Surprise Discovery of X-ray Bright Supermassive Black Hole Jet in the Early Universe
We are pleased to welcome a guest blogger, Aurora Simionescu, who led the study that is the subject of our latest press release, about a distant X-ray jet. Originally from Romania, Aurora completed her PhD at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, before moving to Stanford University as an Einstein Postdoctoral Research Fellow. She is currently working as an International Top Young Fellow at the Institute of Space and Astronautical Sciences of the Japan Aerospace Exploration Agency. Besides being a high-energy astrophysicist, she is also a part-time travel and nature photographer with a skiing addiction who loves ballroom dancing and the color pink.
Around March 2014, my colleague, Lukasz Stawarz, who was then sharing an office with me at the Japan Aerospace Exploration Agency, showed me a very odd astronomical object he and his collaborators had found by searching though archived radio observations from the Very Large Array (VLA).
Lukasz and his fellow researchers had been looking for extended jets from distant supermassive black holes, and one of their targets had been a quasar called B3 0727+409. Quasars are very distant, very luminous objects, emitting up to a thousand times more energy than our own Milky Way galaxy. It is thought that the only way this amount of energy can be produced is when normal matter undergoes immense stresses right before being swallowed by a supermassive black hole. Many such objects are known, and B3 0727+409 did not seem to be very special at first — at least, Lukasz’s colleagues did not find the extended radio jet they were searching for. All of the light seemed to come from just one, compact point.
Instead, near B3 0727+409, they had discovered something else that was unexpected: two extended, comet-shaped radio sources that were separated from the quasar and did not seem to correspond to anything else in the optical data they had ploughed the archives for. Because I had been working with X-ray telescopes for many years, Lukasz asked me whether I would be willing to help him and his team to propose for a short snapshot with the Chandra X-ray Observatory to see if we might get some more information in order to identify what these mysterious objects were. Some very old X-ray data from the ROSAT satellite in the 1990s showed that there might be X-ray emission coming from the space somewhere between the quasar and these unidentified radio “comets,” but the data quality was very far from sufficient to draw any conclusions.
Our proposal for Chandra time was approved, and when I got the data, I opened the file eagerly, and was very soon disappointed by what I saw: there was nothing in the X-ray image at the location of either one of the mysterious radio features! So, when Lukasz came to talk to me about the results of our observation, I told him that, unfortunately, there was nothing interesting in the data. Luckily, he had the inspiration to then ask me, “well, then what is it that we saw in the old ROSAT data?” To which, I casually replied, “oh, that quasar has an X-ray jet. We can easily see it in the Chandra image”.
It was only when Lukasz’s eyes went very big with excitement that I even realized this was anything noteworthy. The idea of black holes having jets was not at all new to me: the first articles that I ever wrote while I was a PhD student were about the galaxy M87, whose central supermassive black hole emits a very famous jet that has been studied across all wavelengths, from radio to X-ray, and it is even possible to see it in optical light with large amateur telescopes.
Every time I explain this to a non-astronomer, though, I inevitably get asked “But wait, black holes have jets?!” Most people think of black holes as exotic objects that “suck in” all of the matter around them, and from which nothing, not even light, can escape. While this it is true that, once anything has fallen in “over the edge” or so called “event horizon” of the black hole, it can never leave it again, a lot of interesting physics happens right before matter gets swallowed forever. Nobody understands the exact details of how this happens, but probably through a twisted geometry of the magnetic fields, some of the material swirling around just on the verge of falling into the black hole is sometimes accelerated in really energetic jets moving away from the black hole instead. This is what we see in M87, and many other nearby galaxies— so, when I first saw the X-ray jet in B3 0727+409, I did not think it was a surprising discovery.
What I had not realized at first, and what my collaborators then pointed out, was that, while many X-ray emitting black hole jets are well studied in the nearby Universe, the number of such objects known in the early Universe is very low. This is a big problem if we want to understand how these systems work, how they shine, and how they evolve as the cosmos grows older. With light from B3 0727+409 having been emitted when the Universe was only a fifth of its present age, that placed this jet among only a handful of similar systems to ever be found!
What is even more remarkable about the B3 0727+409 jet, though, is that while we can see it clearly in the X-ray image, it does not show up almost at all in the radio band. We have a good understanding of why black hole jets shine in radio waves: it has to do with electrons ejected at very high speeds, close to the speed of light, moving around in the magnetic field of the jet. Radio images are also relatively easier to obtain, because we can use large ground-based antennae, while X-ray observations require longer exposure times and the use of space-based telescopes because Earth’s atmosphere is opaque to this type of light. So, typically, until now, black hole jets were first found in radio data and then followed up with Chandra. But B3 0727+409 shows barely any hint of a radio jet, while the X-ray image reveals a spectacular, narrow streak at least 300,000 light years in length.
Scientists are still debating about how exactly black hole jets emit X-rays. One possibility is that the very fast moving electrons in the jet interact with the cosmic microwave background radiation, the glow left over from the Big Bang. When an electron moving near the speed of light encounters a microwave photon, it can impart some of its energy to this photon, boosting it from microwave to X-ray wavelengths. As we look back in time, we know that the glow left over from the Big Bang was more intense. So, if this model for how the X-ray emission from black hole jets is produced is the right one, we would expect that these jets become comparatively brighter and brighter in X-rays the further back in time we look, while the radio emission becomes harder and harder to see because the jets are farther away. This model therefore predicts the existence of an object like B3 0727+409, an X-ray bright black hole jet in the early Universe that is undetected in radio data; this is the first time such an object has actually been discovered. It seems that if we did a more thorough search for such black hole jets around very distant quasars using X-ray rather than radio telescopes, we might be able to find a lot more of these systems, and that would help scientists tremendously in trying to map the growth of black holes over cosmic time.
In the meantime, we plan to take a much deeper look at B3 0727+409, using both longer radio and longer X-ray observations. But the initial mystery that led to this project still has not been solved. What are those extended, unidentified comet-shaped radio sources that appear in projection relatively close by (within the same Chandra field of view)? Perhaps the new data will tell us something about that puzzle as well.
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