The Event Horizon Telescope Gazes into the Heart of a Distant Quasar

Oftentimes in astronomy, it takes a village of telescopes and people to make an amazing find. In the case of the quasar NRAO 530, it took a planet full of radio dishes ganged together to peer into its heart. Then, it took a major collaboration of scientists to figure out what the instruments were telling them.

The Event Horizon Telescope, a collection of nearly a dozen antennas installed across Earth’s surface, recently focused its full operational power onto the heart of this object. The goal: understand what’s happening in that distant object.

This bright and powerful object lies about 7.5 billion light-years away from us. Not only is it a quasar, but astronomers classify it as a blazar. That means it has an active core with a jet pointed almost directly at us. NRAO 530 is also optically violent—it has a history of bright flares and outbursts. But, until the observations with EHT, astronomers didn’t have details about structures at the heart of the quasar. Now they have maps of magnetic fields near the core, as well as a first look at what’s happening there.

Peering into the Quasar

NRAO 530 is the most distant object imaged with EHT and those observations should help astronomers understand more about it. There’s a supermassive black hole at the heart of the quasar. Plus, there’s a jet that funnels accelerated particles and radiation out across space. Occasionally it spews out more material than normal, which probably accounts for the bright flare-ups.

Astronomers still aren’t quite sure exactly how the jet forms. The physics is extremely complicated. It would help if they could get a good “picture” of what’s happening around the black hole. In particular, they want to know what’s happening in the region where the jet originates. This is where the Event Horizon Telescope comes in handy. It offers an extremely high angular resolution view of the previously unseen structures in the heart of NRAO 530.

In the case of these observations, the clue lies in a specific type of light emitted at the source. It’s called “polarized light”. Astronomers have used it in other studies of black holes to probe the physical conditions in their extreme environments. It uncovers clues about the strength of magnetic fields near such a monster. It also shows how those fields are oriented in space. In some regions, polarized light can reveal information about any material that lies between the EHT and the objects emitting the signals detected by the EHT.

A Primer on Polarized Light

A short video explanation of polarized light and how EHT studies it. Courtesy EHT Collaboration

EHT’s Polarized View of the Quasar Jet

Images of NRAO 530 were obtained by the EHT Collaboration using several different imaging methods, with the quasar core located toward the bottom-left part of the image, and the jet extending upwards (north). The contours show the structure in total (solid black) and polarized (dotted) light; dashes represent the direction of the observed polarization (Courtesy: EVPA).

Obviously, these don’t look like a regular “point and shoot” view of the quasar. Optical images normally show a quasar as a bright almost point-like source like quasar 3C 273 appears (below). Often enough, a quasar’s brightness overwhelms the light from the galaxy where it resides.

Quasar 3C 273, 2003 in an HST image.(Credit: NASA/J.Bahcall(IAS))

So, to tease out details, astronomers observe quasars in other wavelengths of light. EHT’s images of NRA 530 map the intensity of polarized light at a frequency of 230 GHz. They reveal some substructure: the core plus a very bright feature located at the southern end of a jet stretching out from the core. That’s essentially where the jet begins as seen in that set of frequencies. Astronomers have also observed it at millimeter wavelengths of light using the Very Large Baseline Interferometer.

The jet stretches across a distance of about 1.7 light-years and shows evidence of helical structure in the jet’s magnetic field. “The outermost feature has a particularly high degree of linear polarization, suggestive of a very well-ordered magnetic field,” said Dr. Svetlana Jorstad, a senior scientist at Boston University, USA, who leads the NRAO 530 project.

What’s also notable here is that this is a very small region to be observed across such a huge expanse of space. “With the power of the EHT, we see the details of the source structure on a scale as small as a single light-year,” added Dr. Maciek Wielgus, a scientist at the Max Planck Institute for Radio Astronomy in Bonn, Germany, co-leading the study.

What Powers the Quasar Jet?

You would think that an object like a black hole would be sucking in material, not allowing it to escape via a high-speed jet. This is where observations of polarized light near the event horizon of the central black hole provide some answers. The jet is basically a high-speed plasma flow passing through strong magnetic fields near the black hole. That polarizes the light, and somehow it interacts with aligned magnetic fields (that is, fields aligned with the flow direction of the jet). It could overcome the strong gravitational pull of the central supermassive monster that powers the quasar and allow the jet of material to escape.

Of course, there’s more work to be done to figure out the fine details of all the action at the heart of NRAO 530. Future EHT observations of this quasar will continue to focus on the jet, particularly its source and innermost features. As with other observations of magnetic fields it has made at objects such as M87, it will continue to characterize those fields at NRAO 530. In addition, astronomers will be looking at how and why the jets could be connected to the production of high-energy photons. In particular, they’re interested in what’s causing this quasar to send out huge bursts of highly energetic gamma rays. What they find could provide unique insights into many other quasars with jets, as well.

For More Information

Peering into the heart of a distant quasar with the Event Horizon Telescope
The Event Horizon Telescope Image of the Quasar NRAO 530
The Event Horizon Telescope Image of the Quasar NRAO 530 (ArXiv version)

Carolyn Collins Petersen

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