  The three pentaquark states, $P_{c}(4312)$, $P_{c}(4440)$, and $P_{c}(4457)$,
discovered by the LHCb Collaboration in 2019, can be nicely arranged into a
multiplet of $\bar{D}^{(\ast)}\Sigma_{c}^{(\ast)}$ of seven molecules dictated
by heavy quark spin symmetry. In this work we employ the effective Lagrangian
approach to investigate the two decay modes of $P_{c}(4457)$, $P_{c}(4457) \to
P_{c}(4312) \pi$ and $P_{c}(4457) \to P_{c}(4312) \gamma$, via the triangle
mechanism, assuming that $P_{c}(4457)$ and $P_{c}(4312)$ are
$\bar{D}^{\ast}\Sigma_{c}$ and $\bar{D}\Sigma_{c}$ bound states but the spin of
$P_{c}(4457)$ can be either 1/2 or 3/2. Our results show that the spin of
$P_{c}(4457)$ can not be discriminated through these two decay modes. The decay
widths of $P_{c}(4457) \to P_{c}(4312) \pi$ and $P_{c}(4457) \to P_{c}(4312)
\gamma$ are estimated to be of order of 100 keV and 1 keV, respectively. The
ratio of the partial decay widths of $P_{c}(4457) \to P_{c}(4312) \pi$ to
$P_{c}(4457) \to P_{c}(4312) \gamma$ is similar to the ratio of $D^{\ast}\to
D\pi$ to $D^{\ast}\to D\gamma$, which could be used to check the molecular
nature of $P_{c}(4457)$ and $P_{c}(4312)$ if they can be observed in the
future.
