Radio echoes of a black hole feasting on a doomed star

An artist’s impression of a high-energy jet blasting away from the inner accretion region of a black hole as it consumes a destroyed star. Image: ESO/L. Calçada

Astronomers studying emissions from a supermassive black hole as it consumed a passing star have found a correlation between X-rays produced by stellar material falling into the hole and a powerful jet of high-energy particles being blasted back into space.

“This is telling us the black hole feeding rate is controlling the strength of the jet it produces,” said Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research. “A well-fed black hole produces a strong jet, while a malnourished black hole produces a weak jet or no jet at all. This is the first time we’ve seen a jet that’s controlled by a feeding supermassive black hole.”

The conclusion is based on detection of a tidal disruption flare – an outburst of electromagnetic energy that is generated when a black hole tears apart a passing star – on 11 November, 2014, by the All-Sky Automated Survey for Supernovae, or ASASSN, a global network of telescopes. Reviewing 180 days of data, Pasham and collaborator Sjoert van Velzen of Johns Hopkins University calculated the X-ray emitting region, the area where material is heated to extreme temperatures before being sucked into the black hole, was about 25 times the size of the sun.

They found a corresponding pattern of radio waves, from a region 400,000 times the radius of the sun, that showed up in the data 13 days after the X-ray outburst. The radio signals showed a 90 percent match with the X-ray data.

“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,” Pasham said. “Clearly there’s a causal connection between this small region producing X-rays, and this big region producing radio waves.”

In a paper this week in the Astrophysical Journal, Pasham and van Velzen suggest radio waves produced by a jet of high-energy particles began streaming away from the black hole soon after it started consuming material from the destroyed star. Because the region where the jet originated was tightly packed with electrons, the radio waves were immediately absorbed. But when electrons traveled far enough “downstream” of the jet, they could escape and generate the signal that was eventually detected.

The strength of the jet, they conclude, is controlled by how fast the black hole is consuming the stellar material generating X-rays as it is pulled in.

Understanding how such jets form and evolve could provide deeper insights into galactic evolution. Jets from supermassive black holes are thought to raise the background temperature of a galaxy and thus affect star formation rates.

“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,” Pasham said. “So this is hinting toward some bigger picture.”