EP240702a

W. Chen (NAOC, CAS), C. Y. Dai (NJU), S. X. Wen, Z. X. Ling, W. Yuan, 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, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, 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, 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 EP240702a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-02T00:50:05 (UTC) and triggered the WXT on-board processing unit. The position of the source is R.A. = 328.203 deg, DEC = -38.980 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 50 seconds and has a peak absorbed flux of ~1.2 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(+/-0.2) (with the column density fixed at the Galactic value of 2.0 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 5.4(+1.5/-1.2) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. 

We triggered a Swift target of opportunity observation. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 

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.

W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farrah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240702a by the Einstein Probe (Chen et al., GCN 36801
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 ), we initiated observations of the fast X-ray transient location starting on July 2 at 6:10 UT (~5 hours after the trigger) in the r band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Cerro Tololo Inter-American Observatory in Chile.
No new optical source was detected in the co-added images within the EP/WXT error box down to ~21.5 mag.
These observations were taken as part of the Global Supernova Project.

W. Chen, X. P. Xu (NAOC, CAS), C. Y. Dai (NJU), S. X. Wen, Z. X. Ling, W. Yuan, 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, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, 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, 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 EP240702a by the Einstein Probe (Chen et al., GCN 36801
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 ), we performed a Swift target of opportunity observation (ObsID: 00016693001). The XRT onboard Swift began the observation at 2024-07-02T09:03:28 UTC, about 8 hours after the WXT detection, with an exposure time of 1950 s in the Photon Counting mode. 

No significant X-ray source was detected within the 3 arcmin region around the WXT position. Assuming an absorbed power-law model with a Galactic column density of 2.0 x 10^20 cm^-2 and a photon index of 1.1, the estimated flux upper limit in 0.3-10 keV at the 90% confidence level is 1.58 x 10^-13 ergs/cm^2/s.

We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible. 

S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), S.Q. Jiang, J. An, X. Liu, S.Y. Fu, Z.P. Zhu, D. Xu, Z. Fan, W.X. Li, N.C. Sun, Y.N. Wang (NAOC) report on behalf of a large collaboration:

We observed the field of EP240702a (Chen et al., GCN 36801
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 ) using the 0.7-m telescope of the Thai Robotic Telescope (TRT) network, located at Spring Brook Observatory, Australia. Observations started at 14:09:46 UTC on 2024-07-03, i.e., 1.55 day after the EP trigger, and 5 x 360 s  frames in R- & I-band were obtained, respectively.

No uncatalogued optical transient is detected in the stacked images within the 3-arcmin EP/WXT error circle, down to 5-sigma limiting magnitudes of R ~ 21.8 and I ~ 20.7, calibrated with SkyMapper sources in the field.

Gregory S.H. Paek (SNU ARC/SNU), Myungshin Im (SNU ARC/SNU), Hyeonho Choi (SNU ARC/SNU), Seo-Won Chang (SNU ARC/SNU), and Ji Hoon Kim (SNU ARC/SNU) report on behalf of the 7-Dimensional Telescope collaboration

We searched for the optical counterpart of a fast X-ray transient, EP240702a (Chen et al., GCN #36801
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 ) using the 7-Dimensional Telescopes (7DT). Approximately 26 hours following the initial detection (2024-07-02T00:50:05 UTC), we targeted the localization center provided by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission at RA, Dec = 328.203 deg, -38.980 deg with an uncertainty of 3 arcmins. Observations were made with eleven 7DT units in twenty medium-band filters, denoted as m400, m425, then through m875, in which the numeric values indicate their central wavelengths in nanometers. Each medium-band filter has a bandwidth of 25nm.

No significant transient event was identified in the preliminary result. Photometric flux calibration was performed using synthetic photometries derived from the Gaia DR3 XP catalog (Gaia Collaboration et al. 2022) within the AB magnitude system. The 5-sigma upper limits (AB) range from 16.3 to 19.6 mag in the medium-band filters. To improve the depth for detection, we combined all images taken with the medium-band filters from m400 to m875. The combined image was treated as an r-band equivalent, and photometric measurements were performed. This approach yielded a 5-sigma upper limit of 20.2 AB magnitudes. Despite the increased depth, no significant transient detection was identified. Observations were conducted under suboptimal conditions, potentially limiting our search sensitivity.
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Filter Date-obs[UT] Exp.time[s] Depth(5sigma)
m400 2024-07-03T03:01:13.000 300 19.181
m425 2024-07-03T03:06:51.000 300 19.510
m450 2024-07-03T03:01:15.000 300 19.643
m475 2024-07-03T03:06:45.000 300 18.943
m500 2024-07-03T03:01:13.000 300 19.643
m525 2024-07-03T03:06:42.000 300 19.497
m550 2024-07-03T03:01:13.000 300 18.947
m575 2024-07-03T03:06:41.000 300 19.092
m600 2024-07-03T03:01:19.000 300 19.009
m625 2024-07-03T03:06:53.000 300 18.688
m650 2024-07-03T03:01:14.000 300 18.777
m675 2024-07-03T03:06:42.000 300 18.853
m700 2024-07-03T03:01:08.000 300 18.502
m725 2024-07-03T03:06:34.000 300 18.062
m750 2024-07-03T03:01:15.000 300 17.998
m775 2024-07-03T03:06:43.000 300 17.450
m800 2024-07-03T03:01:19.000 300 17.131
m825 2024-07-03T03:06:44.000 300 16.989
m850 2024-07-03T03:01:15.000 300 16.692
m875 2024-07-03T03:06:42.000 300 16.306

The 7-Dimensional Telescope (7DT), located in Chile and comprising 20 wide-field telescopes equipped with 40 medium-bandwidth (~25nm) filters, aims to detect optical counterparts of GW sources and conduct the 7-Dimensional Sky Survey (7DS) of the Southern Hemisphere. Further information about the 7DT is available at http://gwuniverse.snu.ac.kr/.