STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from\n Microseconds to Years

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy\nX-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It\ncombines huge collecting area, high throughput, broad energy coverage, and\nexcellent spectral and temporal resolution in a single facility. STROBE-X\noffers an enormous increase in sensitivity for X-ray spectral timing, extending\nthese techniques to extragalactic targets for the first time. It is also an\nagile mission capable of rapid response to transient events, making it an\nessential X-ray partner facility in the era of time-domain, multi-wavelength,\nand multi-messenger astronomy. Optimized for study of the most extreme\nconditions found in the Universe, its key science objectives include: (1)\nRobustly measuring mass and spin and mapping inner accretion flows across the\nblack hole mass spectrum, from compact stars to intermediate-mass objects to\nactive galactic nuclei. (2) Mapping out the full mass-radius relation of\nneutron stars using an ensemble of nearly two dozen rotation-powered pulsars\nand accreting neutron stars, and hence measuring the equation of state for\nultradense matter over a much wider range of densities than explored by NICER.\n(3) Identifying and studying X-ray counterparts (in the post-Swift era) for\nmultiwavelength and multi-messenger transients in the dynamic sky through\ncross-correlation with gravitational wave interferometers, neutrino\nobservatories, and high-cadence time-domain surveys in other electromagnetic\nbands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle\nand high time resolution to characterize the behavior of X-ray sources over an\nunprecedentedly vast range of time scales. STROBE-X's formidable capabilities\nwill also enable a broad portfolio of additional science.\n