EP240918b

Y. F. Liang (PMO, CAS), Z. J. Zhang (HKU), M. J. Liu, X. P. Xu, C. C. Jin, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, 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, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Y. F. Xu, H. N. Yang, 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 two fast X-ray transients designated EP240918b and EP240918c, respectively, by the Einstein Probe (EP) mission. 

EP240918b was detected by EP-WXT at 2024-09-18T15:40:00 (UTC). The WXT position is R.A. = 258.66 deg, DEC = 66.739 deg (J2000) with an uncertainty of about 2.9 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts around 200 seconds. The absorbed average flux in 0.5-4 keV is 2.6(+4.1/-1.2) x 10^(-10) erg/s/cm^2 with a freely fitted column density value of 3.2(+6.2/-3.2) x 10^21 cm^-2 and the photon index of 1.5(+1.8/-1.3). We note that a star, TYC 4210-373-1, is located within 3 arcmin around the source position. 
 
EP240918c was detected by EP-WXT at 2024-09-18T18:06:47 (UTC). The WXT position is R.A.=281.338 deg, DEC=-13.167 deg (J2000) with an uncertainty of about 2.3 arcmin (radius, 90% C.L. statistical and systematic), corresponding to Galactic l, b = 20.503, -4.638. The WXT light curve lasts around 100 seconds with a peak flux of ~ 5.8 x 10^(-9) erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.6(+0.8/-0.8) (with column density fixed at the Galactic value 3.7 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.5(+0.6/-0.2) x 10^(-9) erg/s/cm^2. The quoted 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.

Following the WXT detections, we performed two follow-up observations with the Follow-up X-ray Telescope (FXT) on board EP. 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 onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).

C.-H. Lai, A. Aryan (both NCU), A. K. H. Kong (NTHU), Y. J. Yang, T.-W. Chen, H.-Y. Hsiao, C.-S. Lin (all NCU), J. Gillanders (Oxford), S. Yang (HNAS), S. J. Smartt (Oxford/QUB), A. Sankar. K, W.-J. Hou, C.-C. Ngeow, M.-H. Lee, Y.-C. Pan, H.-C. Lin, J.-K. Guo (all NCU), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, T. Moore, K. W. Smith, C. Angus, A. Aamer (all QUB), A. Schultz and M. Huber (both IfA, Hawaii) report:

We observed the field of two fast X-ray transients (FXTs), namely EP240918b and EP240918c reported by Liang et al. (GCN 37555
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 ), using the 40cm SLT and 1m LOT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first SLT epoch of observations for EP240918b started at 13:43 UTC on the 19th of September 2024 (MJD = 60572.571), 22.02 hr after the EP-WXT detection. The first LOT epoch of observations for EP240918c started at 13:42 UTC on the 19th of September 2024 (MJD = 60572.571), 19.59 hr after the EP-WXT detection.

We utilized the astroalign (Beroiz et al., 2020, A&C, 32, 100384) and astropy (Astropy Collaboration et al., 2022, ApJ, 935, 167) packages to align and stack the individual frames for each FXT. We did not detect any evidence of a new uncataloged source within the EP-WXT error circles reported in Liang et al. (GCN 37555
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 ) for both FXTs.

Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the photometry on our stacked frames. The details of the observations and measured 3-sigma upper limit  (in the AB system) for each FXT are as follows:

EP240918b: 

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
SLT | r | 60572.571 | 22.02 hr | 7 * 300 s | >19.0 | 2".79 | 2.08

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EP240918c:

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60572.571 | 19.59 hr | 5 * 300 s | >18.5 | 2".05 | 2.03

The presented magnitudes for each FXT were calibrated using field stars from the Pan-STARRS1 catalog, and were not corrected for the expected Galactic foreground extinction of E(B-V) = 0.037 and 0.86 mag (Schlafly & Finkbeiner 2011), respectively, in the directions of EP240918b and EP240918c. The sky conditions were unstable during the EP240918c observations, resulting in a shallower limit.

M. J. Liu (NAOC ,CAS), Y. F. Liang (PMO, CAS), Z. J. Zhang (HKU), X. P. Xu, C. C. Jin, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, 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, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Y. F. Xu, H. N. Yang, 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 have reported the detection of two fast X-ray transients EP240918b and EP240918c by the Einstein Probe (EP) mission (Liang et al. GCN 37555
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 ). 

The follow-up observation with the EP-FXT for EP240918b was performed, starting at 2024-09-19T11:25:26 (UTC), about 20 hours after the detection of the source by WXT. No X-ray source was detected within the 3 arcmin radius region around the WXT position of EP240918b.

The follow-up observations with the EP-FXT for EP240918c were performed, starting at 2024-09-19T12:40:28 (UTC) and 2024-09-24T13:01:28 (UTC), respectively. Two X-ray sources were detected within the 3 arcmin radius region around the WXT position of EP240918c in each observation. EP J184515.3-131115 is detected at R.A. = 281.3138 deg, DEC = -13.1875 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). EP J184516.2-130819 locates at R.A. = 281.3187 deg, DEC = -13.1371 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic), being consistent with the position of the star TYC 5704-6-1. No rapid dimming was found for two sources.

Further follow-up observations are encouraged to identify the nature of EP240918b and EP240918c.

Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).