Global well-Posedness for a higher-order Benjamin-Ono-Schrödinger system in the energy space

We study the Cauchy problem associated with the higher-order Benjamin-Ono-Schrödinger system \begin{equation*} \begin{cases} \partial_{t}r-a\partial_{x}^{3}r-b$\mathcal{H}$\partial_{x}^{2}r =cr\partial_{x}r -d\partial_{x}(r$\mathcal{H}$\partial_{x}r+$\mathcal{H}$(r\partial_{x}r)) +β\partial_{x}(|q|^{2}), \quad x,t\in $\mathbb{R}$,\ i\partial_{t}q-α\partial_{x}^{2}q=-βqr , \end{cases} \end{equation*} where $b,c,d,\alpha ,\beta$ are positive constants and $a\mathrm{\ne }0$ is a real constant. This system was introduced by Kairzhan, Kennedy, and Sulem in \cite{Higher-order-Benjamin-Ono-NLS-System-Sulem}. We prove that this system is locally well-posed in the energy space $H^1(\mathbb{R})\times H^1(\mathbb{R})$. Furthermore, in the case $a<0$, this result extends globally for initial data $(r_0,q_0)$ with sufficiently small $H^1\times H^1$-norm. The proof combines compactness arguments with energy methods. To provide smooth solutions, we have to deal with the lost of the derivatives phenomenon introduced by higher-order derivatives and the Hilbert transform in the nonlinear terms when performing energy estimates. This is overcome by introducing a modified energy functional that cancels the problematic terms arising in the standard energy estimates. Once this is done, we extend the method put forward by Molinet and Pilod in \cite{HOBOinH1-Didier-Molinet} to study a single higher-order Benjamin-Ono equation. Their procedure includes the use of a gauge transformation of Tao’s type \cite{GWP-BO-Tao}, and delicate bilinear estimates in Bourgain type spaces.