The detection of bright optical emission measured with good temporal resolution during the prompt phase of GRB 060111B makes this GRB a rare event that is especially useful for constraining theories of the prompt emission .
For this reason an extended multi-wavelength campaign was performed to further constrain the physical interpretation of the observations .
In this work , we present the results obtained from our multi-wavelength campaign , as well as from the public Swift/BAT , XRT , and UVOT data .
We identified the host galaxy at R \sim 25 mag from deep R -band exposures taken 5 months after the trigger .
Its featureless spectrum and brightness , as well as the non-detection of any associated supernova 16 days after the trigger , enabled us to constrain the distance scale of GRB 060111B within 0.4 \leq z \leq 3 in the most conservative case .
The host galaxy spectral continuum is best fit with a redshift of z \sim 2 , and other independent estimates converge to z \sim 1 - 2 .
From the analysis of the early afterglow SED , we find that non-negligible host galaxy dust extinction , in addition to the Galactic one , affects the observed flux in the optical regime .
The extinction-corrected optical-to-gamma-ray SED during the prompt emission shows a flux density ratio F _ { \gamma } / F _ { opt } = 10 ^ { -2 } -10 ^ { -4 } with spectral index \beta _ { \gamma,opt } > \beta _ { \gamma } , strongly suggesting a separate origin of the optical and gamma-ray components .
This result is supported by the lack of correlated behavior in the prompt emission light curves observed in the two energy domains .
The temporal properties of the prompt optical emission observed during GRB 060111B and their similarities to other rapidly-observed events favor interpretation of this optical light as radiation from the reverse shock .
Observations are in good agreement with theoretical expectations for a thick shell limit in slow cooling regime .
The expected peak flux is consistent with the observed one corrected for the host extinction , likely indicating that the starting time of the TAROT observations is very near to or coincident with the peak time .
The estimated fireball initial Lorentz factor is \Gamma \geq 260 - 360 at z = 1 - 2 , similar to the Lorentz factors obtained from other GRBs .
GRB 060111B is a rare case of a GRB with both a bright , well-observed optical counterpart and a ‘ canonical ’ early X-ray light curve , thus providing a good test case of the reverse shock emission mechanism in both energy ranges .