Context : At the dynamical center of the Milky Way , there is the closest supermassive black hole : Sgr A* .
Its non-flaring luminosity is several orders of magnitude lower than the Eddington luminosity , but flares can be observed in the infrared and X-rays .
This flaring activity can help us to understand radiation mechanisms from Sgr A* .

Aims : Our aim is to investigate the X-ray flaring activity of Sgr A* and to constrain the physical properties of the X-ray flares and their origin .

Methods : In March and April 2011 , we observed Sgr A* with XMM-Newton with a total exposure of \approx 226 \mathrm { ks } in coordination with the 1.3 mm Very-Long-Baseline Interferometry array .
We performed timing analysis of the X-ray emission from Sgr A* using a Bayesian-blocks algorithm to detect X-ray flares observed with XMM-Newton .
Furthermore , we computed X-ray smoothed light curves observed in this campaign in order to have better accuracy on the position and the amplitude of the flares .

Results : We detected two X-ray flares on March 30 and April 3 , 2011 , which for comparison have a peak detection level of 6.8 and 5.9 \sigma in the XMM-Newton/EPIC ( pn+MOS1+MOS2 ) light curve in the 2 - 10 keV energy range with a 300 s bin .
The former is characterized by two sub-flares : the first one is very short ( \sim 458 s ) with a peak luminosity of L \mathrm { { } ^ { unabs } _ { 2 - 10 ~ { } keV } } \sim 9.4 \times 10 ^ { 34 } \mathrm { erg s ^ { -1 } } , whereas the second one is longer ( \sim 1542 s ) with a lower peak luminosity ( L \mathrm { { } ^ { unabs } _ { 2 - 10 ~ { } keV } } \sim 6.8 \times 10 ^ { 34 } \mathrm { erg s ^ { -1 } } ) .
The comparison with the sample of X-ray flares detected during the 2012 Chandra XVP campaign favors the hypothesis that the 2011 March 30 flare is a single flare rather than two distinct subflares .
We model the light curve of this flare with the gravitational lensing of a simple hotspot-like structure , but we can not satisfactorily reproduce the large decay of the light curve between the two subflares with this model .
From magnetic energy heating during the rise phase of the first subflare and assuming an X-ray photons production efficiency of 1 and a magnetic field of 100 G at 2 r _ { \mathrm { g } } , we derive an upper limit to the radial distance of the first subflare of 100 ^ { +19 } _ { -29 } r _ { \mathrm { g } } .
We use the decay phase of the first subflare to estimate a lower limit to the radial distance of 4 r _ { \mathrm { g } } from synchrotron cooling in the infrared .

Conclusions : The X-ray emitting region of the first subflare is located at a radial position of 4 - 100 ^ { +19 } _ { -29 } and has a corresponding radius of 1.8 - 2.87 \pm 0.01 in r _ { \mathrm { g } } unit for a magnetic field of 100 G at 2 r _ { \mathrm { g } } .