Absolute Flux Calibration for STIS First-Order, Low-Resolution Modes

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1 Instrument Science Report STIS Absolute Flux Calibration for STIS First-Order, Low-Resolution Modes Ralph Bohlin, Space Telescope Science Institute Nicholas Collins, Hughes STX/LASP/GSFC Anne Gonnella, Space Telescope Science Institute February 1998 ABSTRACT Point source sensitivity curves are derived for the first order modes G140L, G230L, G230LB, G430L, and G750L, which collectively span a wavelength range from 1140Å to 10320Å. The curves are determined by comparing wide slit (52" x 2") observations of the spectrophotometric standard star GD153 to a pure hydrogen white dwarf model. The calibration is checked by comparing a calibrated STIS observation of the spectrophotometric standard star G191B2B to its pure hydrogen model. Except for G140L, there is no good evidence for changes in the sensitivity beyond ~1% through the end of Except for the G140L and the long wavelength part of G750L, the absolute flux calibration is accurate for observations in 1997 to ~3% for stars centered in the 52x2" slit. Preliminary results on the vignetting analysis for the first order modes suggest that the smooth changes in sensitivity as a function of distance along the slit are less than ±2 % for all first order modes, except for the extremities of G140L, where 5% differences in detector QE are observed. While the 52x2" slit is ideal for photometric continuum determinations, a narrower slit should be used for line profile measurements in order to minimize the contamination from the impure light in the wings of the PSF. For the narrowest slits, local inhomogeneities in the slit width cause a variable throughput along the slit (see the STIS Foibles page on the STIS Instrument WWW pages). 1. Introduction A sensitivity curve, i.e. an absolute flux calibration, is used to determine the flux of an observed point-source spectrum by F = C / S 1

2 where F is the calibrated absolute flux spectrum, C is the observed spectrum in counts pixel -1 second -1, and S is the sensitivity as a function of wavelength (Bohlin, Lindler, & Keyes 1995). The sensitivity is derived by S = C std / F std where C std is the countrate spectrum of a spectrophotometric standard star F std is the known absolute flux spectrum of the same standard in erg cm -2 s -1 Å -1. The pixel in the units for C refers to the dispersion direction, while the point source response is integrated over a fixed extraction height in the cross-dispersion direction on a two-dimensional detector. Before the 1998 February delivery of these throughputs that are based on in-flight observations, the STScI CALSTIS pipeline utilized pre-launch estimates in the photometric throughput tables to flux calibrate data. When the rootname_sx2 files for low resolution first order data calibrated with the prelaunch estimates are used to find the stellar flux as described in the Data Handbook (page 23-5 version 3.0) or when a point source spectrum is extracted from the flat fielded data with the X1D IRAF task, the fluxes are generally within a factor of 1.5 of the correct fluxes. For accurate fluxes, GO s should recalibrate their data (X2D or X1D IRAF tasks) using the latest reference files. 2. STIS Observations Observations of the pure hydrogen white dwarf GD153 (Bohlin, Colina, & Finley 1995) are used to define the sensitivity curve, since these data are the only set of observations of a fundamental standard in all five low-resolution spectral modes. GD153 is a preferred calibration standard, because the only lines are from HI and because Bohlin (1996) defined the FOS calibration with a set of four pure hydrogen WD models that includes GD153. An input spectrum with many lines can confuse the derivation of a sensitivity curve, if the reference spectral line profiles do not perfectly match those of the observed spectrum. All input spectra were obtained using the clear 52"x2" aperture. Relative transmission corrections are required to compute absolute fluxes for other apertures. Two observations comprise the average input spectrum for the far-uv MAMA mode G140L. Only one near-uv (G230L) observation of GD153 exists in the 52x2 aperture. For each of the three CCD modes (G230LB, G430L, and G750L), seven observations obtained at the end of SMOV produce high signal-to-noise average spectra. The position of these seven spectra repeated to 1 pixel on the CCD; and the full range of scatter of relative response is better than 1% in broad bands. However, spectra obtained at other times and at non-standard locations on the CCD show an additional scatter of 1 2%. The component spectra that make up the average spectrum for each mode are listed in Table 1; and 2

3 the spectral extraction heights are 11 pixels for G140L and G230L, and 7 pixels for G230LB, G430L, and G750L (Leitherer & Bohlin 1997). The background is extracted 300 pixels above and below the spectrum and is typically much less than 1% of the continuum signal. Each of the seven component G750L spectra is corrected for the long wavelength CCD fringing with a tungsten flat obtained in the 0.3x0.09 slit. This correction starts with the extracted 1-D spectrum of the tungsten lamp for the three observation that have a 0.3x0.09 contemporaneous flat, O3TT46040, O3TT47040, and O3TT The average of these three spectra is normalized to unity with a spline fit; the amplitude of the fringes longward of 6620 Å is reduced by 11%; an offset shift with respect to each stellar spectrum is computed; and this shifted 1-D tungsten flat is divided into each 1-D spectrum of the star. Below 6620 Å, the tungsten flat is set to unity, since the noise level is worse when the average flat is applied. This procedure works, because all of the seven GD153 spectra lie within a pixel of the same location on the CCD. However, the noise level for the earliest of the seven, O3TT42040, is larger than for the other six, so that the final average GD153 spectrum for G750L is composed of only six observations. Details of the general applicability of this procedure, including a quantitative comparison with alternative techniques will be presented in a fringe ISR. 3

4 Table 1. Observations with 52x2 arcsec slit Rootname Observation Date Target Optical Element Exposure Time (sec) Mean Counts px -1 Prop ID O43J01QAM 09/07/97 GD153 G140L O3ZX08HHM 13/07/97 GD153 G140L O3ZX08HLM 13/07/97 GD153 G230L O3TT /05/97 GD153 G230LB , O3TT /05/97 GD153 G230LB , O3TT /06/97 GD153 G230LB , O3TT /06/97 GD153 G230LB , O3TT /06/97 GD153 G230LB , O3TT /06/97 GD153 G230LB , O3TT /07/97 GD153 G230LB , O3TT /05/97 GD153 G430L , O3TT /05/97 GD153 G430L , O3TT /06/97 GD153 G430L , O3TT /06/97 GD153 G430L , O3TT /06/97 GD153 G430L , O3TT /06/97 GD153 G430L , O3TT /07/97 GD153 G430L , O3TT /05/97 GD153 G750L , O3TT /06/97 GD153 G750L , O3TT /06/97 GD153 G750L , O3TT /06/97 GD153 G750L , O3TT /06/97 GD153 G750L , O3TT /07/97 GD153 G750L ,

5 3. Reference Standard Star Flux The reference spectrum, GD153_MOD_002, is a pure hydrogen white dwarf model normalized to Landolt s visual photometry (Bohlin 1996). The spectrum can be obtained from the Calibration Data Base System (CDBS) at: astronomical_catalogs_alt.html 4. Sensitivity Results For each optical mode, the reference spectrum is integrated to match the resolution of each co-added observation and is then divided into the observed spectrum to produce a sensitivity curve in units of counts pixel 1 second ergs second 1 cm 2 Å 1 At the shortest wavelengths of each mode, the sensitivity drops rapidly with wavelength, so that the uncertainty in the flux calibration in dominated by uncertainties in wavelength. In order to derive the proper sensitivity calibration, the wavelength scale must be as observed in the instrumental frame of rest, i.e., with NO velocity correction. The standard star model spectrum must be shifted to coincide with the observations, i.e., the reference spectrum wavelength scale must be shifted by the radial velocity of the star plus the heliocentric correction for the earth and HST velocity vectors. Consequently, the application of derived sensitivity curves must be to the counts/sec with the instrumental rest frame wavelengths. The observed radial velocity of G191B2B is +22 km/s, which includes the gravitational component (Reid & Wegner, 1988), while a value of km/s for GD153 is estimated from the STIS observations of the Balmer lines. From the proper motion and the photometric distance of 77 pc, Finley (private communication) calculates a tangential velocity of 73 km/s for GD153. Splines with evenly spaced nodes are fit to each ratio in order to obtain a smooth sensitivity curve. Each fit is refined by adding nodes at wavelengths where the slope is large. Table 2 lists the number of nodes and the nominal wavelength range for each mode. The wavelength region 1200 Å 1225 Å in mode G140L is masked to exclude the residuals at the strong Ly α feature from the fit. Small residuals at the Balmer lines on G430L and G750L are caused by slight differences in resolution between STIS and the model spectrum. The sensitivity curves and their residuals are shown in Figures 1 and 2. The spline fits are represented by the dashed line in each plot, while the spline nodes are plotted with diamonds. The large residual at Ly α is caused by neighboring continuum light in the far 5

6 wings of the PSF. Figure 3 is a plot of this line, where the model line profile (smooth solid line) agrees well with FOS observations (dashed line). However, the central depth of the line in the STIS spectrum (heavy jagged solid line) is about 10% higher in units of the continuum. Except at wavelengths beyond ~8500Å where there is considerable scattering within the substrate of the CCD, the scattered light level shown in Figure 3 approaches a worst case, because the PSF from the OTA has more energy in the wings from the microroughness of the mirrors, because the effect of the voltage on the repeller wire in the FUV MAMA is to add signal to the PSF wings, and because the wide 2" slit includes the OTA PSF wings. See Figure 6 in Leitherer & Bohlin (1997) for plots of the STIS PSF in the direction perpendicular to the dispersion. Since the fit in Figure 1 ignores this contaminating light, the shape of the sensitivity curve in the 1216Å region is the same as a pointsource continuum calibration based on a STIS spectrum corrected for the impure light. However, at wavelengths shortward of ~1160Å and at the short wavelength cutoff of G230L, out-of-band light dominates the observed spectrum and makes absolute fluxes unreliable. Out-of-band light is substantially reduced with the 0.2" and smaller slits. Additional data are required to quantify the impure light vs. slit width relation and guide the GO in choosing the right slit width for measuring line profiles and equivalent widths. A subsequent ISR will discuss the choice of the optimum slit in detail; and in the interim, GOs are directed to the STIS Foibles page on the STIS WWW page for the latest information. Preliminary results on the vignetting analysis for the first order modes suggest that the smooth changes in sensitivity as a function of distance along the slit are less than ±2 % for all first order modes, except for the extremities of G140L, where ±5% differences in detector QE are observed. While the 52x2" slit is ideal for photometric continuum determinations, a narrower slit should be used for line profile measurements in order to minimize the contamination from the impure light in the wings of the PSF. For the narrower slits, local inhomogeneities in the slit width can cause a variable throughput along the slit (e.g., see the STIS Foibles page on the STIS Instrument WWW pages). 5. Uncertainties The residuals shown in Figures 1 and 2 are determined by dividing each curve by its spline fit and are plotted for the wavelength ranges listed in Table 2. Table 2 lists the percent root-mean-square residuals averaged over all wavelengths for each mode. Average RMS residuals are listed for three wavelength ranges for mode G750L to show how the scatter increases in the long wavelength region, where fringing is the worst. Unless a peakup is done in a small aperture, pointing errors can cause wavelength errors of up to a pixel (~2-3 σ) on the MAMAs and up to a half pixel on the CCDs. The error in absolute fluxes that is caused by wavelength errors depends of the slope of the sensitivity curve vs. wavelength but can be over a percent near the steep sensitivity cutoffs, 6

7 Table 2. Nodes and Average Percent RMS Deviation for Each Mode Optical Element Wavelength Range (Å) Spline Nodes Avg σ rms G140L , a G230L G230LB G430L G750L b 0.44 G750L G750L a. Residuals in the region of the strong Ly α feature ( Å) are not included in the computation of σ rms. b. Number of nodes for full G750L wavelength range. for example below ~1200Å on G140L. If no wavecal spectrum is taken with the data before moving the Mode Select Mechanism, wavelength errors of a few pixels are possible because of MSM repeatability limits. Wavelength errors can often be corrected, using the stellar radial velocity and a spectral feature of known wavelength in the data spectrum. In general, wavelength uncertainties and the counting statistics of the calibration observations, result in negligible errors in the smooth STIS sensitivities derived here for the 1997 epoch. Uncertainties in the absolute sensitivities are dominated by systematic uncertainties in the reference stellar spectra, which are based on model atmosphere calculations for pure hydrogen WDs (Bohlin, Colina, and Finley 1995). Assuming no error in the model atmosphere calculation, the uncertainty is determined by the shape of the model spectrum vs. wavelength and by the normalization to the V magnitudes of Landolt, as explained in (Bohlin, Colina, and Finley 1995). To recap: Landolt photometry is repeatable to 1σ of ~0.004 mag. Allowing for some uncertainty in the normalization of a flux distribution by the broadband V filter photometry, the reference standard star spectra and derived STIS calibration should be accurate to ~1% (1σ) in the V band wavelength region. Additional uncertainty at other wavelengths is introduced by uncertainty in the continuum slope of the model spectrum, which is dominated by uncertainty in the temperature of the model. For the worst case of GD71 at 1150Å, the formal 1σ uncertainty is 0.35% due to the temperature uncertainty of 100K. The main uncertainty in the physics is the Hummer- Mihalas occupation probability formalism, which causes uncertainties of up to 1% between the Balmer lines. So an assignment of 1σ=1% to flux uncertainties that are caused by slope error is conservative and leads to to a formal 3σ=4% uncertainty outside the V band region and 3% at Å. Future revisions to the STIS flux calibration 7

8 should be based on three stars, instead of just GD153, so that the uncertainties would be formally reduced by a factor of 1.7. There is no evidence for changes in the sensitivity or scatter in the STIS spectrophotometry beyond ~1% through the end of 1997, except for G140L where initial results suggest a drop in sensitivity by up to 7% per year. Fringing in the CCD limits photometric precision to ~2% longward of 8500Å. By mid-1998, the sensitivity monitoring observations should be sufficient to more precisely quantify any changes in sensitivity. 6. Application of the Sensitivity Curves to Observations of G191B2B Figures 4a-c show the residuals of STIS fluxes of GD153 and G191B2B calibrated with the sensitivity curves described in this paper. The comparison is with the standard star models from Bohlin (1996). Residuals for the G191B2B fluxes confirm the above estimates of uncertainty in STIS fluxes; but since GD153 provides the flux calibration, its residuals are unity, except for noise and small differences in the Balmer line cores. The small (<1%) systematic deviations from unity for the G230LB and G430L observations of G191B2B in the lower panels of Figures 4a-b may be caused by sensitivity changes over the approximately 5 months between the epochs of the GD153 baseline observations in SMOV and the G191B2B observations in 1997 October-November or else by different sensitivities at the different locations on the CCD. This difference in position is 5 6 px in the dispersion direction and 2-3 px in the perpendicular direction. The systematic deviations from unity for G750L have additions to the list of possible causes: Beyond 6220 Å, contemporaneous flats are used with a normalization to unity that may be in error by ~1%. (No flat field correction is done below 6220 Å, for G750L.) Beyond the Paschen jump at 8200 Å, the models that define the standard stars are uncertain by an additional 1 2%. Acknowledgments We thank Don Lindler and Phil Plait for essential data reduction advice. D. Finley and R. Saffer found the reference for the radial velocity of G191B2B. Thanks to S. Baum for careful review and suggested improvements to this ISR. References Bohlin, R. C. 1996, AJ, 111, Bohlin, R. C., Colina, L., & Finley, D. S. 1995, AJ, 110, Leitherer, C. & Bohlin, R., 1997, Instrument Science Report, STIS 97-13, (Baltimore: STScI). Reid, N., & Wegner, G., 1988, ApJ, 335,

9 Figures Figure 1: Sensitivity curve and residuals for optical element G140L, derived from 2 averaged observations of GD153. 9

10 Figure 2a: Sensitivity curve and residuals for optical element G230L 10

11 Figure 2b: Sensitivity curve and residuals for optical element G230LB 11

12 Figure 2c: Sensitivity curve and residuals for optical element G430L 12

13 Figure 2d: Sensitivity curve and residuals for optical element G750L at central wavelength 7751 Å 13

14 Figure 3: Line profile at Ly α of GD153, illustrating the good agreement of the model (solid line) and FOS (dashed line) in comparison to the STIS data (heavy jagged line) through a wide 2 slit. The excess scattered light will be much less for observations through narrow STIS slits. 14

15 Figure 4a-4c: Residuals of STIS fluxes for GD153 and G191B2B in comparison with the standard star model for G230LB (4a), G430L (4b), and G750L (4c). Since the flux calibration is based on GD153, its residuals are unity, except for noise and small differences in the Balmer line cores. Figure 4a Figure 4b 15

16 Figure 4c 16

17 17

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