GCN 44087: EP260321a: Early TRT rising followed by JinShan decaying and then re-rising

X. Liu, D. Xu (NAOC), S. Tinyanont, R. Anutarawiramkul, P. Butpan, K. Noysena (NARIT), J. An, Z.P. Zhu, N.C. Sun, W.X. Li, Z. Fan, S.Q. Jiang, L.B. He (NAOC), S.Y. Fu (HUST), J.Z. Liu (XAO) report on behalf of a large group:

Upon the alert of EP260321a detected by Einstein Probe (Huang et al., GCN 44068), we observed the field of EP260321a using the 0.7-m telescope of the Thai Robotic Telescope network (TRT), located at New South Wales, Australia (SBO), and then using the 100B telescope of the JinShan project, located at Altay, Xinjiang, China.

The TRT observation started at 12:52:13 UT on 2026-03-21, i.e., 21.9 mins after the EP-WXT trigger. No uncatalogued optical transient is detected within the EP-FXT error cicle (Huang et al., GCN 44068) down to R ~ 20.5 mag.

We noticed the brightening of the catalogued blue point-like source (R.A. 09:59:42.880, Dec. 00:25:06.29, J2000), if compared with PanSTARRS and Legacy Survey, in the outskirt of the galaxy, SDSS J095942.88+002506.2, with a redshift z = 0.0343 in the DESI Legacy Survey DR1 spectroscopic catalogue, mentioned by Kinder (Lee et al., GCN 44070) and then spectroscopically confirmed at VLT/MUSE (Tanvir et al., GCN 44082). The point source has R ~ 19.0 (Vega) at a median time of 27.6 mins post-trigger, calibrated with PanSTARRS in which the source has r = 19.85 +/- 0.03 mag (AB).

The JinShan 100B telescope observed the field in g- and r- bands in the nights of 2026-03-21 and 2026-03-22.

In the first night, the blue source decayed a bit to g ~ 18.9 (AB and hereafter) at 2.3 hrs post-trigger and r ~ 19.3 mag at 2.8 hrs post-trigger. And the g-band brightening, if subtracted using the PanSTARS templates, is more prominent than in the r-band, suggesting the transient itself is also quite blue. Together with the rather deep ZTF pre-EP upper limit (Ahumada et al., GCN 44084), the TRT observation, and the Kinder first observation (Lee et al., GCN 44070), the transient shows a fast rising around the EP trigger time and then starts to decay around the first Kinder observation.

The decaying behavior is consistent with the cooling of Shock Break-Out (SBO) of a SN-like event (Huang et al., GCN 44075) and a thermal-like spectrum at VLT/MUSE (Tanvir et al., GCN 44082).

In the second night, the blue source re-brightened to g ~ 18.5 at 28.5 hrs post-trigger and r ~ 18.8 mag at 29.0 hrs post-trigger, which are also brighter than the second Kinder observation that is a bit earlier than the second JinShan (Aryan et al. GCN 44081).

This indicates that a SN-like main component is emerging and will likely continue to brighten to a peak in the coming days.