Einstein Probe detection of a new X-ray outburst from Aql X-1

ATel #16821; M. J. Liu, X. P. Xu (NAOC, CAS), Y. F. Liang (PMO, CAS), Z. J. Zhang (HKU), A. Marino (ICE-CSIC), C. C. Jin, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, 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, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, J. Zhang, 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 19 Sep 2024; 14:42 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)

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We report on the detection of a new strong outburst in X-rays from the neutron star low-mass X-ray binary Aql X-1 by Einstein Probe (EP). The rise of its X-ray flux was first detected by EP-WXT at 2024-09-14 T12:22:59 (UTC). Its X-ray spectrum, when fitted with a power-law model, has an unabsorbed flux of 2.1(+6.5/-1.1)e-10 erg/s/cm^2 in 0.5-4 keV, an absorption column N_H=1.6(+0.9/-0.7)e22 cm^-2 and a photon index of 2.7(+1.6/-1.1). At 2024-09-18T15:53:42, the flux increased to 6.0(+1.1/-0.7)e-10 erg/s/cm^2 with N_H =1.0(+0.1/-0.2)e22 cm^-2 and an index of 1.9(+0.2/-0.3). Alternatively, these spectra can also be fitted with a multi-color blackbody model; for the spectrum at 2024-09-14 T12:22:59, the unabsorbed flux in 0.5-4 keV is 1.1(+0.6/-0.3)e-10 erg/s/cm^2 with N_H =1.1(+0.3/-0.4)e22 cm^-2 and an inner-disk temperature of 0.8(+0.6/-0.3) keV, whereas for the spectrum at 2024-09-18T15:53:42, the unabsorbed flux is 4.7(+0.4/-0.4)e-10 erg/s/cm^2 with N_H =0.7(+0.1/-0.1)e22 cm^-2 and a temperature of 1.1(+0.2/-0.1) keV.

We have triggered a monitoring campaign with EP-FXT, and also requested NICER to follow the evolution of this outburst in more detail. For coordinating multi-wavelength observations with our ongoing monitoring campaign, please contact us at EP science center (ep_ta@bao.ac.cn). The contact transient advocate for this source is Mingjun Liu.

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).