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

TL;DR

MAXI J1744-294 is a newly detected Galactic center X-ray transient that the authors classify as a stellar-mass BH-LMXB through a comprehensive, multiwavelength campaign. Using broadband X-ray spectra (3–79 keV) from NuSTAR alongside soft X-ray data and high-resolution spectroscopy, they model the continuum, Fe K$\\alpha$ lines, and relativistic reflection with relxillCp, constraining a high spin $a\sim0.95$ and an inclination $\theta\sim28^{\circ}$ in the hard state. The outburst exhibits a unique evolution, transitioning from soft toward a bright hard state and then re-brightening in 2025, with a notable April 7 hard flux spike that coincided with enhanced reflection and Fe-line changes. Radio observations indicate unusually strong jet activity for the X-ray luminosity, and the aggregate properties (no NS signatures, high $L_X$, cusp-like GC location) support a BH-LMXB interpretation within ~1 pc of Sgr A*, contributing to the evidence for a dense BH cusp in the Galactic center. This work demonstrates the effectiveness of coordinated, time-domain, multiwavelength studies for diagnosing accretion physics and compact-object demographics in crowded galactic environments.

Abstract

For the first time in nearly a decade, a bright new transient was detected in the central parsec (pc) of the Galaxy. MAXI J1744-294 was never observed in outburst prior to 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. Based on the spectral and temporal characteristics of the source, we identify MAXI J1744-294 as a candidate black hole (BH) low-mass X-ray binary (LMXB) -- the fourth candidate BH transient discovered within a (projected) distance of one pc from the Galactic supermassive black hole Sgr A*. This discovery 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.

Figures (24)

• Figure 1: Top and center: MAXI J1744-294 light curves in the 2-4 keV and 4-8 keV energy bands, respectively. Swift/XRT data are shown in cyan, with larger markers for WT-mode observations and smaller markers for PC-mode data (scaled $\times10$ for visualization). Also shown are NuSTAR (black), NICER (silver; scaled down by a factor of 10), and XMM-Newton/EPIC (green) count rates. The dates of XRISM, Chandra, IXPE, MeerKAT, and Keck observations are marked by purple solid, orange dotted, yellow solid, dark blue dashed, and pink dashed lines, respectively. Though plotted for all data points, errorbars are smaller than most markers. Bottom panel: hardness ratios calculated from NuSTAR fluxes in the $2-10$ keV (soft) and $10-50$ keV (hard) bands. A dashed horizontal line denotes the hard state threshold.
• Figure 2: NuSTAR FPMA images of the Galactic center region. The 2017 observation (left) was used to model the underlying background at the location of MAXI J1744-294 and the AX J1745.6-2901 background region (regions 2A and 2B, respectively). Region 1B (right) was used to model the contamination from the AX J1745.6-2901 outburst. See Section \ref{['sec:bkg']} for more.
• Figure 3: XMM-Newton/EPIC PN detector images from observations conducted on March 3 (left) and Sept. 6 (center and right), 2025.
• Figure 4: Left: Swift XRT PC-mode image of the Galactic center from 2025 February 2. MAXI J1744-294 is the bright source in the center of the image with AX J1745.6-2901 lying $\sim80$ to the SW. Annular extraction regions are shown for MAXI J1744-294 (cyan) and background (yellow) spectra (see Section \ref{['subsec:swift']} for details). Right: Chandra/ACIS-S HETG image of MAXI J1744-294. The location of MAXI J1744-294 was determined by finding the intersection between the readout streak and the MEG dispersion streak (dashed lines). See Section \ref{['subsec:chandra']} for more.
• Figure 5: NIR follow-up observations. Top left: JWST NIRCam image from 2022. Top right: Keck continuum-subtracted Br-$\gamma$ subtracted. Bottom: Keck Kp (left), Br-$\gamma$ (center) and K-cont (right). The large dashed cyan circle represents the original MeerKAT position of MAXI J1744-294 based on the earliest observations Grollimund2025, while the small solid circle marks the refined MeerKAT position after months of monitoring. There is no evidence of any IR excess in either location with respect to the JWST reference image.