XRISM spectroscopy of a crowded Galactic center region -- II. Narrow emission lines in the Black Hole candidate MAXI J1744-294/Swift J174540.2-290037

Abstract

Narrow, highly ionized X-ray emission lines in black hole low-mass X-ray binaries (BH-LMXBs) are rare and have been observed in only a few sources, during unusual, heavily obscured accretion states. We report on a detailed high-resolution spectral analysis of emission line features from the first XRISM observation of a BH-LMXB candidate in a bright soft state, MAXI J1744-294/Swift J174540.2-290037, in the central parsec region of our galaxy. The source was observed as part of an extensive, coordinated multi-wavelength campaign on its recurring X-ray outburst in early 2025. By carefully modeling the contributions of multiple point sources and diffuse emission within the XRISM/Resolve field of view, and combining these data with broadband X-ray coverage from XMM-Newton and NuSTAR (Paper I), we identified a narrow ($σ\sim 500-1000$ km s$^{-1}$), static emission component intrinsic to the system. This component likely arises from a highly ionized (log $ξ\gtrsim 5.5$) photoionized plasma in the inner disk atmosphere, and is accompanied by a weak, narrow Fe I K$α$ line at 6.4 keV. We also detected at least three narrow emission features at atypical energies between 6.7 and 7.1 keV. The lack of corresponding rest-frame atomic transitions points toward highly ionized blueshifted Fe lines with outflow velocities of $-1300$ to $-6000$ km s$^{-1}$, which we model with multiple layers of photoionized or collisional plasma. We explore scenarios in which these unprecedented features are produced by multiple phases in a jet and/or a disk wind, and discuss potential similarities between MAXI J1744- 294 and the exotic microquasar SS 433.

Figures (28)

• Figure 1: 2-10 keV Resolve lightcurve computed from the "small" MAXI J1744-294 region, and 0.3-10 keV Xtend lightcurve computed from the MAXI J1744-294 region, both with a 256s binning.
• Figure 2: Spectrum, ratio and residuals for the Xtend MAXI J1744-294 region in the DDT observation, in the 0.4-10 keV band, after the empirical continuum modeling described in Section \ref{['sub:empi_xtend']}, before (left) and after (right) fitting additional emission lines. Following the colormaps of Paper I, components from MAXI J1744-294 and Sgr A East are shown in shades of green and purple, respectively. The spectrum is rebinned with the optimized scheme of Kaastra2016_binning_opt.
• Figure 3: Resolve spectra and residuals for the "big" MAXI J1744-294 region and AX J1745.6-2901 region in the DDT observation, after the first step of their common continuum modeling, and in the entire $2-10$ keV band. Both spectra are visually rebinned at a 20$\sigma$ significance for readability, and model components at a 3$\sigma$ significance level.
• Figure 4: Zoomed spectrum, ratio, and residuals for the "big" MAXI J1744-294 region in the DDT observation, in the 6.3-7.1 keV band, after the empirical continuum-only modeling, including the edge readjustment and lines above 7.8keV in AX J1745.6-2901. The spectrum is visually rebinned at a 10$\sigma$ significance level for readability, and model components at a 3$\sigma$ significance level.
• Figure 5: Zoomed spectrum, ratio, and residuals for the "big" MAXI J1744-294 region in the DDT observation, in the 6.3-7.1 keV band, after the full empirical modeling, including all significant lines. The spectrum is visually rebinned at a 10$\sigma$ significance level for readability, and model components at a 3$\sigma$ significance level.