Comments (1)
Emma Alexander and I have noticed that the "noise" in RM cubes has very nasty properties, e.g. apparently randomly varying from pixel to pixel despite the maps being oversampled, and often with higher values than on-source. That is, sources of genuine polarized emission appear as islands of low polarized intensity surrounded by random spikes of much higher polarized intensity in off-source pixels. We attribute this to deriving spectra from (Q/I) and (U/I) where I is a non-significant quantity that frequently passes through zero. The solution to this would be to divide by a model power-law fitted to the I values rather than the raw values. We think this would also stabilise the RM-fitting where there is genuine signal of relatively low significance. A model power law would by definition not change sign. In regions of noise the amplitude will often be negative but that does not matter since there is actually no signal and we are only interested in the noise. On-source, a power-law fit would nearly always be an adequate approximation for data taken in a single band, covering a few hundred MHz. For fitting over larger spectral ranges the best approach would be a low-order polynomial in log-log space: the order of polynomial could be one of the input parameters. Obviously fitting a power law would require a non-linear least-squares fit, but this can be done efficiently by initially doing a linear least-squares fit in log-log space and using the results as the starting estimate for the non-linear fit.
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Related Issues (20)
- QUfitting requests X display HOT 4
- RMsynth1D: does not specify Stokes I function in outputs HOT 1
- QU fitting incompatible with Py3.5 HOT 2
- Migrate QU fitting away from pymultinest HOT 3
- Adjust Rmsynth3D workflow to save memory
- Possible typo HOT 2
- Rmsynth 3D returning zero output HOT 5
- QU fitting running for hours on Modelled data. HOT 3
- Fracpol not bias-corrected
- New Stokes I fitting routine
- Noise in Q and U, and MAD vs theoretical HOT 3
- Progressbar slows progress HOT 1
- Speedup loops with JIT HOT 1
- error message of "TypeError: Object of type float32 is not JSON serializable" on RMtools_1D/do_RMsynth_1D.py HOT 5
- A minor problem on RMtools_3D/mk_test_cube_data.py HOT 1
- RM-Clean does too many iterations
- FDF cubes should have properly set (degenerate) Stokes axis HOT 1
- QU-fitting Stokes I model failing for many double-component RM models (m11 and m4) HOT 4
- Suggest deleting swap file HOT 1
- Odd behaviour in peak outputs from RMsynth3d HOT 3
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