NICER / ISS Science Nugget
for April 17, 2025

Awakening eruptions

The core of a galaxy known as SDSS J1335+0728, which had never previously distinguished itself in any way, flared brightly in visible light in 2019 and again in 2022, with notable increases also in the infrared and ultraviolet bands in the interim. This event was nicknamed "Ansky" (derived from the designation given to it by the discoverers of the early flaring). By 2023, X-rays were detected, and the full spectrum of emission strongly suggested that accretion onto the supermassive black hole at the galaxy's center had "awakened," that it was drawing in substantial amounts of material, likely through a hot accretion disk. These findings motivated NICER observations beginning in May 2024, in an intensive 60-day campaign that lasted until Ansky went behind the Sun. The resulting discovery of quasi-periodic eruptions (QPEs) in soft X-rays, more extreme than any of the QPE emitters yet discovered, was published last week by L. Hernández-García (Univ. de Valparaíso, Chile) and collaborators in the peer-reviewed journal Nature Astronomy.

QPEs are a newly recognized phenomenon, discovered in 2019 and with only 10 QPE systems known to date. The leading model for their origin posits a stellar-mass body (likely a compact object such as a white dwarf, neutron star, or small black hole) orbiting a million-solar-mass black hole (believed to anchor most galaxies), and interacting with - crashing through - the accretion disk surrounding it. Once or twice each orbit, depending on particulars of the disk and orbital geometries, the collision produces a debris cloud that expands rapidly and contracts, or disperses, slowly. This model is supported by the energy distributions, i.e., the spectra, of the measured X-ray photons, as well as the absence of eruption emission at any other wavelength of light. The X-rays appear to be thermal in origin, the glow of a hot bubble of gas evolving in temperature and radius in a way that can readily be modeled. Because Ansky exhibited the brightest and longest-duration eruptions that had ever been seen, these measurements could be made exceptionally well, providing an unprecedented view of the process driving the emission during the eruptions, and lending support to the scenario of a collision between a low-mass body and an accretion disk. In the case of Ansky, the authors argue that the "awakening" of the black hole created a new accretion disk that grew and intersected the orbit of a preexisting orbiting body, which would not otherwise have been detectable. If this model proves to be correct, a future capability - the joint ESA-NASA Laser Interferometer Space Antenna (LISA) mission slated to launch in the 2030s - is expected to directly measure the gravitational waves generated by such extreme mass-ratio inspiral (EMRI) systems, providing valuable tests of our understanding of the extreme gravity around supermassive black holes.