Exploding Stars in Black Hole Disks

This artist’s impression shows a flash of light produced within the accretion disk that surrounds a supermassive black hole. A new study explores whether we could see the signatures produced by exploding stars embedded in these disks. Credit: NASA/JPL-Caltech

The swirling disks of material that surround supermassive black holes are likely home to massive stars, neutron stars, and black holes. A new study explores whether we can detect the signatures of fiery explosions produced by these uniquely situated stars and stellar remnants.

Artist’s illustration of two merging black holes embedded in the gas disk surrounding a supermassive black hole.

Credit: Caltech/R. Hurt (IPAC) 

An Unusual Home

Recently, scientists detected gravitational waves from the merger of unexpectedly large black holes. One proposed explanation — that these monsters grew to their large sizes while embedded within the accretion disk surrounding an even larger supermassive black hole — has piqued interest in studying the evolution of stars hosted within the violent disks of these active galactic nuclei (AGN).

AGN accretion disks are dense, turbulent environments that produce bright, high-energy radiation as disk material spirals inwards toward the black hole. Yet these seemingly hostile surroundings may still host stars that arise either in situ — the gas within accretion disks can become unstable and fragment into self-gravitating clumps that become stars — or are captured from the nuclear star cluster that surrounds an AGN.

Explosive Ends

Once stars form or are trapped in an AGN disk, the dense environment increases the likelihood that the stars pair off into binaries. As disk-hosted stars evolve, some fraction of them should end their lives in spectacular explosions — either as long gamma-ray bursts (GRBs) caused by the deaths of massive stars, or as short GRBs produced when two evolved stellar remnants collide.

The possibility of these relativistic explosions occurring within AGN disks is intriguing. Does the unique environment of the disk influence the explosion? If so, could we expect to see specific, identifiable features from GRBs produced within the disks around supermassive black holes?

A team of scientists led by Rosalba Perna (Stony Brook University and Flatiron Institute) has explored these questions by modeling how the properties of GRB explosions are changed when they occur within disks.

Schematic illustrating the location of an exploding star in an AGN disk, shown in cross section. Bottom: Illustration of relevant radii in observed GRBs. RIS is the location of internal shocks that usually powers prompt emission, and RES marks the location where the expanding outflow runs into the surrounding medium, powering the afterglow. The relative locations of these radii can change in a dense surrounding environment, leading to different emission signatures. Credit: Perna et al. 2021

Searching for Signatures

Perna and collaborators explore a standard model of a GRB in which prompt emission is produced first as a series of internal shocks are driven by colliding shells of speeding material. The prompt emission is then followed by a long, decaying afterglow as this relativistic outflow is slowed when it plows into the surrounding matter.

The authors show that the properties of the AGN disk environment can change the behavior of both of those emission components. The high density of the disk material can cause a powerful reverse shock to be driven backwards early in the explosion, powering the prompt emission in place of internal shocks. And the later afterglow of the GRB can end up brighter and peaking earlier than is the case for typical GRBs observed in a low-density environment like the interstellar medium.

These features and other signatures identified by Perna and collaborators may help us to determine whether future observed GRBs exploded in typical environments, or instead in the extreme surroundings of an AGN disk. This will help us to better understand how some stars may be evolving in their unusual homes around supermassive black holes.

Citation

“Electromagnetic Signatures of Relativistic Explosions in the Disks of Active Galactic Nuclei,” Rosalba Perna et al 2021 ApJL 906 L7. doi:10.3847/2041-8213/abd319

 

Source: American Astronomical Society/ASS Nova

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