CHIPS Systems Engineering

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Ongoing Results from the Cosmic Hot Interstellar
Plasma Spectrometer (CHIPS) Mission

• Mark Hurwitz,a Timothy P. Sasseen,b and Martin
  Sirka
• CHIPS Mission Overview
• Understanding the On-Orbit Performance
• Fields Observed to Date
• Data Reduction
• Results to Date
• aSpace Sciences Laboratory, University of
  California, Berkeley
• bDept. of Physics, University of California,
  Santa Barbara
• Crucial support from David Pierce (WFFC/GSFC),
  Randy Kimble (GSFC)
• NASA Grant NAG5-5213

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CHIPS Observational / Science Goals

• Perform spectroscopic sky mapping at EUV
  wavelengths (90 - 260 Å) of diffuse emission from
  hot plasma in the local bubble.
• Characterize properties of the local hot plasma
  (temperature, abundances, ionization state /
  history, density / morphology, etc.).
• Test models of the cooling rate of hot plasma in
  the local bubble, and by inference, elsewhere.

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Emission Line Spectrum of 106 K Gas with Local
ISM Attenuation
Emission spectrum of 106 K gas attenuated by
local cloud of 2E18 cm-2. Solar abundance and
collisional ionization equilibrium are assumed.
Brightest lines Fe X 174.6Å, Fe IX 171.1Å, Fe
XI180.4Å, Fe XII 186.9Å
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Science Instrument Layout
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Science Instrument Layout
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CHIPS Diffraction Grating Array
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CHIPS Detector Door
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CHIPS Spectral Format
Wavelength
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Calibration Image Spectrum Coordinate System
Field of view filled w/ collimated beam sweeps
multiple source spectra coadded
Wavelength
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Calibration Spectrum
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Response Curve
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Satellite During Environmental Testing at KAFB
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Launch VAFB 12 January 2003
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Field of View
4.5 º
25 º
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Flight Detector Image (Narrow Slits)
He II Lyman ? and ? (256, 243 Å)
Zr/Al
Artifacts
Poly/Al
Hot spot removed in ground s/w
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Background Spectra (Raw Counts Poly/Al Filter)
White Particles Only Red Particles, Geocoronal
Emission, Stray Light
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Stability of CHIPS Calibration Pre-Flight and
On-Orbit

• Pre-flight calibration of components showed
  fair-to-average efficiencies not unrealistic /
  overly optimistic values
• Pre-flight end-to-end calibration in excellent
  agreement (15) with product of components
  despite months of abuse (repeated vibration
  tests of optical subsytem, etc.)
• Instrument purged with N2 virtually continuously
  after end-to-end calibration. TQCM data show
  negligible contamination during thermal-vacuum
  test.
• On-orbit response to geocoronal He II Lyman
  series consistent with nominal response curve
• On-orbit response to stray He I (in aluminum
  panels) consistent with nominal response curve
• On-orbit response to full moon consistent with
  nominal response curve

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Lunar Spectrum (Poly/Al) (Narrow Slits)
White Counts per 0.5 Å inPoly/Al Red Scaled
particle background plus reflected solar model
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Lunar Spectrum (Zr/Al)
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CHIPS long look positions(NGP)
Boron band map of McCammon et al. 1983
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Narrow Slit Fields (Since Early April, 2003)
Coadded Regions HIGH (b gt 45º) LOW (b lt
-45º) PLANE
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Spectral Analysis

• Histogram spectra over useful detector area
• Adjust preflight wavelength scale by 0.9 Å for
  overall best fit of He II 256.3 Å geocoronal
  feature
• Correct spectra for events excluded for hot
  spot regions (v. small effect)
• At wavelengths of interest, sum counts within
  1.4x the nominal 80 EE width (comparable to
  1.4x the FWHM)
• Calculate background by scaling particle
  spectrum to nearby wavelengths on each side
• Intent is to correct for detector artifacts
  true source background is strongly dominated by
  particles.
• Caveat True source background includes some
  stray light, detector dark events could create
  or mask very weak features.
• Subtract background from signal, track
  statistics, divide by Aeff?

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High Latitude Fields Coadded (Poly/Al plus Zr/Al)
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Results Coadded Regions
Best-Fit Line Fluxes Photons / (cm2 sec ster)
and 1? Uncertainty 1subject to transient
uncorrected detector hot spot 2possible
contamination by Vela SNR moderate latitude
sight lines
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Comparison to Predictions for Hot Local Bubble Gas

• Diffuse soft X-ray background reveals local
  component (local hot bubble).
• Distance typ. 100 pc, temperature approx. 106
  K, mean absorption (NHI) typ. 4x1018 cm-2. See,
  e.g., Kuntz Snowden, ApJ 543, 195 (2000).
  Emission measure ranges from 0.0018 0.0058 cm-6
  pc. See, e.g., Snowden, Proc. IAU 166 (Garching
  1997).

(Fe XII excluded)
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B3 Spectra Poly/Al and Zr/Al
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Tentative Detections of Iron Line Features in
Poly/Aluminum
Best-Fit Line Fluxes Photons / (cm2 sec ster)
and 1? Uncertainty Caveat Poly/Al detections
not confirmed in Zr/Al Caveat2 Zr filter
response less well determined than Al
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Potential Implications of the CHIPS Results

• Canonical hot local bubble with significant
  depletion of Fe
• Bubble is too young to evaporate grain cores?
• Hot gas is behind moderate absorption
• Emission arises at edge / shell, not throughout
  the volume
• Emission measure lower than and/or temperature
  far from canonical values
• Solar wind / charge exchange contribution to
  SXRB
• Nonequilibrium ionization effects
• Emission very patchy CHIPS hasnt seen entire
  sky yet
• Instrumental sensitivity significantly
  overestimated
• Fe IX / Fe X ratio, if real, suggests
  surprisingly low temperature (105.8 K) for
  nearby / unabsorbed hot interstellar gas

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Limits on Continuum Break at 228 Å
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Summary / Conclusions

• CHIPS is on orbit and operating nominally
• We set tight constraints on strength of iron
  emission line near 170 180 Å
• Canonical sun-centered local hot bubble seems
  ruled out unless abundances and/or ionization
  conditions anomalous
• Preliminary constraints well below the reported
  detection of iron line complex by McCammon et al.
  (Ap.J. 576, 188)
• Observational challenges posed
• Properties of hot gas in the local cavity
• What ionizes the helium and other elements in
  warm interstellar clouds?
• Ongoing observations and analysis will enable us
  to map more sky, characterize backgrounds, search
  for or limit faint continuum emission, and
  continue to refine limits on emission line
  strengths