JWST Absolute Flux Calibration

Presentation to the internal STScI Cross-Mission Calibration Working Group

Transcript

1. EXPANDING THE FRONTIERS OF SPACE ASTRONOMY JWST Absolute Flux Calibration Karl D. Gordon, Cross-Mission Calibration WG, 11 Aug 2022
2. Absolute flux calibration working group
3. Basics Calibrate all JWST instruments – 0.6 to 28.3 microns – Inherit high-quality cross-calibration between instruments Requirement – 5% for imagers (coronagraphs 5-15%) – 10-15% for spectrographs – AbsFlux program contributes a portion of this requirement Goal: better than 5% (for all?) Test for systematics in calibrators – Different types needed (3 minimum) Cross-calibration with Hubble and Spitzer (& others) Clear and documented methodology (flux prediction & calfactor)
4. Targets Stars – “Simple”, “straightforward” to model (stellar atmospheres) – Heritage (used by Hubble, Spitzer, ground-based) Hot stars (white dwarfs and OB stars) – Simple atmospheres (Hubble primary calibrators) A Dwarfs – Simple atmospheres (Spitzer primary calibrators) Solar Analogs – Solar analogs (Spitzer, strong heritage in IR community)
5. Good calibrator characteristics • Single star • “Easy” to Model star • Previously used as a standard (if possible) • Not variable (< 0.25% 1-sigma) • Not a close binary • No stellar disks (gas or dust) • Low stellar rotation (if possible) • Low or well modeled interstellar extinction • Brightness matched to JWST instrument sensitivities
6. Example spectra Flux density in Rayleigh-Jeans units
7. Stars
9. Stars
10. Stars
11. Stars
12. Cycle 1 “slim” program 1) Calibrate all instrument modes Observe one star of each type in all modes (3 total) Reliance on a single star has not worked well (e.g., Vega) 2) Establish the average calibration Observe larger sample with selected modes NIRCam/MIRI Imaging, NIRSpec/NIRISS/MIRI spectroscopy Diagnose/correct for variations not in models Min of 3 stars per type (must include all from part 1) 3) Repeatability Observe one star approximately once/month Empirically measure detector/instrument/telescope uncertainty Only one filter/grating per detector
13. “Slim” is just the beginning Slim provides – ~3 stars per mode (some modes have more) STScI end-to-end error budget – Assumes 2% uncertainties for an individual star • Each star is unique at some level • From Hubble calibration experience – Number of standards needed for budget • 12 NIRCam/MIRI/NIRISS • 5 NIRSpec Achieving error budget – Requires more stars/observations in future cycles
14. Which stars for “slim”? Picked to optimize the overlap between instruments and modes – Observing efficiency and cross-calibration Fewest targets possible to achieve the slim program goals Use S/N target of 200 – 0.5% measurement uncertainty
15. Summary NIRCam – Imaging • 1 WD, 1 A, 1 G (all) • 3 WD, 3 A, 3 G (wide filters) – WFSS: 1 WD, 1 A, 1 G – Coronagraphy: 1 WD, 1 A, 1 G NIRSpec – IFU: 1 WD, 1 A, 1 G – FS • 1 WD, 1 A, 1 G (all IFU/FS_S1600A1) • 3 WD, 3 A, 3 G (Prism) NIRISS – SOSS: • 1 WD, 1 A, 1 G (all) • 5 A, 4 G (2.5 um) – WFSS: 1 WD, 1 A, 1 G – Imaging: 1 WD, 1 A, 1 G – AMI: 1 A, 1 G MIRI – Imaging: 3 Hot, 8 A, 7 G – MRS: 3 A, 3 G – LRS slitless: 3 A, 3 G – LRS slit: 2 A, 1 G – Coronagraphy: 2A, 2 G
16. Cross-mission calibration All standards have or will be observed – Hubble/STIS (UV+optical) Most with – Spitzer/IRAC 3.6 micron Subset with – Hubble/WFC3 NIR grism (1–1.6 micron) – Spitzer/IRAC and MIPS (4.5 to 24 micron) – TESS ~weeks variability measurements
17. Total time ~318 hours Parts 1 and 2 (calibration+average) – White dwarfs: ~78 h – A-type stars: ~102 h – G-type stars: ~94 h Part 3 (repeatability) – 10 repeats: ~44 h AbsFlux program inherently multi-instrument – Improves fidelity of calibrators – Improves observing efficiency
18. JWST AbsFlux program High quality flux calibration for all instruments Inherently cross-instrument & cross-observatory Will quantify the random & systematic uncertainties Based on expertise from the absolute flux community
19. Thanks
20. Additional information Reports – STScI End-to-end Error Budget • Gordon, Boyer, Muzerolle, Sloan, & Volk, 2019, JWST-STScI-001007 Rev C – JWST Absolute Flux Calibration II: Expanded Sample of Primary Calibrators • Gordon & Bohlin, 2012, JWST-STScI-002540 – JWST Absolute Flux Calibration I. Proposed Primary Calibrators • Gordon, Bohlin, Fullerton, Beck, & Robberto, 2009, JWST-STScI-001855 Selected Papers – A New Stellar Atmosphere Grid and Comparisons with HST/STIS CALSPEC Flux Distributions • Bohlin, Szabolcs, Gordon, et al. 2017, ApJ, 153, 234 – Spectral Calibration in the Mid-Infrared: Challenges and Solutions • Sloan, Herter, et al. 2015, AJ, 149, 11 – Techniques and Review of Absolute Flux Calibration from the Ultraviolet to the Mid-Infrared • Bohlin, Gordon, & Tremblay 2014, PASP, 126, 711 – Absolute Flux Calibration of the IRAC Instrument on the Spitzer Space Telescope Using Hubble Space Telescope Flux Standards • Bohlin, Gordon, Rieke, et al. 2011, AJ, 141, 173 – Absolute Physical Calibration in the Infrared • Rieke, Blaylock, Decin, et al. 2008, AJ, 135, 2245 – Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. II. 70 μm Imaging • Gordon, Engelbracht, et al. 2007, PASP, 119, 1019 – On the calibration of the IRAS low-resolution spectra • Volk & Cohen 1989, AJ, 98, 1918
21. MIRI imaging error budget