Wavelength Calibration Errors in Keck-HIRES/VLT-UVES and Variation in the Fine-Structure Constant

Mathematics – Logic

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Scientific paper

There have been published results claiming that both the VLT-UVES and Keck-HIRES spectrographs have measured a change in the fine-structure constant (alpha) over cosmological timescales. These measurements rely on precise measurements of the relative spacing of QSO absorption lines. In an effort to provide an independent check on the accuracy of the standard thorium-argon wavelength calibration pipelines, we analyzed spectra of several QSO and star exposures taken with the iodine cell on each instrument. We developed a new method to use the iodine cell as a way to accurately probe distortions to the standard wavelength calibration across echelle orders. We found a couple of types of wavelength shifts needed to correct the Th/Ar wavelength scale in both instruments, and in particular we found shifts within echelle orders (intra-order) distortions that have the potential to affect the claimed detections. For VLT-UVES, we find intra-order velocity distortions of 100 m/s up to more than 200 m/s. These calibration errors are similar to, but smaller than, those found in the Keck-HIRES spectrometer (300 m/s up to 800 m/s). Using the results of Monte Carlo mock experiments, we find errors in the relative change in alpha of between 1e-6 Nsys^(-1/2) and 3e-6 Nsys^(-1/2), where Nsys is the number of systems used and the range is due to dependence on how many metallic absorption lines in each system are compared. We discuss the possible origins and further implications of these miscalibrations as a systematic error in the published claims of a measurement of a relative change in alpha (current value - past value) / current value. Finally, we possibly present results from recent attempts to provide a simple method to correct the miscalibrations in QSO exposures using a reference star exposure.

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