GCN 44196: EP260324A / AT 2026hir: COLIBRÍ optical observations
2026-04-03T05:33:45.711Z | rev 0
Noémie Globus (UNAM), Sarah Antier (IJCLAB), Rosa L. Becerra (UNAM), Alan M. Watson (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Marion Guelfand (CPPM), Asuka Kuwata (UNAM), Massimiliano Lincetto (CPPM), Nikos Mandarakas (LAM), Diego López-Cámara (UNAM), Francesco Magnani (CPPM), Enrique Moreno Méndez (UNAM), Margarita Pereyra (UNAM), Ny Avo Rakotondrainibe (LAM), Fredd Sánchez Álvarez (UNAM), Benjamin Schneider (LAM) and Antonio de Ugarte Postigo (LAM) report:
We observed AT2026hir/ZTF26aapviim, the optical counterpart (Anumarlapudi et al., GCN Circ. 44120, Stein et al., GCN Circ. 44194) associated to EP260324a (Wu et al., Atel #17728) with the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-04-03 03:09:08 to 03:26:20 UTC (from 9.95 to 9.96 days after the discovery time reported in the TNS entry and from 9.84 to 9.85 days after the EP trigger) and obtained 5 minutes of exposure in the g, r, i filters, and 15 minutes of simultaneous exposure in the z filter.
In the stacked image, we detect the optical counterpart. The data were reduced and coadded with the ASU pipeline. The photometry was calibrated using nearby stars from the PanSTARRS DR1 catalog, is in the AB system, and is not corrected for Galactic extinction.
The preliminary PSF magnitudes revealed after performing image subtraction using PanSTARRS DR1 as template are:
r = 17.73 +/- 0.01,
z = 18.26 +/- 0.02.
Further observations are planned.
We thank the staff of the Observatorio Astronómico Nacional at Sierra de San Pedro Mártir, as well as the technical and engineering teams at CEA, CPPM, IRAP, LAM, OHP, OSU Pytheas, and UNAM.
COLIBRÍ is an astronomical observatory developed and operated jointly by France (AMU, CNES and CNRS) and Mexico (UNAM and SECIHTI). It is located at the Observatorio Astronómico Nacional on the Sierra de San Pedro Mártir, Baja California, Mexico.
GCN 44194: EP260324A / AT 2026hir: Spectroscopic Observations with the Next Generation Palomar Spectrograph
2026-04-03T00:03:49.568Z | rev 0
Robert Stein (UMD/JSI/NASA GSFC), Kaustav Das (Caltech), Christoffer Fremling (Caltech), Tomas Ahumada (NOIRLab), Akash Anumarlapudi (UNC CH), Ryan Chornock (UC Berkeley),Michael C. Davis (UMN), K-Ryan Hinds (Caltech), Mansi Kasliwal (Caltech), Jillian Rastinejad (UMD), Gokul Srinivasaragavan (UMD) and Yuhan Yao (UC Berkeley) report, on behalf of the ZTF collaboration:
We observed AT2026hir/ZTF26aapviim, the optical counterpart (Anumarlapudi et al, GCN #44120) to EP260324A (Wu et al., ATEL #17728), with the Next Generation Palomar Spectrograph (NGPS; AstroNote 2024-340) on the Palomar 5.1m Hale Telescope (P200). Our observations began on 2026-03-27T03:51:37 (~3 days after the EP trigger), and data were reduced using the standard NGPS pipeline.
The spectrum was extremely blue, as also noted by Zhu et al. (Astronote #2026-87). From our spectrum, in addition to the continuum, we see an excess that would be consistent with broad He II with a velocity of ~15000 km / s. However, we have not performed host galaxy subtraction, and we caution that the SNR is insufficient to be certain whether this broad feature is real.
In any case, given the nuclear location (GCN #44120) and blue spectrum which is either featureless or has broad He, our observations would be consistent with a TDE-F or TDE-He classification for this source. However, we plan for and encourage additional multi-wavelength observations to secure the classification. The transient is bright (mg=17.3 in the latest ZTF data), so remains accessible to a wide variety of ground-based telescopes.
GCN 44156: EP260324a / AT2026hir: g- and r-band optical follow-up with the PKU 60cm Telescope
2026-03-30T13:44:34.609Z | rev 0
Yacheng Kang, Zexuan Wu, Qiang Wang, Ruize Shi, Xinmiao Zhao, Zhuokai Liu, Chenxi Bao, Yiming Dong, Ziming Wang, Fangzhou Ren, Lijing Shao, Zhuo Li, Subo Dong, Xian Chen (PKU) report on behalf of the PKU HiTF (High-energy Transients Follow-up) group:
We observed the field of EP260324a (Wu et al., ATel 17728; T0 = 2026-03-24T07:04:20) using the PKU 60cm telescope at Xinglong Observatory, NAOC. Observations began on 2026 Mar 27, corresponding to ~ 3.4 days after the EP-WXT detection, and spanned two epochs.
In the stacked g- and r-band images, we detect the optical counterpart in the difference image at a position consistent with that of ZTF26aapviim / AT2026hir (Anumarlapudi et al., GCN 44120; Mohan et al., GCN 44128). Preliminary photometry is calibrated against nearby Pan-STARRS field stars and reported in the AB magnitude system, without correction for Galactic extinction.
The observation log is summarized below:
| Date | Start_UT | T_mid − T0 (days) | Filter | Exposure (s) | Magnitude (AB) |
| :--------: | :---------: | :---------------: | :----: | :----------: | :------------: |
| 2026-03-27 | 16:33:34.32 | 3.41 | g | 4 * 600 | 17.88 +/- 0.02 |
| 2026-03-27 | 17:16:14.28 | 3.44 | r | 4 * 600 | 18.22 +/- 0.03 |
Further analysis and follow-up observations are ongoing.
The PKU 60cm Telescope is operated by the Department of Astronomy, Peking University. The PKU HiTF group is dedicated to rapid follow-up observations of high-energy transients.
GCN 44128: EP260324a: GROWTH-India Telescope optical observation
2026-03-27T10:14:57.570Z | rev 0
T. Mohan (IITB), V. Swain (IITB), D. Eappachen (IIA), A.P. Saikia (IITB), S. Patil (IITB), V. Bhalerao (IITB), G.C. Anupama (IIA), S. Barway (IIA) and R. Norbu (IAO) report on behalf of the GIT team:
We observed the field of EP260324a (Wu et al., ATel 17728), with 0.7m GROWTH-India Telescope (GIT). We started the observation at 2026-03-26 18:34:45 (UTC), i.e., 2.48 days after the trigger, and obtained multiple exposures in the r' and g' filters. We detect the optical counterpart in our difference image at position reported by Anumarlapudi et al., GCN 44120. (ZTF26aapviim / [AT 2026hir](https://www.wis-tns.org/object/2026hir)):
| MJD (mid) | Filter | tmid-t0 (days) | Exposure Time (sec) | Magnitude (AB) |
| ------------ | ------ | -------------- | ------------------- | -------------- |
| 61125.7835185| g' | 2.489 | 1x300 | 18.08 +- 0.09 |
| 61125.7871875| r' | 2.493 | 1x300 | 18.34 +- 0.09 |
The measurement is calibrated against PanSTARRS DR1 (Chambers et al., 2016) and not corrected for Galactic extinction.
Our magnitude for the optical counterpart is consistent with other optical observations (Anumarlapudi et al., GCN 44120).
The GROWTH India Telescope (GIT; Kumar et al. 2022) is a 70-cm telescope with a 0.7-degree field of view, set up by the Indian Institute of Astrophysics (IIA) and the Indian Institute of Technology Bombay (IITB) with funding from DST-SERB and IUSSTF. It is located at the Indian Astronomical Observatory (Hanle), operated by IIA. We acknowledge funding by the IITB alumni batch of 1994, which partially supports the operations of the telescope. Telescope technical details are available at https://sites.google.com/view/growthindia/.
GCN 44120: EP260324a: ZTF discovery of the optical counterpart ZTF26aapviim/AT2026hir
2026-03-26T21:32:04.695Z | rev 0
Akash Anumarlapudi (UNC CH), Robert Stein (UMd), Tomas Ahumada (NOIRLab), Jillian Rastinejad (UMd), Mansi Kasliwal, Lin Yan (Caltech), D. Y. Li, C.C.Jin, W. Yuan (NAO, CAS) report, on behalf of the ZTF collaboration.
Einstein Probe detected a long-lived X-ray transient (EP260324a; Wu et al., ATel #17728) beginning on 2026-03-24T07:04:20 (hereafter 't0') that triggered the Wide-field X-ray Telescope (WXT) multiple times over the following day. Subsequent observations with the Follow-up X-ray Telescope (FXT) proposed a likely association of this transient with the galaxy SDSS J084412.34+820110.9.
Here, we report the Zwicky Transient Facility (ZTF; Bellm et al. 2019, Graham et al. 2019) discovery of the optical counterpart, ZTF26aapviim / AT2026hir. AT2026hir was detected by the ZTF survey on 2026-03-26T04:51:22, but forced photometry revealed an earlier detection at 2026-03-24T04:17:28. This first detection is ~3 hours before the first EP detection. The source thus exhibited a moderately fast rise (0.7 mag per day).
ZTF light curve of AT2026hir is provided below:
| Time (days since t0) | Band | Magnitude | Error | Limiting magnitude |
| -------- | -------- | -------- | -------- | ------- |
-0.12 | r | 20.14 | 0.15 | 20.52 |
0.07 | g | -- | -- | 17.93 |
1.91 | r | 18.71 | 0.11 | 19.42 |
2.07 | g | 18.38 | 0.08 | 19.49 |
We note that AT2026hir was also reported to TNS by ATLAS (Tonry et al. 2018), with detections beginning at 2026-03-26T07:32:13.920 (~2 days after the EP discovery).
The transient is 0.3" away from the nucleus (PS1 position) of the host galaxy identified by ATel #17728 (Wu et al.), so it may be nuclear. The host galaxy itself does not display any obvious evidence of being an AGN (W1-W2=0.2, no detection in milliquas (Flesch et al. 2023), and no previous significant ZTF detections).
We are pursuing and encouraging follow-up observations to classify the source and validate the X-ray and optical association.
Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO) and Caltech/IPAC.