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GCN Circular 36027

Subject
GRB/FXT 240402B: Kinder optical counterpart candidate
Date
2024-04-04T15:37:12Z (16 days ago)
From
Janet Chen at National Central University <janetstars@gmail.com>
Via
Web form
S. Yang (HNAS), T.-W. Chen, H.-Y. Hsiao, W.-J. Hou, A. Sankar.K, Y.-C. Pan, M.-H. Lee, C.-H. Lai, C.-C. Ngeow, C.-S. Lin, H.-C. Lin, J.-K. Guo (all NCUIA), S. J. Smartt,  J. Gillanders, L. Rhodes, S. Srivastav (all Oxford) and T. Moore (QUB) report:

Following the detection of an X-ray transient (LXT 240402A) by LEIA (Xu et al. GCN 36016) and FXT on board ​Einstein Probe (Jia et al. GCN 36022), and an associated GRB 240402A by GECAM-C (Xue et al. GCN 36017), we used several telescopes at the Lulin Observatory, Taiwan as part of the Kinder collaboration (Chen et al., AstroNote 2021-92) for optical counterpart searches. Our followup facilities includes the Lulin One-meter Telescope (LOT) with g and r bands, and the 40cm Super Light Telescope (SLT) with r and i bands, and our observing details were below:

Telescope | Filter | MJD | t-t0 | Explosure | avg. Seeing | med. Airmass
SLT | r | 60403.644 | 1.28 day | 300 sec * 15 | 1".8 | 1.33 
SLT | i | 60403.713 | 1.35 day | 300 sec * 44 | 1".8 | 1.03 
LOT | r | 60403.720 | 1.35 day | 300 sec * 12 | 1".2 | 1.08
LOT | g | 60403.771 | 1.40 day | 300 sec * 10 | 1".2 | 1.04 

(1). LXT 240402A

We used the Kinder pipeline (Yang et al. A&A 646, A22) to stack the images with same pointing and subtract the stacked images with the LEGACY SURVEY template images. We do not detect significant optical counterpart candidates within the 1.5 arcmin error area of LXT 240402A. In 2.5 sigma level, we obtained the following preliminary upper limits (in the AB system): 

SLT r > 22.74, 
SLT i > 22.61, 
LOT r > 23.57 and 
LOT g >  23.58 mag.

Our result is consistent with GWAC-F50A (Xin et al., GCN 36018), MASTER (Lipunov et al., GCN 36019) and AST3-3 (Sun et al., GCN 36023), and our limits are deeper than these reports.

(2). FXT/GRB 240402B 

Furthermore, we noticed that EP-FXT announced the detection of the X-ray afterglow (Jia et al., GCN 36022) of LXT240402A, which is 2.4 arcmin away from the LEIA source. We thus repeat the procedure detailed above on this specific position and found a clear optical counterpart at the location of RA = 16:21:48.225, Dec = 25:45:47.57 in the differencing images. We measured the PSF magnitudes of this source after subtracted with the LEGACY SURVEY templates and obtained (all in the AB system):

g = 22.86 +/- 0.09 and
r =  22.28 +/-  0.10 mag.
No deep i-band template images were available in the LEGACY SURVEY archive, so we do not report the measurements in this note. Further analysis is on-going.

The given magnitudes are derived based on calibrating against SDSS field stars and is not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the burst (Schlafly & Finkbeiner 2011).

We note that this source is difficult to resolve from a star (i.e. SDSS J162148.18+254546.5, r = 20.6 mag) in our images. Upon visual inspection, the star appears to have a broader profile.

We independently identified this optical counterpart candidate while drafting this report, and we noticed that this counterpart has also been reported in Levan et al., GCN 36025.

We thank the Einstein Probe team for useful communication.

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