X-ray outburst from the black hole MAXI J0637-043. Image: NASA/Goddard/University of Arizona/MIT/HarvardThe OSIRIS-REx spacecraft set out on September 8, 2016 to study the asteroid Bennu. But last fall, one of its experiments detected something surprising: a flare from a black hole.OSIRIS-REx launched with a suite of instruments, including the Regolith X-Ray Imaging Spectrometer, or REXIS.…