The Echo of a Black Hole’s Feast 200 Years Ago
Black holes are cosmic monsters that devour anything that comes too close to them, including stars, planets and gas. They are also extremely dark and silent, emitting no light or sound. But sometimes, they can reveal their presence by producing powerful flashes of X-rays that can be detected by telescopes.
One such flash was detected on October 19, 2019, by NASA’s Neil Gehrels Swift Observatory. It was a gamma-ray burst (GRB), a brief but intense burst of high-energy radiation that is usually associated with the death of massive stars or the merger of compact objects like neutron stars. This GRB, named GRB 191019A, was unusual because it came from the center of our own galaxy, the Milky Way, where a supermassive black hole called Sagittarius A* (Sgr A*) lurks.
The Mystery of GRB 191019A: A Black Hole’s Awakening
What caused this GRB? Was it a sign that Sgr A* had awakened from its long slumber and devoured a star or a planet? Or was it something else?
An international team of scientists, led by Frédéric Marin from Radboud University in the Netherlands and including researchers from Northwestern University, investigated this question using data from various telescopes, including NASA’s Imaging X-ray Polarimetry Explorer (IXPE), which can measure the polarization of X-ray light with high precision.
They found that GRB 191019A was not caused by a single event near Sgr A*, but by an echo of an event that happened about 200 years ago. At that time, Sgr A* had a brief period of activity, during which it consumed some gas and dust that came within its reach. This produced a powerful X-ray emission that lasted for about a year. The X-rays then traveled outward in all directions, reaching different parts of the galaxy at different times.
Some of the X-rays reached us directly in 2019, creating the GRB. But some of them were reflected by molecular clouds near Sgr A*, creating an echo that also reached us in 2019. By measuring the polarization of the X-ray light, the scientists were able to distinguish between the direct and reflected signals and trace them back to their source: Sgr A*.
The Discovery of a Magnetar: A Black Hole’s Legacy
The scientists also discovered something else: GRB 191019A produced a magnetar, a highly magnetized neutron star that spins rapidly and emits intense beams of radiation. Magnetars are rare and exotic objects that can also result from massive star explosions or neutron star mergers. The formation of a magnetar in this case adds another layer of complexity and mystery to this remarkable event.
How did Sgr A* create a magnetar? The scientists are not sure yet, but they have some ideas. One possibility is that Sgr A* swallowed a star or a planet that had a strong magnetic field and spun it up to create a magnetar. Another possibility is that Sgr A* collided with another black hole or a neutron star and ejected some material that formed a magnetar.
The scientists are continuing their work on Sgr A* to try to determine the physical mechanisms required for a black hole to switch from a quiescent state to an active one and to create a magnetar.
“For every hundred events that fit into the traditional classification scheme of gamma-ray bursts, there is at least one oddball that throws us for a loop,” said Northwestern astrophysicist and study co-author Wen-fai Fong. “However, it is these oddballs that tell us the most about the spectacular diversity of explosions that the universe is capable of.”
The study was published on June 21, 2023, in the journal Nature Astronomy12.
