  Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov-
Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic, and
Infrared Remote Imaging System) cameras since March 2014 using both the Wide
Angle Camera (WAC) and the Narrow Angle Camera (NAC). We use images from the
NAC camera to study a bright outburst observed in the southern hemisphere on
July 29, 2015. The high spatial resolution of the NAC is needed to localize the
source point of the outburst on the surface of the nucleus. The heliocentric
distance is 1.25 au and the spacecraft-comet distance is 186 km. Aiming to
better understand the physics that led to the outgassing, we used the Direct
Simulation Monte Carlo (DSMC) method to study the gas flow close to the nucleus
and the dust trajectories. The goal is to understand the mechanisms producing
the outburst. We reproduce the opening angle of the outburst in the model and
constrain the outgassing ratio between the outburst source and the local
region. The outburst is in fact a combination of both gas and dust, in which
the active surface is approximately 10 times more active than the average rate
found in the surrounding areas. We need a number of dust particles 7.83
$\times$ 10$^{11}$ - 6.90 $\times$ 10$^{15}$ (radius 1.97 - 185 {\mu}m), which
corresponds to a mass of dust 220 - 21 $\times$ 10$^{3}$kg.
