Prior to J1820, light echoes on internal accretion disks were only visible in supermassive black holes of millions to billions of solar masses. Those bigger black holes change slowly. Stellar-mass black holes like J1820 evolve faster, allowing observations to be observed on human time scales. According to NASA, J1820 is about 10,000 light-years from Earth and faces Leo. The distance measurement is attributed to ESA's Gaia mission, which provides data that allows the estimation of the distance of the J1820 companion.
The most exciting part of this is that until March 11, 2018, astronomers knew what the J1820 was, and discovered an X-ray burst that made it the brightest source of X-rays in the sky. One. NASA scientists say that although NICER is designed to look for neutron stars, it has proven to be useful for studying X-ray very bright stellar mass black holes.