EP240626a

Q.Y. Wu(NAOC, CAS), Q. C. Shui (IHEP, CAS), Z.Z. Lv, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, W. Chen, 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, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, 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 

We report on the detection of a fast X-ray transient, designated EP240626a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-06-26T06:28:28 (UTC) and triggered the WXT on-board processing unit. The position of the source is R.A. = 263.023 deg, DEC = -13.051 deg (J2000) with an uncertainty of 2 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 160 seconds and has a peak unabsorbed flux of ~6 x 10^-9 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 0.95(+/-0.54) and the column density fixed at the Galactic value of 1.8 x 10^21 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 1.70(+0.73/-0.50) x 10^-9 erg/s/cm^2. The uncertainties are given at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 2 arcmin region around the source position.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP. 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.

I. Perez-Garcia, S.-Y. Wu, , E. Fernandez-Garcia, M. D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy and A. J. Castro-Tirado (IAA-CSIC, Granada), C. Perez del Pulgar, A. Castellon, A. Reina (Univ. de Malaga),Y.-D. Hu (INAF-OAB, Brera), L. Hernandez-Garcia (Univ. de Valparaiso), M. Gritsevich (Univ. of Helsinki), D. R. Xiong (Yunnan Observatories of CAS), B.-B. Zhang (Nanjing Univ.) and A. Maury (Space, San Pedro de Atacama), on behalf of a larger collaboration, report:

Following the detection of the fast X-ray transient EP240626a by the Einstein Probe (Wu et al. GCNC 36766), the 0.6m BOOTES-7 robotic telescope at San Pedro de Atacama (Chile) observed the fast X-ray transient location starting on June 26, 08:11 UT (~ 1.7 h after trigger) in different optical bands. Conditions were non-optimal as the target was at low altitude above horizon (and setting). No new optical source is detected on the co-added images (clear-filter) within the EP/WXT 2 arcmin radius error box down to 19.0 mag.

We thank the staff at San Pedro de Atacama Celestial Explorations observatory for their excellent support.

Q.Y. Wu, Z. Z. Lv (NAOC, CAS), Q. C. Shui (IHEP, CAS), Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, W. Chen, 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, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, 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, J. Zhang, 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 EP240626a (Wu et al., GCN 36766
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 ), we performed an observation of EP240626a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-06-26T08:58:37 (UTC), about 2.5 hours after the EP-WXT trigger. The exposure time is 6266 seconds. A faint source was detected at R.A. = 263.0171 deg, DEC = -13.0490 deg, with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic). The spectrum can be fitted with an absorbed power-law model with a photon index of 1.2(+0.7/-0.6) and the column density fixed at the Galactic value of 1.8 x 10^21 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 1.5(+1.2/-0.7) x 10^-13 erg/s/cm^2 (90% C.L.). There is no cataloged X-ray source within the error circle, suggesting this source being associated with EP240626a.
 
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). 

WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and
Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:

The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at
Lick Observatory, responded to the fast X-ray transient EP240626a
from the Einstein Probe (Wu et al. GCNC 36766) starting at 1.01 hours
after the trigger. A set of 30x60s images were obtained in the clear
(roughly R) filters. Preliminary analysis do not reveal any optical
counterpart candidate within the EP-FXT position error circle
(Wu et al. GCNC 36770), neither in single image, nor in the co-add
images. The typical limiting magnitude of our single clear image is
about 19.0 mag calibrated to the PS1 catalog.

S. Leonini, M. Conti, P. Rosi, L.M. Tinjaca Ramirez (Montarrenti Observatory, Siena, Italy, part of UAI/SSV-GRB section), M.G. Dainotti (National Astronomical Observatory of Japan), Y. Niino (Tokyo University, Institute of Astronomy), K. Kalinowski (Aarhus University, Department of Physics and Astronomy) and B. De Simone (Universita' degli Studi Di Salerno) report:

we observed the field of the possible optical countepart of the X-ray transient EP240626a (Einstein Probe, GCN 36766
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 , Wu et al.; GCN 36767
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 , Perez-Garcia et al.; GCN 36773
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 , Zheng et al.)  with the automated and remoted 0.53m Ritchey-Chretien telescope at Montarrenti Observatory (Siena, Italy, IAU code C88).

The observations were started at 2024-06-26 21:22:28 UT (approximately 15 hours after the discovery of the X-ray transient) stacking 24x40s Rc-band CCD images.
 
We have not found any optical transient candidate within the error-box of the EP mission position with the following upper limit: 

MJD           Filter   Limit      
60488.3964    Rc      > 19.40    

Magnitude was calibrated with the nearby PanSTARRS stars converted using Lupton (2005) equations. No correction for galactic dust extinction was applied.