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A multiwavelength study of the Galactic center black hole candidate MAXI J1744-294

This paper presents a multi-wavelength study of the newly discovered bright transient MAXI J1744-294, confirming its classification as a black hole low-mass X-ray binary and providing further evidence for a concentration of such objects near the Galactic center.

Original authors: Shifra Mandel, Kaya Mori, Paul A. Draghis, Mark Reynolds, Chichuan Jin, Maxime Parra, Benjamin Levin, Eric Miao, Noa Grollimund, Anna Ciurlo, Sean A. Granados, Gaurava K. Jaisawal, Lorenzo Marra, Matt

Published 2026-02-10

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CC BY 4.0

Original authors: Shifra Mandel, Kaya Mori, Paul A. Draghis, Mark Reynolds, Chichuan Jin, Maxime Parra, Benjamin Levin, Eric Miao, Noa Grollimund, Anna Ciurlo, Sean A. Granados, Gaurava K. Jaisawal, Lorenzo Marra, Matteo Bachetti, Fiamma Capitanio, Nathalie Degenaar, Charles J. Hailey, JaeSub Hong, Sara Motta, Gabriele Ponti, Michael M. Shara, Megumi Shidatsu, John A. Tomsick, Randall Campbell, Stéphane Corbel, Rob Fender, Andrea Ghez, Jonathan Grindlay, Daryl Haggard, Matthew W. Hosek, Ziqian Hua, Ole König, Kai Matsunaga, Romana Mikušincová, Melania Nynka, Grace Sanger-Johnson, Giovanni Stel, Antonella Tarana, Rudy Wijnands, Shuo Zhang

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

The Galactic Center’s "Ghost" Returns: A Story of a Cosmic Heavyweight

Imagine you are looking at a massive, crowded city at night from a high-altitude plane. The city is glowing with millions of lights—streetlamps, skyscrapers, and car headlights. In the very center of this city, there is a massive, dark skyscraper that is so heavy it actually warps the ground around it. This is our Galactic Center, and that heavy skyscraper is Sgr A*, the supermassive black hole at the heart of our Milky Way.

For a long time, this central part of the city was relatively quiet. But in January 2025, a sudden, bright flash of light appeared near the center. It was like a massive neon sign suddenly flickering to life in a dark alleyway. Astronomers named this flash MAXI J1744-294.

This paper is the "detective report" written by a team of scientists who used every high-tech "camera" we have (X-ray telescopes like NuSTAR and Chandra, and radio telescopes like MeerKAT) to figure out exactly what that light was.

1. The Identity Crisis: A New Resident or an Old Regular?

When the light first appeared, scientists weren't sure if they were seeing a brand-new resident moving into the neighborhood or an old regular coming back for a visit.

Think of it like seeing a specific car driving down a street. Is it a new car you've never seen before, or is it the same car that drove by three years ago? By using incredibly precise "GPS" (the Chandra telescope), the scientists realized the light was coming from almost the exact same spot where a different flash occurred in 2016.

The Verdict: This isn't a new stranger; it’s a "repeat offender." This object has a habit of waking up, throwing a massive light show, and then going back to sleep.

2. The "Heavyweight" Reveal: Black Hole or Neutron Star?

The biggest mystery was: What is actually making the light? In space, bright flashes usually come from one of two things:

A Neutron Star: A small, incredibly dense ball (like a heavy bowling ball).
A Black Hole: A much larger, invisible monster that swallows everything (like a cosmic vacuum cleaner).

To tell them apart, the scientists looked at the "flavor" of the light.

The Radio Test: They checked the radio waves. Black holes are "loud" in radio; neutron stars are "quiet." MAXI J1744-294 was screaming in radio waves.
The Spin Test: They looked at how the light was being warped by gravity. This is like watching how water swirls around a drain. The way the light "swirled" suggested the object was spinning incredibly fast—so fast that only a black hole could pull it off.

The Verdict: This is definitely a Black Hole. Specifically, a "stellar-mass" black hole, which is much smaller than the giant at the center of our galaxy, but still a heavyweight.

3. The Strange Behavior: A Cosmic "Flip-Flop"

Most black holes follow a predictable "dietary" pattern. They start bright and "soft" (smooth light), then fade into a "hard" state (jagged, high-energy light).

But MAXI J1744-294 is a bit of a rebel. It followed a weird path. It stayed bright and "hard" for much longer than expected, almost like a person who starts a diet but then decides to have a massive feast right in the middle of it. It even had a strange "glitch" in April where the light suddenly shifted in a way that scientists are still trying to fully explain.

4. Why Does This Matter?

Why spend years studying one flickering light in a crowded city?

Because of where it lives. This black hole is hanging out in the "inner city" of our galaxy, very close to the supermassive black hole. Scientists believe that the gravity of the big boss (Sgr A*) acts like a cosmic magnet, pulling these smaller black holes into a tight cluster.

By studying this one "rebel" black hole, we are learning about the hidden population of monsters that live in the heart of our galaxy—a population that helps us understand how galaxies are built and how gravity rules the universe.

In short: A "ghostly" black hole woke up in the center of our galaxy, put on a massive, weird, and loud light show, and by studying its "performance," scientists confirmed it is a fast-spinning heavyweight that belongs to a secret club of black holes living near the galactic center.

Technical Summary: A Multiwavelength Study of the Galactic Center Black Hole Candidate MAXI J1744-294

1. Problem and Context

The central parsec (pc) of the Milky Way is a high-density environment surrounding the supermassive black hole Sgr A*. Theoretical models predict a "cusp" of stellar-mass black holes (BHs) in this region, yet identifying them is difficult due to extreme interstellar absorption ( $N_H \gtrsim 10^{23} \text{ cm}^{-2}$ ), bright diffuse background emission, and source confusion. While many neutron star (NS) low-mass X-ray binaries (LMXBs) have been identified, the population of BH-LMXBs in the central pc remains sparsely sampled. The study aims to characterize the 2025 outburst of MAXI J1744-294, a bright transient discovered in the Galactic center, to determine its nature (BH vs. NS) and its evolutionary behavior.

2. Methodology

The authors employed a comprehensive, multi-wavelength, and multi-epoch approach, spanning from February to September 2025.

X-ray Spectroscopy & Timing: The core of the study utilizes broadband X-ray data (3–79 keV) from NuSTAR to capture Compton reflection and high-energy tails. This is complemented by soft X-ray data (<10 keV) from Swift/XRT, XMM-Newton, and Chandra to constrain absorption ( $N_H$ ) and disk temperature ( $kT_{in}$ ). NICER provided high-cadence timing, and XRISM offered high-resolution spectroscopy of the Fe K $\alpha$ complex.
Polarimetry: IXPE data were used to provide independent constraints on the system's inclination ( $\theta$ ).
Radio & Infrared: MeerKAT (radio) and Keck (near-infrared) observations were used to study the radio-X-ray luminosity correlation and search for NIR counterparts.
Modeling Techniques:
- Relativistic Reflection: The relxillCp model was used to model the accretion disk atmosphere and relativistic distortions (spin, inclination, and inner disk radius).
- Background Subtraction: Complex modeling was required to account for the nearby active NS-LMXB (AX J1745.6-2901) and the dust-scattering halo using the axjdust model.
- Mass Estimation: A method combining diskbb normalization and relxillCp inner radius ( $r_{in}$ ) was used to estimate the compact object's mass.

3. Key Contributions

Source Identification: The study concludes that MAXI J1744-294 is a repeat outburst of the 2016 transient Swift J174540.2-290037, based on sub-arcsecond positional agreement between Chandra and MeerKAT.
Characterization of Outburst Evolution: The paper documents a unique evolutionary path, including a "bright/hard" state in September 2025 that deviates from the canonical "q-shaped" hardness-intensity diagram (HID).
Relativistic Parameter Estimation: The authors successfully broke several degeneracies to provide high-confidence measurements of the compact object's properties.

4. Results

Black Hole Confirmation: Multiple lines of evidence confirm the source is a stellar-mass BH:
- Spin & Inclination: Relativistic reflection modeling in the hard state yields a high dimensionless spin $a > 0.92$ and an inclination $\theta \approx 28^\circ$ .
- Mass: Spectral modeling suggests a mass preference $> 2\text{--}3 \, M_\odot$ , consistent with a BH.
- Radio-Loudness: The source is significantly more radio-loud than typical NS-LMXBs at comparable X-ray luminosities.
- Luminosity & Timing: The peak $L_X \gtrsim 10^{37} \text{ erg s}^{-1}$ and the lack of Type I X-ray bursts or pulsations further support the BH classification.
Spectral Dynamics: The inner disk radius ( $r_{in}$ ) was observed to increase (disk truncation) as the source transitioned from the soft to the intermediate state.
Anomalous Behavior: The April 7, 2025, observation revealed a "flip-flop" style event where a drop in soft X-rays was followed by a significant hardening and an increase in radio flux, suggesting complex coupling between the accretion disk and the jet.

5. Significance

This study provides the third piece of evidence for a cusp of BH-LMXBs within the central parsec of the Galaxy. By demonstrating that a BH transient can undergo a highly unusual "bright/hard" state and by providing precise relativistic measurements in a crowded field, the work establishes a high-standard model for future time-domain multi-wavelength astrophysics in the Galactic center. It reinforces the theoretical prediction that stellar-mass black holes remain dynamically bound to the supermassive black hole Sgr A*.