Impact of instrumental systematic errors on fine-structure constant measurements with quasar spectra

TL;DR

This paper introduces a supercalibration method to detect and quantify systematic wavelength calibration errors in high-resolution spectrographs, revealing their potential to mimic or obscure signals of varying fine-structure constant in quasar spectra.

Contribution

The study develops a novel supercalibration technique using solar twin spectra to measure long-range distortions in spectrograph wavelength scales, impacting previous claims of varying alpha.

Findings

Systematic wavelength distortions are common and substantial in UVES and HIRES spectrographs.

These distortions can significantly affect measurements of the fine-structure constant from quasar spectra.

The distortions can explain much of the apparent variation in alpha previously reported.

Abstract

We present a new `supercalibration' technique for measuring systematic distortions in the wavelength scales of high resolution spectrographs. By comparing spectra of `solar twin' stars or asteroids with a reference laboratory solar spectrum, distortions in the standard thorium--argon calibration can be tracked with $\sim$10 m s$^{-1}$ precision over the entire optical wavelength range on scales of both echelle orders ($\sim$50--100 \AA) and entire spectrographs arms ($\sim$1000--3000 \AA). Using archival spectra from the past 20 years we have probed the supercalibration history of the VLT--UVES and Keck--HIRES spectrographs. We find that systematic errors in their wavelength scales are ubiquitous and substantial, with long-range distortions varying between typically $\pm$200 m s$^{-1}$ per 1000 \AA. We apply a simple model of these distortions to simulated spectra that characterize the large UVES and HIRES quasar samples which previously indicated possible evidence for cosmological variations in the fine-structure constant, $\alpha$. The spurious deviations in $\alpha$ produced by the model closely match important aspects of the VLT--UVES quasar results at all redshifts and partially explain the HIRES results, though not self-consistently at all redshifts. That is, the apparent ubiquity, size and general characteristics of the distortions are capable of significantly weakening the evidence for variations in $\alpha$ from quasar absorption lines.