Attendance:

Notes by: Karl

• Brief Update: Foreground Workshop - Raphael
• Status of spectrometer decision; Please be prepared to opine on whether we should keep a spectrometer on the spacecraft at the possible expense of a reduction in the Imager aperture and sensitivity
• Emphasis of efforts in the near future

Hi Al,

Can you pose this question to the group: what spectrometer would it take to get to the silk damping signal at >5 sigma? What aperture or set of apertures? Is the measurement feasible given the foregrounds? What are the other challenges to the measurement? Would you gain anything by sacrificing the polarization sensitivity?

Shaul

Hi Shaul,

Max and Colin have run some analyses to see what is required for a spectrometer to detect the spectral distortion (mu distortion) from Silk damping at better than 5 sigma. For specificity, the sensitivities are calculated relative to PIXIE 12 month observation. Sensitivity scales as sqrt(time) and sqrt(etendu).

• If there were no foregrounds, reaching 5 sigma requires 13x PIXIE sensitivity. This could be PIXIE with 153 months of observations, or a larger Super-PIXIE spectrometer with (say) 4 sets of mirrors and 54 months observation.

• Foregrounds seriously degrade the prospects of detection. A rough scaling is that the synchrotron amplitude and spectral index need to be measured to 0.1%. Achieving this with PIXIE's baseline 15 GHz spectral bin width requires 390x PIXIE sensitivity for a worst-case scenario (no help from spatial information) and ~80x PIXIE sensitivity for a more realistic scenario with foreground cleaning such as SMICA that includes spatial information. This could be achieved using some combination of longer integration time (10 year mission like WMAP), fatter beams (7 deg tophat like FIRAS) and multiple cloned FTS assemblies (12 copies).

• Foreground subtraction is substantially improved by adding channels below 30 GHz. Fourier transform spectroscopy is not ideal for such low-frequency observations, so this would better be addressed by adding an absolute photometer of some sort, sharing the same calibrator as the FTS. Adding a low-frequency photometer could reduce the sensitivity requirement of the FTS from the 390x PIXIE sensitivity above to perhaps 30x PIXIE. One such scenario would have 4 years integration, 5 deg beams, and 3 copies of the FTS assembly.

Much of this analysis is captured in https://arxiv.org/pdf/1705.01534.pdf. It seems that a Super-PIXIE that includes a low-frequency photometer could indeed get a reliable detection of the Silk damping signal.

  Cheers,
  
      -- Al