EP240617a
GCN Circular 36691
Subject
EP240617a: EP-WXT detection of a fast X-ray transient
Date
2024-06-18T13:30:06Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Zhou (PMO, CAS), W. Chen, H. Sun, W. D. Zhang, J. W. Hu, D. Y. Li, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), J. Yang, C. Y. Dai (NJU) Y. F. Liang (PMO), 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
We report on the detection of a fast X-ray transient, designated EP240617a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-06-17T12:19:13 (UTC). The position of the source is R.A. = 285.030 deg, DEC = -22.561 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for over 300 seconds before the observation was interrupted due to earth occultation. The lightcurve of the transient exhibits a multi-peak profile. The peak unabsorbed flux is around 1.4(+/-0.1) x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.1(+/-0.1) (with the column density fixed at the Galactic value of 1.7 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 3.5(+/-0.3) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No previously known bright X-ray sources are found within the error circle around the source position. Further follow-up observations are encouraged to identify the nature of this X-ray transient.
The above observations were 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 36692
Subject
EP240617a is likely a GRB event
Date
2024-06-18T15:51:31Z (a year ago)
From
jyang@smail.nju.edu.cn
Via
Web form
Jun Yang (NJU), Yi-Han Iris Yin (NJU), Binbin Zhang (NJU), Hui Sun (NAOC), Qinyu Wu (NAOC) and Xuefeng Wu (PMO) report on behalf of large collaboration:
Our team has followed up on EP's X-ray flare detection at 2024-06-17T12:19:13 UTC (EP240617a; GCN Circular 36691) and discovered a weak, untriggered gamma-ray transient in the Fermi/GBM data within the occurrence time interval of EP240617a. This transient lasted approximately 100 seconds and its location aligns with that of EP240617a.
Considering the factors that the gamma-ray transient is consistent with EP240617a in terms of event time and location, we are inclined to claim that this is likely a GRB event. A preliminary spectral analysis indicates that its peak energy is below one hundred keV, which suggests its classification as an X-ray rich GRB.
We strongly recommend further follow-up on EP240617a to confirm its physical nature and to detect any afterglows.
GCN Circular 36693
Subject
EP240617a: BOOTES-6/DPRT optical upper limit
Date
2024-06-18T21:56:11Z (a year ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
legacy email
I. Perez-Garcia, E. Fernandez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu and A. J. Castro-Tirado (IAA-CSIC, Granada), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS) and C. J. Perez del Pulgar (UMA, Malaga), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240617a (a likely GRB) by the Einstein Probe (Zhou et al. GCNC 36691, Yang et al. GCNC 36692), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) observed the fast X-ray transient location starting on June 18, 19:59 UT (~ 31.6 h after trigger) in different optical bands. No new optical source is detected on the co-added images (clear-filter) within the EP/WXT error box down to 20.4 mag.
We thank the staff at Boyden Observatory for their excellent support.
GCN Circular 36707
Subject
EP240617a: STEP/T80S optical limits
Date
2024-06-19T23:42:54Z (a year ago)
From
André Santos at Centro Brasileiro de Pesquisas Físicas (CBPF) <andsouzasanttos@gmail.com>
Via
Web form
A. Santos (CBPF), C. R. Bom (CBPF), C. D. Kilpatrick (Northwestern), L. Santana-Silva (CBPF), P. Darc (CBPF), C. Mendes de Oliveira (IAG-USP) report on behalf of the STEP collaboration:
We conducted optical follow up of the fast X-ray transient EP240617a by the Einstein Probe (GCN [36691](https://gcn.nasa.gov/circulars/36691), [36692](https://gcn.nasa.gov/circulars/36691)) with the T80S 0.8-m robotic telescope using the 1.4 x 1.4 field-of-view T80S-Cam imager (see Santos et al., 2024, MNRAS, 529, 59 for details). The observations started on June 19, 05:10 UT (~41 hours after the trigger). We obtained 360s (3x120s) images in both i and z bands with the T80S camera centered at R.A.=19:00:07.29 and Decl.=-22:33:39.60 (J2000). Subtracting Pan-STARRS 3pi template images from the T80S frames using photpipe (Rest et al., 2005), we do not detect any sources in our difference images and derive 3-sigma limiting magnitudes of z > 20.6 and i > 20.9 mag for any optical transient.
GCN Circular 36722
Subject
EP240617a: tentative Swift/XRT detection of the afterglow
Date
2024-06-21T09:36:34Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Sun, W. Chen (NAOC, CAS), H. Zhou (PMO, CAS), W. D. Zhang, D. Y. Li, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, 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, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. 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
Following the detection of the fast X-ray transient EP240617a by the Einstein Probe (Zhou et al., GCN 36691), which is likely a gamma-ray burst based on Fermi/GBM data (Yang et al., GCN 36692), we triggered a Swift target of opportunity observation (ObsID: 00016669001). The XRT onboard Swift began the observation at 2024-06-20T17:57:41 UTC, about 77 hours after the WXT detection, with an exposure time of 1.69 ks in the Photon Counting mode.
Within the 3 arcmin error radius of the WXT position, we have tentatively detected a weak signal with a significance of 5.3 sigma (as evaluated by the Li-Ma formula, Li & Ma 1983, ApJ, 272, 317) at R.A.=19:00:06.0, DEC=-22:33:29.9 (J2000) with an uncertainty of 7.5 arcsec (radius, 90% C.L. statistical and systematic). The position of the source is 0.3 arcmin away from the WXT detection and is thus spatially consistent with EP240617a, suggesting that we detected the X-ray afterglow. The source has an average count rate of 5.1 x 10^-3 cps. Assuming an absorbed power-law model with a Galactic column density of 1.7 x 10^21 cm^-2 and a photon index of 2, the estimated flux in 0.3-10 keV is 1.9 x 10^-13 ergs/cm^2/s.
Further follow-up observations are encouraged to search for the multi-band afterglows.
We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible.