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We consider the Cauchy problem for the classical Hirota equation on the line with decaying initial data. Based on the spectral analysis of the Lax pair of the Hirota equation, we first expressed the solution of the Cauchy problem in terms of the solution of a Riemann-Hilbert problem. Further we apply nonlinear steepest descent analysis to obtain the long-time asymptotics of the solution in the critical transition region $|\frac{x}{t} - \frac{\alpha^2}{3\beta}|t^{2/3} \leq M$, $M$ is a positive constant. Our result shows that the long time asymptotics of the Hirota equation can be expressed in terms of the solution of Painlev\'{e} $\mathrm{II}$ equation. Keywords: Hirota equation, steepest descent method, Painlev\'{e} $\mathrm{II}$ equation, long-time asymptotics., Comment: 17 pages

The Sasa-Satsuma equation with $3 \times 3 $ Lax representation is one of the integrable extensions of the nonlinear Schr\"{o}dinger equation. In this paper, we consider the Cauchy problem of the Sasa-Satsuma equation with generic decaying initial data. Based on the Rieamnn-Hilbert problem characterization for the Cauchy problem and the $\overline{\partial}$-nonlinear steepest descent method, we find qualitatively different long time asymptotic forms for the Sasa-Satsuma equation in three solitonic space-time regions: (1)\ For the region $x<0, |x/t|=\mathcal{O}(1)$, the long time asymptotic is given by $$q(x,t)=u_{sol}(x,t| \sigma_{d}(\mathcal{I})) + t^{-1/2} h + \mathcal{O} (t^{-3/4}). $$ in which the leading term is $N(I)$ solitons, the second term the second $t^{-1/2}$ order term is soliton-radiation interactions and the third term is a residual error from a $\overline\partial$ equation. (2)\ For the region $ x>0, |x/t|=\mathcal{O}(1)$, the long time asymptotic is given by $$ u(x,t)= u_{sol}(x,t| \sigma_{d}(\mathcal{I})) + \mathcal{O}(t^{-1}).$$ in which the leading term is $N(I)$ solitons, the second term is a residual error from a $\overline\partial$ equation. (3) \ For the region $ |x/t^{1/3}|=\mathcal{O}(1)$, the Painleve asymptotic is found by $$ u(x,t)= \frac{1}{t^{1/3}} u_{P} \left(\frac{x}{t^{1/3}} \right) + \mathcal{O} \left(t^{2/(3p)-1/2} \right), \qquad 4