EP240408a

J. W. Hu, D. H. Zhao, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. 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 (NAOC, 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 EP240408a at 2024-04-08T17:56:30 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 158.840 deg, DEC = -35.749 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the source shows a duration of approximately 10 seconds. The peak flux reaches ~1.4 x 10^-8 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged spectrum can be fitted by an absorbed power-law with NH fixed at the Galactic value of 6.23 x 10^20 cm^-2 and a photon index of 1.1(-0.7/+0.8). The derived average unabsorbed 0.5-4.0 keV flux is 4.0(-1.3/+1.3) x 10^-9 erg/s/cm^2. 

No previously known bright X-ray sources have been found within the 3 arcmin region around the source position. The derived source parameters may be subject to larger uncertainties than those quoted here since in-orbit calibration of the instrument is still in progress. 

Further follow-up observations are strongly 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.

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

Following the detection of the fast X-ray transient EP240408a by the Einstein Probe (Hu et al. GCNC 36053), the BOOTES-6/DPRT 0.6m robotic telescope at Boyden Observatory in Maselspoort (South Africa) observed the fast X-ray transient location starting on Apr 9, 18:54 UT (~ 25 h after trigger) in different optical bands. No new optical source is detected on the co-added images (clear-filter) within the EP/WXT error box down to 20.5 mag.

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

J. W. Hu, D. H. Zhao, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. 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 (NAOC, 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 up on the new X-ray flare EP240408a detected by Einstein Probe (Hu et al., GCN 36053
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 ), we performed a target of opportunity observation with Swift. The Swift XRT observation began at 2024-04-10T02:57:40 (UTC) with an exposure time of 966 seconds in the Photon Counting mode, about 33 hours after the burst detected by EP-WXT. An X-ray source was detected at R.A. = 158.851 deg, DEC = -35.747 deg, with an uncertainty of 3.68 arcsec (radius, 90% C.L. statistical and systematic), 32 arcsec away from the position of EP240408a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 6.23 x 10^20 cm^-2 and a photon index of 1.9(-0.2, +0.2). The derived average unabsorbed 0.5-4.0 keV flux is 1.2(-0.1,+0.3) x 10^-11 erg/s/cm2 (90% C.L.). There is no catalogued X-ray source within the error circle, suggesting that this XRT source is most likely associated with EP240408a. No significant counterpart was detected in the UVOT image within the error circle of the XRT position. 

Further follow-up observations are encouraged to identify the nature of this X-ray transient. 

We thank the Neil Gehrels Swift Observatory team for making the ToO observation possible. 

Chen-Wei Wang, Shao-Lin Xiong, Jia-Cong Liu, Cheng-Kui Li and Chao Zheng, report on behalf of the GECAM team:

GECAM-B was observing normally and covered the sky region of EP240408a at event time 2024-04-08T17:56:30 (UTC). The smallest incident angle of GECAM-B GRD detectors is about 27.3 deg for the WXT location (RA=158.840 deg, Dec=-35.749 deg).

There was no GECAM-B in-flight trigger around the event time of EP240408a. An automated, blind search for gamma-ray bursts of GECAM-B data found no burst candidates. The targeted search was run within +/-150 s around event time, and also identified no counterpart candidates.

With the three typical GRB spectral models, integration time of 10 s (which is the duration of the EP240408a) and the WXT localization, the 3-sigma upper-limits of fluence (15 - 300 keV, incident energy) are reported below:

Band model 1 (alpha=-1.9, beta=-3.7, Ep=70 keV):   2.4e-6erg cm^-2
Band model 2 (alpha=-1.0, beta=-2.3, Ep=230 keV):  3.5e-6 erg cm^-2
Band model 3 (alpha=0.0, beta=-1.5, Ep=1000 keV):   4.3e-6 erg cm^-2

All measurements above are made with the GRD detector which has the smallest incident angle to this source.

We note that these results are preliminary and refined analysis will be reported later.

Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM) mission originally consists of two micro-satellites (GECAM-A and GECAM-B) launched in Dec. 2020. As the third member of GECAM constellation,  GECAM-C was launched onboard SATech-01 experimental satellite in July 2022. GECAM mission is funded by the Chinese Academy of Sciences (CAS).

Arne Rau (MPE):

We observed the position of EP240408a (Hu et al., #GCN 36053; #36057) with the Gamma-ray Burst Optical Near-ir Detector (GROND, Greiner et al. 2008) starting on April 10, 2024, 04:14:09 UTC. A faint source is detected in the Swift/XRT error circle located at  RA = 10:35:24.30 Dec = -35:44:47.9 with an uncertainty of 0.2” in both coordinates.

The source is detected in the J and H bands at the following AB magnitudes:

J= 19.9 +/- 0.3
H= 20.5 +/- 0.5

The photometric uncertainties are dominated by a very bright point source ~20 arcsec from the optical/NIR counterpart.

We note that the source is also visible in the z-band data of the Legacy Survey DR10 (https://www.legacysurvey.org/).

We thank the 2.2m staff member, Angela Hempel, for the rapid execution of the observation.

W. X. Li, S. J. Xue, D. Xu (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 EP240408a by the Einstein Probe (Hu et al. GCN 36053
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 ), we initiated observations of the fast X-ray transient location starting on April 9 at 10:46 UT (~17 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 Siding Spring Observatory in Australia.
We report that no new optical source was detected in the co-added images within the EP/WXT error box down to ~21.5 mag.
These observations were taken as part of the Global Supernova Project.