TITLE: GCN CIRCULAR NUMBER: 45295 SUBJECT: SN 2026stf / GRB 260708A: ATLAS forced photometry DATE: 26/08/06 00:07:04 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias I. Pérez-Fournon (IAC and ULL), F. Poidevin (IAC and ULL), A. López-Oramas (IAC and ULL), D. Aguado (IAC and ULL), N. Abreu García (ULL), D. Cano-Morales (ULL), I. Correa-Plasencia (ULL), A.E. Hernández-Díaz (ULL), E. Lekaroz-Urriza (ULL), M. Magdalena-Rodríguez (ULL), M. Quintana-Ansaldo (ULL), D. Rodríguez-Fernández (ULL), and J. Uríbarri-Dermit (ULL) We report results of ATLAS forced photometry of the type Ic-BL SN 2026stf at z = 0.055 (Corcoran et al., GCN 45283) which appears associated with the LONG GRB 260708A detected by Fermi-GBM, Fermi-LAT, GECAM, and Glowbug (Fermi-GBM Team, GCN 45115; Holzmann Airasca et al., GCN 45117; Malacaria, GCN 45124; Ren et al., GCN 45126; Cheung et al., GCN 45130) and EP/FXT (Li et al., GCN 45150). The optical counterpart (AT 2026stf, MASTER OT J150237.02-435307.7) was discovered by MASTER (Shilova et al., GCN 45123; and Zhirkov et al., TNS Astronomical Transient Report No. 313535), with other reported optical detections (Freeberg et al., GCN 45127, Shi et al., GCN 45140, Sheng et al. GCN 45147, Shi et al., GCN 45280, Corcoran et al., GCN 45283, and O'Neill et al., TNS photometry report). Using the ATLAS forced-photometry server (Shingles et al. 2021, Transient Name Server AstroNote 2021-7), we find four detections of the fading afterglow in the ATLAS orange filter starting on MJD = 61230.82579, at about 1.40 days after the Fermi trigger, at an ATLAS orange magnitud of o = 18.567 +/- 0.106, and more detections close to the minimum of the light curve and in 10 more epochs that include the rebrightening in the SN Ic-BL phase. The last ATLAS non detection was on MJD = 61228.84782, about 0.58 days before the Fermi trigger. The following table gives the ATLAS forced photometry since MJD = 61200. We note that according to the methods of the ATLAS forced photometry, if the flux is negative, then a magnitude is still quoted and denoted with a negative sign (see https://fallingstar-data.com/forcedphot/resultdesc/). Magnitudes are in the AB system and are not corrected for interstellar medium absorption. Fluxes are in uJy. MJD. | mag | e_mag |flux |e_flux| filter| observation ---------------------------------------------------------------------------------------------- 61203.257379 18.634 0.491 128.0 63 o 04a61203o0384o 61203.260575 -18.648 0.712 -126.0 90 o 04a61203o0391o 61203.285519 20.077 0.581 34.0 20 o 04a61203o0444o 61203.29178 20.792 1.566 18.0 27 o 04a61203o0457o 61206.11809 21.107 4.412 13.0 58 o 04a61206o0248o 61206.12176 -18.955 0.647 -95.0 62 o 04a61206o0256o 61206.127177 -19.91 1.685 -39.0 66 o 04a61206o0268o 61206.137231 -18.837 0.787 -106.0 83 o 04a61206o0290o 61208.003738 -21.432 0.984 -10.0 10 c 03a61208o0094c 61208.006902 21.842 1.825 7.0 12 c 03a61208o0101c 61208.03234 -22.219 2.429 -5.0 11 c 03a61208o0156c 61208.037748 -23.402 7.111 -2.0 11 c 03a61208o0168c 61215.101062 20.405 0.87 25.0 22 o 04a61215o0341o 61215.106041 -20.875 1.571 -16.0 26 o 04a61215o0352o 61215.109687 21.063 1.366 14.0 19 o 04a61215o0360o 61215.116029 20.644 1.086 20.0 22 o 04a61215o0374o 61215.125947 23.114 8.294 2.0 17 o 04a61215o0396o 61220.18041 20.739 2.089 18.0 38 o 04a61220o0405o 61220.212007 -19.219 0.46 -74.0 34 o 04a61220o0473o 61220.215569 -20.976 2.384 -15.0 35 o 04a61220o0480o 61220.244229 -20.8 2.407 -17.0 42 o 04a61220o0541o 61221.106872 -20.685 1.464 -19.0 28 o 04a61221o0347o 61221.110508 19.599 0.571 52.0 30 o 04a61221o0355o 61221.114247 19.799 0.677 44.0 30 o 04a61221o0363o 61221.119822 19.554 0.429 55.0 24 o 04a61221o0375o 61221.129752 20.192 1.137 30.0 35 o 04a61221o0394o 61224.134068 17.646 2.756 317.0 875 o 04a61224o0172o 61224.137394 17.479 1.786 370.0 661 o 04a61224o0179o 61224.143895 -18.04 2.122 -221.0 468 o 04a61224o0193o 61224.153723 14.528 0.353 5608.0 1982 o 04a61224o0214o 61224.850689 -22.279 2.743 -4.0 12 o 03a61225o0364o 61224.854295 20.341 0.58 26.0 15 o 03a61225o0372o 61224.860177 20.367 0.6 26.0 16 o 03a61225o0385o 61224.870545 -21.838 2.262 -7.0 15 o 03a61225o0408o 61225.953387 -23.99 15.857 -1.0 15 o 03a61226o0036o 61225.957069 21.685 1.891 8.0 14 o 03a61226o0044o 61225.962523 20.444 0.561 24.0 14 o 03a61226o0056o 61225.982975 20.19 0.653 30.0 20 o 03a61226o0100o 61227.828562 21.292 0.881 11.0 10 o 03a61228o0314o 61227.831716 22.195 2.425 5.0 12 o 03a61228o0321o 61227.837861 21.672 1.562 8.0 12 o 03a61228o0334o 61227.847773 0.0 21.251 -0.0 12 o 03a61228o0356o 61228.828314 20.573 0.548 21.0 12 o 03a61229o0313o 61228.831947 23.404 7.462 2.0 12 o 03a61229o0321o 61228.837418 -21.735 1.435 -7.0 10 o 03a61229o0333o 61228.847822 23.144 4.533 2.0 9 o 03a61229o0356o 61230.825786 18.567 0.106 136.0 14 o 03a61231o0303o 61230.829421 18.463 0.089 150.0 13 o 03a61231o0311o 61230.83483 18.536 0.1 140.0 14 o 03a61231o0323o 61230.847461 18.6 0.098 132.0 13 o 03a61231o0351o 61233.132315 19.314 0.21 68.0 14 o 04a61233o0234o 61233.152559 19.689 0.312 48.0 15 o 04a61233o0277o 61233.155718 19.645 0.277 50.0 14 o 04a61233o0284o 61233.168334 19.367 0.185 65.0 12 o 04a61233o0311o 61234.819699 19.127 0.166 81.0 14 o 03a61235o0253o 61234.822959 19.42 0.243 62.0 15 o 03a61235o0260o 61234.828055 19.497 0.231 58.0 13 o 03a61235o0271o 61234.840874 19.112 0.161 82.0 13 o 03a61235o0299o 61236.79229 18.959 0.239 95.0 23 o 03a61237o0226o 61236.795085 19.905 0.853 40.0 34 o 03a61237o0232o 61236.801591 19.14 0.382 80.0 31 o 03a61237o0245o 61236.811886 18.648 0.173 126.0 22 o 03a61237o0267o 61237.809594 19.061 0.123 86.0 11 o 03a61238o0262o 61237.812764 19.156 0.163 79.0 13 o 03a61238o0269o 61237.8201 19.148 0.147 80.0 12 o 03a61238o0285o 61237.830145 19.069 0.121 86.0 10 o 03a61238o0307o 61240.79296 19.048 0.147 87.0 13 o 03a61241o0092o 61240.796119 19.057 0.157 86.0 14 o 03a61241o0099o 61240.801526 18.923 0.138 98.0 14 o 03a61241o0111o 61240.835558 19.297 0.213 69.0 15 o 03a61241o0184o 61242.787705 18.836 0.173 106.0 18 o 03a61243o0213o 61242.793772 18.852 0.164 104.0 17 o 03a61243o0226o 61242.796932 18.821 0.185 108.0 20 o 03a61243o0233o 61242.807162 18.875 0.223 102.0 23 o 03a61243o0255o 61243.802909 18.507 0.214 144.0 31 o 03a61244o0135o 61243.851004 18.728 0.234 117.0 28 o 03a61244o0151o 61243.856046 18.883 0.273 102.0 28 o 03a61244o0162o 61243.868817 18.547 0.206 138.0 28 o 03a61244o0190o 61245.908433 19.092 0.268 84.0 22 o 03a61246o0447o 61245.929595 18.977 0.271 93.0 25 o 03a61246o0492o 61245.935043 19.026 0.263 89.0 23 o 03a61246o0504o 61245.959763 19.081 0.37 85.0 31 o 03a61246o0558o 61247.804195 19.022 0.28 89.0 25 o 03a61248o0239o 61247.807349 18.81 0.239 109.0 26 o 03a61248o0246o 61247.812913 18.911 0.292 99.0 29 o 03a61248o0258o 61247.823386 19.47 0.493 59.0 29 o 03a61248o0281o 61249.247796 19.688 0.712 48.0 34 o 01a61249o0018o 61249.251905 18.651 0.255 126.0 32 o 01a61249o0027o 61249.2583 18.647 0.246 126.0 31 o 01a61249o0041o 61249.274816 18.898 0.294 100.0 29 o 01a61249o0077o 61253.787954 18.938 0.185 96.0 18 o 03a61254o0205o 61253.791569 18.882 0.144 102.0 15 o 03a61254o0213o 61253.797995 18.984 0.164 92.0 15 o 03a61254o0227o 61253.807829 19.042 0.217 88.0 19 o 03a61254o0248o 61256.862169 -20.237 10.875 -29.0 317 o 03a61257o0091o 61256.871251 -16.142 1.031 -1268.0 1308 o 03a61257o0111o This work is part of grant CEX2025-001609-S, awarded to the Instituto de Astrofísica de Canarias under the Severo Ochoa Centre of Excellence program and funded by MICIU/AEI/10.13039/501100011033. This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5).