EP-WXT detection of possible brightening of the LMXB 1A 1744-361 in X-ray
ATel #16714; X. P. Xu (NAOC, CAS), Y. Wang, H. Zhou (PMO, CAS), J. W. Hu, W. Yuan, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, Q. Y. Wu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, S. N. Zhang (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 17 Jul 2024; 16:32 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
Subjects: X-ray, Binary, Neutron Star, Transient
Referred to by ATel #: 16735
We report on the detection of an X-ray source by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) during an observation starting at 2024-07-16 00:47:30 (UTC) and lasting for 8ks. The position of the source, R.A. = 267.061 deg and DEC = -36.134 deg (J2000), with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic), is consistent with that of the neutron star low-mass X-ray binary 1A 1744-361. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 2.3(+1.8/-1.2) with the column density of 3.8(+4.0/-2.7) x 10^21 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 6.6(+13.5/-2.2) x 10^-11 erg/s/cm^2. (The uncertainties are given at the 90% confidence level for the above parameters.) The source was not detected in previous two observations by EP-WXT on July 11 (with an exposure of 38 ks) and 13 (with an exposure of 2.4 ks), 2024, respectively.
The previous outbursts from 1A 1744-361 were reported in Nov. 2003, Apr. 2004, July 2005, June 2008, Nov. 2009, Aug. 2013, and May 2022 (ATels #204, #265, #567, #1587, #2305, #5301, #15407). The last outburst had a peak flux around ~1e-9 erg/cm^2/s, which was two orders of magnitude higher than the flux reported here. We tend to identify this WXT source with 1A 1744-361. If this is the case, 1A 1744-361 is likely brightening again in X-ray. More follow-up observations are encouraged. A follow-up X-ray observation with the EP-FXT will be performed soon.
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.
