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Evidence for Multiple Orbiting Hotspots in the 340 GHz Variability of Sgr A*

By analyzing 11 epochs of ALMA data, the authors propose that the 340 GHz variability of Sgr A* is best explained by a unified model of multiple orbiting hotspots with decaying emission, which accounts for both periodic and non-periodic flux fluctuations near the black hole's innermost stable circular orbit.

Original authors: Kazuki Yanagisawa, Tomoharu Oka, Tatsuya Kotani, Ryo Ariyama, Kazuki Yanagihara, Yuhei Iwata

Published 2026-05-13

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CC BY 4.0

Original authors: Kazuki Yanagisawa, Tomoharu Oka, Tatsuya Kotani, Ryo Ariyama, Kazuki Yanagihara, Yuhei Iwata

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ 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

Imagine the center of our galaxy, the Milky Way, as a cosmic dance floor dominated by a massive, invisible partner: a supermassive black hole called Sagittarius A* (Sgr A*). While this black hole is too dark to see directly, it is surrounded by a swirling disk of superheated gas and dust. Occasionally, this cosmic soup gets stirred up, creating bright, glowing "hotspots" that orbit the black hole like fireflies circling a lantern.

This paper is a detective story about trying to understand the rhythm of these fireflies using a giant radio telescope called ALMA.

The Mystery of the Flickering Light

The researchers looked at 11 different "snapshots" (epochs) of Sgr A* taken over several years. In most of these snapshots, the light from the black hole just wiggles randomly, like static on an old TV. However, one specific snapshot from August 31, 2016, was special. It showed a very distinct pattern of flickering that looked like it had a rhythm.

When they analyzed this flickering, they found two distinct "beats":

One beat happened roughly every 30 minutes.
Another beat happened roughly every 50 minutes.

The "Red Noise" Problem

Here is the tricky part: In astronomy, data is often messy. The researchers had to ask, "Is this rhythm real, or is it just random noise that happens to look like a pattern?"

If they assumed the background noise was simple and white (like static), the rhythm looked incredibly strong and real. But, the universe is rarely that simple. When they accounted for "red noise" (a type of natural, sloshing variability common in black holes), the statistical confidence dropped. The rhythm wasn't strong enough to be called a "statistically proven" discovery in the strictest sense.

However, the authors argue that even if the math isn't perfect, the story the data tells makes sense. The timing of the flickers matches exactly how long it should take for an object to orbit the black hole at its closest safe distance. Furthermore, the brightness of the flickers didn't stay constant; they faded away over time, just like a campfire dying down.

The Solution: A Two-Firefly Dance

To explain what they saw, the team proposed a model involving multiple orbiting hotspots.

Think of it like this:

Phase 1: Imagine one bright firefly (hotspot) is orbiting the black hole. It creates a steady, rhythmic flicker as it moves toward and away from us.
Phase 2: Suddenly, a second firefly appears nearby. Now, you have two fireflies orbiting at different speeds.
The Fade: As time goes on, both fireflies start to lose energy and dim, much like a hot coal cooling down.

The researchers built a mathematical model based on this "two-firefly" idea. It fit the data surprisingly well. The model suggested:

The first hotspot orbited every 50 minutes and took about 70 minutes to fade away.
The second hotspot orbited every 30 minutes and faded away faster, in about 38 minutes.

Why Do They Fade?

Why do these cosmic fireflies die out? The paper suggests two main culprits:

Cooling: The gas in the hotspot is so hot it glows, but as it expands and loses energy, it cools down and gets dimmer (like a hot piece of metal turning from white-hot to red-hot to black).
Stretching: The hotspot might be physically expanding, spreading its energy out over a larger area, making it look dimmer.

The data showed that the "color" of the light changed as the hotspot faded, suggesting that simple cooling isn't the whole story; it's likely a mix of cooling and expansion.

The Big Picture: A Unified Theory

The most exciting takeaway from this paper is a new way to look at the black hole's behavior.

Usually, astronomers treat "rhythmic" flickering (periodic) and "random" flickering (non-periodic) as two different problems. This paper suggests they might be the same thing.

The Theory: The accretion disk is probably full of many hotspots orbiting at once. When there are too many of them, their rhythms clash and cancel each other out, making the light look random and messy.
The Flare: Occasionally, a flare happens, or hotspots die out, leaving just one or two dominant ones. Suddenly, the rhythm becomes clear, and we see the "beats" of the orbit.

Conclusion

In short, this study suggests that the chaotic, flickering light from the center of our galaxy is actually a complex dance of multiple, short-lived hotspots. While we couldn't prove the rhythm with 100% mathematical certainty due to the noisy data, the "two-hotspot" model provides a unified explanation for why the light sometimes looks rhythmic and sometimes looks random. It paints a picture of a dynamic, ever-changing environment right next to the black hole, where structures are constantly being born, orbiting, and fading away.

Technical Summary: Evidence for Multiple Orbiting Hotspots in the 340 GHz Variability of Sgr A*

Problem and Motivation
Sagittarius A* (Sgr A*), the supermassive black hole at the Galactic Center, exhibits flux variability across radio, infrared, and X-ray bands. While transient periodicities with timescales comparable to the orbital period near the innermost stable circular orbit (ISCO, $\sim$ 30 min) have been reported, these signals are often sporadic and lack statistical significance when red noise is accounted for. Previous studies (e.g., I20+, Y25+) identified transient periodicities at 230 GHz, interpreting them as Doppler beaming from orbiting hotspots. However, the physical nature of these hotspots—specifically their lifetimes, decay mechanisms, and whether multiple structures can coexist—remains an open question. This study investigates whether the 340 GHz variability of Sgr A* can be explained by a unified model involving multiple, decaying orbiting hotspots.

Methodology
The authors analyzed 11 epochs of archival Atacama Large Millimeter/submillimeter Array (ALMA) data at 340 GHz, selected based on field of view, frequency band, and integration time (>1 hour). The data reduction involved standard calibration, self-calibration, and the CLEAN algorithm using the CASA package.

The analysis focused on a specific light curve from August 31, 2016 (150 minutes duration), which exhibited clear short-timescale variability. To isolate short-timescale features from long-term trends, the authors employed Gaussian Process Regression (GPR) with a radial basis function (RBF) kernel to model and subtract long-timescale variability. Periodicity searches were conducted using the Generalized Lomb-Scargle (GLS) periodogram. Crucially, the significance of detected peaks was evaluated under two assumptions: white noise only, and a combination of red noise (power spectral density $\propto f^{-2}$ ) and white noise.

To model the variability, the authors proposed a "multiple-hotspot" scenario where a single hotspot orbits initially, followed by the emergence of a second hotspot. Both structures orbit while undergoing energy decay. The flux density model ( $F_{obs}$ ) combines two sinusoidal components with time-dependent amplitudes decaying via a Gaussian function and a sigmoid function governing the emergence of the second hotspot. Parameters were constrained using Markov Chain Monte Carlo (MCMC) fitting.

Key Results

Periodogram Analysis: In the 2016 August 31 light curve, the GLS periodogram revealed two peaks at $\sim$ 34 min and $\sim$ 52 min. Under a white-noise assumption, these peaks were highly significant ( $\sim$ 12.7 $\sigma$ and $\sim$ 25.4 $\sigma$ ). However, when red noise was included, the significance dropped below the 3 $\sigma$ threshold ( $\sim$ 0.9 $\sigma$ and $\sim$ 2.9 $\sigma$ ). Consequently, the authors do not claim these as statistically significant periodic detections.
Model Fitting: Despite the lack of statistical significance in the periodogram, the light curve exhibited phase-dependent structure and amplitude evolution consistent with orbital modulation. The multiple-hotspot model provided a good fit to the data. The best-fit parameters yielded orbital periods of $T_1 = 50.4 \pm 0.3$ min and $T_2 = 30.6 \pm 0.3$ min, consistent with previously reported timescales near the ISCO.
Decay Timescales: The model inferred amplitude decay timescales of $\sigma_1 \approx 70.3$ min and $\sigma_2 \approx 37.7$ min.
Cooling Mechanisms: The authors compared the observed decay timescales with theoretical cooling mechanisms. Synchrotron cooling alone ( $t_{syn} \sim 150$ min) was found to be too slow to explain the observed amplitude decay and the concurrent decrease in the spectral index. Adiabatic expansion, with timescales on the order of $\sim$ 1 hour, offers a more consistent explanation, though the authors note that the decay likely results from a combination of physical processes.

Significance and Claims
The paper argues that while strict periodicity is difficult to confirm due to red noise and the transient nature of the events, the observed variability is well-described by a physical model of multiple orbiting hotspots with decaying emission. The key contribution is the proposal that both periodic and non-periodic variability in Sgr A* may arise from a common physical origin: orbiting structures within the accretion flow.

The authors suggest that under typical conditions, multiple hotspots may coexist, preventing the buildup of coherent periodic signals over short observation windows. Periodic signatures may only emerge clearly when a transient event (such as a flare) reduces the number of active hotspots, allowing a single structure to dominate. This framework provides a unified interpretation for the diverse variability behaviors observed in Sgr A*, linking sporadic periodicity with stochastic fluctuations. The study concludes that millimeter-wavelength variability serves as a probe of the immediate vicinity of the SMBH, though future multi-wavelength and polarization observations are required to further constrain the physical mechanisms (e.g., distinguishing between synchrotron cooling and adiabatic expansion).