  Compact binary mergers that involve at least one neutron star, either binary
neutron star or black hole--neutron star coalescences, are thought to be the
potential sources of electromagnetic emission due to the material ejected
during the merger or those left outside the central object after the merger.
Since the intensity of these electromagnetic transients decay rapidly with
time, one should pay more attention to early emissions from such events, which
are useful in revealing the nature of these mergers. In this work, we study the
early emission of kilonovae, short $\gamma$-ray bursts and cocoons that could
be produced in those mergers. We estimate their luminosities and time scales as
functions of the chirp mass which is the most readily constrained parameter
from the gravitational wave detections of these events. We focus on the range
of chirp mass as $1.3M_{\odot} -2.7M_{\odot}$ which is compatible with one of
the merging component being a so-called `mass gap' black hole. We show that the
electromagnetic observation of these transients could be used to distinguish
the types of the mergers when the detected chirp mass falls in the range of
$1.5M_{\odot}-1.7M_{\odot}$. Applying our analysis to the sub-threshold GRB
GBM-190816, we found that for this particular event the effective spin should
be larger than 0.6 and the mass of the heavier object might be larger than
5.5$M_{\odot}$ for the SFHo equation of state.
