LMXB 1A 1744-361: continued EP-WXT & NICER monitoring

ATel #16735; H. Q. Cheng, T. Y. Lian, X. Pan (NAOC, CAS), R. Z. Li (YNAO, CAS), W. J. Zhang (NAO, CAS), F. Coti Zelati, A. Marino, N. Rea (ICE-CSIC, IEEC), Y. Liu, H. W. Pan, Z. X. Ling, C. C. Jin, W. Yuan, C. Zhang, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), L. Tao, H. Feng, J. Guan, C. K. Li, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team
on 26 Jul 2024; 02:40 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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A possible brightening of the LMXB 1A 1744-361 was detected by EP-WXT on July 16 (ATel #16714). Since then, EP-WXT continued monitoring the field of the source. The observations of the WXT were taken from 2024-07-16T00:47:30 (UTC) to 2024-07-25T06:08:19 (UTC). The light curve of 1A 1744-361 shows a gradual brightening until 2024-07-23T07:36:49 (UTC), followed by a dramatic increase in flux in the last observation. The latest observation began at 2024-07-25T06:08:19 (UTC) with an exposure time of 2045 seconds. The average 0.5-4 keV spectrum can be fitted by an absorbed power-law model, yielding a column density of 5.7(-1.1/+1.2) x 10^21 cm^-2 and a photon index of 1.4(-0.2/+0.2). The derived average unabsorbed 0.5-4 keV flux is 3.9(-0.4/+0.5) x 10^-9 erg/s/cm^2, which is over 50 times brighter than that reported in ATel #16714. The uncertainties are at the 90% confidence level for the above parameters.

We also performed a series of follow-up observations of the source with the Neutron star Interior Composition Explorer (NICER) which started on 2024-07-20T08:45:20 (UTC) and is still ongoing. The brightening of the source was also detected by NICER. The count rate of the source evolved from ~40 on 2024-07-20 to ~440 counts per second on 2024-07-25 in the 0.3-10 keV band within which the source dominates. We conducted a preliminary search for periodic signals in the power spectrum, as well as an acceleration search, and did not find anything significant in the 0.1-2000 Hz frequency range. The 0.3-10 keV averaged power spectrum is consistent with white noise (i.e., is flat). A type-I burst was detected at 2024-07-22T19:58:13 (note that this is a rough estimation), which lasted for ~70 seconds with a peak count rate of ~400 counts per second (0.3-10 keV). In addition, the spectral shape of NICER data also shows some evolutionary trend as indicated by the hardness ratio, defined as the ratio of the count rate within 0.3-2 keV band and that within 2-10 keV band. Specifically, the spectrum underwent a gradual softening during the observations between 2024-07-20 and 2024-07-24 (except for the sudden hardening during the type-I burst), whereas it hardened as the flux increased dramatically from late 2024-07-24.

We thank the NICER team for making the ToO observation possible. EP will keep monitoring the source in the following days. Multi-band 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. NICER is a 0.2-12 keV X-ray telescope operating on the ISS. The NICER mission and portions of the NICER science team activities are funded by NASA.