EP240816a

H. N. Yang, W. J. Zhang, W. X. Wang, W. Yuan, Z. X. Ling, C. C. Jin, Y. Liu, 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, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, M. Zhang, W. D. 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 EP240816a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-08-16T03:28:42 (UTC) and triggered the WXT on-board processing unit (trigger ID: 01709034199). The light curve lasts for more than 200 seconds. The WXT position of the source is R.A. = 292.925 deg, DEC = -54.412 deg (J2000) with an uncertainty of 2.66 arcmin in radius (90% C.L. statistical and systematic). The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.1(+0.6/-0.5) (with a frozen column density value of 4.2 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 3.0(+0.5/-0.5) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

An automated observation by the Follow-up X-ray Telescope (FXT) on board EP was triggered at 2024-08-16T05:57:58 (UTC). Three uncatalogued X-ray sources, a relatively bright one and two faint sources, are detected within the error circle of the WXT source position. Their parameters derived from the data taken with the two identical FXT units (FXT-A and FXT-B) are summarized in the table. For the brightest source, EP J193152.4-542420, the average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 1.87(+0.40/-0.39) (with a frozen column density value of 4.2 x 10^20 cm^-2), giving an average unabsorbed flux in 0.5-10 keV of 1.2(+0.5/-0.3) x 10^-13 erg/s/cm^2. Given this source is significantly brighter than the other two, we tentatively suggest it to be likely associated with the transient EP240816a detected by WXT. However, it cannot be ruled out that the transient is associated with one of the other two faint FXT sources. Further follow-up observations are encouraged to identify the nature of this X-ray transient. 

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Source |   R.A.  |   Dec.  | FXT-A Significance | FXT-B Significance | Estimated Flux
       | (J2000) | (J2000) |                    |                    |  (erg/s/cm^2) 
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EP J193152.4-542420 | 292.9686 | -54.4060 | 7.990 | 12.836 | 1.2 x 10^-13 
EP J193128.6-542502 | 292.8693 | -54.4173 | 3.560 |    -   | 4.7 x 10^-14 
EP J193132.7-542607 | 292.8862 | -54.4353 |   -   |  3.788 | 2.1 x 10^-14
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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). 

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

Following the detection of EP240816a by EP-WXT (Yang et al., GCNC 37188), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) observed the fast X-ray transient location starting on Aug. 16, 16:25 UT (~ 13 hrs after trigger) in different optical bands. No new optical source is detected on the co-added images (6 x 120 s, clear filter) within the EP error box down to 19.8 mag.  Further imaging is ongoing.

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

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

Following the Einstein Probe detection (GCN 37188
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 ), we observed the fast X-ray transient field in both the J and H filters with PRIME ~22 hours after the EP/WXT detection.  The total exposure time in J band was 1800s, while the total exposure time in H band was 1000s.  

At the candidate positions given by EP/FXT, we detect no uncatalogued sources in either J or H band. Using nearby VISTA Hemispherical Survey (VHS) and 2MASS stars for preliminary calibration we derive the following limiting magnitudes, not corrected for Galactic extinction:  

Filter | Mag(AB)        | Total exposure time (s) 
-------|----------------|-------------------------
J      | >20.9          | 1800
H      | >21.0          | 1000

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.