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

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

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

Spica variability

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

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

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)

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

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

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

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

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

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

Date: 2009-315 Loss-of sensitivity around Lyman-alpha looks like the FUV occultation slit. Spatial average Spectral average starburn undercorrected Lyman-alpha

Row normalized detector images of Spica slew-scans time

Row normalized detector images of Spica slew-scans time

Some spectral features appear in the normalized image

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

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

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

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 2005-295 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

HSP sensitivity decline from Miodrag