Improved Photometry for G750L

SPACE TELESCOPE SCIENCE INSTITUTE Operated for NASA by AURA Instrument Science Report STIS 2015-01(v1) Improved Photometry for G750L R.C. Bohlin 1, C.R. Proffitt 1 1 Space Telescope Science Institute, Baltimore, MD 08 June, 2015 ABSTRACT The repeatability of STIS G750L spectrophotometry at the longer wavelengths is significantly worse than that at shorter wavelengths or for the STIS G430L grating. The cross-dispersion width increases beyond the extent of the standard seven-pixel extraction width and leads to an increased sensitivity to variations in the width of the spectral profile. This problem is quantified and one solution using an extraction width of 11 pixels instead of the standard width of seven is explored in detail here. The onesigma repeatability in a 400 Å broad band centered at 9700 Å is improved from 0.9 to 0.5%, the cross-calibration agreement with ACS is improved, and the agreement of STIS SEDs with stellar models is better. Even for a flux level as low as 1.3x10-16 erg s -1 cm -2 Å -1, the noise level is increased by only 8%, i.e. the S/N is reduced from 5.9 to 5.4; and even in the limiting case where the signal approaches the background level, the noise increases by only 25% for the larger width of 11. Contents Introduction (page 2) Observations (page 2) Required Corrections for Calculating STIS Flux Distributions (page 3) Results (page 14) Conclusions and Recommendations (page 19) References (page 19) Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration

1. Introduction Photometric calibration uncertainties are the dominant source of error in current type Ia supernova dark energy studies and other forefront cosmology efforts, e.g., photoredshifts for weak lensing mass tomography. Modern surveys require a network of calibration stars with SEDs that are accurate to ~1% in absolute flux. Precision STIS flux distributions simultaneously determine effective temperature, log g, log z, and reddening of stars lying within a good model atmosphere grid (e.g. Bohlin et al. 2014). The best fitting models can be used as flux standards at wavelengths beyond the range of HST and constitute the gold-standards with which to directly calibrate JWST and other facilities. The precision HST photometric heritage can be extended to existing and upcoming surveys, standardizes (spectro)photometry across observatories, and directly addresses one of the current barriers to understanding the nature of dark energy. The STIS low and medium dispersion spectra are extracted with default widths of 11 pixels from MAMA images and 7 pixels from CCD images (Leitherer & Bohlin 1997). These widths (EXTRSIZE) are a compromise between better photometry with wider extractions that collect signal from the PSF wings and better S/N and spectral purity with smaller widths. When noise dominates the signal, wider widths add mostly noise. However for our goal of 1% and requirement of 2% photometric precision, the sevenpixel width is insufficient at the longer wavelengths of G750L. At the expense of somewhat lower S/N, an EXTRSIZE=11 improves the 1σ repeatability from 0.89% to 0.51% in the 9500-9900 Å G750L band for the 63 observations of the STIS monitor standard star AGK+81 266. Stellar SEDs with the larger extraction width agree with theoretical model atmosphere calculations better at the crucial region below one µm, where model atmosphere SEDs are fit to measured STIS flux distributions in order to extend flux estimates beyond the STIS one µm wavelength limit. 2. Observations 2.1 Spectral PSF Width Changes over Time Observations in the wide 52X2 arcsec slit of the star AGK+81 266 are used to monitor changes in the STIS CCD low dispersion sensitivities for G230LB, G430L, and G750L (Stys, Bohlin, & Goudfrooij 2004). Figure 1 illustrates the change in the width of the spectrum over the STIS lifetime. The IDL function gaussfit.pro is used to fit each of the 1024 columns of each of the 63 AGK+81 266 observations. Outliers are removed with a five point median filter before computing the mean FWHM of the PSF at short (columns 50-150) and long (columns 900-1000) wavelengths for each observation. These means appear in Figure 1, where the short wavelength PSF is constant; but the long wavelength PSF width grows at a rate of 0.033 px/yr for a total increase of 0.6 px to date. If the width increase is due to CTE losses to the trailing pixels of the readout, then there should be increasing asymmetry over time with larger values of the PSF shape at lower row numbers. However, there is no obvious asymmetry, which leaves the cause a mystery. Other possible causes include instrumental deformation that could degrade the long wavelength focus of G750L and de-lamination of the G750L replica grating, Instrument Science Report STIS 2015-01(v1) Page 2

which is suspected as the cause of the STIS echelle grating blaze changes. The increase in scatter after SM4 in 2009 May also argues for an environmental effect, instead of CTE, which monotonically degrades. For example, degradation of the external thermal blankets induces stronger thermal transients in the instrumentation. Figure 1 The STIS PSF FWHM at the short wavelengths (average over columns 50-150) below and at long wavelengths (columns 900-1000) above. The approximate wavelengths for these two regions are 5519-6007 and 9668-10156 Å. The larger widths and increasing scatter at the long wavelengths require a larger extraction height to maintain photometric reproducibility. Dashed lines are linear fits to the data points that are connected by solid lines. 2.2 Extraction Widths The changes in measured sensitivity vs. time for several wavelength bins are fit with linear segments for each grating mode; and these corrections are evaluated at the date of observation and interpolated in wavelength. An example of the longest wavelength 9700 Å bin (9500-9900 Å) of G750L appears in Figure 2 for EXTRSIZE=7, where three dashed line segments are the piece-wise linear fits to the data points. The rms 1σ scatter of the data points is 0.89% around the fitted dashed lines. Thus, the expected 1σ difference in the residuals between any two points is 1.26%; but the ~4% difference between the data points for oc4il3060 at 2013.47 and oceil1060 at 2013.91 is more than 3σ. Figure 3 is the same as Figure 2, except for a wider EXTRSIZE of 11. The overall rms scatter is reduced to 0.51%, and few residuals exceed 1%. The rms for Instrument Science Report STIS 2015-01(v1) Page 3

three separate time periods in the 9700 Å bin are 0.43, 0.54, and 0.56%, respectively for side 1 operations through 2001 May 16, side 2 until its failure in 2004 August, and for post SM4 after 2009 May. Temperature control was lost with the side 1 electronics failure, which causes slightly poorer repeatability after 2001 May. When using the narrower EXTRSIZE of 7, these values become 0.99, 0.79, and 0.89% for the same time periods. Perhaps, some of the earliest observations were poorly focused. The reduced noise with EXTRSIZE=11 makes the choice of the piece-wise linear line segments more obvious; and a new break at 1999.3 has been added for both G430L and G750. The break in slope at 1993.3 is even more obvious in the plots for G430L with EXTRSIZE=11, which correspond to Figure 3 and will be discussed in a future ISR. The ratio of oc4il3060 to oceil1060 as a function of wavelength is illustrated in Figure 4 for three EXTRSIZE values, where the EXTRSIZE=11 ratio is unity within ~1%. With a separation of only five months, any change in the CTE correction is negligible compared to the differences between oc4il3060 and oceil1060. Figure 5 compares the rms scatter about the fits for EXTRSIZEs of 7 and 11 for all 11 wavelength bins of 400 Å width. The ratio for a height of 11 in Figure 4 dips below unity by ~0.6% in the 6500-7500 Å range, which is ~2σ for the difference between these two extreme spectra. Table 1 lists the rms scatter of the linear fits vs. time for the 9700 Å bin and four different EXTRSIZE choices. The corresponding noise in the spectra of two faint WDs is also tabulated, as discussed in Section 4.4. A new EXTRSIZE=11 is a suitable compromise choice for the G750L data reduction pipeline. The optimum extraction height depends on specific scientific goals and data quality. As long as the background noise is negligible, the S/N improves with larger extraction heights that extract more signal. Apparently for AGK+81 266, the height of 15 is picking up more noise than signal in comparison to EXTRSIZE=11; but the extreme value of 0.51% for the 9700 Å point in Figure 5 suggest that EXTRSIZE=13 might reduce the rms scatter further for that one bin. However, EXTRSIZE=11 is sufficient to bring the repeatability within our 1% goal for photometric precision. Instrument Science Report STIS 2015-01(v1) Page 4

Figure 2. The change of sensitivity with time for the 9500-9900 Å band of G750L. The diamonds are the average signal in the band for EXTRSIZE=7 and the three dashed lines are linear piece-wise fits to the data, where everything is normalized by the fit value at time zero, i.e. 1997.38. STIS failed in 2004 Aug but was repaired during Servicing Mission 4 in 2009 May. Instrument Science Report STIS 2015-01(v1) Page 5

Figure 3. As in Figure 2 but for EXTRSIZE=11. Notice the reduction of scatter about the fitted dashed lines. At 2015.0, the correction is 0.935 vs. 0.922 for EXTRSIZE=7 in Figure 2. Instrument Science Report STIS 2015-01(v1) Page 6

Table 1. rms scatter for 62 AGK+81 266 Obs. at 9700Å and S/N for 8720-9390Å. EXTRSIZE rms (%) S/N LDS749B WD1657 7 0.89 60 5.9 9 0.61 61 5.7 11 0.51 62 5.4 15 0.58 62 4.9 In principle, an optimal extraction (Horne 1986) should be superior to a fixed width extraction, especially for faint objects. The Optimal Spectrum Extraction Package for IDL of J. Harrington 1 was tried but failed at Step 5 in trying to fit the normalized spectrum as a function of wavelength with a polynomial. Apparently, the STIS CCD has manufacturing geometric distortions at the pixel level, which makes smooth fits impossible. 3. Required Corrections for Calculating STIS Flux Distributions In addition to sensitivity changes as a function of time, STIS CDD data require a correction for temperature and for charge transfer losses (CTE) in the aging CCD (Bohlin & Goudfrooij 2003, Goudfrooij & Bohlin 2006, Goudfrooij et al. 2006). The correlation of G750L signal (electrons s -1 ) vs. temperature is 0.12 ±0.02 % C -1 for EXTRSIZE=11, instead of the 0.06 ±0.02 % C -1 for EXTRSIZE=7. As these correction coefficients differ by <3σ, the difference could be just statistical. For the best precision, the CTE correction should also be updated. However, the STIS signal is not much larger in a EXTRSIZE=11 extraction, as shown in Figure 6; so that the existing CTE correction algorithm for EXTRSIZE=7 should not have a large error. An improved pixel-based CTE correction is under development in order to correct entire STIS CCD images (Lockwood et al. 2013) using the technique developed for ACS (Anderson & Bedin 2010). To compute absolute fluxes (erg s -1 cm -2 Å -1 ), the correction for EXTRSIZE=11 vs. EXTRSIZE=7 is done via the *pct.fits reference files (Bohlin 1998), which account for the systematic difference in signal so that the standard absolute sensitivity function for EXTRSIZE=7 can be applied. The pct corrections are averages of AGK+81 266 observations in the 52x2 arcsec slit and should be updated, because many more observations are available. For example, new G750L 52x2 pct corrections are illustrated in Figure 7 and may be compared to the original figure 8 of Bohlin (1998), where 12 wavelength nodes were used, instead of the 10 for the new reference file. Figure 8 compares the original and new corrections for EXTRSIZE=11. While the original ISR shows 12 wavelength nodes, that fit was revised to 8 nodes in 1999; and the new G750L corrections utilize a ten-node fit. However, some of the EXTRSIZE ratios show a systematic trend as a function of time, so that a more straightforward 1 http://physics.ucf.edu/~jh/ast/software/optspecextr-0.3.1/doc/ Instrument Science Report STIS 2015-01(v1) Page 7

approach is adopted to follow the same procedure as used for calculating fluxes for EXTRSIZE=7, i.e. the EXTRSIZE=11 sensitivity function is the basis of the G750L absolute flux calibration. Figure 9 shows examples of average stellar flux changes for the switch from EXTRSIZE=7 to 11. While Figures 4 and 6 illustrate an extreme difference in flux of almost 4% for the EXTRSIZE of 7 vs. 11 for a pair of individual spectra, the change is always within 2.5% of unity for any particular CALSPEC 2 standard star. In addition to changes of order 1% due random variation of the PSF width, there is a systematic flux decrease in later observations (e.g. HD205905 at 2011.4) relative to earlier observations (e.g. HD93521 at 1998.2) because of the larger time corrections for EXTRSIZE=7 vs. 11, as illustrated for the 9700 Å region in Figures 2-3. For example, Figures 2-3 show a 1/0.925 correction for EXTRSIZE=7 vs. 1/0.937 for EXTRSIZE=11 at 2014.0 and 9700 Å. The second and third stars in time sequence are BD+17 4708 at 2002.9 and Vega at 2003.6. The contribution of the new temperature correction coefficients to flux changes is generally minor, e.g. <0.1% for HD205905 in Figure 9. An explanation for the more significant changes illustrated in Figure 9 is the difference in extracted signal relative to the average. For example, Figure 6 shows a range for the EXTRSIZE=11/7 ratio of >3% at some wavelengths. Thus, the changes shown in Figure 9 can be envisioned mainly as a systematic decrease of ~1% at later times superposed on random fluctuations of ~1%. For the heavily saturated observations of Vega and Sirius (Bohlin & Gilliland 2004, Bohlin 2014) that require large EXTRSIZEs to collect all the signal, the special pct calibration files are updated and utilized to convert the extracted signal to values appropriate for application of the standard sensitivity functions. 2 http://www.stsci.edu/hst/observatory/crds/calspec.html Instrument Science Report STIS 2015-01(v1) Page 8

Figure 4. The ratio in 400 Å bins of two 432s exposures of AGK+81 266: oc4il3060 obtained 2013Jun20 and oceil1060 observed 2013Nov27. The pairs of spectra are extracted with EXTRSIZE=7 (dash), =9 (dots), and =11 (solid). Instrument Science Report STIS 2015-01(v1) Page 9

Figure 5. The rms scatter about the fits for the time changes in the eleven 400 Å bands of G750L with EXTRSIZE=7 (black) and =11 (red). At the longer wavelengths, the larger EXTRSIZE produces better photometry with less scatter, because a larger and more stable fraction of the signal lies within the 11 pixel width. Figure 6. The increase in extracted signal for EXTRSIZE=11 vs. the standard 7 pixel width for a wider spectrum (oceil1060 dots) and a better focused narrower case (oc4il3060 solid). Instrument Science Report STIS 2015-01(v1) Page 10

Figure 7. The G750L pct corrections for different EXTRSIZEs from 3 to 600 relative to unity for a EXTRSIZE=7 and the 52x2 arcsec slit. The error bars at many of the spline nodes (diamonds) are smaller than the symbol size. The curve for EXTRSIZE=200 pixels is the dotted line, while the highest correction for EXTRSIZE=600 represents the infinite width required for the diffuse object specific intensity absolute calibration. Instrument Science Report STIS 2015-01(v1) Page 11

Figure 8. The pct correction for EXTRSIZE=11 relative to EXTRSIZE=7 for G750L. The dotted line is the old locus, while the solid line is the newly revised curve. Instrument Science Report STIS 2015-01(v1) Page 12

Figure 9. Change in flux for the new EXTRSIZE=11 calibration relative to EXTRSIZE=7 for the largest increase (HD 93521) and largest decrease (HD 205905) of the CALSPEC stars and also for five other important CALSPEC standard stars. Instrument Science Report STIS 2015-01(v1) Page 13

4. Results 4.1 Primary WD Residuals A useful check on the use of EXTRSIZE=11 is to compare the flux residuals of the primary calibrators for the two different extraction heights. The modeled standard star flux distributions for the three primary WDs (G191B2B, GD153, and GD71) define the HST absolute flux scale (Bohlin, et al. 2014). For the 52X2 arcsec slit of STIS, the average response for 12, 14, and 17 G750L observations of G191B2B, GD153, and GD71, respectively, define the STIS low dispersion sensitivities, when each star is weighted equally. After applying the sensitivities to compute fluxes, the average STIS flux is compared to the defining model flux SED; and the residuals appear in Figures 10-11 for EXTRSIZE=7 and 11, respectively. The worst error in the three demarcated bands is a positive offset of 0.7% for G191B2B in the 8688-10015 Å band for EXTRSIZE=7 in Figure 10, while this residual declines to 0.3% for EXTRSIZE=11 in Figure 11. For the same band in GD71, there is a residual in Figure 11 of -0.3%. Such low residuals increase confidence that the WD models of Bohlin et al. (2014) are correct and are a necessary (but not sufficient) requirement for achieving the goal of STIS SEDs with sub-percent precision. 4.2 Model Fits for O Stars Bohlin et al. (2014) tabulate preliminary results for model fits to STIS SEDs with a chi-squared minimization technique. However, one flaw in those preliminary results is poor fits at the longer STIS wavelengths, especially for the hottest stars. For the three hottest OB stars, the chi-square residuals for the fits over the 1700-10000 Å range are reduced by about a factor of 2 with the STIS EXTRSIZE=11 SEDs. Instrument Science Report STIS 2015-01(v1) Page 14

Figure 10. Residuals for a complete STIS flux calibration using the current default of EXTRSIZE=7. The vertical dotted lines define three regions (5764-7179, 7227-8556, and 8688-10015 Å) for which the average level and rms scatter appear at the bottom of each panel. G191 in the top panel indicates G191B2B. Instrument Science Report STIS 2015-01(v1) Page 15

Figure 11. Residuals for a complete STIS flux calibration using EXTRSIZE=11, instead of the 7 pixels used for Figure 10. Instrument Science Report STIS 2015-01(v1) Page 16

4.3 ACS Photometry For the three longest wavelength ACS filters, Bohlin (2012) found a >3σ discrepancy of 1-1.6% between ACS/WFC photometry and STIS synthetic photometry for BD+17 4708, P330E, and KF06T2, where agreement would be improved for lower STIS fluxes. The reduction shown in Figure 9 by up to 1% in STIS flux over the relevant wavelength region should reduce most of the discrepancies for these three stars to <2σ. 4.4 Faint Stars A disadvantage to using a wider extraction EXTRSIZE is extra noise from the additional pixels included in the signal, especially when the signal is near the background level. The larger width increases the enclosed detector background by 57%, which for a vanishingly faint target would increase the noise by about 25%. Figure 12 quantifies this extra noise for two relatively faint stars LDS749B (1.0x10-15 ) and WD1657+343 (1.3x10-16 ), where their flux (erg s -1 cm -2 Å -1 ) at 9000 Å is in parentheses. Each of the four panels show the ratio of the measured STIS flux to their model fluxes from the CALSPEC database, where the average ratio and rms scatter per resolution element are written in each panel for three separate bins. At the top of each panel, the extraction height of 7 or 11 pixels is indicated Only in the two longer wavelength bins for the fainter WD1657+343 is there any appreciable increase in the noise level, e.g. from 17.0% for EXTRSIZE=7 to 18.4% for EXTRSIZE=11 in the bin centered near 9100 Å, i.e. the S/N decreases from 5.9 to 5.4, as compiled in Table 1. At shorter wavelengths the S/N is much better because of the much higher signal from this hot star. For the brighter LDS749B, the S/N actually improves with wider extractions because of relatively high signal level, while the background noise dominates the small, added signal for the faint WD1657+343. Instrument Science Report STIS 2015-01(v1) Page 17

Figure 12. Residuals for STIS fluxes of two WDs in comparison to their models. The bottom two panels compare the EXTRSIZE=7 and 11 residuals for LDS749B, while the upper two panels are the same comparison for the fainter WD1657+343. Pairs of the average ratio and the rms noise in percent are indicated for three bins delineated by the vertical dotted lines. Instrument Science Report STIS 2015-01(v1) Page 18

5. Conclustions and Recommendations Given the improvements discussed above for the absolute flux accuracy and repeatability, consideration should be given to changing the default pipeline spectral extraction height for the G750L from 7 to 11 pixels. The EXTRSIZE would change in the appropriate row of the 1dx EXTRACTAB table, which contains the 1D extraction parameters; and the temperature correction would change from 0.06 to 0.12 % C -1. The throughput curve for the G750L requires updating in the PHOTTAB file (Photometric Conversion Table). The G750L pct corrections for the 52X2 aperture in the PCTAB, which contains the photometric correction as a function of extraction box height and wavelength, is significantly improved by considering a greatly expanded dataset of AGK+81 266, 52X2 observations. While pct corrections for other apertures might be similarly updated by analyzing larger archival datasets, there is no known systematic error in the existing pct tables. To complete the list of required pipeline changes, the tds corrections for the G750L change in sensitivity vs. time must be updated in the TDSTAB file per the discussion of Figures 2-3 above. The accuracy requirements promised in the STIS IHB are 5% for absolute photometry and 2% for relative photometry for the L modes. Even though the difference between the two AGK+81 266 observations shown in Figure 4 reaches almost 4% in violation of our 2% requirement, the decision might be made that maximizing S/N at low flux levels is more important than improving the photometric repeatability. For the weakest signals, an EXTRSIZE of 3-5 would be better than 7. However, if the decision is made to keep the default pipeline extraction width at 7 pixels, the fundamental flux calibration should not be based on the wider extraction width of 11 pixels, because the flux calibration and TDS corrections must be implemented as a pair. The TDS correction for EXTRSIZE=7 partially compensates for the increasing width of G750L spectra at the longer wavelengths. However, the improved G750L 52X2 PCTAB should be implemented in any case. References Anderson, J., & Bedin, L. R. 2010, PASP, 122, 1035 Bohlin, R. 1998, Instrument Science Report, STIS 98-01, (Baltimore:STScI) Bohlin, R. C. 2012, Instrument Science Report, ACS 2012-01, (Baltimore:STScI) Bohlin, R. C. 2014, AJ, 147, 127 Bohlin, R. C., & Gilliland, R. L. 2004, AJ, 127, 3508 Bohlin, R. C., Gordon, K.D., & Tremblay, P.-E. 2014, PASP, 126, 711 Bohlin, R. & Goudfrooij P. 2003, Instrument Science Report, STIS 03-03R, (Baltimore:STScI) Goudfrooij, P., & Bohlin, R. C. 2006, Instrument Science Report, STIS 2006-03, (Baltimore:STScI) Goudfrooij, P., Bohlin, R., Maiz-Apellaniz, J., & Kimble, R. 2006, PASP, 118, 1455 Horne, K. 1986, PASP, 98, 609 Leitherer, C., & Bohlin, R. 1997, Instrument Science Report, STIS 97-13, (Baltimore:STScI) Lockwood, S. A., Debes, J. H., Anderson, J., Aloisi, A., Proffitt, C. R. 2013 American Astronomical Society Meeting Abstracts #221, 221, #344.10 Instrument Science Report STIS 2015-01(v1) Page 19

Stys, D. J., Bohlin, R. C. & Goudfrooij, P. 2004, Instrument Science Report, STIS 2004-04, (Baltimore:STScI) Instrument Science Report STIS 2015-01(v1) Page 20