GCN Circular 36301
Subject
LIGO/Virgo/KAGRA S240422ed: X-ray Upper Limits from EP-FXT for the Potential Electromagnetic Counterparts Reported by Swift and DECam
Date
2024-04-25T13:26:26Z (13 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
C. K. Li (IHEP, CAS), C. C. Jin (NAOC, CAS), H. S. Zhao, M.Y. Ge, Y. Chen, S.M. Jia, W. W. Cui, J. Guan, H. Feng, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, D. W. Han, S. N. Zhang, X. F. Zhao (IHEP, CAS), H. Q. Cheng, J. W. Hu, Q. Y. Wu, H. Y. Liu, D. Y. Li, H. Sun, W. J. Zhang, H. N. Yang, Y. L. Wang, X. Pan, X. P. Xu, J. J. Jin, M. J. Liu, Y. Liu, Z. X. Ling, C. Zhang, M. H. Huang, H. W. Pan, Y. F. Xu, W. Yuan, M. Zhang(NAOC, CAS), Q. C. Shui(IHEP, CAS), A. Li(Beijing Normal Univ.), B. Zhang (UNLV), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau, P. Friedrich, N. Meidinger, V. Burwitz (MPE), B. Cordier (CEA), on behalf of the Einstein Probe team
We report on the X-ray upper limits from the ongoing observations of the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission, for the potential electromagnetic counterparts reported by Swift (Evans et al. GCN 36278) and DECam (Hu et al. GCN 36273).
Through our preliminary analysis of the FXT data, we found no significant X-ray counterparts at the positions of these sources. The upper limits of the count rate are calculated using the method provided by Ruiz et al. 2022 [1]. Assuming an absorbed power-law model with a column density of 2 x 10^21 cm^-2 and a photon index of 2 (Zhang et al. 2022 [2]), the flux upper limits at the 90% confidence level are obtained, which is detailed as follows.
Source | R.A | Dec. | Obs time |Exposure| FXTA Upper limit | FXTB Upper limit
| | | | (s) | (erg/s/cm^2) | (erg/s/cm^2)
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AT 2024hdk |130.625724|-15.59953| 2024-04-24T07:05:53 | 447 | 1.9 x 10^-13 | 2.0 x 10^-13
AT 2024hdn |127.302473|-21.50639| 2024-04-24T07:24:02 | 515 | 1.3 x 10^-13 | 2.1 x 10^-13
AT 2024hdm |126.65801 |-20.83375| 2024-04-24T07:15:58 | 434 | 1.1 x 10^-13 | 1.6 x 10^-13
AT 2024hdl |125.47270 |-30.10220| 2024-04-24T07:43:56 | 509 | 1.3 x 10^-13 | 7.2 x 10^-14
AT 2024hdo |120.866797|-26.01212| 2024-04-24T08:48:53 | 507 | 8.6 x 10^-14 | 1.1 x 10^-13
AT 2024hdr* |120.804560|-24.33268| 2024-04-24T07:34:01 | 510 | 3.5 x 10^-12
AT 2024hdp |120.543455|-27.25787| 2024-04-24T08:40:25 | 447 | 1.4 x 10^-13 | 1.6 x 10^-13
AT 2024hdq* |115.811388|-25.76403| 2024-04-24T08:58:34 | 512 | 6.4 x 10^-12
S240422ed_X61 |120.3245 |-28.1765 | 2024-04-24T10:41:07 | 4472 | 1.4 x 10^-14 | 1.7 x x10^-14
S240422ed_X101 |122.3954 |-24.3272 | 2024-04-24T13:29:32 | 2946 | 1.8 x 10^-14 | 2.2 x 10^-14
S240422ed_X190 |121.2770 |-26.1203 | 2024-04-24T09:22:15 | 574 | 7.6 x 10^-14 | 6.4 x 10^-14
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Note: For AT 2024hdr and AT 2024hdq, which are reported to be unrelated to S240422ed (GCN 36285), the averaged flux of FXTA and FXTB is given.
The above observations were made with the FXT instrument during the commissioning phase of EP. 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.
References:
[1] Ruiz, A., et al. "The RapidXMM upper limit server: X-ray aperture photometry of the XMM-Newton archival observations." Monthly Notices of the Royal Astronomical Society 511.3 (2022): 4265-4284.
[2] Zhang, Juan, et al. "Estimate of the background and sensitivity of the follow-up X-ray telescope onboard Einstein Probe." Astroparticle Physics 137 (2022): 102668.