NICER Observation of the Galactic Center Transient MAXI J1744-294

A new X-ray transient near the Galactic Center, MAXI J1744-294, was reported by MAXI on January 2, 2025, with a flux level of 250 mCrab (ATels #16975, #16983, #17009). Follow-up Swift/XRT observations identified two sources near Sgr A*: a fainter source consistent with the position of the low-mass X-ray binary AX J1745.6-2901 (within 10") and a bright uncataloged target at coordinates 17:45:41.93, -29:00:35 (J2000), with a positional uncertainty of 6.4" (ATel #17010). The latest NuSTAR observation on February 6, 2025, confirmed activity from the new source, MAXI J1744-294 (ATel #17031).

We observed MAXI J1744-294 with NICER on February 11-12, 2025, for a total exposure of 2.7 ks. The 0.5-10 keV light curve showed an average count rate of 105 cts/s without any significant variability in the power density spectrum in 0.01-100 Hz.

We fitted the NICER spectrum using an absorbed power-law model with a disk blackbody component in the 2-10 keV range. We detect a broad emission line with a centroid energy of 6.6 ± 0.9 keV, a line width of 0.4 ± 0.2 keV, and line normalization of (2 ± 0.7) × 10-3, consistent with both neutral or ionized Fe. Other best-fit spectral parameters are: nH = (1.1 ± 0.1) × 1023 cm-2, inner disk temperature = 0.57 ± 0.03 keV, inner disk normalization of (6100 ± 1800), and photon index of 3.1 ± 0.4 with a power-law normalization of (2.3 ± 1.3). The unabsorbed and absorbed 2-10 keV fluxes are (3.9 ± 0.2) × 10-9 erg/s/cm2 and (8.8 ± 0.1) × 10-10 erg/s/cm2, respectively. The uncertainties on parameters are reported for the 68% confidence range.

Because NICER is a non-imaging instrument with a 30 arcmin2 field of view, the observation may be affected by AX J1745.6-2901, located approximately 1.7 arcmin from MAXI J1744-294. However, our measured spectral parameters match well (within uncertainties) with those from NuSTAR, suggesting that MAXI J1744-294 is likely in a soft state of its ongoing outburst. Moreover, we note that the Galactic Ridge X-ray Emission may contribute to the iron band (Yamauchi et al. 2009, PASJ, 61, 225). Follow-up observations are encouraged.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.