Aql X-1: the X-rays in recent outburst keep brightening indicated by LEIA observations

ATel #16170; H. N. Yang (NAOC, CAS), H. Y. Liu (NAOC, CAS), Y. Liu (NAOC, CAS), Z. X. Ling (NAOC, CAS), C. Zhang (NAOC, CAS), H. Q. Cheng (NAOC, CAS), C. Z. Cui (NAOC, CAS), D. W. Fan (NAOC, CAS), H. B. Hu (NAOC, CAS), J. W. Hu (NAOC, CAS), M. H. Huang (NAOC, CAS), C. C. Jin (NAOC, CAS), D. M. Li (BNU), D. Y. Li (NAOC, CAS), Q. X. Li (BNU), M. J. Liu (NAOC, CAS), H. Sun (NAOC, CAS), H. W. Pan (NAOC, CAS), C. Y. Wang (THU), W. X. Wang (NAOC, CAS), Y. L. Wang (NAOC, CAS), Q. Y. Wu (NAOC, CAS), X. P. Xu (NAOC, CAS), Y. F. Xu (NAOC, CAS), M. Zhang (NAOC, CAS), W. D. Zhang (NAOC, CAS), W. J. Zhang (NAOC, CAS), Y. J. Zhang (THU), D. H. Zhao (NAOC, CAS) and W. Yuan (NAOC, CAS), on behalf of the LEIA and Einstein Probe team
on 3 Aug 2023; 09:37 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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The recurrent outburst of Aql X-1 was detected by XB-NEWS and MAXI since July 23, 2023 (ATel #16147). A follow-up observation by NICER showed that Aql X-1 was in the hard state during this outburst (ATel #16153). The increasing X-ray brightness was also reported by LEIA (Lobster Eye Imager for Astronomy; Atel #16158). Meanwhile, the observations by the 1-m and 0.5-m SAO RAS optical telescopes also shows an optical brightening in R band (Atel # 16169) , and its brightness is close to the peak in 2014 (ATel #6447).

We report the latest observations on Aql X-1 by LEIA, which show a significant increase in the flux of 0.5-4 keV. The spectra are fitted with an absorbed power-law with Galactic N_H=4.3e21 cm^-2. The results are listed in the table below.

  
Time              	Unabsorbed Flux      	Photon Index     	Exposure 
                        (0.5-4keV)                               	(seconds) 
                        (10^-9 erg/s/cm2) 
2023-07-23T15:05:59 	0.15               	1.43 (fixed)    	671 
2023-07-26T13:46:39 	0.58               	1.43 (+0.42/-0.41) 	688 
2023-08-02T01:15:21 	11.4               	1.41 (+/-0.19)  	191 
2023-08-02T02:49:34 	11.8               	1.07 (+/-0.15)  	269 

The observations on 2023-08-02 show a flux comparable to the peak of the outburst in 2014 (ATel #6447). The brightening in X-ray and optical indicates this outburst is still in progress and more observations are encouraged.

LEIA (Zhang et al, ApJL, 941, 2) is a soft X-ray Lobster-eye imager (0.5 - 4.0 keV) with a FoV of 340 square degrees aboard the SATech-01 satellite of CAS, launched on July 27, 2022.