NuSTAR follow-up observation of the new Galactic center transient MAXI J1744-294

ATel #17031; S. Mandel, K. Mori, C. Hailey (Columbia U.), M. Bachetti (INAF), N. Degenaar (U. Amsterdam), P. Draghis (MIT), J. Grindlay, J. Hong (Harvard CfA), G. Ponti (INAF, MPE), M. Reynolds (U. Michigan), J. Tomsick (UCB), R. Wijnands (U. Amsterdam)
on 11 Feb 2025; 18:06 UT
Credential Certification: Shifra Mandel (ss5018@columbia.edu)

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We report on a follow-up NuSTAR observation of the new Galactic center transient MAXI J1744-294, which was detected by MAXI on Jan. 2, 2025 (Atels #16975, #16983), followed by NinjaSat on Jan. 16-23 (Atel #17009), Swift/BAT on Jan. 14-16 and Swift/XRT on Feb. 1 (ATel #17010).

We requested a NuSTAR ToO observation, which was performed on Feb. 6, 2025 with an exposure of 22.7 ks. Two bright sources are clearly visible in the NuSTAR images, one of which is consistent with the position of the neutron star LMXB AX J1745.6-2901 (Atel #16642). The second, brighter source appears to be ≲20″ from the position of Sgr A*. We identify this source as the counterpart to MAXI J1744-294.

We extracted FPMA and FPMB spectra from a circular region of radius 20″; the narrow aperture size was selected to avoid contamination from AX J1745.6-2901. The FPMA and FPMB spectra each show net count rates of ~2.50 +/-0.01 cts/s over the full 3-79 keV NuSTAR band. Both an absorbed disk-blackbody and an absorbed power-law model yielded very poor fits (χ²/ν ≳ 5). A combined absorbed disk-blackbody plus power-law model gave a much better fit, which was further improved (χ²/ν ~1.2) by the addition of a Gaussian component to fit an emission line (LineE = 6.63 keV; equivalent width = 0.2 keV) evident between 6-7 keV. The best-fit model parameters are n_H = (1.3 +/-0.5)*10²³ cm^-2, photon index Γ = 2.27 +/-0.07 and disk temperature kT_in = (0.65 +/-6e-3) keV (all errors quoted are at 90% C.L.) Residuals at energies ≳ 30 keV indicate the presence of an additional hard reflection component. The 2-10 keV unabsorbed flux is estimated at ~2*10^-9 erg/s/cm², corresponding to L_X ~1.5*10³⁷ erg/s at a distance of 8 kpc. The spectral parameters of MAXI J1744-294 are consistent with those of a black hole LMXB in the soft state.

Further observations with XMM-Newton, Chandra, and NuSTAR will take place in the coming weeks.

We thank the NuSTAR team for their quick response in scheduling this observation.