  HEX-P is a probe-class mission concept that will combine high spatial
resolution X-ray imaging (<10" FWHM) and broad spectral coverage (0.2-80 keV)
with an effective area far superior to current facilities (including XMM-Newton
and NuSTAR) to enable revolutionary new insights into a variety of important
astrophysical problems. With the recent discoveries of over 40
ultra-high-energy gamma-ray sources (detected above 100 TeV) and neutrino
emission in the Galactic Plane, we have entered a new era of multi-messenger
astrophysics facing the exciting reality of Galactic PeVatrons. In the next
decade, as more Galactic PeVatrons and TeV gamma-ray sources are expected to be
discovered, the identification of their acceleration and emission mechanisms
will be the most pressing issue in both particle and high-energy astrophysics.
In this paper, along with its companion papers (Reynolds et al. 2023, Mori et
al. 2023), we will present that HEX-P is uniquely suited to address important
problems in various cosmic-ray accelerators, including Galactic PeVatrons,
through investigating synchrotron X-ray emission of TeV-PeV electrons produced
by both leptonic and hadronic processes.
