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JWST Absolute Flux Calibration

Karl Gordon
August 11, 2022

JWST Absolute Flux Calibration

Presentation to the internal STScI Cross-Mission Calibration Working Group

Karl Gordon

August 11, 2022
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Transcript

  1. EXPANDING THE FRONTIERS OF SPACE
    ASTRONOMY
    JWST Absolute Flux Calibration
    Karl D. Gordon, Cross-Mission Calibration WG, 11 Aug 2022

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  2. Absolute flux calibration working group

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  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)

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  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)

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  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

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  6. Example spectra
    Flux density in
    Rayleigh-Jeans
    units

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  7. Stars

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  9. Stars

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  10. Stars

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  11. Stars

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  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

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  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

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  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

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  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

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  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

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  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

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  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

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  19. Thanks

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  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

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  21. MIRI imaging
    error budget

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