Detection and Evolution of Linear Polarization of the Galactic Center Transient MAXI J1744-294

TL;DR

This study reports the first detection of variable linear polarization in the Galactic-center transient MAXI J1744-294, revealing a large Faraday rotation measure and providing evidence of its location within the Galactic center region.

Contribution

First measurement of linear polarization and Faraday rotation in MAXI J1744-294, linking it to the Galactic center and suggesting jet-related magnetic activity.

Findings

Detected a large RM of -63,606 rad/m^2, the third largest in the Galaxy.

Found a secondary polarized component with RM ~ -6000 rad/m^2 on one epoch.

Implied magnetic field strength of 15-30 gauss from synchrotron cooling assumptions.

Abstract

MAXI J1744$-$294, likely a low-mass X-ray binary system, is a Galactic-center transient source, detected at radio and X-ray wavelengths, located approximately $19''$ southeast of Sgr A*. We report the first detection of its variable linear polarization in four epochs spanning 2025 Apr 04--09. The normalized 33 and 43 GHz Stokes parameters $q$ and $u$ over the four epochs imply a common Faraday rotation screen with a rotation measure RM $=-63\,606^{+844}_{-861}$ radians m$^{-2}$, the third largest RM detected within the Galaxy. The RM is consistent with that of the Galactic center magnetar PSR J1745$-$2900, giving the first direct evidence that MAXI J1744 lies within the Galactic center region, is bound to Sgr A*, and therefore, is part of the nuclear star cluster. The uniformity in the Galactic center Faraday screen suggests that Sgr A*'s $\approx-10^5$ rad m$^{-2}$ RM is intrinsic rather than originating from an unrelated line-of-sight source. On 2025 Apr 06, we detected a secondary polarized component with an additional RM $\approx-6000$ rad m$^{-2}$, which was not seen at any other epoch. Assuming this secondary component primarily cools by synchrotron radiation, the implied local magnetic field strength is $\sim$15--30 gauss. In the context of a jetted X-ray binary progenitor, the additional RM screen and magnetic field strength are explainable with a short-lived knot in a putative jet.