EP240820a

Y. F. Liang (PMO, CAS), H. Y. Liu, X. Mao, C. C. Jin, Y. Liu, Z. X. Ling, 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, M. J. Liu, Z. Z. Lv, T. Y. Lian, 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, 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 EP240820a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-08-20T00:54:47(UTC). The WXT position of EP240820a is R.A.= 16.221 deg, DEC = -34.698 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts around 250 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.2(-0.7, +0.8) (with a column density fixed at the Galactic one of 1.62 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.2(-0.5, +0.7) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP. The observation began at 2024-08-20T09:48:47 (UTC), and the exposure time is about 1.8 ks. FXT-B detected an uncataloged weak souce at R.A. = 16.2459 , DEC = -34.6941 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). Given the limited counts, the FXT spectrum is fitted using an absorbed power law and the parameters are fixed to those obtained by the EP-WXT spectral fitting. The derived flux is about 2.0 x 10^(-13) erg/s/cm^2. 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.Q. Jiang (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan (NARIT), X. Liu, S.Y. Fu, Z.P. Zhu, S.Y. Gao, 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 EP240820a (Liang et al., GCN 37214
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 ) using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at Cerro Tololo Inter-American Observatory, Chile. Observations started at 03:32:49 UTC on 2024-08-21, i.e., 1.11 days after the EP/WXT trigger, and 4 x 240 s  frames in R-band were obtained.

No new optical source is detected in the stacked image within the 10-arcsec EP/FXT error circle (Liang et al., GCN 37214
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 ), down to a 5-sigma limiting magnitude of R ~ 19.6, calibrated with SkyMapper stars in the field.

S. Atri(U Rome), J. Durbak (UMD), O. Guiffreda (UMD), A. S. Kutyrev (NASA/GSFC), E. Troja (U Rome), K. De (MIT), S. B. Cenko (NASA/GSFC)

Following the Einstein Probe detection (GCN 37214
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 ), we observed the fast X-ray transient field in J-band with PRIME ~51 hours after the EP/WXT detection.  The total exposure time in J-band was 1800s.

At the candidate position given by EP/FXT, we detect no uncatalogued sources in J-band. Using nearby VISTA Kilo-Degree Infrared Galaxy Survey (VIKING) stars for preliminary calibration we derive the following limiting magnitudes, not corrected for Galactic extinction:  


Filter | Mag (Vega)     | Total exposure time (s) 
-------|----------------|-------------------------
J      | >21.3          | 1800

There is a galaxy within the FXT error radius found in the Dark Energy Survey (DES) catalogs located at 16.2450, -34.6941 (J2000) with a photo-z of 0.402+/-0.088.

The observed J-band brightness of this galaxy is 19.356 +/- 0.19 Vega, which is marginally brighter than the cataloged VIKING brightness of 19.784 +/- 0.0948 Vega.

Further observations to assess variability are planned.

PRIME is a 1.8m telescope with 1.56 square degree FOV (0.5 arcsec/pixel) located in Sutherland, South Africa at the South African Astronomical Observatory (SAAO) (Kutyrev et al. 2023, Yama et al. 2023).

We thank the Osaka University observers at PRIME and the staff at SAAO for their support with these observations.