Astronomers baffled by a radio ‘echo’ detected from distant galaxy

13 days after an X-ray emission was detected from a distant galaxy, an almost identical radio signal was detected


An accretion disc usually surrounds a black hole, providing it with the fuel to produce such intense radiation. Image credit: ESO/L. Calçada

On the 11 November 2014, a collection of telescopes spanning all over the globe collected unusually intense signals from over 300-million-light-years away. These intense signals were the result of a star passing by a black hole and being ripped apart in the process, also known as a tidal disruption flare. Since this discovery astronomers have been trying to learn more about this event and how black holes devour mass and regulate the growth of galaxies.

Scientists from the Massachusetts Institute of Technology (MIT) and Johns Hopkins University, Maryland, both in the United States, have now detected radio signals coming from the event which appear very similar to the X-ray emissions detected from the same flare 13 days earlier. The researchers involved suspect that that these radio ‘echoes’, showing a similarity of 90 per cent to the event’s X-ray emissions, are more than just a coincidence. Instead they believe that this is evidence of an enormous stream of highly energetic particles streaming out from the black hole as a star is being eaten.

Dheeraj Pasham, a postdoctoral research associate at MIT’s Kavli Institute for Astrophysics and Space Research, claims the power of the jets emitted from the black hole are controlled by the rate at which the stellar material is being devoured.

“This is telling us the black hole feeding rate is controlling the strength of the jet it produces,” Pasham says. “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.”

Pasham also suggests that scientists think that the black hole’s jets are powered by their accretion rate, but there has never been any evidence from a single event in order to confirm this relationship. “You can do this only with these special events where the black hole is just sitting there doing nothing, and then suddenly along comes a star, giving it a lot of fuel to power itself,” Pasham says. “That’s the perfect opportunity to study such things from scratch, essentially.”

Thanks to theoretical models of black hole evolution and observations of distant galaxies, scientists have a fairly good idea of what occurs during tidal disruption events. For instance, when the star approaches the black hole it is understood that the black hole’s gravity causes tidal friction within a star, similar to what occurs between the Earth and the Moon. However, the black hole’s gravity is so powerful that it completely deforms the star, stretching it into a pancake and tearing it apart. The departing mass is drawn towards the black hole’s accretion disc, which is a ring of gas and dust that fuels the black hole.

This event produces large amounts of energy which can be seen across a range of wavelengths, from radio to X-ray sections of the electromagnetic spectrum. X-ray emissions are thought to originate from the innermost region of the accretion disc, whereas material much further out emits more optical and ultraviolet light, but will eventually join the inner section of the accretion disc. With this being said, scientists are not entirely sure where the radio emissions come from during this tidal disruption flare.

“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,” Pasham says. “The debate has been: where are these really energetic electrons coming from?”

It has been suggested that moments after the stellar explosion, a shockwave permeates the cosmos and energises the surrounding material, thus releasing the radio signal. In this case however, the radio signal would appear extremely different to the X-rays produced from the in-falling stellar debris. “What we found basically challenges this paradigm,” Pasham says.

Black holes have an incredibly powerful gravitational effect that can completely tear apart a star. Image credit: ESO, ESA/Hubble, M. Kornmesser

Pasham and his collaborator Sjoert van Velzen of Johns Hopkins University looked through data recorded from a tidal disruption flare discovered in 2014 by the global telescope network All-Sky Automated Survey for Supernovae (ASASSN). After its initial discovery, several telescopes were focused on this event, henceforth known as ASASSN-14li, looking at it in different wavelengths. Three telescopes collected radio data over a period of 180 days.

After comparing the radio and X-ray data the researchers could clearly see a resemblance. After overlaying and shifting the datasets around, two fluctuations with a similarity of 90 per cent appear, with the radio signals detected 13 days after the X-rays. “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 says.

In the same piece of work, the researchers calculated the X-ray-emitting region to be roughly 25-times the size of the Sun, while the radio-emitting region was roughly 400,000-times the solar radius.

“It’s not a coincidence that this is happening,” Pasham says. “Clearly there’s a causal connection between this small region producing X-rays and this big region producing radio waves.”

The team proposed that the radio waves were a result of high-energy particles, in the form of a jet, streaming out from the black hole shortly after it began feeding off the star. The region of the jet where radio waves first formed was extremely dense, and other electrons immediately absorbed a majority of the radio waves.

Only when the electrons travelled downstream of the jet could the radio waves escape, producing the echo. These theories will ultimately help astronomers characterise the physics of black hole jets. This is essential for understanding the evolution of black holes and how they hinder star formation by creating a hot and inhospitable environment for stars to be born.

“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 says. “So this is hinting toward some bigger picture.”

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