Transients and variable objects pervade the X-ray universe, some as spectacular outbursts. Observations and interpretations of these phenomena have greatly advanced, or even revolutionised, our understanding of the universe and its underlying physical laws. They provide ideal laboratories to explore the limits of contemporary physics and to study matter under extreme conditions, which are otherwise hard to realize in ground-based laboratories. A rich variety of transients and variables, with diverse timescales from sub-seconds to years, have been discovered and extensively studied since the early days of X-ray astronomy, thanks to successive all-sky monitors (ASM) in X-rays. Yet many key questions remain unanswered. New phenomena continue to be discovered and appeal for observations on a large scale for characterisation, others are highly expected and await discovery. Time-domain astronomy will see its golden era towards the end of this decade with the advent of major facilities across the electromagnetic spectrum and in the multi-messenger realms of gravitational wave and neutrinos. This makes monitoring the X-ray sky even more exciting.

The Einstein Probe (EP) mission is a small scientific satellite dedicated to time-domain high-energy astrophysics. It aims at discovering transients and monitoring variable objects in 0.5-4 keV X-rays, for which it will employ a wide-field X-ray telescope (WXT) with a large instantaneous field-of-view (60°×60°, or ~1 steradian), along with moderate spatial resolution (FWHM ~5arcmin) and energy resolution. Its wide-field imaging capability is achieved by using established technology of novel micro-pore “lobster-eye” optics, thereby offering unprecedentedly high sensitivity and large Grasp superseding previous and existing X-ray all-sky monitors and survey missions. To complement this powerful ability to monitor and discover sources over a wide area, the Einstein Probe will also carry a smaller field-of-view (1°×1°) X-ray telescope — capable of much larger light-collecting power and better energy resolution than the main survey telescope — with which to perform follow-up observations of newly-discovered transients. Public transient alerts will also be issued rapidly, in order to trigger multi-wavelength follow-up observations from the world-wide community.

The satellite will be on a near-Earth circular orbit at an altitude of 650km with a low inclination < 30°. During each of the 97-minute orbit, five adjacent patches of the night-side of the sky will be observed in pointing mode, each with an 11-minute exposure. Over three orbits almost the entire night sky will be covered, with cadences ranging from 5 to 25 times per day for a give sky position.

The key science objectives of the Einstein Probe mission are to:

  1. Reveal quiescent black holes over almost all astrophysical mass scales and study how matter fall onto them, by detecting their rare X-ray transient events, particularly stars being tidally-disrupted by otherwise dormant massive black holes in galactic centers.
  2. Detect and locate the electromagnetic-wave counterparts of gravitational-wave events (such as binary neutron-star mergers) found with the next generation of gravitational-wave detectors.
  3. Perform systematic all-sky surveys for a diverse set of X-ray transients/bursts and the variability of known X-ray sources over a wide range of timescales and at high cadence, including supernova shock breakout, black hole X-ray binaries, neutron stars, active galactic nuclei, gamma-ray bursts, and stellar coronal activity, etc.

The mission will address questions such as the prevalence of black holes in the universe, how black holes accrete mass and launch jets, the astrophysical origins and underlying processes of gravitational wave bursts, and details of the physics which operates in extreme conditions of strong gravity. All these objects, phenomena and conditions are predicted and dealt with by Einstein’s general theory of relativity.

Einstein Probe has been selected as one of the candidate missions for advanced studies under the pilot space science programme of the Chinese Academy of Sciences (CAS). The proposed launching date is around 2020/2021 and the nominal lifetime is 3 years. The Einstein Probe mission is a CAS-led project and is open to international collaboration, currently involving China (NAOC, IHEP and Tsinghua Univ.) and UK (Leicester Univ.).