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

For the first time in nearly a decade, a bright transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294, or – as it was known in its previous life – Swift J174540.2-290037, was discovered in outburst by the MAXI telescope in January 2025. We present the results of a broadband, multi-wavelength study of MAXI J1744-294, including data from the NuSTAR, Chandra, XMM-Newton, Swift, and NICER X-ray telescopes, as well as complementary radio and near-infrared observations. We analyze the changing X-ray emission as the outburst evolved from the high/soft to the low/hard state. Using relativistic reflection features in the data, we estimate a spin of $a>0.92$ and viewing inclination $θ=28^{+3}_{-4}$ deg, consistent with the parameters measured for Swift J174540.2-290037. Based on the spectral and temporal characteristics of MAXI J1744-294, we reaffirm its classification as a candidate black hole (BH) low-mass X-ray binary (LMXB) – the third candidate BH transient discovered within 20 arcsec of the Galactic supermassive black hole Sgr~A*. This work provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as argued for in previous observational studies and suggested by analytical and theoretical work. Our ongoing multi-wavelength study, involving a complementary range of observatories and spanning different outburst states, can serve as a model for future time domain astrophysics research.

💡 Research Summary

This paper presents a comprehensive, multi-wavelength study of the X-ray transient MAXI J1744-294, a bright outburst discovered in the Galactic Center in January 2025—the first such event in nearly a decade. The study combines data from a suite of X-ray observatories (NuSTAR, Chandra, XMM-Newton, Swift, NICER) as well as complementary radio (MeerKAT, VLA) and near-infrared (Keck) observations to analyze the source’s evolution and physical properties.

A key finding is the identification of MAXI J1744-294 as a recurrent outburst of the previously known transient Swift J174540.2-290037, which was active in 2016. This conclusion is based on precise astrometry from Chandra and MeerKAT, which show the positions of the two transients are coincident within less than an arcsecond. This makes MAXI J1744-294/Swift J174540.2-290037 the first black hole candidate low-mass X-ray binary (BH-LMXB) in the Galactic Center observed to outburst more than once.

The analysis tracks the source’s spectral evolution over several months, from February to September 2025. The light curves and hardness ratios, derived primarily from NuSTAR data, show a clear state transition. The source began in a high/soft state and subsequently evolved through high/intermediate and low/intermediate states, eventually settling into a canonical low/hard state. This progression is characteristic of outbursting black hole X-ray binaries.

A major technical achievement of the work is overcoming the significant observational challenges posed by the crowded Galactic Center environment. These include high line-of-sight absorption, bright diffuse background emission, contamination from nearby sources (notably the simultaneously active neutron star LMXB AX J1745.6-2901), and a dust scattering halo. The team employed a multi-faceted strategy: NuSTAR’s high-energy coverage (>10 keV) was less affected by absorption and scattering, Chandra provided precise localization and background assessment, while XMM-Newton and Swift data constrained the absorption column and disk temperature. Careful background modeling using archival data was crucial to account for contamination.

Spectral modeling, focusing on relativistic reflection features from the inner accretion disk, yielded estimates for fundamental black hole parameters. The analysis constrains the black hole’s dimensionless spin parameter to a > 0.92 and the viewing inclination to θ = 28^{+3}_{-4} degrees. These values are consistent with those previously measured for Swift J174540.2-290037, providing further strong evidence that the two outbursts originate from the same object.

Based on its spectral and temporal characteristics, the study reaffirms the classification of MAXI J1744-294 as a strong black hole candidate LMXB. It is noted as the third such candidate transient discovered within 20 arcseconds of the supermassive black hole Sgr A*. This finding provides further observational evidence supporting the theoretical prediction of a “cusp” or over-density of BH-LMXBs in the central parsec of our Galaxy, as suggested by earlier analytical and theoretical work.

The paper concludes that this multi-wavelength campaign, spanning different outburst states and utilizing a complementary array of observatories, serves as a model for future time-domain astrophysics research. It successfully demonstrates how to extract detailed physical insights from a transient source in one of the most complex and challenging regions of the sky.