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UVIS Calibration Update

Explore recent UVIS calibration observations including an increase in starburn depth, Spica variability, and stellar flux comparisons with SOSLTICE. Understand the impact of changes in the FUV starburn depth and Spica's non-eclipsing double-lined binary system on data analysis.

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UVIS Calibration Update

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  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?)

  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

  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 4.01454 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]) http://observatory.sfasu.edu

  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=0.822 (wavelength dependent “photometric distortion”) M2/M1 = 1/1.59 (ratio of masses) R = 7.6 Rsun = 5.2858e6 km (polar radius of primary) D = 1.92916e7 km (mean separation between stars) e = 0.14 (orbital eccentricity) TA (true anomaly) T0 = 4.01454 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 • 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) New data

  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 130-180 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

  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

  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 SOLSTICEwith 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)

  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

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