GCN 44790: EP260602a: COLIBRÍ optical observations
2026-06-03T06:45:07.121Z | rev 0
Alan M. Watson (UNAM), Marion Guelfand (CPPM), Rosa L. Becerra (UNAM), Stéphane Basa (UAR Pytheas), William H. Lee (UNAM), Edilberto Aguilar-Ruiz (UNAM), Jean-Luc Atteia (IRAP), Camila Angulo (UNAM), Dalya Akl (NYUAD), Sarah Antier (IJCLAB), Nathaniel R. Butler (ASU), Damien Dornic (CPPM), Jean-Grégoire Ducoin (CPPM), Francis Fortin (IRAP), Leonardo García García (UNAM), Ramandeep Gill (UNAM), Noémie Globus (UNAM), 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 imaged the field of EP260602a (Li et al., GCN Circ. 44770) using the DDRAGO two-channel wide-field imager on the COLIBRÍ telescope. We observed from 2026-06-03 03:38 to 05:17 UTC (from 19.45 to 21.10 hours after the trigger) and obtained 15, 30, 29, and 74 minutes of exposure in the g, r, i, and z filters, respectively.
The data were reduced and coadded with the COLIBRÍ 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.
We detect a faint source at a position consistent with that of the counterpart reported by Xu et al. (GCN Circ. 44781). This source is not seen in Pan-STARRS DR2 or Legacy Survey DR10 (Dey et al. 2019) images. The preliminary magnitudes of the source are, after subtracting the PS images to remove the 20-mag source 3.5 arcsec to the south, are:
g = 23.27 +/- 0.40
r = 23.26 +/- 0.21
i = 23.28 +/- 0.24
z = 22.98 +/- 0.39
The colors in our photometry are moderately red and do not suggest a high-redshift counterpart. Xu et al. report a very red color of VT_B - VT_R ≈ 2.0 +/- 0.3. Comparing griz photometry to VT photometry is not straightforward, but approximating VT_B ≈ g + r and VT_R ≈ i + z, we estimate much less extreme colors of VT_B - VT_R ≈ 0.13 +/- 0.32.
One possible explanation for this difference is that Xu et al. observed a high-redshift or highly-reddened afterglow which has now faded, and we are observing the host or an intervening galaxy. However, the lack of detection of a corresponding source in both Pan-STARRS and Legacy Survey images argues against this. We currently do not have an explanation for this difference.
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 44772: EP260602a: Optical upper limits with Kinder observations
2026-06-02T17:23:06.293Z | rev 0
A. Aryan, T.-W. Chen, W.-J. Hou, C.-S. Lin (all NCU), A. K. H. Kong (NTHU), S. J. Smartt, J. Gillanders (both Oxford), S. Yang (HNAS), Y.-H. Lee, A. Sankar.K, M.-H. Lee, A. Dutta, Y.-C. Pan, C.-C. Ngeow, K. N.-T. Ho, C.-H. Lai, H.-C. Lin, H.-Y. Hsiao, J.-K. Guo (all NCU), Z. N. Wang, D. C. Qiang, L. L. Fan (all HNAS), Y. J. Yang (NYUAD), H.-W. Lin (UMich), H. F. Stevance, S. Srivastav, K. W. Smith, L. Rhodes (all Oxford), M. Nicholl, M. Fulton, C. Angus, A. Aamer (all QUB), T. Moore (STScI), A. Schultz and M. Huber (both IfA, Hawaii) report:
We observed the field of the FXT EP260602a (Li et al., GCN 44770) using the 40cm Seisi Lulin Telescope (SLT) at the Lulin observatory, as part of the Kinder collaboration (Chen et al. 2025, ApJ, 983, 86, doi:10.3847/1538-4357/adb428). The first SLT epoch of observations in r-band started at 15:31 UTC on the 2nd of June 2026 (MJD 61140.5068), 7.34 hours after the EP-WXT detection.
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. We utilized the Python-based package AutoPhOT (Brennan & Fraser, 2022, A&A, 667, A62) to perform template subtraction with the DESI Legacy Survey (Dey et al. 2019, AJ 157, 168) DR10 image using the 'SFFT' (Hu et al. 2022, ApJ, 936, 157) algorithm. Neither in the stacked nor in the difference image did we detect any signature of any new or uncatalogued source.
Moreover, we further used AutoPhOT to perform PSF photometry. The details of the observations and the 3-sigma upper limit (in the AB system) are as follows:
Telescope | Filter | MJD (start) | t-t0 (hr) | Exposure (s) | Magnitude| avg. Seeing | Med. Airmass SLT | r | 61140.5068 | 7.34 | 300 * 6 | >20.0 | 1".77 | 1.13
The presented upper limit is calibrated using the field stars from the PanSTARRS catalog and is not corrected for an expected galactic extinction of A_r = 0.06 mag in the direction of the transient (Schlafly & Finkbeiner 2011). The methodology, details on the Lulin observatory telescopes, and a compilation of our optical follow-up campaign for FXTs discovered within the first year of operation of the Einstein-Probe mission are presented in Aryan et al. 2025, ApJS, 281, 20, doi:10.3847/1538-4365/adfc69.
GCN 44770: EP260602a: Einstein Probe detection of a fast X-ray transient
2026-06-02T16:35:29.297Z | rev 0
Z. X. Li, G. L. Huang, J. Y. Cao (IHEP, CAS), Y. Wang (PMO, CAS; UCB), G. J. Yang, W. D. Zhang (NAO, CAS) on behalf of the Einstein Probe (EP) team:
We report on the detection of a fast X-ray transient by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission, designated EP260602a. The transient triggered EP-WXT (ID: 01709266359) at 2026-06-02T08:11:20 (UTC). The WXT position of the source is R.A. = 240.886 deg, DEC = 50.563 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The WXT spectrum can be fitted with an absorbed power-law, with the absorption fixed at the Galactic value of 2.03 x 10^20 cm^-2, and a photon index of 1.7 (-0.5, +0.5). The unabsorbed 0.5-4 keV flux is 9.0 (-2.4, +3.2) x 10^-11 erg/s/cm2.
A follow-up observation with the Follow-up X-ray Telescope (FXT) onboard EP is scheduled.
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).