The Universe as seen in X-rays is highly dynamic, pervaded with transients and variables of various types involving release of extremely high energies. Exploration of these violent phenomena has greatly advanced, and even revolutionized, our understanding of the Universe and its underlying physical laws, and is expected to keep surprising us with even more profound discoveries. 

The Einstein Probe (EP) is a small mission of the Chinese Academy of Sciences (CAS) dedicated to time-domain high-energy astrophysics. Its primary goals are to discover high-energy transients and monitor variable objects. To achieve this, EP employs a very large instantaneous field-of-view (3600 square degrees), along with moderate spatial resolution (FWHM ~5 arcmin) and energy resolution.  Its wide-field imaging capability is achieved by using established technology of novel lobster-eye optics, thereby offering unprecedentedly high sensitivity and large Grasp, which would supersede previous and existing X-ray all-sky monitors. To complement this powerful capability to discover and monitor sources over a wide area, EP also carries a conventional X-ray focusing telescope with a larger effective area to perform follow-up characterization and precise localization of newly-discovered transients. Public transient alerts will be issued rapidly to trigger multi-wavelength follow-up observations from the world-wide community. The satellite has a weight of ∼1050 kg and average power of ∼825W in total. In the normal survey mode, during one orbit of 97 minutes three fields will be observed on the night-side of the sky with a ~20 min pointing each. Over three orbits almost the entire night sky will be sampled, with cadences ranging from several to a few ten revisits per day depending on the sky location. The mission is aimed for launch by the end of 2022 with a nominal lifetime of 3 years (5 years as a goal).

The primary science objectives are: (1) Discover and characterize cosmic X-ray transients, particularly faint, distant and rare X-ray transients, in large numbers. (2) Discover and characterize X-ray outbursts from otherwise normally dormant black holes.  (3) Search for X-ray sources associated with gravitational-wave events and precisely locate them. These populations of cosmic high-energy transients will be characterized over wide time-scales and at high cadences, revealing new insights into a diverse set of systems including dormant black holes, neutron stars, supernova shock breakouts, active galactic nuclei, X-ray binaries, gamma-ray bursts, stellar coronal activity, and electromagnetic-wave sources and gravitational-wave events. Meanwhile, EP will also monitor the variability of various types of X-ray sources in large samples all over the sky. In light of the multi-messenger and multi-wavelength all-sky monitoring capability highly anticipated in the next decade, EP will produce a legacy data set in the X-ray band that are key to characterizing and understanding the nature of cosmic transients and variables, by working in synergy with the sky surveys by other facilities.

The mission will address some of the key questions in astrophysics and cosmology, and details of the physics which operates in extreme conditions of strong gravity. These include the prevalence of massive black holes in the Universe and how they formed and evolved; how black holes accrete mass and launch jets; the astrophysical origins and underlying processes of gravitational wave events; the progenitors and processes of supernovae; when and where did the first stars form in the early Universe and how they re-ionize the Universe.