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STUDIES OF RADIAL VELOCITY SYSTEMATICS IN NEWSIPS (High Dispersion)

Myron Smith

The following are results of several studies performed to evaluate the wavelength accuracy of NEWSIPS high dispersion data. These studies were prompted by a report by J. Holberg (Holberg, Barstow, Sion, ApJS, 119, 207, 1998) that high-dispersion SWP spectra of several white dwarfs processed through NEWSIPS give rise to different radial velocities than those obtained from well-calibrated optical spectra.

Most of these studies are focused on spectra obtained with the SWP camera. The study includes the velocity behavior with both time and wavelength, the differences with respect to the Copernicus system for tau Sco, with respect to IUESIPS processing, and finally between large and small apertures. We chose as a standard reference the sharp-lined B0V star tau Scorpii for several of these tests. We also exhibit RV differences between SWP and LWP/LWR cameras of tau Sco.

In all cases, the measurements discussed below were made by comparing pairs of various spectra using the IUEDAC analysis tool CRSCOR.

Index

  • Study 1:Radial Velocity of tau Sco & White Dwarf Images with epoch (SWP)
  • Study 2:Radial Velocity Difference with Wavelength, IUESIPS - NEWSIPS
  • Study 3:Radial Velocity Difference with Wavelength, IUESIPS - Copernicus
  • Study 4:Velocity Offsets Between Large and Small SWP Aperture
  • Study 5:Radial Velocity with Time: NEWSIPS - Copernicus (tau Sco, SWP)
  • Study 6:Radial Velocity Difference for Small & Large Apertures for LW Cameras
  • Study 7:Radial Velocity Difference Between SWP and LW Cameras
  • An Appendix is provided listing the spectra used in these studies and acknowledgments.

Study 1
Radial Velocity of tau Sco & White Dwarf Images with Epoch (SWP)

Figure 1

Figure 1 shows the IUESIPS - NEWSIPS radial velocity difference with time computed from several large-aperture, high-dispersion SWP images of tau Scorpii and the white dwarfs BD 2111+49, R2214-49, HD340611, G191-B2B for the 1190--1720 Angstrom region. The times of a number of changes implemented into the IUESIPS are denoted by verticle dotted lines. These changes are expected to have affected the IUESIPS wavelengths to greater or lesser degrees (courtesy, C. Imhoff).

Note the correspondence of various discontinuities and trends in the velocity difference with the times of modifications to the IUESIPS software noted. This correspondence implies that most or all of the RV fluctuations time are caused by updates to the IUESIPS wavelength calibration. This plot, including the positive during the middle of the mission and the complicated time dependence, is very similar to that found in a study by (Holberg, Barstow, & Sion, ApJS, 119, 207, 1998); our white dwarf images comprise a subset of their dataset.

The implied constancy of the SWP NEWSIPS wavelength system with time is confirmed in Study 5 (Fig. 7), which compares NEWSIPS results with those high-quality data from the Copernicus atlas of tau Sco.

Study 2
Radial Velocity Difference with Wavelength, IUESIPS - NEWSIPS

Figure 2
Figure 2 exhibits the variation of radial velocity with wavelength in the range 1170--1410 Angstroms for large-aperture images of tau Sco and white dwarfs.

Because the zero point of the IUESIPS RVs is a function of epoch, we used an arbitrary zero point appropiate to 14 white dwarf images studed by Holberg et al. (for listing, see Appendix). The radial velocities of the tau Sco images have been shifted to match the mean curve for the white dwarfs. (The mean difference in RVs for these source types differs because of the different distributions of times at which the images were taken (STUDY 1).)

Study 3
Radial Velocity Difference with Wavelength, IUESIPS - Copernicus

Figure 3
Figure 3 exhibits a plot similar to Figure 2, and for the same large- aperture images of Tau Sco but processed with IUESIPS. The large range in RV offsets is a consequence of calibration errors in the zero points calculated by IUESIPS, which are as a function of epoch (Study 1), of and different centerings of the image in the large aperture. The important point from this plot is that no wavelength dependence exists in the velocities. This suggests that the distribution of wavelength assignments (within a zero point offset) is the same for IUESIPS and Copernicus. Click here for a discussion of probable wavelength systematics.

Study 4
Velocity Offsets Between Large and Small SWP Aperture

Figure 6Figure 7
This study shows equivalent RV differences for pairs of contemporaneous images, in the sense large-aperture minus small aperture. These are shown as a function of wavelength for images from tau Sco and zeta Oph in Figures 6 & 7, respectively. In each case an offset of -5 to -7 km/s is evident. See also Study 6.

Study 5
Radial Velocity with Time: NEWSIPS - Copernicus (tau Sco, SWP)

Figure 8
Figure 8 shows the mean velocities for a number of SWP large-aperture (dashed line) and small-aperture (solid line) images processed through NEWSIPS. Notice that subsequent to the year 1980 the wavelength system is stable for images in this camera. An equivalent velocity difference of about -6 km/s is also evident for LGAP - SMAP comparisons. This study is consistent with the results in Studies 1, 3, and 4. Together they show that the NEWSIPS LGAP data for Tau Sco has a bias of about -6 km/s which is possibly attributed to the offset of this target during its observation through the large aperture (Study 6). This may also be at least part of the discreprancy noted by Sion, Barstow, & Holberg. As a result we recommend that radial velocities derived from SWP images be obtained from NEWSIPS data. A correction of about +6 km/s should be further applied to LGAP images.

Study 6
Radial Velocity Difference for Small & Large Apertures For LW Cameras

Figure 9
Figure 9 shows a plot with wavelength of the equivalent radial velocity differences for large-aperture minus small-aperture image pairs taken of tau Sco at the same epochs. The solid lines denote the equivalent RV differences for a pair of LWP images; dashed lines signify RV differences for pairs of LWR images.

If one LWP result is excepted, the large-aperture minus small-aperture RV offsets are virtually identical for the two long-wavelength cameras.

C. Imhoff has pointed out (priv. comm.) that one can predict a difference in offsets due to a systematic centering error for the "FES underlap mode" used to guide on bright stars. This amounts to an equivalent RV difference, LGAP - SMAP, of -5 km/s for SWP and +5 km/s for LWP/LWP. These results appear to be consistent with our results for tau Sco for all three cameras (Studies 4 & 6) and zeta Oph for SWP (Study 4).

Study 7
Radial Velocity Difference Between SWP and LW Cameras

Figure 10Figure 11

A small wavelength overlap range (nearly 150Å) between SWP and LWP/LWR spectra can be used to provide a wavelength comparison for images of hot stars. In Figures 10 and 11 we show the equivalent velocity shift of pairs of common-epoch, small-aperture images of tau Sco. The results show an equivalent velocity shift, relative to SWP, of -5 km/s for LWP and +2 km/s for LWR, i.e., an LWR - LWP offset of +7 ±2 km/s.