EP240708a

W. Chen, Y. L. Wang, J. W. Hu, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, T. Y. Lian, Z.Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. 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 EP240708a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-08T23:28:23 (UTC) and triggered the WXT on-board processing unit. The position of the source is R.A. = 345.963 deg, DEC = -22.840 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 1300 seconds and has a peak flux of ~1.1 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 1.57(+0.63/-0.54) with the column density fixed at the Galactic Absorption of 2.0 x 10^20 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 7.7(+3.4/-2.6) x 10^-11 erg/s/cm^2. The uncertainties are given at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 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 and proposed a Swift target of opportunity observation. 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.

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

We observed the field of the fast X-ray transient EP240708a (Chen et al., GCN 36838
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 ) using the One-meter Telescope (LOT) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first LOT epoch of observations started at 16:38 UT on 09th of July 2024 (MJD = 60500.693), 17.16 hrs after the EP trigger. 

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. We  subtracted the stacked images with the Pan-STARRS1 template image using hotpants (Becker A., 2015, ascl.soft. ascl:1504.004). The subtracted image did not provide any evidence of a new source. Moreover, we utilized the python based package, AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the PSF photometry on our stacked frame. 

We did not detect any optical counterpart in the errorbox of 3 arcminute around the reported position of the EP240708a in the stacked and subtracted frames. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follow:

Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | r | 60500.693 | 17.16 hrs | 300 sec * 6 | > 22.6 | 1".86 | 2.16
LOT | g | 60500.719 | 17.78 hrs | 300 sec * 6 | > 22.4 | 1".67| 1.82

The presented magnitudes are calibrated using the field stars from the Pan-STARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the transient (Schlafly & Finkbeiner 2011).

J. W. Hu, W. Chen, Y. L. Wang, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, T. Y. Lian, Z.Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. 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 EP240708a (Chen et al., GCN 36838
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 ), we performed an observation of EP240708a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-07-09T03:45:47(UTC), about 4 hours after the EP-WXT trigger. The exposure time is 1603 seconds. A faint source was detected at R.A. = 345.9656 deg, DEC = -22.8428 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 a photon index of 1.62(-0.60/+0.65) and the column density fixed at the Galactic value of 2.0 x 10^20 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 2.37(+2.36/-1.08) x 10^-13 erg/s/cm^2 (90% C.L.). 
 
We also triggered a Swift target of opportunity observation (ObsID: 00016707001). The XRT onboard Swift began the observation at 2024-07-09T16:44:14 UTC, about 17 hours after the WXT detection, with an exposure time of 1843 s in the Photon Counting mode. No significant X-ray source was detected within the 3 arcmin region around the WXT position. Assuming an absorbed power-law model with a Galactic column density of 2.0 x 10^20 cm^-2 and a photon index of 1.62, the estimated flux upper limit in 0.3-10 keV at the 90% confidence level is 2.67 x 10^-13 ergs/s/cm^2. 

We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible. 

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). 

Chao Wu (NAOC),  Xuhui Han (NAOC),  Zhe Kang (CHO), Liping Xin (NAOC),
Pinpin Zhang (NAOC)，Xiaomeng Lu (NAOC), Zhenwei Li (CHO)，Ruosong Zhang
(NAOC)，Yujie Xiao(NAOC),  Yulei Qiu (NAOC)，You  Lv (CHO)，Jing
Wang(GXU/NAOC), and Jianyan Wei (NAOC) on behalf of SVOM GRB team

We observed the field of the fast X-ray transient EP240708a (Chen et al.,
GCN 36838
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 ) on 18:00:53 UT, July, 9th, 2024, about 18.54 hours after the
Einstein Probe (EP) mission trigger with C-GFT (Chinese Ground Follow-up
Telescope in SVOM mission) in System Test Mode (STM). C-GFT is located at
Jilin (long.=126.33 deg, lat.= 43.8243778 deg), Changchun Observatory,
National Astronomical Observatories, CAS. It has FOV of 1.28 deg X 1.28 deg
with a 4k*4k CMOS detector mounted on the primary focus of
1.2-meter-aperure telescope.

A series of g ,r,and i band images were obtained. The exposure time was
12*10 seconds for each band.

No optical afterglow was detected within the enhanced FXT error box (Hu et
al., GCN 36840
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 ) down to a limit magnitude of 18.2,18.9 and 18.9 for g,r,and
i band respectively, which was consistent with the report (Lai et al.,
36839; ) at a similar time range.

The photometry was calibrated with nearby UCAC 4 catalogs.

We thank the observation assistant Guangsheng Zhang at Jilin observatory
for their excellent support.

S.Q. Jiang, X. Liu, Z.P. Zhu, S.Y. Fu, J. An, L.B. He, 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 EP240708a detected by EP/WXT (Chen et al., GCN 36838
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 ) using the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 5 x 300 s frames in the Sloan r-band at a median time of 4.20 hr fter the EP trigger.

No credible uncatalogued optical transient is detected in our stacked image within the EP/FXT 10 arcsec error circle (Hu et al., GCN 36840
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 ) as well as in its neighboring region, down to a limiting magnitude of r ~ 24.1 mag, calibrated with nearby PanSTARRS stars and not corrected for the Galactic extinction.

We note that there is a marginal detection of an uncatalogued source in the NOT image at coordinates

R.A.(J2000) = 23:03:50.556
Dec.(J2000) = -22:50:47.99

with an uncertainty of ~0.5 arcsec. It has r ~ 24.0 +/- 0.3 mag and is away from the center of the FXT error circle by ~ 21.7 arcsec, and not present in the archival Pan-STARRS or Legacy Survey. Given the low S/N, this source could be due to background fluctuation.

WeiKang Zheng (UCB), Xuhui Han (NAOC), Pinpin Zhang (NAOC) and

Alexei V. Filippenko (UCB) report on behalf of the KAIT GRB team:



The 0.76-m Katzman Automatic Imaging Telescope (KAIT), located at

Lick Observatory, responded to the fast X-ray transient EP240708a

from the Einstein Probe (Chen et al. GCNC 36838) starting at 9.13

hours after the trigger. A set of 30x60s images were obtained in

the clear (roughly R) filters. Preliminary analysis do not reveal

any new optical counterpart candidate within the 10 arcsec radius

EP-FXT error box (Hu et al. GCN 36840
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 ) neither in single image,

nor in the co-add images, consistent with the reports from

Lai et al. (GCN 36839
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 ) and Wu et al. (GCN 36841
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 ), though shallower

than Jiang et al. (GCN 36842
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 ). The typical limiting magnitude of

our single clear image is about 19.5 mag, and about 20.5 mag in the

co-add image.

W. X. Li, S. J. Xue (NAOC), M. Andrews, J. Farrah, D. A. Howell, M. Newsome, E. Padilla Gonzalez, C. McCully, and G. Terreran (Las Cumbres Observatory), on behalf of a larger collaboration, report:
Following the detection of the fast X-ray transient EP240708a by the Einstein Probe (Chen et al., GCN 36838
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 ), we initiated observations of the fast X-ray transient location starting on July 9 at 5:03 UT (~5.5 hours after the trigger) in the r band. These observations were conducted using the 1-meter telescope at the Las Cumbres Observatory node located at Cerro Tololo Inter-American Observatory in Chile.
No new optical source was detected in the co-added images within the EP/WXT error box down to ~21.0 mag.
These observations were taken as part of the Global Supernova Project.

J. Quirola-Vásquez (Radboud Univ.), P.G. Jonker (Radboud Univ), J. van Dalen (Radboud Univ.) ,F. E. Bauer (PUC), A.J. Levan (Radboud Univ. & Warwick Univ.), D. B. Malesani (DAWN/NBI and Radboud Univ.), Maria E. Ravasio (Radboud Univ.), D. Mata Sanchez (IAC), M.A.P. Torres (IAC), report on behalf of a larger collaboration:

We observed the location of the X-ray counterpart of the fast X-ray transient EP240708a reported by the Einstein Probe WXT instrument and FXT (Chen et al. GCN 36838
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 ; Hu et al. GCN 36840
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 ) using the Swift-XRT Observatory ToO program (ObsID: 00016707002). The observation started at 2024-07-15T23:58:44 UT and lasted for ~2.0 ksec, using the PC mode. We do not detect an X-ray counterpart to the transient. Three photons were detected in a ~10" radius centered on R.A. = 345.9641 deg, DEC = -22.8430 deg. Using Kraft et al. (1991) this leads to a 90% upper limit on the source count of ~3.5E-3 cnt/s. Using an absorbed power law model with photon index 1.62 and column density 2.0E20 cm-2 following the detected parameters of Hu et al. (GCN 36840
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 ), we obtain a flux upper limit of 1.5E-13 erg cm-2 s-1 (0.3-10 keV).

We thank the Neil Gehrels Swift Observatory team for making the X-ray observation possible.

This was a duplicate submission of GCN 37002
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 .