Observational prospects for magnetars with GECAM

Magnetars are a group of very young neutron stars with extremely strong magnetic fields. They provide a unique laboratory for discovering and testing physics in extreme conditions. Magnetars exhibit relatively slow rotation compared with other young pulsars, and they undergo fast spin down. Both properties suggest the existence of a very strong dipole magnetic field, of the order of 1014 G. Including candidates, there are currently 29 known magnetars. They are discovered by detecting persistent/transient X-ray emission or soft gamma-ray bursts or flares. In general, the magnetar model can explain energetic phenomena associated with the strong magnetic field. However, many puzzles of magnetars remain: the energy budget of a burst-active period, the connection between the burst or flares and the outburst or glitches in the persistent emission, and the evolution between magnetars and other neutron stars. Additional surprises come with the discovery of each new source. In order to answer these questions, a gamma-ray burst monitor with a full-sky field of view and an ability to locate a burst with adequate precision is required. The Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission consists of two satellites operating at opposite ends of the diameter of a near-Earth orbit. This design enables the field of view to cover the entire sky. Moreover, about 40% of the sky is monitored by both satellites at the same time. Localization of bursts can thus be improved by triangulation between the two satellites. From simulations, a typical short burst from a magnetar can be located to within a region that can be covered by about six pointed observations with X-ray telescopes (e.g., the XMM-Newton). The localization can be improved even further if the burst is also detected by other gamma-ray monitors (e.g., the Gamma-ray Burst Monitor on the Fermi Gamma-ray Space Telescope or the Burst Alert Telescope on the Neil Gehrels Swift Observatory). Therefore, GECAM is a powerful instrument for discovering new magnetars; fully recording the burst-active episodes of magnetars; attempting to detect soft gamma-ray outbursts of magnetar persistent emission and studying their timing properties; and for guiding multi-wavelength follow-up observations. All these are important observational goals that can help us to understand the physical properties of magnetars and the new physics that underlies them.