Einstein Probe observations of the new Galactic transient MAXI J1744-294

ATel #17068; Yilong Wang (NAO, CAS; ICE, CSIC-IEEC), Francesco Coti Zelati (ICE, CSIC-IEEC), Nanda Rea (ICE, CSIC-IEEC), Chichuan Jin (NAO, CAS), Alessio Marino (ICE, CSIC-IEEC), Jingwei Hu, Minghao Zhang, Tianying Lian (NAO, CAS), Cuiyuan Dai (NJU), Yaqi Zhao (USTC; PRIC), Wenda Zhang (NAO, CAS), Erik Kuulkers (ESA), Weimin Yuan (NAO, CAS), Sebastien Guillot (IRAP, CNES), Huaqing Cheng (NAO, CAS), Guobao Zhang (YNAO, CAS), Albert Kong (NTHU), Zhaosheng Li (XTU) on behalf of the Einstein Probe team:
on 7 Mar 2025; 15:58 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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MAXI J1744-294 is a new X-ray transient near the Galactic Center detected by MAXI (ATels #16975 and #16983) on January 2, 2025. Following the discovery, the source location was observed by multiple instruments, including NinjaSat (ATel #17009), Swift/XRT (ATel #17010), NuSTAR (ATel #17031), NICER (ATel #17040) and XRISM (ATel #17063) in X-rays, along with MeerKAT (ATel #17045) in the radio band. These observations suggest that this source is likely a new Galactic transient, possibly an X-ray binary in a soft state (e.g., ATel #17040), but other interpretations cannot be excluded at present.

The Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) observed the source location starting on 2025-03-02 at 04:25:20 (UTC) with an on-source exposure of 3071 s. A bright source was detected near the reported position of MAXI J1744-294. The derived absorbed and unabsorbed fluxes in the 0.5-4 keV energy band are 7.8 (+0.8/-0.7) x 10^-10 erg/s/cm^2 and 3.1 (+3.0/-1.2) x 10^-9 erg/s/cm^2, respectively. Uncertainties on parameters are quoted at 90% confidence level in this report.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board EP. The observation started at 2025-03-05 04:25:45 (UTC), with an on-source exposure time of 2195 s. Two sources were detected near the WXT position. The brighter source is located at R. A. = 266.4189 deg, Dec. = -29.0147 deg (J2000) and the fainter one is at R. A. = 266.3993 deg, Dec. = -29.0281 deg (J2000). Both positions have an uncertainty of ~10 arcsec (radius, 90% C.L. statistical and systematic). The brighter source is identified as MAXI J1744-29 while the fainter one is possibly associated with a known low-mass X-ray binary, AX J1745.6-2901. These results are consistent with previous reports (e.g. ATel #17063). The 1-10 keV FXT spectrum of MAXI J1744-294 can be fitted with an absorbed blackbody model. The best fit yielded a strong absorption with a neutral hydrogen column density of NH = (10.1+/-0.2) x 10^22 cm^-2 which is consistent with the expected value in this direction, and a blackbody temperature of (0.59+/-0.01) keV. The absorbed and unabsorbed FXT fluxes in the 1-10 keV band are estimated to be (7.1+/-0.1) x 10^-10 erg/s/cm^2 and (4.9+/-0.3) x 10^-9 erg/s/cm^2, respectively. When fitting the spectrum with an absorbed disk blackbody model, the best-fit parameters are NH = (11.3+/-0.2) x 10^22 cm^-2, T_in = (0.69+/-0.01) keV and a normalization of 2801 (+422/-360). Assuming a distance of ~ 8 kpc and a binary inclination of 60 degrees, we estimate an inner disc radius of ~ 60 km. We note that the FXT data suffer from mild pile-up effects above ~ 5 keV, and that a significant dust scattering halo is visible around MAXI J1744-294. A more accurate spectral analysis is ongoing.

We also performed a search for coherent signals in the power spectra from both detectors. No statistically significant signal was found; we set a 3 sigma upper limit of around 10% on the pulsed fraction for any signal within the frequency range of 10 Hz to 1 kHz (assuming a sinusoidal pulse shape).

Multi-wavelength follow-up observations are encouraged.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.