  NaOsO$_3$ has recently attracted significant attention for the strong
coupling between its electronic band structure and magnetic ordering. Here, we
used time-resolved magnetic X-ray diffraction to determine the timescale of the
photoinduced \afm dynamics in NaOsO$_3$. Our measurements are consistent with a
sub-100~fs melting of the \afm long-range order, that occurs significantly
faster than the lattice dynamics as monitored by the transient change in
intensity of selected Bragg structural reflections, which instead show a
decrease of intensity on a timescale of several ps.
