Artist's impression of a luminous quasar surrounded by a swirling accretion disk.
Credit: ESA/Hubble, NASA, M. Kornmesser; CC BY 4.0
Credit: ESA/Hubble, NASA, M. Kornmesser; CC BY 4.0
Title: X-Ray Investigation of Possible Super-Eddington Accretion in a Radio-Loud Quasar at z = 6.13
Authors: Luca Ighina et al.
First Author’s Institution: Harvard–Smithsonian Center for Astrophysics and Italian National Institute for Astrophysics
Status: Published in ApJL
Quasars are some of the most extreme objects in the entire universe. Despite being as far as tens of billions of light-years away, they nonetheless can appear as bright as some stars in our own Milky Way. To put their brightness in perspective, one just has to consider the Sun. Compared to puny human
scales, our star is a truly gargantuan object. It’s a fully functional fusion reactor, churning hydrogen into helium in its core and outputting an unfathomable amount of energy in the process. This glowing furnace is so bright that it can cook us with the heat of an oven during the day, even at a distance of more than 90 million miles (about 8 light-minutes). That distance dramatically dilutes the radiation of the Sun by the time it reaches us, as we only receive a small sliver of its total output, and yet we can still feel its heat pounding down on us when we stand beneath a clear summer sky.
However, if one were to move the Sun about 30 light-years away, it would appear completely unremarkable. You’d need to be under relatively dark skies to even spot it! The vastness of space would simply crush the output of our solar engine; however, if you place a luminous quasar at that same 30 light-year distance, its searing radiation would seem to effortlessly cross the cosmic gulf looming between the stars, scorching us relentlessly with the same heat as the Sun does today. The power of a quasar simply puts our star to shame.
Still, even quasars have limits. The source of power for these objects is a supermassive black hole. The supermassive black hole that lives at the center of our Milky Way is currently dormant. However, many supermassive black holes (especially in the early universe) actively gulp down matter, releasing tremendous energy that escapes in the form of radiation across the electromagnetic spectrum. The escaping photons bump into particles on their way out, exerting an outward pressure. If enough light is unleashed by the quasar, this pressure will actually balance against the pull of gravity, cutting off the food supply for the black hole. This negative feedback loop means that a given quasar has an upper limit to its brightness, called the Eddington luminosity, and to the speed at which it accretes matter, called the Eddington rate. The authors of today’s bite examine a particularly misbehaved quasar that seems to violate even these extreme limits.
However, if one were to move the Sun about 30 light-years away, it would appear completely unremarkable. You’d need to be under relatively dark skies to even spot it! The vastness of space would simply crush the output of our solar engine; however, if you place a luminous quasar at that same 30 light-year distance, its searing radiation would seem to effortlessly cross the cosmic gulf looming between the stars, scorching us relentlessly with the same heat as the Sun does today. The power of a quasar simply puts our star to shame.
Still, even quasars have limits. The source of power for these objects is a supermassive black hole. The supermassive black hole that lives at the center of our Milky Way is currently dormant. However, many supermassive black holes (especially in the early universe) actively gulp down matter, releasing tremendous energy that escapes in the form of radiation across the electromagnetic spectrum. The escaping photons bump into particles on their way out, exerting an outward pressure. If enough light is unleashed by the quasar, this pressure will actually balance against the pull of gravity, cutting off the food supply for the black hole. This negative feedback loop means that a given quasar has an upper limit to its brightness, called the Eddington luminosity, and to the speed at which it accretes matter, called the Eddington rate. The authors of today’s bite examine a particularly misbehaved quasar that seems to violate even these extreme limits.
It’s a Bird! It’s a Plane! It’s a Jet?
The authors observed the quasar RACS J032021.44−352104.1 (RACS J0320−35 for short) with several radio observatories, including the Giant Metrewave Radio Telescope, the Australia Telescope Compact Array, and the Australian Large Baseline Array. Combining this data with publicly available observations, they find that the source is “radio-loud,” or bright at very long wavelengths. Typically, this kind of emission is expected to be generated by powerful jets that are ejected from the poles of a quasar. For example, the authors compare this radio-loud quasar to similar sources, which show significant variability in the X-ray part of the spectrum. This is a tell-tale sign that the X-ray emission is also generated by these jets (which might come out in fits and spurts and thus cause fluctuations in brightness over time).
However, when the authors examine X-ray data of RACS J0320−35 taken by the Chandra X-ray Observatory, they find surprising results. The quasar is extremely luminous in X-rays, making it one of the brightest in the early universe. But despite the fact that it pumps out a huge number of these energetic photons, it seems to preferentially emit only the lower-energy band of X-rays and completely lacks the highest-energy emission that characterizes similar sources. In technical terms, the X-ray spectrum of RACS J0320−35 is incredibly “soft.” Moreover, this X-ray emission seems to be constant on the timescale of months, though further observations will be required to test if it varies on longer timescales. Still, the softness of the spectrum and weak variability of this system mean that its X-ray emission is unlikely to be produced by a jet.
However, when the authors examine X-ray data of RACS J0320−35 taken by the Chandra X-ray Observatory, they find surprising results. The quasar is extremely luminous in X-rays, making it one of the brightest in the early universe. But despite the fact that it pumps out a huge number of these energetic photons, it seems to preferentially emit only the lower-energy band of X-rays and completely lacks the highest-energy emission that characterizes similar sources. In technical terms, the X-ray spectrum of RACS J0320−35 is incredibly “soft.” Moreover, this X-ray emission seems to be constant on the timescale of months, though further observations will be required to test if it varies on longer timescales. Still, the softness of the spectrum and weak variability of this system mean that its X-ray emission is unlikely to be produced by a jet.
The authors carefully consider a particular variety of jet — one that is sharply angled towards us. Since quasar jets often travel at incredible speeds, the special theory of relativity kicks in and causes strange behaviors. In particular, an effect called relativistic beaming can cause a source to appear much brighter if traveling towards the observer at extremely high speeds. However, they find that although relativistic beaming can explain the mysterious X-ray properties of RACS J0320−35, such a scenario is incompatible with its observed radio emission. Moreover, a lack of gamma-ray emission and weak variability are further pieces of evidence against the jet origin of the X-ray emission.
Figure 1: A plot showing the observed energy spectrum of RACS J0320−35 and the theoretical spectra predicted by several models. In particular, the green circles (visible light and ultraviolet (UV) emission), blue diamonds (X-ray emission), and red squares (radio emission) representthe real observations. The shaded pink region represents the predictions of a model that simulates a black hole spinning very slowly and accreting at super-Eddington rates. The model seems to match the observed X-ray and visible/UV emission excellently. However, explaining the observed radio emission might require conditions that violate the assumptions behind this scenario. Adapted from Ighina et al. 2025.
Limits Are Meant to Be Broken
The article presents one more fascinating possibility for the origin of the X-rays in this source. Some theoretical work and simulations show that if a black hole breaks the Eddington limit and starts accreting matter at “super-Eddington rates,” the X-ray spectrum that results can be incredibly soft. The authors find excellent agreement between the observations and the predictions from a particular model that simulates a very slowly spinning black hole (see Figure 1). This scenario seems to be a promising explanation for the X-ray emission in RACS J0320−35, which is extremely exciting for several reasons.
Many astronomers are considering super-Eddington accretion to explain the masses of early quasars and their fainter counterparts, called active galactic nuclei. Because supermassive black holes are supposed to have a cap on their accretion rate (the Eddington limit), the earliest black holes in the universe should only have been able to reach large sizes after sufficient time had passed. Astonishingly, observations from JWST are finding massive active galactic nuclei everywhere in the early universe, which seems to violate this — these black holes appear astonishingly massive despite existing for only a fraction of the universe’s current age. However, if early black holes could grow faster than expected by undergoing super-Eddington accretion, this tension might be resolved.
Despite the promising initial results of this model, there are a few caveats to the results presented in today’s article. For example, the radio jets observed from this source require a rapidly spinning black hole, which conflicts with the model assumptions. The authors note that a radio jet can be very far from a black hole, tracing its past activity. In fact, this jet may have actually spun down the central supermassive black hole by extracting energy from it, a fascinating possibility that will require both further observations and simulations to explore. Today’s bite shines a brilliant light on this possibility by analyzing RACS J0320−35, a fascinating quasar in the early universe and a stunning example that cosmic limits are meant to be broken.
Many astronomers are considering super-Eddington accretion to explain the masses of early quasars and their fainter counterparts, called active galactic nuclei. Because supermassive black holes are supposed to have a cap on their accretion rate (the Eddington limit), the earliest black holes in the universe should only have been able to reach large sizes after sufficient time had passed. Astonishingly, observations from JWST are finding massive active galactic nuclei everywhere in the early universe, which seems to violate this — these black holes appear astonishingly massive despite existing for only a fraction of the universe’s current age. However, if early black holes could grow faster than expected by undergoing super-Eddington accretion, this tension might be resolved.
Despite the promising initial results of this model, there are a few caveats to the results presented in today’s article. For example, the radio jets observed from this source require a rapidly spinning black hole, which conflicts with the model assumptions. The authors note that a radio jet can be very far from a black hole, tracing its past activity. In fact, this jet may have actually spun down the central supermassive black hole by extracting energy from it, a fascinating possibility that will require both further observations and simulations to explore. Today’s bite shines a brilliant light on this possibility by analyzing RACS J0320−35, a fascinating quasar in the early universe and a stunning example that cosmic limits are meant to be broken.
Original astrobite edited by Veronika Dornan
About the author, Ansh Gupta:
I’m an astronomy graduate student at the University of Texas at Austin working with Steven Finkelstein. I use data from JWST to study the formation and growth of the first galaxies and black holes in the universe. In my spare time, I enjoy playing piano, reading, and making YouTube videos.
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