tokens: and started observations in two polarization filters 60 s after notice time and 79 s after trigger time at 2015-03-01 19:39:23 ut . The very first frame showed a bright (15.3 m) optical source with the coordinates and an error of 1 arcsecond (Buckley et al. 2015 ). The Swift XRT X-ray telescope and UVOT ultraviolet telescope also detected X-ray and optical emission, respectively, from GRB 150301B 82 and 85 s after the BAT trigger (Lien et al. 2015 ). Later, 5.12 h after the burst, the Very Large Large Telescope (Paranal Observatory, Chile) and X-shooter measured the redshift of this burst, which was found to be z = 1.5169 (de Ugarte Postigo et al. 2015 ). MASTER observations. MASTER robotic telescope observed this gamma-ray burst in the alert mode in two polarization planes with variable exposures (see Section 2.2 ). As a result of our observations, the following light curve was obtained (see Fig. 1 and Table 1 ). Figure 1. GRB 150301B MASTER-SAAO light curve in two telescope tubes with different polaroid orientations (45° and 135° ). Table 1. Photometry data for GRB 150301B by MASTER in the unfiltered band with two orthogonal polarizing filters. MASTER-SAAO telescope observations of GRB 150301B lasted from 2015-03-01 19:39:23 ut through 2015-03-01 20:48:11, i.e. for about 1 h beginning with the 79th second after the trigger. Observations were then interrupted because of heavy clouds. We observed well-defined optical emission from GRB 150301B up until 2000 s after the trigger while the source remained brighter than the limiting magnitude on our frames. Unfavorable weather conditions prevented us from extending the light curve by obtaining deep stacked frames. Throughout the entire time interval from 79 to 2000 s the source faded in accordance with the power law F ∼ t −1.16 ± 0.05 which, in general, is typical of GRB that time (Mészáros 2006 ). 3.1 Spectrum from the optical to X-ray Observations of the gamma-ray burst with MASTER telescopes were performed synchronously with observations on the Swift /XRT X-ray telescope (Fig. 2 ). Optical data corrected for absorption N H = 6.2 × 10 20 cm −2 (Maselli et al. 2015 ) which is consistent with the Galactic value (Willingale et al. 2013 ). Note that variability observed in X-rays in the form of three local maxima in the 88, 191 and 282 s does not occur in the optical wavelength, that has already been observed previously (for example, Ziaeepour et al. 2008 ; Li et al. 2012 ). Figure 2. Optical, X-ray and gamma observation light curve of GRB 150301B. The Swift /XRT 0.3–10 keV and Swift /BAT 15–150 keV light curves are available from the Swift Burst Analyser (Evans et al. 2010 , http://www.swift.ac.uk/burst_analyser/ ). 3.2 Polarimetric measurements The technique of polarimetric measurements used on MASTER network telescopes allows detecting the optical emission polarization at a level of 5 per cent or higher for objects brighter than 16 m or estimating a lower limit for polarization (if the object
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