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

TL;DR

This study presents high-resolution XRISM spectral analysis of the black hole candidate MAXI J1744-294, revealing rare narrow emission lines likely from highly ionized plasma and blueshifted Fe lines indicating outflows, in a crowded galactic center region.

Contribution

First detailed spectral analysis of MAXI J1744-294 in a bright soft state, identifying unique narrow emission features and modeling outflows with multiple plasma layers.

Findings

Detected narrow, static, highly ionized emission component from the inner disk atmosphere.

Identified blueshifted Fe lines indicating outflows with velocities of -1300 to -6000 km/s.

Modeled features as arising from multiple phases of jet and disk wind plasma.

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 ($\sigma \sim 500-1000$ km s$^{-1}$), static emission component intrinsic to the system. This component likely arises from a highly ionized (log $\xi \gtrsim 5.5$) photoionized plasma in the inner disk atmosphere, and is accompanied by a weak, narrow Fe I K$\alpha$ 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.