NICER / ISS Science Nugget
for March 28, 2024

A supermassive black hole's "hiccups"

On 20 December 2020, the All-Sky Automated Survey for SuperNovae (ASAS-SN) - a ground-based search for visible-light transients - detected brightening of the nucleus of a galaxy at a distance of approximately 800 million light years. NASA's Swift observatory detected X-ray emission from the transient, named ASASSN-20qc, some 52 days after the visible flare. The combination of the flare and the soft (thermal) X-ray spectrum suggested that a new accretion disk had been formed from the disruption of a star passing too close to the 30 million solar-mass black hole in the galaxy's core.

Prompted by the Swift X-ray detection, a NICER Guest Observer investigation was triggered for a high-cadence monitoring campaign: 1-2 observations per day for the duration of the ASASSN-20qc outburst, more than 4 months. From the outset, the NICER data showed a departure from the simple thermal spectrum expected of an accretion disk: a broad absorption-like feature at higher photon energies. This feature could be successfully modeled with an ultra-fast outflow - ionized gas moving toward us at roughly 30% the speed of light - and, for the first time, its properties could be tracked over the entire outburst. Surprisingly, the strength of the absorption dip in the NICER spectra varied with a regular, repeating pattern. The implication was that this system was launching ultra-fast outflows roughly once every 8.3 days.

In a peer-reviewed paper published this week in Science Advances, D. Pasham (MIT) and collaborators report these results and use state-of-the-art theoretical simulations to present a model for this peculiar new black-hole behavior. As described in their press release, they propose that the quasi-periodic outflows (described in the media as "hiccups") can be produced by a smaller black hole orbiting the central supermassive black hole at a mean separation of about 70 times the Earth-Sun distance. This secondary, intermediate-mass black hole (anywhere from 100 to 10,000 times the mass of our Sun) regularly interacts with the accretion disk of its supermassive counterpart, pushing gas toward the poles where the magnetic field then launches the outflows, which we see in absorption against the hot accretion disk. If this is indeed the correct model, then this new phenomenon of supermassive black-hole hiccups could be a novel avenue for identifying tight black-hole binaries lurking in the centers of galaxies.