Detection of a strong X-ray flare from the quasar PHL 1811 by Einstein Probe
ATel #16763; A. Li (BNU), Y. L. Wang, W. X. Wang, T. Y. Lian, H. W. Pan, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, S. X. Wen, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, J. Guan, C. K. Li, W. W. Cui, H. Feng, 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 9 Aug 2024; 07:59 UT
Credential Certification: Yuan Liu (liuyuan@bao.ac.cn)
We report on the detection of a recent strong X-ray flare from the quasar PHL 1811 by the Einstein Probe (EP). The flare was first detected by the Wide-field X-ray Telescope (WXT) on board EP as a possible new X-ray transient in a single pointing observation (with an exposure time of 9.24 ks) at 2024-08-03T05:15:42 (UTC). The derived unabsorbed flux in 0.5-4 keV is 1.2(+/-0.7) x 10^(-11) ergs/cm^2/s. An observation with the Follow-up X-ray Telescope (FXT) on board EP was triggered at 2024-08-04T14:35:25, 33 hours after the WXT observation, confirming the flare to be associated with PHL 1811. The FXT spectrum can be well modeled by an absorbed power law with a photon index of 1.85 (+/-0.29), and the derived unabsorbed 0.5-10 keV flux is 5.25 (+/-0.15) x 10^(-13) ergs/cm^2/s. The uncertainties are given at the 90% confidence level for the above parameters. A further target of opportunity (ToO) observation with the Swift/XRT was triggered at 2024-08-06T20:45:02. No significant source was detected within the error circle of FXT. An upper limit flux of 1.0e-13 ergs/cm^2/s in 0.3-10 keV is set by the non-detection of XRT data.
As a famous 'X-ray weak' weak-line quasar (WLQ, e.g. Wu et al. 2011), PHL 1811 was observed by XMM-Newton on November 1, 2004, with a flux of 6.8(+/-0.4) x 10^(-14) ergs/cm^2/s within 0.2-12 keV, and later by Swift/XRT on October 22, 2005 with a flux of 4.2(+/-1.0) x 10^(-13) ergs/cm^2/s within 0.3-10 keV. Therefore, the recent X-ray flare detected by EP was almost two orders of magnitude brighter than its previous flux, suggesting that this quasar just underwent a strong X-ray flare on a timescale of days.
We thank the Swift team for performing the ToO observation. Multi-wavelength follow-up observations are also 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.
