EP240625a

X. Pan, Z. Z. Lv (NAOC, CAS), Y. C. Fu (BNU), Y. Liu, Z. X. Ling, 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, H. Sun, 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 
 
We report on the detection of a fast X-ray transient, designated EP240625a, 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 2024-06-25T01:48:23 (UTC). The position of the source is R.A. = 310.760 deg, DEC = -15.966 deg (J2000) with an uncertainty of 2 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 300 seconds and has a peak unabsorbed flux of ~1.3 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 1.8(+/-0.3) (with the column density fixed at the Galactic value of 3.85 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.9(+0.7/-0.5) x 10^-10 erg/s/cm^2. The uncertainties are at the 1 sigma 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.

X. Pan, Z. Z. Lv (NAOC, CAS), Y. C. Fu (BNU), Y. Liu, Z. X. Ling, 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, H. Sun, 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 EP240625a (Pan et al., GCN 36757
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 ), we performed an observation of EP240625a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-06-25T03:10:00, about 1 hour after the EP-WXT detection. The exposure time is 5780 seconds. A slowly fading X-ray source was detected at R.A. = 310.7308 deg, DEC = -15.9760 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 3.85 x 10^20 cm^-2 and a photon index of 1.7(+/-0.4). The derived unabsorbed flux in 0.5-10 keV is 1.4(+0.8/-0.5) 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 EP240625a.
 
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). 

S.Y. Fu, J. An, S.Q. Jiang, X. Liu, Z.P. Zhu, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI), N. Koivisto (NOT) report on behalf of a large collaboration:

We observed the field of EP240625a detected by EP/WXT (Pan et al., GCN 36757
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 ) using the the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera. We obtained 16x30 s Sloan z-filter images at a median time of 1.52 hr after the EP trigger.

An uncatalogued optical transient (OT), within the 10 arcsec EP/FXT error circle (Pan et al., GCN 36760
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 ), is marginally detected in the stacked z-band image at coordinates:

R.A. (J2000)= 20:42:55.47
Dec. (J2000)= -15:58:24.00

with an uncertainty of ~ 0.2 arcsec. It has z = 20.7 +/- 0.2 mag , calibrated against nearby Pan-STARRS stars and not corrected for Galactic extinction. Given the low S/N for the OT, we cannot completely rule out it being due to background fluctuation. 

Further observations are scheduled.

D. Akl (AUS), A. Le Calloch (Berkeley), Y.Rajabov (UBAI), M. Molham (KAO), S. Antier (OCA), S. Karpov (FZU), M. Coughlin (UMN), C. Andrade (UMN), I. Tosta e Melo (UniCT-DFA), P. Hello, N. Leroy (IJCLAB), P-A Duverne (APC), D. Turpin (CEA-Saclay/Irfu), T. Pradier (Unistra/IPHC), N. Guessoum (AUS), M. Masek (FZU), M. Prouza (FZU), A. Kaeouch (OUCA), M. Freeberg (KNC), N. Leroy (IJCLAB) on behalf of the GRANDMA collaboration:

We performed observations with GRANDMA and its citizen science project Kilonova-catcher (KNC) on the X-ray transient EP240625a detected by Einstein Probe (EP) WXT on T0 = 2024-06-25T01:48:23 (Pan et al., GCN 36757
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 ). We monitored the source with three telescopes, including FRAM-CTA-N, KNC-T32, KNC-ASO, from 2024-06-26T00:33:03.632 to 2024-06-26T16:15:48 UTC, starting 0.95 days after the EP trigger. 

We did not find any new optical counterpart in any of our frames within 10 arcsec radius centered around Ra/Dec: 310.7308, -15.9760.

All our observations and upper limits, which correspond to the depth of the full image (given in the AB system), can be found here:
https://skyportal-icare.ijclab.in2p3.fr/public/sources/EP240625a/version/fbcf272073852fb0c34826deb0914978

Below is the version that can be downloaded from the link above:
+--------------+----------+----------------+-------------------+
|          MJD | Filter   |        Lim.Mag |       Instrument  |
+==============+==========+================+===================+
| 60487.008465 | L        |          20.04 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.061847 | G        |          19.84 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.034262 | R        |          19.46 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.091242 | B        |          20.13 | KNC-ASO           |
+--------------+----------+----------------+-------------------+
| 60487.182141 | R        |          16.95 | FRAM-CTA-N        |
+--------------+----------+----------------+-------------------+
| 60487.626447 | V        |          19.88 | KNC-T32           |
+--------------+----------+----------------+-------------------+

We note that bessellv and bessellr filters correspond to V and R Johnson Cousin filters, respectively.Images obtained in Johnson Cousin filters were calibrated using the Gaia DR3 Synphot catalog while images taken with Sloan filters were calibrated using the Pan-STARRS DR1 Catalog.

We also note that some amateur telescopes use green filters, which were calibrated with the Gaia eDR3 catalog.Generic telescopes correspond to KNC telescopes.

We performed force photometry on our images at Ra, Dec: 20:42:55.47, -15:58:24.00 (S.Y. Fu et al., GCN 36761
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 ). We did not detect a source at that position at 20.7 upper limit in the V-band (Vega, 5 sigma).

All the data have been reduced by a single data processing pipeline STDPIPE (Karpov et al., 2022). We use the Skyportal application (skyportal.io) to monitor our observational campaign.

We thank the Einstein Probe team for useful communication.

GRANDMA is a worldwide telescope network (grandma.ijclab.in2p3.fr) devoted to the observation of transients in the context of multi-messenger astrophysics (Antier et al. 2020 MNRAS 497, 5518). Kilonova-Catcher (KNC) is the citizen science program of GRANDMA (http://kilonovacatcher.in2p3.fr/).