Five March observations by Chandra captured the first recorded X-rays from AT 2016blu, adding evidence for a companion-star model of its repeated supernova-like eruptions, researchers say.
The paper reports a signal in all five observing periods from March 24-27, 2026 (UTC), totaling 95.4 kiloseconds of effective exposure. NASA said optical monitoring showed an outburst earlier that year, prompting the X-ray observations. The March X-ray brightness was at least 100 times that of the 2001-02 Chandra observations, which showed no source at the star’s position, the paper reports.
AT 2016blu is a luminous blue variable, a massive star whose brightness changes, in the spiral galaxy NGC 4559, about 30 million light-years from Earth. NASA estimates the star has at least 33 times the Sun’s mass.
Researchers counted 27 detected outbursts from 2012 through June 2026, with the flashes recurring about every 113 days. These events are called supernova impostors because they resemble supernova explosions but leave the star intact.
The study authors propose that a compact object draws in material from AT 2016blu during close orbital passes. As the material heats, it emits X-rays; those X-rays warm gas around the system, which then glows in visible light. The available observations cannot determine whether the companion is a neutron star or a black hole.
NASA said the optical record drew on the Katzman Automatic Imaging Telescope, the Las Cumbres Observatory Global Supernova Project and the American Association of Variable Star Observers. The observations included data from more than 30 amateur astronomers.
The paper compares the proposed orbital interactions with activity discussed for Eta Carinae and pre-supernova outbursts from SN 2009ip.
NASA describes AT 2016blu as a candidate ultra-luminous X-ray source (ULX). The paper’s combined analysis of the March data fell short of the formal luminosity threshold, and the authors could not establish whether the source reached a brighter peak. The classification therefore remains a candidate.
