EP240420a
GCN Circular 36194
Subject
EP240420a: EP-WXT detection of a fast X-ray transient
Date
2024-04-20T14:22:52Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
W. J. Zhang (NAOC, CAS), Y. J. Zhang (THU), S. X. Wen, W. X. Wang, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. 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
Subjects: X-ray, Transient
We report on the detection of a fast X-ray transient, designated EP240420a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-04-20T12:04:28 (UTC). The position of the source is R.A. = 228.713 deg, DEC = 14.796 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The flare lasted for more than 100 seconds and reached a peak flux of ~1 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. Since only the basic data of the alert was transmitted to the ground, the information on the source property is limited. Further information may be provided once the full scientific telemetry data are received at the EP science center.
No previously known X-ray sources at a similar flux level are found within 3 arcmin around the source position.
The above observation was made with the WXT 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.
GCN Circular 36202
Subject
EP240420a: NOT optical counterpart candidate and potential host
Date
2024-04-21T05:16:55Z (a year ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
email
J. An, Z.P. Zhu, X. Liu, S.Q. Jiang, S.Y. Fu, D. Xu (NAOC), D. B. Malesani (DAWN/NBI and Radboud), P. Jonker (Radboud), M. A. Diaz Teodori (NOT) report on behalf of a large collaboration:
We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT (Zhang et al., GCN 36194) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 5 x 200 s frames in the Sloan z-band with a median time of 2024-04-21T01:01:05.287, i.e., 0.54 day after the EP trigger.
The stacked z-band image has a 5-sigma limiting magnitude of z ~ 22.3. Within the 3 arcmin error circle of the EP/WXT, we identified an optical transient (OT) localized at
R.A. = 15:14:55.08 (J2000)
Dec. = +14:48:17.8 (J2000)
with an uncertainty of radius ~ 0.3" and z ~ 21.4 mag, calibrated with the PanSTARRS field.
The OT is ~ 1” south-west to a known source in the Legacy Survey (LS) which has r ~ 23.3 , z ~ 22.9, and a photometric redshift of z_phot ~ 0.82. This LS source could be the host of the OT.
Further observations are encouraged.
GCN Circular 36203
Subject
EP240420a: detection of fading X-ray emission with refined position by EP-FXT
Date
2024-04-21T05:26:43Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
D. W. Han (IHEP, CAS), W. J. Zhang (NAOC, CAS), C. K. Li, 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, S. N. Zhang, H. S. Zhao, X. F. Zhao (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), P. Friedrich, V. Burwitz, N. Meidinger, K. Nandra, A. Rau (MPE), B. Cordier (CEA), S. X. Wen, W. X. Wang, Z. X. Ling, Y. Liu, H. Sun, D. Y. Li, H. Y. Liu, W. Yuan, C. Zhang (NAOC, CAS) on behalf of the Einstein Probe team
Following the detection of the fast X-ray transient EP240420a (Zhang et al., GCN 36194) by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (EP), we performed an observation with EP's Follow-up X-ray Telescope (FXT). The observation started at 2024-04-20T14:05:01, about 2 hours after the WXT detection of the X-ray flare, with a net exposure time of 7.1ks.
An X-ray source was clearly detected at R.A. = 228.7291 deg, Dec. =14.8024 deg (R.A. = 15:14:54.98, Dec = 14:48:08.6), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is 1 arcmin away from the reported WXT position of EP240420a. There is no cataloged X-ray source within the error circle. The FXT count rate of the source has dropped by a factor of 50% over the observation duration of 14 ks. Considering this source being within the error circle of the WXT transient and being fading away, we suggest it being the late-time X-ray emission from EP240420a.The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 2.5e20 cm-2 and a photon index of 1.9(+/-0.1). The derived unabsorbed flux in 0.5-10 keV is 1.2(+/-0.1)e-12 erg/s/cm2. Assuming this source is indeed associated with EP240420a, its X-ray flux has faded by about more than 3 orders of magnitude in X-ray flux since the WXT detection. The quoted errors above are at the 90% confidence level.
Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. 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.
GCN Circular 36206
Subject
EP240420a: GWAC-F50A optical upper limit
Date
2024-04-21T07:47:22Z (a year ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
legacy email
L. P. Xin(NAOC), Y. G. Yang(HNU), C. WU(NAOC), Y. L. Qiu(NAOC), J. Wang(GXU/NAOC),
H. L. Li(NAOC), C. Wu(NAOC), E. W. Liang(GXU), X. H. Han(NAOC), X. M. Lu(NAOC),
R. S. Zhang(NAOC), Y. Xu(NAOC), Y.J . Xiao(NAOC), P. P. Zhang(NAOC), L. Lan(NAOC),
J. Y. Wei(NAOC) on behalf of the SVOM/GWAC team:
We began to observe X-ray transient EP240420a (Zhang et all., GCN 36194)
detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission,
with GWAC-F50A telescope at Xinglong station, China, at 13:58:11 (UT), 20. April. 2024,
about 1.9 hours after the burst.
The weather was not good.
After stacking 100*100s seconds, the candidate reported (An et al., GCN 23602)or
any other new sources were not detected within the errorbox of EP-FXT (Han et al., GCN 23603)
in our stacked image down to a limit magnitude of 19.5 mag
in R band comparing to several nearby USNO B1.0 stars.
We acknowledge the excellent support from observation assistant Yangtong ZhengandWenlongDong.
GCN Circular 36207
Subject
EP240420a: Nanshan optical counterpart observations
Date
2024-04-21T09:48:59Z (a year ago)
Edited On
2024-04-21T14:47:07Z (a year ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Edited By
Leo P. Singer at NASA/GSFC <leo.p.singer@nasa.gov> on behalf of Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
email
U. Mahmut, A. Iskandar, J.Z. Liu, Y. Zhang (XAO), J. An, Z.P. Zhu, X. Liu, S.Q. Jiang, S.Y. Fu, D. Xu (NAOC) report on behalf of a large collaboration:
We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT (Zhang et al., GCN 36194) using the 1-m Nanshan Optical Wide-field Telescope (NOWT) and the Half-Meter-Telescope (HMT) located at Nanshan, Xinjiang, China.
At series of 300 s and 200 s NOWT frames in the V-band were obtained starting at 16:00:05 UT on 2024-04-20 and the median time is 4.58 hr after the EP trigger. An optical transient (OT) is marginally detected at the counterpart candidate position reported by NOT (An et al., GCN 36202) with V ~ 21.0 mag, calibrated with the PanSTAR field.
At HMT 40 x 60 frames without any filter were obtained and the median time is 4.32 hr after the EP trigger. No credible source is detected at the NOT position, down to a 3-sigma limiting magnitude of r ~ 20.0, calibrated with the PanSTAR field.
The NOT candidate is within the EP/FXT error circle (Han et al., GCN 36203). Comparison of the Nanshan observation with the NOT observation shows that the OT is decaying, being consistent with the behavior of GRB optical afterglows.
We therefore think the OT is the optical counterpart of EP 240420a, which could be a GRB.
GCN Circular 36209
Subject
EP240420a: Xinglong decaying optical counterpart
Date
2024-04-21T10:19:24Z (a year ago)
Edited On
2024-04-21T14:46:57Z (a year ago)
From
Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>
Edited By
Leo P. Singer at NASA/GSFC <leo.p.singer@nasa.gov> on behalf of Dr Jie Zheng at Xinglong Observatory, National Astronomical Observatories ( NAOC), CAS <jiezheng@nao.cas.cn>
Via
Web form
B. C. Wang, J. J. Jin, Z. Fan, H. Wu, M. He, H. Y. Mu, L. Ge, J. B. Zhang, J. Zheng, Y. Zhang, X. Li, Y. G. Sun, J. J. Jia, W. X. Li, S. J. Xue, H. Zou, X. N. Guan, D. Xu report on behalf of a large collaboration:
We observed the field of the X-ray transient, EP 240420a, detected by EP/WXT using the Xinglong 0.8-m telescope located at Xinglong, Hebei, China. We obtained 3 x 600 s clear-band frames with a median time of 2024-04-20T14:48:24, i.e., 2.7 hr after the EP trigger, we luckily found the optical counterpart of EP 240420a in the stacked image. The stacked clear-band image using a 0.8-m telescope has a 3-sigma limiting magnitude of ~ 21 mag, calibrated with the Pan-STARRS g band field. Inspection of the whole EP/WXT 3 arcmin error circle produces a few low S/N optical counterpart candidates. We detected no optical transient candidate during April 20T13:00-15:00 with the 60/90 cm Schmidt Telescope in Xinglong, Hebei, China. The upper limit is ～19.5 m in the g-band. Then we performed clear-band photometry on the Xinglong 2.16-m telescope located at Xinglong, Hebei, China, and obtained 3x180s clear-band frames with a median time of 2024-04-20T20:20:30, i.e., 8.2 hr after the EP trigger. The stacked clear-band image using the 2.16-m telescope has a 5-sigma limiting magnitude of ~21.7 mag, calibrated with the Pan-STARRS g-band field. Inspection of the whole EP/WXT 3 arcmin error circle produces no optical counterpart candidates.
Within the 10 arcsec error circle of the EP/FXT, we identified an optical transient (OT) localized at
R. A. = 15:14:55.072 , Dec. = 14:48:18.096 (J2000).
This OT has m = 20.657 +/- 0.4 (AB) in the Xinglong 0.8-m telescope image calibrated with the Pan-STARRS g band field.
We summarize our observation results as follows:
Obs. No. | Start Time (UTC) | End Time (UTC) | Exposure Time (s) | Apparent mag (AB) | Telescope Name
1 | 2024-04-20 13:00:00 | 2024-04-20 15:00:00 | | > 19.5 | Xinglong Schmidt 60/90-cm Telescope
2 | 2024-04-20 14:38:24 | 2024-04-20 14:58:20 | 3x600 s | 20.657 +/- 0.4 | Xinglong 0.8-m Telescope
3 | 2024-04-20 20:17:20 | 2024-04-20 20:23:41 | 3x180 s | > 21.7 | Xinglong 2.16-m Telescope
Further observations are encouraged.
GCN Circular 36210
Subject
EP240420a: TNT upper limit to its optical counterpart
Date
2024-04-21T14:20:00Z (a year ago)
From
Wenxiong Li <liwenxiong1992@gmail.com>
Via
Web form
J. Mo, X. F. Wang, H. W. Peng (THU), X. Li (BJP), Y. G. Sun, W. X. Li (NAOC) report the optical follow-up observations of EP240420a (Zhang et al., GCN 36194, Han et al., GCN 36203). These observations were conducted on 2024 April 20.75 UT in g and r bands, at ~4 hours after the EP-WXT detection, using the 80-cm Tsinghua-NAOC Telescope (TNT) located at Xinglong Observatory, Hebei Province, China. No optical counterpart candidates were detected.
The 3-sigma upper limits are reported as below:
g ~ 21.6 mag
r ~ 21.9 mag
Further followup observations are encouraged.