1. UVIS Calibration Update
Greg Holsclaw, Bill McClintock
June 8, 2009

2. Outline Recent calibration observations Increase in starburn depth
More Spica variability
Stellar flux comparisons with SOSLTICE

3. Recent UVIS Calibrations
FUV2009_108_19_52_14_UVIS_109IC_ALPVIR001_PRIME
Data from first ~1/3 of slew along the slit was lost
FUV2009_121_01_12_45_UVIS_109IC_ALPVIR002_PRIME

4. Total signal vs position
Total FUV signal from Spica vs position of the star within the slit for all obsevations in the last four years
Each curve is normalized to the value at row 20
Significant increase in starburn depth is seen
time

5. Starburn in both channels
EUV
FUV
This recent increase in depth of the FUV starburn is also seen in the EUV

6. Sharp change in response within the starburned rows
This plot shows the total signal from Spica as a function of time for when the stellar image is centered at row 20 and row 31
A slow decline in overall sensitivity is evident in row 20

7. Depth of Starburn vs time
This shows the ratio of the signal when the star is centered at row 31 to that of when the star is centered at row 20
This is measure of the increasing depth of the starburn area
Were there a lot of occultation measurements in late 2008 / early 2009 where a star was located in the starburned area?

8. Spectral ratio of row 31 to row 20
This shows the ratio of the spectrum in row 31 to that in row 20
This looks like the inverted signal from a star (Spica?) 8

9. Spectral ratio of row 31 to row 20
This shows the ratio of the spectrum in row 31 to that in row 20
This looks like the inverted signal from a star (Spica?)
With Flat-field 9

10. Spica variability

11. Background on Alpha Vir (Spica)
Spica is a non-eclipsing double-lined spectroscopic binary system
Though not spatially resolvable, each component is detectable through measurements of out-of-phase Doppler shifts in the constituent spectral lines
Non-eclipsing due to large apparent orbital inclination of ~70 degrees
Both stars are of a similar spectral class:
Primary: B1V
Secondary: B4V
Spica is the brightest rotating ellipsoidal variable star
The stars have a distorted ellipsoidal shape due to mutual gravitation effects
As the components revolve, the visible area (and thus the observed flux) changes with orbital phase
Since this is a geometric effect, it should be roughly wavelength-independent
Orbital period is days
Amplitude of flux variation in V-filter ~3%
The primary of Spica is a Cepheid variable
Periodic variation in the pulsating primary star is much shorter than the system’s orbital period and about a factor of 2 less in magnitude
Period is 4.17 hours
Amplitude of flux variation in V-filter ~1.5%
This short-term variation, identified in 1968, became undetectable in the early 1970’s (but may return again due to precession of the primary’s rotation axis relative to the orbital plane, which has a period of 200 years [Balona, 1986])

12. Ellipsoidal variation model
Variation in flux is given by [Shobbrook, 1969; Sterken et al, 1986]:
dE = A M2/M1 (R/D)3 (1+e cos(TA+Φ))3 (1-3cos2(TA+TA0+Φ) sin2i )
Where:
A= (wavelength dependent “photometric distortion”)
M2/M1 = 1/1.59 (ratio of masses)
R = 7.6 Rsun = e6 km (polar radius of primary)
D = e7 km (mean separation between stars)
e = 0.14 (orbital eccentricity)
TA (true anomaly)
T0 = days (orbital period)
TA0 = 150 degrees (apparent angle to line of apsides in
year 2005, has precession period of 128 years)
i = 65.9 degrees (orbital inclination)
Φ = empirical phase shift, a free parameter to match with data
One period of the expected variation in flux from Spica

13. Normalized signal vs time
New data
The left plot shows the total FUV signal vs time (normalized to the mean), with a line fit
The right plot shows the same data with this linear trend removed, along with a theoretical model of the Spica ellipsoidal variation that has been fit to the curve (optimizing only the magnitude and phase offset parameters)

14. Data vs model
The Spica model continues to be consistent with the observed variability

15. Stellar flux comparisons
At the last meeting, an effort was made to compare the stellar spectra from UVIS with that derived from the SORCE-SOLSTICE instrument
In order to include as many stars as possible, it was necessary to use spectra from when the star was located in the star-burned rows of the detector
The next few slides summarize those results

16. Ratio of UVIS to SOLSTICE
This plot shows the ratio of UVIS spectra (1.1nm bins) to SOLSTICE over nm
Here, UVIS spectra were reduced using the flat-field
While the magnitude varies, a consistent shape in the ratio is apparent
WITH flat-field

17. Ratio of UVIS to SOLSTICE
Normalized to a mean value of one

18. Ratio of UVIS to SOLSTICE
This plot shows the average ratio of UVIS to SOLSTICE for all stars observed (except Alp PsA)
No normalization has been done here 18

19. Ratio of UVIS to SOLSTICE
This plot shows the average ratio of UVIS to SOLSTICE for all stars observed (except Alp PsA)
No normalization has been done here 19

20. Comparison of UVIS to SOLSTICE away from the starburn
Only two stars have along-slit slew scans and are sufficiently comparable: Alp Vir (Spica) and Eta Uma
The left plot shows the absolute spectra from UVIS and SOLSTICE
The right plot shows the ratio of UVIS to SOLSTICE
Most of the shape in the ratio appears consistent with the previous results with the stars located in the starburn

21. Ratio of UVIS to SOLSTICE
This shows the ratio of spectra where the star is located away from the starburn
The dashed lines are from where the star was located in the starburn (row 31)

22. Ratio of UVIS to SOLSTICE with flat-field applied
This shows the ratio of spectra where the star is located away from the starburn
The dashed lines are from where the star was located in the starburn (row 31) 22

23. Lots more to do
Quantify sensitivity decrease at and around Lyman alpha, include in calibration routine
Develop sensitivity correction based on comparisons to SORCE-SOLSTICE
Compare with Don’s sensitivity curve
Develop a flat-field corrector based on the hypothesis of spatially variant PSF

24. Summary
Significant increase in the depth of the starburn in the last 6 months
Both in FUV and EUV
More stellar occultations recently?
Spica variability model continues to match the observations
Comparisons of UVIS spectra with SOLSTICE suggest that a shape correction in the FUV is required
UVIS Calibration paper
Work continues, but not quite ready to distribute a draft

25. Ratio of UVIS to SOLSTICE
This plot shows the average ratio of UVIS to SOLSTICE for all stars observed (except Alp PsA)
Average of all stars

26. Ratio of UVIS to SOLSTICE
This plot shows the average ratio of UVIS to SOLSTICE for all stars observed (except Alp PsA)
Expanded vertical scale