Astronomers have detected radio echoes of a supermassive black hole feeding on a star, suggesting the black hole emits a jet of energy proportional to the stellar material it gobbles up.
This artist’s impression shows a supermassive black hole at the center of PGC 43234 accreting mass from a star that dared to venture too close to the galaxy’s center. Image credit: ESA / C. Carreau.
Based on theoretical models of black hole evolution and observations of distant galaxies, astronomers have a general understanding for what transpires during a so-called tidal disruption event: as a star passes close to a black hole, the black hole’s gravitational pull generates tidal forces on the star, similar to the way in which the moon stirs up tides on Earth.
However, a black hole’s gravitational forces are so immense that they can disrupt the star, stretching and flattening it like a pancake and eventually shredding the star to pieces. In the aftermath, a shower of stellar debris rains down and gets caught up in an accretion disk — a swirl of cosmic material that eventually funnels into and feeds the black hole.
This entire process generates colossal bursts of energy across the electromagnetic spectrum.
Astronomers have observed these bursts in the optical, UV, and X-ray bands, and also occasionally in the radio end of the spectrum.
The source of the X-ray emissions is thought to be ultrahot material in the innermost regions of the accretion disk, which is just about to fall into the black hole. Optical and UV emissions likely arise from material further out in the disk, which will eventually be pulled into the black hole.
However, what gives rise to radio emissions during a tidal disruption flare has been up for debate.
“We know that the radio waves are coming from really energetic electrons that are moving in a magnetic field — that is a well-established process,” said MIT researcher Dr. Dheeraj Pasham.
“The debate has been, where are these really energetic electrons coming from?”
On November 11, 2014, a tidal disruption flare was discovered by the All-sky Automated Survey for Supernovae (ASASSN) in the center of PGC 43234, a galaxy approximately 290 million light-years away. Soon after the discovery, multiple electromagnetic telescopes focused on the event, dubbed ASASSN-14li.
Dr. Pasham and his colleague, Dr. Sjoert van Velzen of Johns Hopkins University and New York University, looked through data recorded from ASASSN-14li.
They looked through the compiled radio data and discovered a clear resemblance to patterns they had previously observed in X-ray data from the event.
When they fit the radio data over the X-ray data, and shifted the two around to compare their similarities, they found the datasets were most similar, with a 90% resemblance, when shifted by 13 days. That is, the same fluctuations in the X-ray spectrum appeared 13 days later in the radio band.
“The only way that coupling can happen is if there is a physical process that is somehow connecting the X-ray-producing accretion flow with the radio-producing region,” Dr. Pasham said.
From these same data, the team calculated the size of the X-ray-emitting region to be about 25 times the size of the Sun, while the radio-emitting region was about 400,000 times the solar radius.
“It’s not a coincidence that this is happening. Clearly there’s a causal connection between this small region producing X-rays, and this big region producing radio waves,” Dr. Pasham noted.
The authors propose that the radio waves were produced by a jet of high-energy particles that began to stream out from the ASASSN-14li black hole shortly after the black hole began absorbing material from the exploded star.
Because the region of the jet where these radio waves first formed was incredibly dense (tightly packed with electrons), a majority of the radio waves were immediately absorbed by other electrons.
It was only when electrons traveled downstream of the jet that the radio waves could escape — producing the signal that the team eventually detected.
“Thus, the strength of the jet must be controlled by the accretion rate, or the speed at which the black hole is consuming X-ray-emitting stellar debris,” the scientists said.
Ultimately, the results may help astronomers better characterize the physics of jet behavior — an essential ingredient in modeling the evolution of galaxies.
“It’s thought that galaxies grow by producing new stars, a process that requires very cold temperatures,” Dr. Pasham said.
“When a black hole emits a jet of particles, it essentially heats up the surrounding galaxy, putting a temporary stop on stellar production. Our new insights into jet production and black hole accretion may help to simplify models of galaxy evolution.”
“If the rate at which the black hole is feeding is proportional to the rate at which it’s pumping out energy, and if that really works for every black hole, it’s a simple prescription you can use in simulations of galaxy evolution. So this is hinting toward some bigger picture.”
The findings are published in the Astrophysical Journal.
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Dheeraj R. Pasham & Sjoert van Velzen. 2018. Discovery of a Time Lag between the Soft X-Ray and Radio Emission of the Tidal Disruption Flare ASASSN-14li: Evidence for Linear Disk–Jet Coupling. ApJ 856, 1; doi: 10.3847/1538-4357/aab361
