EP240918a
GCN Circular 37541
Subject
EP240918a: an X-ray transient detected by Einstein Probe
Date
2024-09-18T13:27:51Z (9 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. J. Zhang (HKU), Y. F. Liang (PMO, CAS), H. Y. Liu, W. Chen, T. Zhao , Z. X. Ling, J. W. Hu, W. M. Yuan, C. C. Jin, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), X. F. Zhao, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, 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 an X-ray transient designated EP240918a by the Einstein Probe (EP) mission. The transient event triggered the Wide-field X-ray Telescope (WXT) on-board processing unit at 2024-09-18T11:24:37 (UTC). An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about two minutes later. Afterwards, the FXT on-board processing unit was also triggered, showing that an uncatalogued source was detected at R.A. = 289.3937, DEC = 46.1281 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic).
More information on this source will be updated when the full telemetry data is received. 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 X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 37544
Subject
EP240918a: GMG Optical Upper Limit
Date
2024-09-18T15:44:35Z (9 months ago)
From
wangbaiting@ynao.ac.cn
Via
Web form
B.-T. Wang, R.-Z. Li, F.-F. Song, J. Mao, Y.-F. Fan and J.-M. Bai (YNAO, CAS) report:
We observed the field of EP240918a (Zhang et al., GCN 37541, T0 at 2024-09-18T11:24:37) using the GMG-2.4m telescope at the Lijiang Observatory. The observation began at 2024-09-18T14:02:51, about 2.64 hours after the trigger.
No new credible optical counterpart of EP240918a was detected within the EP-FXT error circle (GCN 37541).
The preliminary analysis results are shown as follows:
+----------------+------------+----------+----------------+
| Tmid-T0 [hr] | Exp. [s] | Filter | 5-sigma U.L. |
+================+============+==========+================+
| 2.72 | 600 | R | 20.5 |
+----------------+------------+----------+----------------+
The given magnitudes are derived based on calibrating against Pan-STARRS1 field stars.
We acknowledge the staff at the Lijiang Observatory for conducting the observation.
GCN Circular 37545
Subject
EP240918a: Optical upper limits with Kinder observations
Date
2024-09-18T15:59:55Z (9 months ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
A. K. H. Kong (NTHU), A. Aryan, T.-W. Chen, C.-H. Lai, H.-Y. Hsiao, C.-S. Lin (all NCU), S. Yang (HNAS), S. J. Smartt (Oxford/QUB), A. Sankar. K, Y. J. Yang, 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, J. Gillanders, 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 the fast X-ray transient EP240918a (Zhang et al., GCN 37541) using the 40cm SLT at Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024arXiv240609270C). The first SLT epoch of observations started at 13:43 UTC on the 18th of September 2024 (MJD = 60571.571), 2.30 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. In the stacked frames, we do not detect any evidence of a new uncataloged source within the FXT error circle of 20" reported by Zhang et al. (GCN 37541).
Moreover, we utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform the photometry on our stacked frame. The details of the observations and measured 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 | Exposure | Magnitude | avg. Seeing | med. Airmass
SLT | r | 60571.571 | 2.30 hrs | 300 sec * 16 | >20.5 | 1".83 | 1.37
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) = 0.08 mag in the direction of the transient (Schlafly & Finkbeiner 2011).
Our result is consistent with Wang et al. (GCN 37544).
We thank the Einstein Probe team for useful communication.
GCN Circular 37546
Subject
EP240918a: YAHPT Optical Upper Limit
Date
2024-09-18T16:00:01Z (9 months ago)
From
Tianrui Sun at Purple Mountain Obs,CAS <trsun@pmo.ac.cn>
Via
Web form
Tian-Rui Sun, Jin-Jun Geng, Jian Chen, Yan-Long Hua, Yi-Fang Liang (Purple Mountain Observatory, CAS) and Lei Hu (CMU) report on behalf of the YAHPT team:
Following the detection of EP240918a by Einstein Probe (Zhang et al., GCN 37541),
we use the Yaoan High Precision Telescope at Yaoan Astronomy Observation Station (Yunnan province, China) to search and follow up.
We observed the target position in both Rc and Ic starting from 2024-09-18T13:13:31 with 180s exposure.
We did not detect any possible new optical counterpart with an uncertainty of 20 arcsec in the coadded image.
UT (start) |5 sigma Upper Limit| Exposure Time | Filter
2024-09-18T13:13:31| 20.3 | 180*5s | Rc
2024-09-18T13:16:34| 19.1 | 180*5s | Ic
This non-detection result of the transient is in agreement with the previous circulars (Wang et al., GCN 37544).
We used the USNO B1.0 catalogue (R2mag and Imag) as the magnitude reference for calibration.
GCN Circular 37547
Subject
EP240918a: JinShan optical upper limits
Date
2024-09-18T16:46:22Z (9 months ago)
From
Dong Xu at NAOC/CAS <dxu@nao.cas.cn>
Via
Web form
S.Y. Fu, X. Liu, S.Q. Jiang, J. An, Z.P. Zhu, D. Xu (NAOC), J.Z. Liu (XAO) report on behalf of a large collaboration:
We observed the field of EP240918a detected by EP/WXT (Zhang et al., GCN 37541), using the 50cm-A/B and 100cm-C telescopes (50A, 50B and 100C) of the JinShan project, located at Altay, Xinjiang, China. Observations started at 13:38:24 UT on 2024-09-18, i.e., 2.23 hr after the EP trigger, and a series of frames were obtained in the Sloan g-, r-, i- and z- bands.
No new optical source is detected within the EP/FXT error circle (Zhang et al., GCN 37541), down to the following 5-sigma upper limits:
T_mid (UT) | T_mid (hr) | Telescope | filter | U.L. (AB)
----------------------------------------------------
2024-09-18T13:53:45.0 | 2.49 | 50B | z | 17.1
2024-09-18T13:55:53.5 | 2.52 | 50A | r | 19.3
2024-09-18T14:24:52.5 | 3.00 | 50B | i | 18.2
2024-09-18T14:26:51.0 | 3.04 | 50A | g | 19.2
2024-09-18T15:52:16.8 | 4.46 | 100C | r | 21.2
calibrated with nearby PanSTARRS stars and not corrected for Galactic extinction.
We acknowledge the excellent support from X. Yao, S.W. Luo, and Z.K. Feng for enabling these observations.
GCN Circular 37548
Subject
EP EP240918a: Global MASTER-Net observations report
Date
2024-09-18T17:18:36Z (9 months ago)
From
Vladimir Lipunov at Moscow State U/Krylov Obs <lipunov@xray.sai.msu.ru>
Via
legacy email
V. Lipunov, V.Kornilov, E. Gorbovskoy, K. Zhirkov, N.Tyurina, P.Balanutsa, A.Kuznetsov, V.Senik, D. Vlasenko,
G.Antipov, D.Zimnukhov, E.Minkina, A.Chasovnikov, V.Topolev, D.Kuvshinov, D. Cheryasov, Ya.Kechin, Yu.Tselik, A. Sosnovskij
(Lomonosov Moscow State University, SAI, Physics Department),
R. Podesta, C.Lopez, F. Podesta, C.Francile
(Observatorio Astronomico Felix Aguilar OAFA),
R. Rebolo, M. Serra
(The Instituto de Astrofisica de Canarias),
D. Buckley
(South African Astronomical Observatory),
O.A. Gress, N.M. Budnev, O.Ershova
(Irkutsk State University, API),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez
(INAOE, Guillermo Haro Astrophysics Observatory),
A. Tlatov, D. Dormidontov
(Kislovodsk Solar Station of the Pulkovo Observatory),
V. Yurkov, A. Gabovich
(Blagoveschensk Educational State University)
MASTER-Tunka robotic telescope (Global MASTER-Net: http://observ.pereplet.ru, Lipunov et al., 2010, Advances in Astronomy, vol. 2010, 30L) located in Russia (Applied Physics Institute, Irkutsk State University) was pointed to the EP EP240918a ( EP Team et al., GCN 37541) errorbox 151 sec after notice time and 7592 sec after trigger time at 2024-09-18 13:31:09 UT, with upper limit up to 18.7 mag. The observations began at zenith distance = 11 deg. The sun altitude is -21.2 deg.
MASTER-Tavrida robotic telescope located in Russia (Lomonosov MSU, SAI Crimea astronomical station) was pointed to the EP EP240918a errorbox 11305 sec after notice time and 18746 sec after trigger time at 2024-09-18 16:37:03 UT, with upper limit up to 18.4 mag. Observations started at twilight. The observations began at zenith distance = 6 deg. The sun altitude is -9.7 deg.
The galactic latitude b = 15 deg., longitude l = 78 deg.
Real time updated cover map and OT discovered available here:
https://master.sai.msu.ru/site/master2/observ.php?id=2606210
We obtain a following upper limits.
Tmid-T0 | Date Time | Site | Coord (J2000) |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________
7623 | 2024-09-18 13:31:09 | MASTER-Tunka | (19h 18m 13.42s , +46d 08m 39.1s) | C | 60 | 18.7 | Coadd
7862 | 2024-09-18 13:35:08 | MASTER-Tunka | (19h 18m 10.43s , +46d 10m 42.6s) | C | 60 | 18.1 | Coadd
8024 | 2024-09-18 13:37:51 | MASTER-Tunka | (19h 18m 10.13s , +46d 08m 47.7s) | C | 60 | 18.1 | Coadd
8187 | 2024-09-18 13:40:33 | MASTER-Tunka | (19h 18m 13.05s , +46d 09m 18.0s) | C | 60 | 18.1 | Coadd
8424 | 2024-09-18 13:44:31 | MASTER-Tunka | (19h 18m 09.57s , +46d 11m 00.5s) | C | 60 | 17.9 | Coadd
8585 | 2024-09-18 13:47:11 | MASTER-Tunka | (19h 18m 09.30s , +46d 09m 05.9s) | C | 60 | 18.1 | Coadd
8747 | 2024-09-18 13:49:53 | MASTER-Tunka | (19h 18m 12.35s , +46d 09m 17.2s) | C | 60 | 18.2 | Coadd
8987 | 2024-09-18 13:53:53 | MASTER-Tunka | (19h 18m 09.73s , +46d 11m 20.2s) | C | 60 | 18.0 | Coadd
9150 | 2024-09-18 13:56:37 | MASTER-Tunka | (19h 18m 08.75s , +46d 09m 26.5s) | C | 60 | 17.7 | Coadd
9313 | 2024-09-18 13:59:20 | MASTER-Tunka | (19h 18m 11.73s , +46d 09m 54.4s) | C | 60 | 17.6 | Coadd
9559 | 2024-09-18 14:03:26 | MASTER-Tunka | (19h 18m 08.14s , +46d 11m 40.4s) | C | 60 | 17.4 | Coadd
9724 | 2024-09-18 14:06:11 | MASTER-Tunka | (19h 18m 08.54s , +46d 09m 46.4s) | C | 60 | 17.1 | Coadd
9885 | 2024-09-18 14:08:52 | MASTER-Tunka | (19h 18m 10.74s , +46d 10m 01.5s) | C | 60 | 16.8 | Coadd
10121 | 2024-09-18 14:12:48 | MASTER-Tunka | (19h 18m 08.25s , +46d 11m 59.0s) | C | 60 | 17.2 | Coadd
10282 | 2024-09-18 14:15:28 | MASTER-Tunka | (19h 18m 06.43s , +46d 10m 03.6s) | C | 60 | 17.2 | Coadd
10442 | 2024-09-18 14:18:08 | MASTER-Tunka | (19h 18m 09.63s , +46d 10m 27.6s) | C | 60 | 17.2 | Coadd
10678 | 2024-09-18 14:22:04 | MASTER-Tunka | (19h 18m 07.08s , +46d 12m 16.2s) | C | 60 | 17.3 | Coadd
10837 | 2024-09-18 14:24:44 | MASTER-Tunka | (19h 18m 06.80s , +46d 10m 20.8s) | C | 60 | 17.2 | Coadd
10997 | 2024-09-18 14:27:24 | MASTER-Tunka | (19h 18m 08.71s , +46d 10m 37.6s) | C | 60 | 17.1 | Coadd
11234 | 2024-09-18 14:31:21 | MASTER-Tunka | (19h 18m 05.10s , +46d 12m 31.8s) | C | 60 | 17.1 | Coadd
18777 | 2024-09-18 16:37:03 | MASTER-Tavrida | (19h 17m 32.41s , +46d 05m 00.9s) | C | 60 | 17.2 |
18777 | 2024-09-18 16:37:03 | MASTER-Tavrida | (19h 17m 33.33s , +46d 09m 15.9s) | C | 60 | 17.4 |
19669 | 2024-09-18 16:51:55 | MASTER-Tavrida | (19h 17m 31.56s , +46d 04m 47.0s) | C | 60 | 18.1 |
19669 | 2024-09-18 16:51:55 | MASTER-Tavrida | (19h 17m 33.22s , +46d 08m 14.1s) | C | 60 | 18.4 |
19920 | 2024-09-18 16:56:07 | MASTER-Tavrida | (19h 17m 32.46s , +46d 02m 40.9s) | C | 60 | 18.0 |
19921 | 2024-09-18 16:56:07 | MASTER-Tavrida | (19h 17m 30.82s , +45d 58m 44.6s) | C | 60 | 18.4 |
20774 | 2024-09-18 17:10:21 | MASTER-Tavrida | (19h 17m 33.63s , +46d 03m 39.5s) | C | 60 | 18.1 |
20775 | 2024-09-18 17:10:21 | MASTER-Tavrida | (19h 17m 35.14s , +46d 07m 37.3s) | C | 60 | 18.4 |
Filter C is a clear (unfiltred) band.
The observation and reduction will continue.
The message may be cited.
GCN Circular 37551
Subject
EP240918a: Swift-XRT observation
Date
2024-09-19T03:02:42Z (9 months ago)
From
Jonathan Quirola at Radboud University <jaquirola1990@gmail.com>
Via
Web form
J. Quirola-Vasquez (Radboud Univ.), P. G. Jonker (Radboud Univ), D. B. Malesani (DAWN/NBI and Radboud Univ.), A. Martin-Carrillo (UCD), A. J. Levan (Radboud Univ. & Warwick Univ.), F. E. Bauer (PUC), J. van Dalen (Radboud Univ.), M. 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 EP240918a reported by the Einstein Probe WXT instrument and FXT (Zhang et al. GCN 37541) using the Swift-XRT Observatory target of opportunity (ObsID: 00016827001). The observation started at 2024-09-18T22:05:49 UT, (i.e. ~10.7 hrs after the X-ray trigger) and lasted for ~1 ksec. The XRT PC mode was used. We do not detect an X-ray counterpart to the transient. The Swift automatic analysis (2007, A&A, 469, 379; 2009, MNRAS, 397, 1177 and 2014, ApJS, 210, 8) results in an upper limit of ~1.5E-2 cnt/s. Assuming an absorbed power law model with photon index 1.5 and column density 2.0E20 cm-2, we obtain a flux upper limit of <6.9E-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.
GCN Circular 37554
Subject
EP240918a: EP observation update
Date
2024-09-19T08:42:10Z (9 months ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Z. J. Zhang (HKU), Y. F. Liang (PMO, CAS), H. Y. Liu, W. Chen, T. Zhao , Z. X. Ling, J. W. Hu, W. M. Yuan, C. C. Jin, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), X. F. Zhao, Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, 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 the fast X-ray transient EP240918a by the Einstein Probe (EP) mission (Zhang et al. GCN 37541). Based on the newly received EP telemetry data of this source, the transient event started at 2024-09-18T11:21:52 (UTC). The lightcurve of the transient obtained by the WXT lasts around 170 seconds and has a peak flux of around 3.2 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 1.7 (-0.6, +0.7) (with the column density fixed at the Galactic one of 9.9 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 7.2 (-1.9, +2.8) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
The EP-FXT autonomous follow-up observation started at 2024-09-18T11:25:55 (UTC). The observation clearly shows an uncatalogued X-ray source located at R.A. = 289.3933 deg, DEC = 46.1278 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The EP-FXT light curve shows a fast decline and the flux decreased to around 1.0 x 10^(-13) erg/s/cm^2 within about 1000 s. 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 X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).
GCN Circular 37568
Subject
EP240918a: 1.6m Mephisto multi-band Optical Upper Limit
Date
2024-09-20T12:21:59Z (9 months ago)
From
Brajesh Kumar at SWIFAR, YNU <brajesh@ynu.edu.cn>
Via
Web form
Jinghua Zhang (SWIFAR, YNU), Xingzhu Zou (SWIFAR, YNU), Guowang Du (SWIFAR, YNU), Tao Wang (SWIFAR, YNU), Jianhui Lian (SWIFAR, YNU), Jiang Runnan (SWIFAR, YNU), Yu Pan (SWIFAR, YNU), Xinlei Chen (SWIFAR, YNU), Yuan Fang (SWIFAR, YNU), Brajesh Kumar (SWIFAR, YNU), Xinzhong Er (SWIFAR, YNU), Xiangkun Liu (SWIFAR, YNU), Xiaowei Liu (SWIFAR, YNU) report on behalf of Mephisto Team:
Simultaneous multi-band photometric observations of the fast X-ray transient EP240918a (Zhang et al., GCN 37541) were conducted using the 1.6m Multi-channel Photometric Survey Telescope (Mephisto) and the Twin 50-cm Telescopes at Yunnan University's Lijiang Observatory. Mephisto was triggered ~2.33 hours post-alert and commenced observations at 13:43:57 UTC on 2024-09-18 under moderate sky conditions, capturing multiple frames across uvgriz bands. Similarly, the two 50-cm Telescopes were activated 2.25 hours after the notice, beginning observations at 13:39:07 UTC, and collected frames in gr bands. No evidence of a new, uncataloged optical source was detected within the FXT error circle of 20" reported by Zhang et al. (GCN 37541). Existing sources within this error circle exhibited no significant optical enhancement. Details of the observations and the measured 3-sigma upper limits (in the AB magnitude system) with Mephisto and 50-cm are listed below:
Time-start Time-end Telescope Filter Exposure U.L.(AB)
14:17:50 14:20:50 Mephisto u 180s*1 20.16
13:43:58 14:33:02 Mephisto v 180s*7 22.05
14:17:51 14:20:51 Mephisto g 180s*1 20.85
13:43:57 14:32:59 Mephisto r 50s*24 22.79
14:17:51 14:20:51 Mephisto i 180s*1 20.61
13:44:02 14:33:05 Mephisto z 79s*15 21.01
13:39:07 13:55:33 50-cm g 300s*4 21.69
13:39:09 13:55:33 50-cm r 300s*4 21.69
The non-detection result of the transient is in agreement with the previous circulars (Wang et al., GCN 37544, Kong et al., GCN 37545, Sun et al., GCN 37546, Fu et al., GCN 37547, Lipunov et al., GCN 37548).
------------------------------------------------------------------------------------------------------
Mephisto (Multi-channel Photometric Survey Telescope) is a 1.6-m wide-field multi-channel telescope, the first of its type in the world, capable of imaging the same field of view in three optical bands simultaneously. It provides real-time, high-quality colors of stellar objects. The on-site telescope assemblage and commissioning were carried out in September 2022. The first light in all three channels
was achieved on 2023 December 21.
------------------------------------------------------------------------------------------------------
GCN Circular 37581
Subject
EP240918a: SVOM/VT upper limit
Date
2024-09-20T22:22:03Z (9 months ago)
From
Liping Xin at NAOC, SVOM <xlp@nao.cas.cn>
Via
Web form
SVOM/VT commissioning team: Y. L.Qiu, L. P. Xin, H. L. Li, C. Wu, X. H. Han, W. J. Xie, H. B. Cai, Y. Xu, Y. J. Xiao, P. P. Zhang, J. S. Deng, L. Lan, X. M. Lu, R. S. Zhang (NAOC), J. Zhang, L. J. Dan, G. Y. Zou, C. J. Wang, Y. F. Du, C. Huang (XIOPM) and Palmerio, J. (CEA).
SVOM JSWG: Jian-Yan Wei (NAOC), Bertrand Cordier (CEA), Shuang-Nan Zhang (IHEP), Stéphane Basa (LAM), Jean-Luc Atteia (IRAP), Arnaud Claret (CEA), Zi-Gao Dai (USTC), Frédéric Daigne (IAP), Jin-Song Deng (NAOC), Andrea Goldwurm (APC), Diego Götz (CEA), Xu-Hui Han (NAOC), Cyril Lachaud (APC), En-Wei Liang (GXU), Yu-Lei Qiu (NAOC), Susanna Vergani (Obs.Paris), Jing Wang (NAOC), Chao Wu (NAOC), Li-Ping Xin (NAOC), Bing Zhang (UNLV)
report on behalf of the SVOM team:
During the commissioning phase of SVOM mission, we observed the field of EP240918a (Zhang et al., GCN 37541；GCN 37554) with SVOM/VT telescope in ToO mode. The observation started at 2024-09-19T14:11:29 UT, about 26.9 hours after the burst, and the total exposure time was 3820 seconds. VT made the observations with two channels simultaneously, VT_B (400nm-650nm) and VT_R (650nm-1000nm).
No any new optical source was detected within the error circle of EP/FXT (Zhang et al., GCN 37541) down to the 3 sigma upper limit of VT_B=24.0 mag and VT_R=23.6 mag in AB magnitude. The non-detection is consistent with the reports (Wang et al., GCN 37544, Kong et al., GCN 37545, Sun et al., GCN 37546, Fu et al., GCN 37547, Lipunov et al., GCN 37548, Zhang et al., GCN 37568).
Second follow-up was performed started at 2024-09-20T11:14:05 UT, about 48 hours after the trigger. No any new optical source was detected with the similar upper limit.
The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT is an optical telescope on board SVOM with an aperture of 44 cm, designed to automatically follow the Eclairs triggers. It has two channels: VT_B and VT_R, covering wavelengths from 400 nm to 650 nm and 650 nm to 1000 nm, respectively. The two bands are observed simultaneously. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC),CAS.