GCN Circular 40375

H. Sun, M. H. Zhang, H. Q. Cheng (NAO, CAS), Y. J. Zhang (THU), W. Yuan (NAO, CAS) report on behalf of the Einstein Probe team:

We report on the detection of a new X-ray outburst of the low-mass X-ray binary (LMXB) MAXI J1957+032 by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The burst triggered EP-WXT (ID: 01709175213) at 2025-05-06T21:07:59 (UTC). The source light curve lasted for 74 seconds before the observation was interrupted by the follow-up observation. The WXT position of the source, inferred from the onground analysis, is R.A. = 299.150 deg, DEC = 3.444 deg (J2000) with an uncertainty of 2.8 arcmin in radius (90% C.L. statistical and systematic). The averaged WXT spectrum in 0.5-4 keV can be fitted by an absorbed power law model with a photon index of 1.82 (-0.56, +0.62) when fixing the column density at 1.7 x 10^21 cm^-2 (Sánchez et al. 2017). The average unabsorbed 0.5-4 keV flux is 7.6 (-1.7,+2.2) x 10^-10 erg/cm^2/s. 

Following the WXT trigger, a follow-up observation with the Follow-up X-ray Telescope (FXT) was performed automatically. The FXT observation started at 2025-05-06T21:08:30 (UTC), around 30 seconds after the trigger. The analysis of the data, with an exposure of 1.6 ks, reveals a detection of an X-ray source at RA = 299.1636 deg, Dec = 3.4449 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). This position is consistent with the known LMXB MAXI J1957+032 (also named as IGR J19566+0326), confirming the WXT detection as the outburst of this X-ray source. 

The averaged FXT spectrum in 0.5-10 keV can be fitted by an absorbed power law model with a photon index of 1.85 (-/+ 0.02) when fixing the column density at 1.7 x 10^21 cm^-2. The average unabsorbed 0.5-10 keV flux is 9.1 (-/+ 0.1) x 10^-10 erg/cm^2/s, which is consistent with the flux detected by the WXT. These parameters derived are at the 1-sigma confidence level.

The previous outburst of MAXI J1957+032 happened at the end of September 2016 and lasted for around 4 days. Its X-ray flux was approximately 7 x 10^-13 erg/cm^2/s in 2022 before the outburst, inferred from the Swift/XRT observation. We thus suggest that MAXI J1957+032 has entered a new outburst phase. 

Contact transient advocate (TA) for MAXI J1957+032 is H. Sun (hsun@nao.cas.cn) and please contact the TA for information regarding the EP observation of this source.
  
Reference: Sánchez et al. 2017, MNRAS, 468, 564 

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).