  We study the first electromagnetic pulse after the gravitational wave chirp
signal, focusing on the profile and duration. It is found that the light curve,
especially the steep decay (SD) phase, can be very different by adopting
different viewing angle $\theta_{\rm view}$ on the jet shell. For an on-axis
jet with a power-law radiation spectrum, the observed flux in the SD is
proportional to $t_{\rm{obs}}^{-2-\beta}$ with $\beta$ being the spectral index
and $t_{\rm{obs}}$ being the observer time. Here, $t_{\rm{obs}}=0$ is set at
the observed time of the jet ejected from the central engine. The SD may become
steep by increasing $\theta_{\rm view}$. We also study the bolometric
luminosity $L$ from a jet shell with a non-power-law radiation spectrum. For an
on-axis jet, $L{\propto}t_{\rm{obs}}^{-3}$ is found in the SD. However, the SD
is steeper than $L{\propto}t_{\rm{obs}}^{-3}$ for the radiation from an
off-axis jet. The higher value of $\theta_{\rm view}$ is, the steeper of SD
would be. Then, we suggest that the SD phase can be used to discriminate an
off-axis jet from an on-axis jet. The reason for above behaviors is discussed.
In addition, we find that the duration of first electromagnetic pulse is close
to its peak time, especially for $\theta_{\rm{view}}\sim20^\circ$. This result
is consistent with that found in GW~170817/GRB~170817A. Thus, the jet
corresponding to the prompt emission of GRB~170817A should be immediately
ejected after the merger. Our results also reveal that the duration of the
first electromagnetic pulse can provide the information of the time to search
gravitational waves.
