🔭 astrophysics

The X-Ray Continuum Emission Region in the Lensed Quasar SDSS J133907.23+131038.6 is Much Smaller than the Accretion Disk

By analyzing microlensing variability in 15 seasons of optical data and 4 epochs of X-ray observations of the lensed quasar SDSS J133907.23+131038.6, the study reveals that its X-ray continuum emission region is significantly smaller than its optical accretion disk, with a size consistent with the innermost stable circular orbit, while also detecting shifted Fe K $\alpha$ lines in the stacked spectrum of image A.

Original authors: Christopher W. Morgan, James B. Margeson, Gilberto Garcia, Xinyu Dai, Luis J. Goicoechea, Vyacheslav N. Shalyapin, George Chartas

Published 2026-08-19

📖 4 min read☕ Coffee break read

CC0 1.0

Original authors: Christopher W. Morgan, James B. Margeson, Gilberto Garcia, Xinyu Dai, Luis J. Goicoechea, Vyacheslav N. Shalyapin, George Chartas

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Quasars are the brilliant, distant hearts of galaxies, powered by supermassive black holes that devour surrounding gas and dust. As this material spirals inward, it heats up and glows with intense light, from the invisible ultraviolet to the powerful X-rays. For decades, astronomers have struggled to understand the physical layout of these glowing regions. The problem is one of scale: the X-ray emitting area is so incredibly tiny and far away that even our most powerful telescopes cannot resolve it directly. It is like trying to see the details of a coin on the Moon with a backyard telescope. To solve this, scientists have turned to a cosmic trick called gravitational lensing. When a massive galaxy sits directly between Earth and a distant quasar, its gravity bends the light, creating multiple images of the same object. As stars within that foreground galaxy move, they act as natural magnifying glasses, causing the brightness of the quasar's images to flicker in a pattern that reveals the size of the source behind them.

A team of astronomers recently applied this technique to a specific quasar known as SDSS J133907.23+131038.6, located billions of light-years away. By combining fourteen years of optical observations with four new, high-precision snapshots taken by the Chandra X-ray Observatory, they managed to measure the physical dimensions of the quasar's glowing regions with unprecedented clarity. Their work focused on two distinct areas: the disk of hot gas that emits ultraviolet light, and the even more compact region that produces X-rays. The researchers found that the ultraviolet-emitting disk is surprisingly small, measuring roughly one hundred times the radius of the black hole's event horizon. While this is already considered tiny in astronomical terms, the X-ray source is even more compact. The measurements indicate that the X-ray emission comes from a region so close to the black hole that it sits just outside the point of no return, the innermost stable orbit where matter can circle the black hole without immediately falling in.

The study relied on a massive computational effort to interpret the flickering light. The team simulated millions of possible scenarios, moving virtual models of the quasar across the complex gravitational landscape created by the foreground galaxy. They compared these simulations against the actual light curves recorded over fifteen years of monitoring. The results showed that the X-ray source is significantly smaller than the ultraviolet disk, with a size ratio of nearly thirty to one. This finding challenges some existing theories that suggest the X-ray emitting region might be much larger, extending far out into the accretion disk. Instead, the data points to a scenario where the X-rays originate from a very tight zone immediately surrounding the black hole, potentially consistent with the size of the innermost stable orbit for a non-spinning black hole, or even smaller if the black hole is spinning rapidly.

A particularly striking discovery emerged from the X-ray data itself. In the spectrum of one of the quasar's images, the researchers detected two distinct shifts in the signature of iron atoms, appearing at energies of 5.9 and 8.9 kiloelectron volts. These shifts are caused by the extreme gravity and the motion of the gas near the black hole, which stretches and compresses the light waves. The fact that these shifted lines were seen in only one of the two images suggests that the region producing them is incredibly small and localized, allowing the foreground stars to magnify it differently than the broader X-ray source. This provides further evidence that the X-ray emission is not spread out over a large area but is concentrated in a tiny, dynamic region.

The implications of these measurements extend beyond just this single object. The fact that the X-ray source is so close to the black hole suggests that the black hole in this system may be spinning, as a spinning black hole allows matter to orbit closer than a stationary one. While the data does not definitively prove the spin rate, it opens a window for future studies to measure it more precisely. The researchers plan to continue monitoring this system with both ground-based telescopes and the Hubble Space Telescope to map the temperature of the accretion disk across different wavelengths. By piecing together the sizes of the ultraviolet, extreme ultraviolet, and X-ray regions, they hope to build a complete, three-dimensional picture of how matter behaves as it falls into a supermassive black hole, finally bridging the gap between theoretical models and the actual physical reality of these cosmic engines.

Technical Summary: The X-Ray Continuum Emission Region in the Lensed Quasar SDSS J133907.23+131038.6

Problem Statement
The physical mechanism generating X-rays in quasars remains unclear because the emission structures are orders of magnitude smaller than the angular resolution limits of current or planned X-ray observatories. While radiative accretion models exist, they predict X-ray emission on vastly different physical scales relative to the black hole's gravitational radius ( $r_g$ ). "Disk-corona" models suggest emission from a hot corona at $r \approx 200 r_g$ , similar to the optical accretion disk, whereas other models propose emission from a compact region near the innermost stable circular orbit (ISCO) at $r \lesssim 3.0 r_g$ . Existing quantitative measurements of X-ray continuum sizes have relied exclusively on gravitational microlensing in lensed quasars, which has consistently indicated emission regions immediately outside the ISCO. The authors focus on SDSS J133907.23+131038.6 (SDSS1339), a doubly-imaged lensed quasar ( $z_s=2.231$ ) previously found to host an anomalously small optical/UV accretion disk ( $\sim 40 r_g$ ). The study aims to determine if the X-ray emission region in this system is similarly compact and to constrain its size relative to the ISCO.

Methodology
The authors employed a Bayesian Monte Carlo analysis to constrain the sizes of the optical and X-ray continuum emission regions using microlensing variability.

Observational Data: The study combined 15 seasons of ground-based optical monitoring (r-band) from the GLENDAMA project (including new data from 2023) with four new epochs of X-ray observations from the Chandra X-ray Observatory (Cycle 24). The X-ray observations were scheduled to align with the system's measured time delay ( $\Delta t_{BA} \approx 47$ days) to isolate microlensing variability from intrinsic quasar variability.
Data Processing: Optical light curves were processed using photometric fitting to deblend the closely spaced images ( $1.7''$ separation). Chandra data were analyzed using CIAO 4.12, with PSF fitting applied to separate the blended X-ray images. Spectral analysis was performed using XSPEC, separating the data into soft ($0.2-1.6$ keV), Fe K $\alpha$ ($1.6-2.1$ keV), and hard ($2.1-8.0$ keV) bands.
Lensing Models: Macroscopic lens models were constructed based on deep I-band imaging, varying the fraction of mass in stars versus dark matter. These models generated 800 magnification patterns using the inverse ray-shooting technique, populated with stars drawn from a Galactic Initial Mass Function.
Monte Carlo Simulation: A joint Monte Carlo analysis was performed. In Phase 1, the authors fitted the optical light curves to constrain the source size and effective transverse velocity. In Phase 2, these successful trajectories were used to fit the X-ray light curves simultaneously. The analysis utilized a Bayesian framework to derive probability densities for the source scale radii, marginalizing over other parameters such as trajectory, velocity, and stellar mass fraction. Physical priors were applied to the effective transverse velocity based on CMB dipole and peculiar velocity models.

Key Results

Optical/FUV Size: The half-light radius of the far-UV source ( $\lambda_{rest} = 1930$ Å) was measured as $\log(r_{1/2, FUV}/\text{cm}) = 15.78^{+0.26}_{-0.28}$ . For a black hole mass of $4.0 \times 10^8 M_\odot$ , this corresponds to a size of $\sim 100 r_g$ , consistent with previous findings of an anomalously small accretion disk for this system.
X-Ray Size: The half-light radius of the full-band ($0.2-8.0$ keV) X-ray continuum emission region was measured as $\log(r_{1/2, Xfull}/\text{cm}) = 14.32^{+0.23}_{-0.31}$ . This size is formally consistent with the ISCO radius in a Schwarzschild metric ( $6 r_g$ ) and is approximately $3.5 \times$ larger than the ISCO of a maximally rotating Kerr black hole ( $1 r_g$ ). The authors note that uncertainties in the hard and soft bands prevented independent size measurements for those specific bands, leading them to rely on the full-band constraint.
Fe K $\alpha$ Lines: Two shifted Fe K $\alpha$ emission lines were detected in the stacked spectrum of image A at $5.90 \pm 0.12$ keV (redshifted) and $8.94 \pm 0.11$ keV (blueshifted) with $>99\%$ significance. No significant lines were detected in image B.
Size Ratio: The ratio of the optical to X-ray region sizes ( $r_{opt}/r_{X,full}$ ) is approximately 29, which falls within the range of ratios found in previous microlensing studies of other quasars.

Significance and Claims
The paper claims that the X-ray continuum emission region in SDSS1339 is significantly smaller than the optical accretion disk, emerging from a region consistent with the immediate vicinity of the black hole's ISCO. The measured X-ray size ( $\sim 3.5 r_g$ ) is the smallest expectation value for an X-ray continuum region measured to date in a lensed quasar.

The authors suggest that the compactness of the X-ray source implies that the supermassive black hole in SDSS1339 may possess a measurable angular momentum (spin), as a non-rotating black hole would have an ISCO at $6 r_g$ . However, they explicitly state that the current constraints are too broad to permit a precise measurement of the black hole's spin parameter.

The detection of the blueshifted Fe K $\alpha$ line at 8.9 keV is noted as extremely rare, even among lensed quasars. The authors discuss potential explanations for why these lines appear in image A but not image B, including differential magnification of the continuum versus the reflection region or the presence of a physically separated, smaller X-ray reflection region. They conclude that while the current data supports the existence of a compact X-ray source, future high signal-to-noise, multi-epoch monitoring is required to refine the spin constraints and differentiate between competing models for the Fe K $\alpha$ line profile.

More like this