Context : The recent discovery of a weak surface magnetic field on the normal intermediate-mass star Vega raises the question of the origin of this magnetism in a class of stars that was not previously known to host detectable magnetic fields .

Aims : We aim to confirm the field detection reported by Lignières et al .
( 2009 ) and provide additional observational constraints about the field characteristics , by modelling the large-scale magnetic geometry of the star and by investigating a possible seasonal variability of the reconstructed field topology .

Methods : We analyse a total of 799 high-resolution circularly-polarized spectra collected with the NARVAL and ESPaDOnS spectropolarimeters during 2008 and 2009 .
Using about 1,100 spectral lines , we employ a cross-correlation procedure to compute , from each spectrum , a mean polarized line profile with a signal-to-noise ratio of about 20,000 .
The technique of Zeeman-Doppler Imaging is then used to determine the rotation period of the star and reconstruct the large-scale magnetic geometry of Vega at two different epochs .

Results : We confirm the detection of circularly polarized signatures in the mean line profiles .
The signal shows up in four independent data sets acquired with both NARVAL and ESPaDOnS .
The amplitude of the polarized signatures is larger when spectral lines of higher magnetic sensitivity are selected for the analysis , as expected for a signal of magnetic origin .
The short-term evolution of polarized signatures is consistent with a rotational period of 0.732 \pm 0.008 Â d. The reconstruction of the magnetic topology unveils a magnetic region of radial field orientation , closely concentrated around the rotation pole .
This polar feature is accompanied by a small number of magnetic patches at lower latitudes .
No significant variability in the field structure is observed over a time span of one year .

Conclusions : The repeated observational evidence that Vega possesses a weak photospheric magnetic field strongly suggests that a previously unknown type of magnetic stars exists in the intermediate-mass domain .
Vega may well be the first confirmed member of a much larger , as yet unexplored , class of weakly-magnetic stars now investigatable with the current generation of stellar spectropolarimeters .