TITLE: GCN CIRCULAR NUMBER: 40771 SUBJECT: GRB 250617B: NUTTelA-TAO early time optical observations DATE: 25/06/18 08:09:56 GMT FROM: Bruce Grossan at LBNL/UCB SSL Bruce_Grossan@lbl.gov
B. Grossan (UCB, NU) , T. Komesh (NU), Z. Maksut (NU), D. Berdikhan (NU), M. Krugov (FAI), G. F. Smoot (HKUST, UCB, NU), E. Abdikamalov (NU) report on behalf of the Energetic Cosmos Laboratory:
The Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO) observed the field of GRB 250617B on receipt of an automated GCN / BAT position alert, observing in Sloan g' and r' bands with the Burst Simultaneous Three-Channel Imager (BSTI; Grossan, Kumar & Smoot 2019, JHEA, 32, 14). We find a fading OT consistent with the position given in Palmer, 2025 (GCN 40760).
We received the alert and started observations at UT 2025-06-17 21:02:48 (58 seconds between GRB trigger time and notice receipt/response time). Stabilized images commenced at 21:03:10, 80 s after trigger. Observations were made under good conditions for more than 1300 s. The early measurements were sufficiently bright that they are visible in our short frames with no co-addition. Below, we give a selection of these measurements(irregular sampling due to changes in exposure time, camera mode,etc.) Note that these data provide essentially full-time coverage, nearly simultaneous in both bands. We report the following results:
---------------------------------------------------------------------------- g' tstart(*) tm-ttrig(s) g' err r' err exposure_time (s) ------------ ---------- ----- ------ ------ ----- ----------------- 21:03:10.4 80.5 15.29 0.06 14.75 0.09 3.0 21:03:40.4 110.5 15.54 0.10 14.90 0.10 3.0 21:04:46 176 16.12 0.09 15.69 0.04 30.0 21:09:46 476 17.40 0.10 17.02 0.07 30.0 21:15:45 835 17.87 0.12 17.67 0.08 60.0 21:18:45 1015 18.36 0.12 17.81 0.09 60.0 21:23:45 1315 UL18.72 18.00 0.11 60.0 (*) Times for r' are about 1.8 s before times for g'; times are UT June 17. "tm-ttrig" gives the mid time of the exposure minus the trigger time. The Sloan filter g', err, r', err are given in magnitudes. UL denotes no detection; 5 sigma upper limit only. Conditions at later times than given in the table showed a significant deterioriation in stability and sensitivity and so are omitted. -----------------------------------------------------
Because of the very good, quite early, time coverage thus far, we encourage observers capable of longer-term observations (12 hours+) to attempt additional g',r' (and other filters) coverage.
We caution the reader that these are preliminary results, without color or other corrections, and will likely change in small measure. Please also note that times are approximate. We welcome requests for additional data and collaboration.
---------------------------------- NU = Nazarbayev University, Nur-Sultan, Kazakhstan UCB = University of California, Berkeley, USA HKUST = Hong Kong University of Science and Technology FAI = Fesenkov Astrophysical Institute, Kazakhstan
The NUTTelA-TAO Team acknowledges the support of the staff of the Assy-Turgen Astrophysical Observatory, Almaty, Kazakhstan, and the Fesenkov Astrophysical Institute, Almaty, Kazkhstan.
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