TITLE: GCN CIRCULAR NUMBER: 5901 SUBJECT: GRB 060926: MASTER: discovery of the optical flare DATE: 06/12/07 17:36:35 GMT FROM: Vladimir Lipunov at Moscow State U/Krylov Obs V. Lipunov, V.Kornilov, D.Kuvshinov, N.Tyurina, A.Belinski, E.Gorbovskoy, A.Krylov, G.Borisov, A.Sankovich, V.Vladimirov, P.Gritsyk Sternberg Astronomical Institute, Moscow Union 'Optic' MASTER robotic system (http://observ.pereplet.ru) responded to GRB060926 (Holland et al., GCN Circ 5612) under the good conditions (V. Lipunov et al., GCN Circ 5632). The first image was started at 2006-09-26 16:49:57 UT, 76 s after the GRB time (355 mm telescope, 6 square degrees FOV, 2.1'' per pix, CCD Alta U16). The unfiltered image is calibrated relative to USNO A2.0 (0.8 R + 0.2 B). We find (Lipunov et al., GCN5632) faint OT on the first and on the coadded images at position: aplha = 17 35 43.66 dec = 13 02 18.3 err = +- 0.7'' wich coincided with the Holland et al.(GCN Circ 5612) OT position. We have refind analysis of the images. We discovery optical flare around 500-700 s after the GRB time (see picture): Start Mean Exposure Magnitude Flux, time time time erg/(sm2*sec*eV) 76 s 91 s 30 s 17.3+-0.3 (1,4+-0,3)E-13 150 s 165 s 30 s 18.5+-0.3 (4,6+-1,1)E-14 165 s 343 s 5x30 s 19.3+-0.3 (2,2+-0,5)E-14 255 s 432 s 5x30 s 18.9+-0.3 (3,2+-0,8)E-14 343 s 519 s 5x30 s 18.5+-0.3 (4,6+-1,1)E-14 432 s 608 s 5x30 s 18.3+-0.3 (5,8+-1,3)E-14 519 s 707 s 5x30 s 18.4+-0.3 (5,1+-1,2)E-14 608 s 804 s 5x30 s 18.7+-0.3 (3,9+-0,9)E-14 707 s 1001 s 5x30 s 20.0+-0.3 (1,2+-0,3)E-14 804 s 1200 s 5x30 s 20.1+-0.3 (1,1+-0,3)E-14 901 s 1298 s 5x30 s > 20.1+-0.3 <(1,1+-0,3)E-14 The light curve is available at http://observ.pereplet.ru/images/GRB060926/light_curve_new.jpg . Between 91s and 255s a power-law decline with a temporal index of -1.40+/-0.24 is estimated. Between 707s and 1200s a power-law decline with a temporal index of -3.30+/-0.70 is estimated. After 1000 s a power-law decline with a temporal index 0.73 +-0.1 was observed (A. Stefanescu et al., GCN 5623). We remember that X-ray flare in GRB 060926 was descovered by XRT team (M. Perri et al., GCN 5622). The X-ray spectrum covering the time period from T+67s to T+878s is well fit by an absorbed power-law with a photon index of 2.1(+/-0.3) and column density of (2.2+/-0.9)e21 cm**-2 (M. Perri et al., GCN 5622). They note the Galactic column density in the direction of the source is 7.3e20 cm**-2 . This means that absorbtion is about 1 magnitude in our band. The optical-xray data is well fit by power-low with a photon index of 1.7+-0.2 during all our time observation. We thanks to Dr.K.A.Postnov for usefull discussion and comments. This work is supported by RFFI 04-02-16411 grant. This message can be cited. Mailto: lipunov@sai.msu.ru