EP240804a

Y. Wang (PMO, CAS), H. Sun, Y. L. Wang, W. D. Zhang, Y. Liu, H. W. Pan, Z. X. Ling, C. C. Jin, C. Zhang, H. Q. Cheng, W. Chen, 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, 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. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, 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 
 
We report on the detection of a fast X-ray transient designated EP240804a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-08-04T21:36:53(UTC) and triggered the WXT on-board processing unit (trigger ID: 01709017305). The WXT position of EP240804a is R.A.= 337.644 deg, DEC = -39.121 deg (J2000) with an uncertainty of 4 arcmin (radius, 90% C.L. statistical and systematic). The lightcurve of the transient observed by the WXT lasts more than 100 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.7(-0.4, +1.2) (with a freely fitted column density value of 1.1(-1.0, +37.0) x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 6.1(-1.6, +2.6) x 10^(-10) erg/s/cm^2. 

The EP-FXT automonous follow-up observation started at 2024-08-04T21:41:15 (UTC). The observation clearly shows an uncatalogued X-ray source located at R.A. = 337.6638 deg, DEC = -39.1243 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows significant variability. The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 1.6(+/-0.1) (with a free column density value of 2.8(+/-2.7) x 10^20 cm^-2), giving an average unabsorbed flux of 2.2(+/-0.1) x 10^(-11) 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. 

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). 

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

We observed the field of EP240804a detected by Einstein Probe (EP, Wang et al., GCN 37034
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 ), using the ALFOSC instrument mounted on the Nordic Optical Telescope (NOT), with 3x300 s photometry in the SDSS-r band. Observations started at 02:55:25 UT on 2024-08-05, i.e. 5.28 hr after the trigger.

An uncatalogued decaying source is detected in the individual as well as the stacked frames within the EP/FXT error circle (Wang et al., GCN 37034
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 ) and localized at coordinates

R.A. (J2000) = 22:30:38.84
Dec. (J2000) = -39:07:20.06

with an uncertainty of ~ 0.5 arcsec. The source has r ~ 20 mag, calibrated with SkyMapper and not corrected for Galactic extinction. There is no known object at the above position by checking MPC.

We thus conclude that the source is the optical counterpart candidate of EP240804a.

Further observations are encouraged.

F. Poidevin, I. Pérez-Fournon, D. S. Aguado, J. A. Acosta-Pulido, A. López-Oramas, D. Nespral (IAC and ULL), and F. Acero (CEA Saclay and IAC)

We report on optical follow-up observations of the Einstein Probe (EP) fast X-ray transient EP240804a (Wang et al., GCN 37034
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 ) using one of the Las Cumbres Observatory Global Telescope Network (LCOGT) 1-m telescopes located at the Cerro Tololo Inter-American Observatory (Chile) LCOGT node, with a single 300 sec exposure in the SDSSr filter. The observation started at 07:03:14 UT on 2024-08-05, i.e. 9.44 hr after the EP trigger.

We detect an uncatalogued source with r = 20.30 +/- 0.25 at a position consistent with the optical counterpart reported by An et al. (GCN 37035
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 ). For the calibration we used the Gaia synthetic photometry generated from the Gaia BP/RP mean spectra (Gaia DR3 Part 6. Performance verification).
The magnitude is not corrected for Galactic extinction.

This work makes use of observations from the Las Cumbres Observatory global telescope network (program IAC2024B-004).

A. Bochenek (LJMU), D. Xu (NAOC), Z. P. Zhu (NAOC), L. Izzo (INAF-OACn & DARK/NBI), J. T. Palmerio (CEA-Irfu & GEPI/Obs. de Paris), A. Saccardi (GEPI/Obs. de Paris), V. D’Elia (ASI-SSDC), A. De Cia (ESO), S. D. Vergani (GEPI/Obs. de Paris), N. R. Tanvir (U. Leicester), J. An (NAOC), report on behalf of the Stargate collaboration:

We obtained observations of the Einstein Probe (EP) fast X-ray transient EP240804a (Wang et al., GCN 37034
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 ), using the X-shooter spectrograph at the VLT. The spectra cover the wavelength range of 3000-21000 AA, and consist of four 600-s exposures. Observations started at 2024-08-05, 06:48:20 UT, or about 9.2 hr after the trigger. The transient is detected at a position consistent with the counterpart reported by An et al. (GCN 37035
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 ).

In a preliminary reduction of the spectra, we clearly detect a continuum over the entire wavelength range and infer a redshift of z = 3.662. We detect a Lya absorption and multiple absorption features, which we interpret as being due to S II, Si II, Si II*, O I, O I*, C II, C II*, Al II, Fe II, Mg II and Al III, Si IV, C IV.


We acknowledge expert support from the ESO staff in Paranal, in particular Claudia Paladini and Felipe Gaete Roman.

W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report the optical follow-up observations of the optical candidate counterpart to the fast X-ray transient EP240804a (Wang et al. 37034; An et al. 37035).

Our observations were conducted on 2024 Aug. 05.3 UT using the 1-meter telescope at the Las Cumbres Observatory node located at Cerro Tololo Interamerican Observatory in Chile. The transient is detected in r-band images:
r = 20.7+-0.1
Further follow-up observations are encouraged.

These observations were taken as part of the Global Supernova Project.

I. Perez-Garcia, E. Fernandez-Garcia, M.D. Caballero-Garcia, R. Sanchez-Ramirez, S. Guziy, S.-Y. Wu, G. Garcia-Segura and A. J. Castro-Tirado (IAA-CSIC, Granada), Y.-D. Hu (INAF-OAB), P. J. Meintjes and H. J. van Heerden (UFS, South Africa), A. Martin-Carrillo and L. Hanlon (UCD, Ireland), C. J. Perez del Pulgar (UMA, Malaga) and D.-R. Xiong (YNAO) on behalf of a larger collaboration, report:

Following the detection of EP240804a by EP-WXT (Wang et al., GCNC 37034), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) observed the transient location starting on Aug. 5, 01:55:49 UT (~ 4.3 hrs after trigger). A series of images in clear filter were gathered and the optical counterpart was detected for which we measure a preliminary magnitude of 20.0 +- 0.1 on the co-added 60 s x 13 image, which is consistent with the detections of NOT (An et al. GCNC 37035), LCOGT (Poidevin et al. GCNC 37036), VLT (Bochenek et al. GCNC 37039) and GSP (Li et al. GCNC 37044). Further imaging is ongoing.

We thank the staff at Boyden Observatory for their excellent support.

D. B. Malesani (DAWN/NBI and Radboud Univ.), J. P. U. Fynbo (DAWN/NBI), J. Verwohlt (DARK/NBI), K. Valeckas (DAWN/NBI), T. Bach Johannessen (Aarhus Univ.), J. K. Mogensen (Aarhus Univ.), D. Xu (NAOC), S. Grund (NOT) report on behalf of a larger collaboration:

We observed the optical counterpart (An et al., GCN 37035
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 ; Poidevin et al., GCN 37038
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 ; Bochenek et al., GCN 37039
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 ; Li et al., GCN 37044
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 ) of the X-ray transient EP240804a (Wang et al., GCN 37034
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 ), identified with GRB 240804B (Frederiks et al., GCN 37071
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 ). using the Nordic Optical Telescope equipped with the ALFOSC imager. A set of three 300-s r-band observations were carried out at large airmass (average 2.75), with mid time Aug 6.081 UT (28.34 hr after the EP trigger).

The optical counterpart is well detected in the stack of our data, for which we measure r = 21.72 +- 0.08 (AB), calibrated against nearby stars from the SkyMapper catalog.