EP240416a
GCN Circular 36138
Subject
EP240416a: EP-WXT detection of a fast X-ray transient
Date
2024-04-16T16:41:11Z (a year ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
H. Q. Cheng, W. X. Wang, W. Yuan, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. 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, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (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 EP240416a at 2024-04-16T02:42:13 (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. = 203.150 deg, DEC = -13.612 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The flare lasted for at least 200 seconds, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux reached ~1.3 x 10^-9 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged 0.5-4.0 keV spectrum can be fitted with an absorbed power law (with the column density fixed at the Galactic value of 4.0 x 10^20 cm^-2) with a photon index of 1.5(-0.6/+0.6). The derived average unabsorbed 0.5-4.0 keV flux is 5.0(-1.6/+2.6) x 10^-10 erg/s/cm^2. The 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. The derived source parameters may be subject to larger uncertainties than those quoted here since the in-orbit calibration of the instrument is still in progress.
Further follow-up observations are encouraged to identify the nature of this X-ray transient.
The above observation was made with EP-WXT during the commissioning phase. 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.
GCN Circular 36139
Subject
EP240416a: Kinder optical counterpart candidate
Date
2024-04-16T19:39:25Z (a year ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
T.-W. Chen, A. Aryan (both NCUIA), S. Yang (HNAS), S. J. Smartt (Oxford/QUB), M. Fulton (QUB), W.-J. Hou, H.-Y. Hsiao, C.-H. Lai, Y.-C. Pan, A. Sankar.K, M.-H. Lee, C.-C. Ngeow, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCUIA), J. Gillanders, L. Rhodes, S. Srivastav, H. F. Stevance (all Oxford), T. Moore and K. W. Smith (both QUB) report:
We observed the field of EP240416a (Cheng et al., GCN 36138) using the Lulin One-meter Telescope (LOT) at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen et al., AstroNote 2021-92). The first LOT epoch of observations started at 16:51 UT on 16 of April 2024 (MJD = 60416.702), 14.15 hours after the EP trigger.
We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and subtract the stacked images with the PanSTARRS1 template images. We detected an optical transient in the difference image, at RA = 13:32:34.514, Dec = -13:37:49.05 (which is 69.6 arcsec away from the reported coordinate of EP240416a by Cheng et al. GCN 36138).
The details of the observations and measured PSF magnitudes (in the AB system) of the possible counterpart of EP240416a are as follows:
Telescope | Filter | MJD | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
LOT | g | 60416.702 | 14.15 hrs | 300 sec * 7 | 22.39 +/- 0.12 | 1".4 | 1.86
LOT | r | 60416.707 | 14.27 hrs | 300 sec * 7 | 22.08 +/- 0.12 | 1".3 | 1.40
LOT | i | 60416.711 | 14.36 hrs | 300 sec * 7 | 22.02 +/- 0.16 | 1".3 | 1.54
The MJD reported is the start time of the first observation in each band. We observed in a sequence of 300 sec in g,r and i, then 6x300 in r, i, g. All measurements were made on the 7-frame co-adds. The presented magnitudes are calibrated using the field stars from the PanSTARRS1 catalog and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.05 mag in the direction of the burst (Schlafly & Finkbeiner 2011).
Further follow-up observations are encouraged to identify the nature of this X-ray transient.
GCN Circular 36144
Subject
EP 240416a: Xinglong and NOT decaying optical counterpart
Date
2024-04-17T07:45:53Z (a year ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
email
J. An, X. Liu, S.Q. Jiang, Z.P. Zhu, J. Zheng, S. Liu, J.J. Jin, S.Y. Fu, T.H. Lu, Z. Fan, D. Xu (NAOC), J.P.U. Fynbo (DAWN/NBI) report on behalf of a large collaboration:
We observed the field of the X-ray transient, EP 240416a, detected by EP/WXT (Cheng et al., GCN 36138) using the 2.16-m telescope located at Xinglong, Hebei, China. We obtained 5 x 360 s R-band frames with a median time of 2024-04-16T17:18:06, i.e., 14.6 hr after the EP trigger.
The stacked R-band image has a limiting magnitude of R ~ 22 mag, calibrated with the PanSTAR field. Inspection of the whole EP/WXT 3 arcmin error circle produces a few low S/N optical counterpart candidates.
We subsequently performed r-band photometry at the 2.56-m Nordic Optical Telescope (NOT) equipped with the ALFOSC camera, and obtained 3 x 300 s frames in the Sloan r-band with a median time of 2024-04-17T01:00:00, i.e., 22.3 hr after the EP trigger.
The stacked r-band image has a limiting magnitude of r ~ 23.5 mag, calibrated with the PanSTAR field. Inspection of the whole EP/WXT 3 arcmin error circle also produces a few optical counterpart candidates.
The candidates from the two epochs are corssmatched in position and only one single optical transient (OT) is left. It is localized at
R.A. = 13:32:34.51 (J2000)
Dec. = -13:37:48.88 (J2000)
with an uncertainty of radius ~ 0.3 arcsec, being consistent with the candidate reported by Kinder (Chen et al., GCN 36139). This OT has m(R) = 21.8 +/- 0.4 (Vega) in the Xinglong image and decayed to m(r) = 22.66 +/- 0.18 (AB) in the NOT image, calibrated with the PanSTARRS field. The decay rate is consistent with that for typical GRB optical afterglows.
Therefore, we suggest that the OT is very likely the optical counterpart of EP 240416a, which is likely a GRB event.
GCN Circular 36155
Subject
EP240416a: BOOTES-2/TELMA Optical Upper Limit
Date
2024-04-18T09:25:48Z (a year ago)
From
Dingrong Xiong at Yunnan Observatories of CAS, China <xiongdingrong@ynao.ac.cn>
Via
legacy email
D. R. Xiong, J. M. Bai, J. R. Mao, Y. F. Fan, K. Ye, C. J. Wang, Y. X. Xin, B. L. Lun, J. J. Zhang, X. H. Zhao, L. Xu, K. X. Lu, X. Ding, X. G. Yu, D. Q. Wang, S. S. He, X. L. Zhang, J. G. Wang (Yunnan Observatories/CAS), A. J. Castro-Tirado, E. Fernandez-Garcia, Y. D. Hu (IAA-CSIC) and C. J. Perez del Pulgar (UMA) on behalf of the BOOTES team report:
We observed the X-ray transient EP 240416a (Cheng et al., GCN 36138), and did not found any optical counterpart within an uncertainty of 3 arcmin in radius compared to the PanSTARRS DR1 image. The upper limits of magnitudes (without being corrected for Galactic extinction) are given as follows.
Tmid-T0 (day) | UT (start) | Upper Limit (error) | Exposure Time | Filter
--------------------------------------------------------------------------------------------------------------
0.96 | 24-04-17T01:50:42 | 19.56 (0.2) | 100s*10 | Clear
-------------------------------------------------------------------------------------------------------------
The Burst Optical Observer and Transient Exploring System (BOOTES) is a world-wide automatic telescope network which aims to repaid follow-up of transient and astrophysical sources in the sky for which the first station was installed in 1998 (Castro-Tirado et al. 1999; Hu et al. 2021). The BOOTES-2/TELMA robotic telescope at IHSM La Mayora (UMA-CSIC) in Algarrobo Costa (Malaga, Spain). We acknowledge the support of these staffs from the BOOTES telescope networks.
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GCN Circular 36172
Subject
EP240416a: J-band observations with WINTER
Date
2024-04-19T05:07:16Z (a year ago)
From
Geoffrey Mo at MIT <gmo@mit.edu>
Via
Web form
Geoffrey Mo (MIT), Viraj Karambelkar (Caltech), Danielle Frostig (MIT), Robert Stein (Caltech), Nathan Lourie (MIT), Tomas Ahumada (Caltech), Robert Simcoe (MIT), and Mansi Kasliwal (Caltech) report:
We observed the field of the EP-WXT X-ray transient EP240416a (Cheng et al., GCN 36138) in the near-infrared J-band with the Palomar 1-m telescope, equipped with the 1 square degree WINTER camera (Lourie et al. 2020).
Observations began at 2024-04-17T14:01:04.438 UTC (~1.5 days after the X-ray transient) and consisted of 15 x 120 s exposures. The images were processed using the WINTER data reduction pipeline (https://github.com/winter-telescope/mirar).
We do not detect the optical counterpart discovered by Kinder and confirmed by Xinglong and NOT (Chen et al., GCN 36139, An et al., GCN 36144, Antipov et al., GCN 36142, Xiong et al., GCN 36155, Pankov et al., GCN 36157, Belkin et al., GCN 36158), setting the following upper limit: J ~ 19.4 mag (AB).
WINTER (Wide-field INfrared Transient ExploreR) is a partnership between MIT and Caltech, housed at Palomar Observatory, and funded by NSF MRI, NSF AAG, the David and Lucille Packard Foundation, and the MIT Kavli Institute for Astrophysics and Space Research.