During the past week, NuSTAR observed the nearby galaxy UGC 4211 in coordination with ESA’s XMM-Newton observatory as part of a broader multi-wavelength campaign including NASA’s JWST, ALMA, the VLBA, and the VLA. UGC 4211 hosts the closest confirmed pair of actively accreting supermassive black holes, separated by just 750 light years, offering a rare opportunity to study black hole growth in the final stages of a galaxy merger. Systems like this are the precursors to the supermassive black hole binaries expected to produce gravitational waves detectable by pulsar timing arrays and future space-based missions. Previous observations have revealed unusually strong millimeter emission relative to its X-ray brightness, making it one of the most extreme outliers among nearby active galaxies and raising questions about whether the system is heavily obscured, highly variable, or powered by additional physical processes. The primary goal of the coordinated NuSTAR and XMM-Newton observations is to measure the high-energy X-ray emission and determine the level of obscuration and intrinsic luminosity of the system, while the accompanying radio, millimeter, and infrared observations provide complementary constraints on the emission from each nucleus. Together, this campaign will provide one of the most complete views to date of a dual active galactic nucleus and help clarify how black holes grow, interact, and eventually merge.

Author: Michael Koss, Research Scientist, Eureka Scientific