During the past week, NuSTAR performed a 21-hour observation of J1622+3521, a newly-discovered “Green Pea” galaxy hosting a powerful accreting supermassive black hole. Green Pea galaxies were first discovered by Citizen Scientists in 2009 as bright green point-like galaxies, which we now believe are primarily compact (<5 kpc size), highly starforming (~10 solar masses per year) galaxies at a specfic distance range (average redshift ~0.25) such that the strong oxygen emission lines from the starburst are shifted into bands that produce an apparent green color. However, the oxygen emission could from processes other than star formation. Green Peas have recently gained renewed interest since the discovery of “Little Red Dots” by NASA’s James Webb Space Telescope. First identified as surprisingly bright red point-like galaxies in James Webb infrared images, scientists have been wrestling with explaining this population. One recent theory is that Little Red Dots are powered by Black Hole Stars -- enormous pristine gas stars reaching millions of solar masses powered by an accreting massive black hole at its center. One of the most powerful probes of accretion onto compact objects is X-ray observations, but so far no X-ray observatory has come close to detecting a Black Hole Star candidate. With many studies now suggesting that Green Pea galaxies could be the low-redshift analogues of Little Red Dots, the NuSTAR observations of J1622+3521 will search for conclusive evidence of a Black Hole Star from the nature of its X-ray spectrum. If J1622+3521 is powered by accretion from a spherical cloud of thick pristine gas, NuSTAR should show the strong signatures of X-ray reprocessing at energies above 10 keV, offering the most sensitive direct X-ray constraints to date on the existence of a Black Hole Star.
Author: Peter Boorman, NewAthena/WFI Project Scientist, Max Planck Institute for Extraterrestrial Physics
