GRB 201015A: From seconds to months of optical monitoring and supernova discovery

We present full photometric coverage and spectroscopic data for soft GRB 201015A with a redshift z = 0.426. Our data spans a time range of 85 days following the detection of GRB. These observations revealed an underlying supernova SN 201015A with a maximum at 8.54 ±1.48 days (rest frame) and an optical peak absolute magnitude $-19.45_{-0.47}^{+0.85}$ mag. The supernova stands out clearly, since the contribution of the afterglow at this time is not dominant, which made it possible to determine SN's parameters. A comparison of these parameters reveals that the SN 201015A is the earliest (the minimum T<SUB>max</SUB>) known supernova associated with gamma-ray bursts. Spectroscopic observations during the supernova decay stage showed broad lines, indicating a large photospheric velocity, and identified this supernova as a type Ic-BL. Thus, the SN 201015A associated with the GRB 201015A becomes the 27th SN/GRB confirmed by both photometric and spectroscopic observations. Using the results of spectral analysis based on the available data of Fermi-GBM experiment, the parameters E<SUB>p, i</SUB> = 20.0 ± 8.5 keV and E<SUB>iso</SUB> = (1.1 ± 0.2) × 10<SUP>50</SUP> erg were obtained. According to the position of the burst on the E<SUB>p, i</SUB>-E<SUB>iso</SUB> correlation, GRB 201015A was classified as a Type II (long) gamma-ray burst, which was also confirmed by the T<SUB>90, i</SUB>-EH diagram.