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We report deep Green Bank Telescope (GBT) spectroscopy in the redshifted NH₃~ (1, 1), CS~1-0 and H₂CO~0 ₀₀-1 ₀₁ lines from the z 0. 685 absorber towards B0218+357. The inversion (NH₃) and rotational (CS, H₂CO) line frequencies have different dependences on the proton-electron mass ratio, implying that a comparison between the line redshifts is sensitive to changes in. A joint 3-component fit to the NH₃, CS, and H₂CO lines yields / = (-3. 5 1. 2) 10^-7, from z 0. 685 to today, where the error includes systematic effects from comparing lines from different species and possible frequency-dependent source morphology. Two additional sources of systematic error remain, due to time variability in the source morphology and velocity offsets between nitrogen-bearing and carbon-bearing species. We find no statistically-significant (3) evidence for changes in, and obtain the stringent 3 constraint, / < 3. 6 10^-7, over 6. 2~Gyrs; this is the best present limit on temporal changes in from any technique, and for any lookback time, by a factor 5.
Nissim Kanekar (Wed,) studied this question.