • He I (1168A).

For each system, we mark on the figures the lines that are detected as well as lines that are commonly seen for comparison.