1. UVIS Calibration Update
Greg Holsclaw, Bill McClintock
Jan. 5, 2010

2. Outline Recent calibration observations
Continued intrinsic variability of Spica
Potential for a modified flat-field corrector

3. Recent UVIS Calibrations
FUV2009_165
FUV2009_276
FUV2009_315
FUV2009_352
Much data lost during downlink due to snow in Madrid

4. Spica variability

5. 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])

6. 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

7. 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)

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

9. Primary UVIS calibration issues
FUV flat-field
Addressed by Andrew Steffl corrector
Increase in sensitivity at FUV long-wavelengths (red-response)
Addressed by time-varying sensitivity
Light-leak in EUV (mesa)
Addressed through change in instrument setup
Decrease in sensitivity at EUV and FUV central rows (starburn)
To be addressed through a modified flat-field corrector
Decrease in sensitivity in FUV around Lyman-alpha
There appear to be two components:
Occultation slit - To be addressed through modified flat-field corrector
Low-res slit - To be addressed through update to time-varying sensitivity

10. Approach to creating a modified flat-field corrector
Create a summed spectrum for each Spica calibration observation
Star is slewed along the slit at a fixed rate
Subtract a background estimate
Apply Andrew Steffl post-starburn flat-field
Normalize each detector column to the mean value in a a select few rows
This creates an image that represents a relative change to these rows
Evaluate how this evolves through time

11. Observation of Spica on 2005-295
Sum of all scans as the star is slewed along the slit.
Spatial average
Spectral average
starburn

12. Application of Andrew Steffl’s flat-field.
Significant reduction in row-to-row and column-to-column variation.
Spatial average
Spectral average
starburn
overcorrected

13. Divide an average spectrum created from rows 18-23, 41-46 into each row.
Spatial average
Spectral average
Reference rows
starburn
overcorrected
Lyman-alpha

14. Date:
Loss-of sensitivity around Lyman-alpha looks like the FUV occultation slit.
Spatial average
Spectral average
starburn
undercorrected
Lyman-alpha

15. Row normalized detector images of Spica slew-scans
time

16. Row normalized detector images of Spica slew-scans
time

17. Some spectral features appear in the normalized image

18. Ratio of row 31 to 20
~40% decrease in sensitivity 125 – 135 nm (relative to row 20)

19. How do we use these results as a corrector?
“First, do no harm”
Make sure that a corrector does not cause more problems than it is solving
Linearly interpolate the corrector values as a function of time?
Need some validation process

20. Absolute spectra of Spica over time
Ratio of UVIS to SOLSTICE
SOLSTICE
Overall sensitivity decline
The time-varying sensitivity is overestimating the adjustment for the most recent observations

21. Change in response due to burn-in at Lyman-alpha due to low-res slit
This shows the ratio of several spectra of Spica acquired over the years, to one obtained on
This represents an average response over several rows
Since these are the “reference” rows used for the previous flat-field modifier, this is an additional correction required

22. HSP sensitivity decline from Miodrag