Prompt, early and afterglow optical observations of five γ-ray bursts: GRB 100901A, GRB 100902A, GRB 100905A, GRB 100906A and GRB 101020A

E. S. Gorbovskoy 2 G. V. Lipunova 2 V. M. Lipunov 2 V. G. Kornilov 2 A. A. Belinski 2 N. I. Shatskiy 2 N. V. Tyurina 2 D. A. Kuvshinov 2 P. V. Balanutsa 2 V. V. Chazov 2 A. Kuznetsov 2 D. S. Zimnukhov 2 M. V. Kornilov 2 A. V. Sankovich 2 A. Krylov 2 K. I. Ivanov 1 O. Chvalaev 1 V. A. Poleschuk 1 E. N. Konstantinov 1 O. A. Gress 1 S. A. Yazev 1 N. M. Budnev 1 V. V. Krushinski 0 I. S. Zalozhnich 0 A. A. Popov 0 A. G. Tlatov 5 A. V. Parhomenko 5 D. V. Dormidontov 5 V. Senik 5 V. V. Yurkov 4 Yu. P. Sergienko 4 D. Varda 4 I. P. Kudelina 4 A. J. Castro-Tirado 3 J. Gorosabel 3 R. S anchez-Ramrez 3 M. Jelinek 3 J. C. Tello 3 0 Kourovka Astronomical Observatory, Physical Department of Ural State University , pr. Lenina 51, Ekaterinburg 620083 , Russia 1 Irkutsk State University , ul. Karla Marxa 1, Irkutsk 664003 , Russia 2 Moscow MV Lomonosov State University, Sternberg Astronomical Institute , Moscow 119992 , Russia 3 Instituto de Astrof sica de Andaluc a (IAA-CSIC) , Glorieta de la Astronom a s/n, 18008 Granada , Spain 4 Blagoveschensk State Pedagogical University , ul. Lenina 104, Amur Region, Blagoveschensk 675000 , Russia 5 Kislovodsk Solar Station of the Pulkovo Observatory RAS , PO Box 45, ul. Gagarina 100, Kislovodsk 357700 , Russia A B S T R A C T We present the results of the prompt, early and afterglow optical observations of five - ray bursts (GRBs): GRB 100901A, GRB 100902A, GRB 100905A, GRB 100906A and GRB 101020A. These observations were made with the Mobile Astronomical System of TElescopeRobots in Russia (MASTER-II Net), the 1.5-m telescope of the Sierra Nevada Observatory and the 2.56-m Nordic Optical Telescope. For two sources, GRB 100901A and GRB 100906A, we detected optical counterparts and obtained light curves starting before the cessation of -ray emission, at 113 and 48 s after the trigger, respectively. Observations of GRB 100906A were conducted in two polarizing filters. Observations of the other three bursts gave the upper limits on the optical flux; their properties are briefly discussed. A more detailed analysis of GRB 100901A and GRB 100906A, supplemented by Swift data, provides the following results and indicates different origins for the prompt optical radiation in the two bursts. The light-curve patterns and spectral distributions suggest that there is a common production site for the prompt optical and high-energy emission in GRB 100901A. The results of the spectral fits for GRB 100901A in the range from optical to X-ray favour power-law energy distributions and a consistent value of the optical extinction in the host galaxy. GRB 100906A produced a smoothly peaking optical light curve, suggesting that the prompt optical radiation in this GRB originated in a front shock. This is supported by a spectral analysis. We have found that the Amati and Ghirlanda relations are satisfied for GRB 100906A. We obtain an upper limit on the value of the optical extinction on the host of GRB 100906A. telescopes - gamma-ray burst; general - gamma-ray burst; individual; GRB 100901A - gamma-ray burst; individual; GRB 100906A - Since 1997, when the optical radiation of -ray bursts (GRBs) was first detected, we have known that these are the most energetic events in the Universe (Kulkarni et al. 1998). Optical emission that is observed hours after a GRB is attributed to the so-called afterglow, which is the result of a shock propagating outward in the surrounding media (Meszaros & Rees 1997). The characteristics of such an emission are defined mainly by conditions in the interstellar medium (ISM) and the amount of released energy, but they depend weakly on the details of the central burst. The nature of GRBs and their emission mechanisms are not completely understood, and more observational data and model analysis are necessary for further understanding. An acknowledged model is that a GRB is a manifestation of the formation of a rotating black hole (or another compact relativistic object) in the course of gravitational collapse. An engine converts the energy of the collapse into emissions of different types, among which there is a high-energy emission that is produced for up to several tens of seconds. The afterglow emission is detected long afterwards, whether the engine is still working or not, which is uncertain. In order to better understand the details of the process, it is necessary to observe the main event itself, at different wavebands, while the engine is at its most active stage. However, it is a challenge to observe prompt optical emission because a GRB usually lasts no more than several tens of seconds. Although the prompt optical observations of GRBs were first carried out in 1998 by Akerlof et al. (2000), successful prompt optical detections remain rare. Evidently, there are two approaches: to observe extensive sky fields and wait for a GRB to occur or to use special robotic telescopes that can be ready to point anywhere with an alert from an orbital -ray observatory (the alert observations technique). The Mobile Astronomical System of TElescope-Robots in Russia (MASTER-II1) uses both techniques (Lipunov et al. 2010). Our paper is dedicated to the alert MASTER observations of five GRBs in Siberia, Ural and North Caucasus. Alert ground-based optical observations of GRBs are a new global physical experiment, which have been available since the last decade. They are made possible because of the implementation of the global Internet network, powerful personal computers and fast optical CCD receivers. The challenge is to quickly accomplish the four following steps as early as possible. (i) A GRB is detected by a -ray telescope on board a spacecraft, such as Swift (Gehrels et al. 2004), Fermi (Atwood et al. 2009), INTEGRAL (Winkler et al. 2003), etc. (ii) After the onboard processing is finished, the location of a GRB is sent to the GRB Coordinates Network (GCN) at the National Aeronautics Space Administration (NASA). The first two steps take 1040 s. (iii) The burst position is then disseminated to ground-based robotic telescopes through the Internet network (in about 0.5 s). (iv) The robotic telescopes are scheduled and pointed to the received positions, which takes 740 s for moderate-size instruments (less than 0.5 m) and from several minutes to hours for 2-m and larger instruments. Whereupon, imaging is performed in the optical and infrared bands. The first Russian robotic telescope MASTER came into operation in 2002 near Moscow, with the help of private funding from 1 The MASTER web site is http://observ.pereplet.ru. the Moscow association Optics.2 Construction of the all-Russia network MASTER began in 2008 (Lipunov et al. 2010). At present, the telescopes of the MASTER-Net are located in the observatories of Moscow State University (in Kislovodsk), Ural State University (in Kourovka), Irkutsk State University (in Tunka near Baikal Lake) and the Blagoveschensk Pedagogical University (in the Blagoveschensk region). These observat (...truncated)