Using Chandra, astronomers discovered a previously hidden black hole or neutron star companion that is triggering repeated outbursts from a giant, unstable star. These outbursts are so extreme that they rival the brightness of a supernova. These images show the star AT 2016blu in X-rays from Chandra along with its host galaxy, NGC 4559, in optical light from the SDSS. AT 2016blu is in the middle of the small square of the labeled version and is seen in both the Chandra and SDSS data. The SDSS data also shows the galaxy and foreground and background objects in red and blue.

Images showing AT 2016blu in outburst and quiescence. The top-left image in the mosaic of four images shows optical data obtained March 14, 2026, by the Katzman Automatic Imaging Telescope (KAIT), when AT 2016blu was in quiescence, or not in outburst, and the top-right image shows KAIT data from March 25, 2026, when the source was in outburst. The bottom-left image shows NASA’s Chandra X-ray data obtained in 2001 and 2002, before AT 2016blu’s first observed outburst in 2012, and the bottom-right image shows Chandra data obtained between March 24 and March 27, 2026.

This tactile plate is a physical relief map based on the intensity of X-ray and optical data, featuring a spiral galaxy with a giant, unstable star at its outer edge. Using Chandra, astronomers discovered the star’s previously hidden companion, which triggers luminous outbursts every 113 days. The spiral galaxy is called NGC 4559. Shown at a sharp angle in the plate, its flat face tilts toward our upper left. Sitting near the outer edge of the dense spiral, near the center of the frame, is a small group of pointed dots. This group is outlined with a raised square.

The smallest dot in this group is AT 2016blu, a massive star some 33 times the size of our Sun. For over 15 years astronomers have been watching this star get bright and then dim again in optical light on a regular basis. Chandra’s X-ray observations suggest that a hidden companion object, either a stellar mass black hole or a neutron star, is pulling material from AT 2016blu every 113 days as they orbit one another. This captured material gets so hot that it shines brightly in X-rays. Other specks are unresolved stars or star clusters, too distant to be related to the movement of material between AT 2016blu and its hidden but influential neighbor.