Rovibronic molecular hydrogen ( H _ { 2 } ) transitions at redshift z _ { abs } \simeq 2.659 towards the background quasar B0642 - 5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio , \mu .
We utilise an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of \sim 35 per 2.5-km s ^ { -1 } pixel at the observed H _ { 2 } wavelengths ( 335–410 nm ) .
Some 111 H _ { 2 } transitions in the Lyman and Werner bands have been identified in the damped Lyman \alpha system for which a kinetic gas temperature of \sim 84 K and a molecular fraction \log f = -2.18 \pm 0.08 is determined .
The H _ { 2 } absorption lines are included in a comprehensive fitting method , which allows us to extract a constraint on a variation of the proton-electron mass ratio , \Delta \mu / \mu , from all transitions at once .
We obtain \Delta \mu / \mu = ( 17.1 \pm 4.5 _ { stat } \pm 3.7 _ { sys } ) \times 10 ^ { -6 } .
However , we find evidence that this measurement has been affected by wavelength miscalibration errors recently identified in UVES .
A correction based on observations of objects with solar-like spectra gives a smaller \Delta \mu / \mu value and contributes to a larger systematic uncertainty : \Delta \mu / \mu = ( 12.7 \pm 4.5 _ { stat } \pm 4.2 _ { sys } ) \times 10 ^ { -6 } .