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Systematic errors of sidereal time in time marginalization

For a PE run with time marginalization, geocent_time is fixed to the start time of data (See this line). This means the sidereal time is calculated at that time rather than the coalescence time of signal. It leads to systematic errors of ~2pi * duration / (24 * 60 * 60) in right ascension.

This could be an issue for BNS, whose duration is long. Below is the p-p plot for BNS signals, which are observed by [H1, L1, V1], whose maximum luminosity distance is 100Mpc, and whose median SNR is ~25.

p-p-plot

The bad p-p plot for right ascension is fixed after the right ascension is incremented by the expected systematic error as follows:

correction = 2. * np.pi * duration / (24. * 60. * 60.)
samples["ra"] = np.fmod(samples["ra"] + correction, 2. * np.pi)

The plot attached below compares the p-p plots of right ascension before and after this correction.

ra_corrected

I should also note that the p-p runs were done with the ROQ likelihood in my local environment, into which I have implemented time marginalization. Since its implementation is almost same as that for GravitationalWaveTransient, I think the same thing happens for GravitationalWaveTransient.

Edited by Soichiro Morisaki