EP241202b

C. Zhou(HUST), G. Y. Zhao(SYSU), X. Mao, R. D. Liang, W. Yuan (NAO,CAS) report on behalf of the Einstein Probe team:

We report on the detection of a fast X-ray transient detected by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, EP241202b (Obs. ID: 08500000223). The transient was first detected with WXT at around 2024-12-02 15:12:55 (UTC) and lasted for over 140 seconds. The WXT position of EP241202b is R.A.= 45.302 deg, DEC = 2.441 deg (J2000) with an uncertainty of 2.6 arcmin (radius, 90% C.L. statistical and systematic). It has a peak flux of 1.4 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 1.06 (+0.43/-0.41) (with a column density fixed at the Galactic value of 9.5 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 5.4 (+2.0/-1.6) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources are found within the error circle around the source position.

We plan to perform a target of opportunity observation with the Follow-up X-ray Telescope (FXT) on board EP. Further follow-up observations are encouraged to explore the origin of EP241202b.

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).

V.Lipunov, E.Gorbovskoy, N.Tiurina,  P.Balanutsa, , D.Vlasenko, I.Panchenko,
A.Kuznetsov,  G.Antipov,  A.Sankovich, A.Sosnovskij, Yu.Tselik, M.Gulyaev, Ya.Kechin,
V.Senik, A.Chasovnikov, K.Labsina, I. Gorbunov (Lomonosov MSU),
O.Gress, N.Budnev (ISU),
C.Francile,  F. Podesta, R.Podesta, E. Gonzalez  (Observatorio Astronomico Felix Aguilar (OAFA),
A. Tlatov, D. Dormidontov (Kislovodsk Solar Station of the Pulkovo Observatory),
A. Gabovich, V.Yurkov (Blagoveschensk Educational StateUniversity)
D.Buckley (SAAO),
R.Rebolo (The Instituto de Astrofisica de Canarias),
L.Carrasco, J.R.Valdes, V.Chavushyan, V.M.Patino Alvarez, J.Martinez,
A.R.Corella, L.H.Rodriguez (INAOE, Guillermo Haro Astrophysics Observatory) 

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 EP241202b ( EP Team et al., GCN 38426
Loading...
 
 ) errorbox  11695 sec after notice time and 68974 sec after trigger time at 2024-12-03 10:22:29 UT, with upper limit up to  19.5 mag. Observations started at twilight.  The observations began at zenith distance = 78 deg. The sun  altitude  is -11.9 deg. 

The galactic latitude b = -46 deg., longitude l = 175 deg.


Real time updated cover map and OT discovered available here: 
https://master.sai.msu.ru/site/master2/observ.php?id=2693898

We obtain a following upper limits.  

Tmid-T0  |      Date Time      |          Site       |             Coord (J2000)          |Filt.| Expt. | Limit| Comment
_________|_____________________|_____________________|____________________________________|_____|_______|_______|________

   69004 | 2024-12-03 10:22:29 |        MASTER-Tunka | (03h 01m 32.26s , +02d 12m 18.8s) |   C |    60 | 17.9 |  Coadd 
   70325 | 2024-12-03 10:44:30 |        MASTER-Tunka | (03h 01m 32.36s , +02d 14m 44.8s) |   C |    60 | 19.2 |        
   71945 | 2024-12-03 11:11:29 |        MASTER-Tunka | (03h 01m 31.26s , +02d 15m 37.9s) |   C |    60 | 19.4 |        
   72017 | 2024-12-03 11:12:41 |        MASTER-Tunka | (03h 01m 26.53s , +02d 14m 42.3s) |   C |    60 | 19.5 |        
   72090 | 2024-12-03 11:13:54 |        MASTER-Tunka | (03h 01m 25.46s , +02d 15m 45.5s) |   C |    60 | 19.5 |        
Filter C is a clear (unfiltred) band. 


The observation and reduction will continue. 
The message may be cited.

C.-C. Ngeow (NCU), S. Yang (HNAS), A. Aryan, T.-W. Chen, Y. J. Yang, Y.-C. Cheng, M.-H. Lee, W.-J. Hou (all NCU), A. K. H. Kong (NTHU), S. J. Smartt (Oxford/QUB), J. Gillanders (Oxford), A. Sankar.K, Y.-H. Lee, H.-Y. Miao, Y.-C. Pan, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, C.-S. Lin, J.-K. Guo (all NCU), Z. N. Wang, L. L. Fan, G. H. Sun (all HNAS), 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 the fast X-ray transient EP241202b (Zhou et al., GCN 38426
Loading...
 
 ) using the 1m LOT at the Lulin Observatory in Taiwan as part of the Kinder collaboration (Chen & Yang et al., 2024 arXiv:2406.09270
Loading...
 
 ). The first LOT epoch of observations started at 10:40 UTC on 3rd December 2024 (MJD 60647.444), 19.45 hr after the EP-WXT trigger.   

We utilized the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images and visually identified a faint, plausible optical transient candidate at RA = 03:01:20.862, Dec = +02:26:27.04, approximately 2.11 arcminutes from the reported coordinates of EP241202b. To confirm this, we subtracted the stacked image from the DESI Legacy Survey (Dey et al. 2019) DR10 template images. The resulting difference image also revealed the trace of a tenuous source characterized by a well-defined Gaussian profile.

The details of the observations and measured aperture magnitude without template subtraction (in the AB system) of the possible counterpart of EP241202b are as follows:
-------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass 
LOT | r | 60647.444 | 19.45 | 300 * 6 | 22.41 +/- 0.16 | 1".51 | 1.56

Following the detection of the plausible optical counterpart candidate, we began another set of observations about 1.47 hr after the first set of observations. The optical counterpart candidate disappeared in the second set. The details of observations and measured 3-sigma upper limit for the second epoch of observations are as follows: 
-------------------------------------------------------------------------------------------------Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude | avg. Seeing | med. Airmass 
LOT | r | 60647.524 | 21.36 | 300 * 6 | > 22.9 | 1".02 | 1.13

The plausible optical counterpart candidate has faded by 0.5 magnitudes over 1.9 hours; however, this variation could be attributed to statistical fluctuations, given the source's faintness.

The presented magnitudes were calibrated using the field stars from the Pan-STARRS1 catalog (Chambers et al., 2016 arXiv:1612.05560
Loading...
 
 ) and were not corrected for the expected Galactic foreground extinction corresponding to a reddening of A_r = 0.21 mag in the direction of the transient (Schlafly & Finkbeiner 2011).   

Further follow-up observations are encouraged to confirm this candidate. 

M. E. Ravasio (Radboud Univ.), E. Burns (LSU), P.G. Jonker (Radboud Univ.), and  M. Hui (NASA MSFC) report on behalf of the Fermi-GBM Team:


Fermi-GBM had full spatial and temporal coverage of the EP-WXT signal of EP241202b (Zhou et al., GCN 38426
Loading...
 
 ). There was no Fermi-GBM onboard trigger around the EP-WXT trigger time T0=2024-12-02 15:12:55 UTC. 

The GBM targeted search [1], the most sensitive, coherent search for GRB-like signals, was run in the time interval [-50;+500] s from the EP trigger time, seeking signals between 64 ms and 32.768 s in duration. A transient was found, but its localization is not consistent with the EP transient’s one. No signal consistent both temporally and spatially is identified, as confirmed by visual inspection of the data. 

Assuming a “normal” spectral template (Band function with Epeak = 230 keV, alpha = -1.0, beta = -2.3), whose alpha value is consistent with the power law index reported by EP (Zhou et al., GCN 38426
Loading...
 
 ), and a duration of 8.192 s, we derive a sky-averaged upper limit of 5.2e-08 erg/cm2/s in the energy band 10-1000 keV.
[1] Goldstein et al. 2019 arXiv:1903.12597
Loading...
 
 

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, observed the field of EP241202b (Zhou et al.,

GCN 38426
Loading...
 
 ) starting at ~14.3h after the trigger and again at

~39.0h after the trigger. A set of 30x60s images were obtained in

the clear (roughly R) filters for each run. We do not detect the

optical counterpart candidate reported by Ngeow et al. (GCN 38433
Loading...
 
 )

in our coadd images with upper limits of 21.5 mag for both runs.