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ISSN 1063-7729, Astronomy Reports, 2007, Vol. 51, No. 12, pp. 1004?1025. c Pleiades Publishing, Ltd., 2007. Original Russian Text c V.M. Lipunov, V.G. Kornilov, A.V. Krylov, N.V. Tyurina, A.A. Belinskii, E.S. Gorbovskoi, D.A. Kuvshinov, P.A. Gritsyk, G.A. Antipov, G.V. Borisov, A.V. Sankovich, V.V. Vladimirov, V.I. Vybornov, A.S. Kuznetsov, 2007, published in Astronomicheski Zhurnal, 2007, Vol. 84, No. 12, pp. 1110?1134. i

Optical Observations of Gamma-Ray Bursts, the Discovery of Supernovae 2005bv, 2005, and 2006ak, and Searches for Transients Using the "MASTER" Robotic Telescope
V. M. Lipunov1 , V. G. Kornilov1 , A. V. Krylov1 , N. V. Tyurina1, A. A. Belinskii1 , E. S. Gorbovskoi2, D. A. Kuvshinov2 , P. A. Gritsyk2 , G. A. Antipov3 , G. V. Borisov1, A. V. Sankovich3, V. V. Vladimirov3 , V. I. Vybornov2 , and A. S. Kuznetsov2
1

Sternberg Astronomical Institute, Moscow State University, Moscow, Russia 2 Moscow State University, Moscow, Russia 3 The Moscow Association "Optika", Moscow, Russia
Received April 24, 2007; in final form,

Abstract--We present the results of observations obtained using the MASTER robotic telescope in 2005? 2006, including the earliest observations of the optical emission of the gamma-ray bursts GRB 050824 and GRB 060926. Together with later observations, these data yield the brightness-variation law t-0.55+0.05 for GRB 050824. An optical flare was detected in GRB 060926--a brightness enhancement that repeated the behavior observed in the X-ray variations. The spectrum of GRB 060926 is found to be FE E - ,where = 1.0 + 0.2. Limits on the optical brightnesses of 26 gamma-ray bursts have been derived, 9 of these for the first time. Data for more than 90% of the accessible sky down to 19m were taken and reduced in real time during the survey. A database has been composed based on these data. Limits have been placed on the rate of optical flares that are not associated with detected gamma-ray bursts, and on the opening angle for the beams of gamma-ray bursts. Three new supernovae have been discovered: SN 2005bv (type Ia)--the first to be discovered on Russian territory, SN 2005ee--one of the most powerful type II supernovae known, and SN 2006ak (type Ia). We have obtained an image of SN 2006X during the growth stage and a light curve that fully describes the brightness maximum and exponential decay. A new method for searching for optical transients of gamma-ray bursts detected using triangulation from various spacecraft is proposed and tested. PACS numbers : 95.55.Cs, 95.75.De, 95.75.Rs, 95.85.Kr, 98.70.Rz, 97.60.Bw, 98.70.Qy DOI: 10.1134/S1063772907120050

1. INTRODUCTION The construction of robotic telescopes, which not only automatically acquire but also automatically process images and choose observing strategies, is a new and vigorously developing area in modern astronomy. MASTER (Mobile Astronomy System of TElescope Robots), the first robotic telescope in Russia, began to be created through the efforts of scientists at the Sternberg Astronomical Institute of Moscow State University and the Moscow "Optika" Association in 2002, and continues to be developed to the present [1, 2]. In its current form, the system has four parallel telescopes on an automated equatorial mount, capable of slewing at a rate of up to 6/s (located near Domodedovo, Moscow region), and two very-wide-field cameras with separate mounts and domes, with one located on the Mountain Solar Station of the Pulkovo Observatory (near Kislavodsk),

roughly 1500 km from the other (which is also near Domodedovo). The parameters of the telescopes and CCD arrays are given in Table 1. The system whose characteristics are closest to the MASTER system (http://observ.perep-let.ru) is the American ROTSE-III system [3] (http:// www.rotse.net). MASTER (Fig. 1) differs in its larger field of view and the presence of several telescopes on a single axis, which makes it possible to obtain images at several different wavelengths simultaneously. The main telescope, Telescope 1 (355 mm diameter, a modified Richter?Slevogt system initially invented by V. Yu. Terebizh) takes images in white light, and is the main search element of the system. An Apogee Alta U16 (4096 ' 4096 pixels) is installed on this telescope, making it possible to obtain images in a six square degree field. A Sony video recorder is mounted on Telescope 2 (200 mm diameter, a Richter?Slevogt system constructed by G. V. Borisov), providing

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THE "MASTER" ROBOTIC TELESCOPE Table 1. Telescopes and receivers of the MASTER system in Domodedovo Telescope 1 2 3 4 5 Optical system Richter?Slevogt Richter?Slevogt Flugge Wright Wide-field camera Diameter, mm 355 200 280 200 25 Light-gathering power F/2.6 F/2.6 F/2.5 F/4 F/1.2 CCD array Apogee U16E Sony LCL 902K Pictor-416 SBIG ST-10XME Foreman Electronics FE-285, Sony ICX 285AL Field of view 2.4 ' 2.4 1 ' 0.7 1 ' 0.7 1 ' 0.7 30 ' 40


1005

Format, Mpixels 16 0.4 0.4 3.2 1.4

images to 13m -14m with a time resolution of 0.05 s. A prism is mounted at the focus of Telescope 3 (280 mm diameter, a Flugge system), providing spectra of objects to 13m in a 30 ' 40 field of view ? with a resolution of 50 A (with a Pictor-416 camera). Telescope 4 (200 mm diameter, a Wright system constructed by A. V. Sankovich) is equipped with a filter cassette and SBIG ST-10XME camera. In addition, MASTER has a very-wide-field camera (50 ' 60 ) that covers the field of view of the HETE orbiting gamma-ray telescope, making it possible to obtain simultaneous observations with HETE to 9m using a separate automated scheme. This widefield equipment enables searches for bright, transient objects. In the Summer of 2006, we installed a widefield camera (MASTER-VWF-Kislovodsk) on the Mountain Solar Station of the Main Astronomical Observatory in Pulkovo, making it possible to continuously monitor a 420-square-degree field of sky to 13m in a five second exposure. Thus, we currently have three automated miniobservatories with the following equipment: --Six telescopes and CCD arrays --Three automated equatorial mounts --Three automated domes --Two cloud-cover and temperature sensors --One GPS receiver --Six control and reduction computers The Kislovodsk and Domodedovo systems are connected via the Internet, and are able to respond to the detection of uncataloged objects (optical transients) within several tens of seconds (including processing time). The results of observations using the MASTER network will be reported separately. MASTER is able to operate in a fully automated regime: automatically, based on the ephemerides (sunset) and the presence of satisfactory weather conditions (the control computer is continuously attached to a weather sensor), the roof (above the main mount and wide-field camera) is opened, the
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telescope is pointed at bright stars and pointing corrections introduced, and, depending on the seeing, it then either goes into a standby regime or begins a survey of the sky using a specialized, fully automated programme. Thus, observations are conducted in two regimes: survey and "alert" (e.g. observations of the locations of gamma-ray bursts based on coordinates obtained). In the former case, the telescope automatically takes three frames of an arbitrary region in succession, with exposures from 30 to 60 s, moves to a neighboring region 2 away and carries out the same procedure, and so on, repeating a given set of three frames every 40?50 min. This makes it possible to avoid artefacts in the data processing, and to locate moving objects. The alert regime is supported by a continuous connection between the control computer and the GCN international gamma-ray burst (GRB) network [4] (http://gcn.gsfc.nasa.gov). After detection of a GRB by a space gamma-ray observatory (SWIFT, HETE, Konus-Wind, INTEGRAL etc.), the telescope obtains the coordinates of the burst region (the socalled coordinate error box), automatically points to this direction, obtains an image of this region, reduces this image, and identifies all objects not present in the computer catalogs. If a GRB is detected during the day, its coordinates are included in the observing program for the next night. A special program package for image reduction in real time has been created, making it possible not only to carry out astrometry and photometry of a frame, but to recognize objects not contained in astronomical catalogs: supernovae, new asteroids, optical transients, and so forth. Over the entire time observations have been obtained on the MASTER system (see the results for 2002?2004 in [1, 2]), images have been obtained for 52 GRB error boxes. In 23 cases, these observations were the first in the world. In three cases, optical emission was detected (this was the earliest detection in Europe for GRB 030329, and the earliest in the world for the two cases reported here).


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4

1

3 2

Fig. 1. The MASTER wide-field robotic telescope in its observatory near Domodedovo (January 2006).

Note that, from February through August 2006, the main matrix of the search telescope was being repaired, so that the sky survey essentially was not conducted during this time. 2. OBSERVATIONS OF GRBS IN 2005?2006 Starting at the beginning of 2005 through October 2006, the Domodedovo MASTER station carried out observations of 31 GRBs (Table 2). In 16 cases, we obtained the first upper limits on the optical fluxes of the GRBs, i.e., fluxes brighter than which no optical candidate for the GRB was detected. Note that, before 2005, only a few alerts for GRBs in the Moscow night-time sky in regions of sky accessible to MASTER were received from the SWIFT orbiting observatory, which provided more than 90% of all detected GRBs in 2005, with one of these occurring during rainy weather. Nevertheless, we were able to report the first optical detections in the world for two of these GRBs. Unless otherwise indicated, we present instrumental magnitudes in white light. Our photometry was carried out in an automated regime using objects in the frame identified with stars in the USNO-A2.0 catalog [51] (there were usually about 2000 stars in

a frame) and the combined USNO-A2.0 R and B magnitudes: m = 0.89R +0.11B. (1) We chose this combination so that our instrumental magnitude was close to the R magnitudes of minor planets, i.e., objects with solar spectra. As our observations show, these magnitudes are in poor agreement with the R magnitudes of GRBs, due to the enhanced red sensitivity of the Apogee Alta U16 array. The processing of a frame begins immediately after it is taken, and requires less than one minute. As a result, the robotic system can attempt to find unidentified objects within the error box and compose the text of a GCN telegram with the indicated brightness limit for an optical transient. In parallel, our full frame with the error box and an enlarged error box (usually 6 -8 ) with an image of the same region in the Palomar Sky Survey are sent to a database over the Internet, together with our frames obtained during the previous survey observations. Thus, an on-duty observer can visually inspect the region to search for objects with low signal-to-noise ratios (two to three). If no object is found in the individual frames, the images are summed. On a good, Moonless night, the
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THE "MASTER" ROBOTIC TELESCOPE Table 2. ( ) - 2005?2006 . Publication in GCN circular 5632 [5] 5619 [6] 5613 [7] 5303 [8] 5056 [9] Time from detection of the GRB 76 s Limiting magnitude 17.5
m

1007

GRB

First observation? 1

Comments

GRB 060926

Optical transient detected, decay law obtained. Pointing 72 s after the burst. No optical candidate detected. SWIFT GRB. Evening sky. Main observations were 82 min later (after sunset) with all MASTER instruments: simultaneous BV R, spectral, and high-time-resolution images of the GRB region obtained. No optical candidate detected. Konus-wind GRB. Observations began 18.5 hr after the GRB in the survey regime (the delay was due to processing of the signals from the IPN triangulation equipment; the IPN error box is a region tens of square degrees in size). The region is located near the Galactic plane. We obtained roughly 20 six-square-degree images. SWIFT GRB. First image obtained on 04.27.2006 at 18:18:07 UT. No new objects brighter than 17.5m detected. IPN triangulation GCN5005. MASTER observed in the survey regime on two nights: 04.26.2006 from 18:17:03 to 19:53:07 UT (1.05?1.12 days after the event [13]) and 04.27.2006 from 18:57:51 to 19:21:20 UT. A total of 72 images were obtained. No new objects brighter than 15.5m were detected. Two minutes before the alert, the roof was closed due to strong cloud-cover. No new objects within the SWIFT-XRT error box brighter than 16.8m (S/N = 3) were detected. Spectral and integrated images of the burst region were obtained. No new objects brighter than 19.5m were found in the SWIFT error box. GRB 060213 was detected by the IPN. No optical candidate brighter than 17.5m was found.

GRB 060712 GRB 060502B

212 s 69 s, 82 m

14 16

1 2

GRB 060427B

5032 [10]

18.5 h

1

GRB 060427

5020 [11]

9h 13 m

17.5

4

GRB 060425

5026 [12] 5008 [13] 5080 [14]

26 h 53 m

17.5

1

GRB 060421

4988 [15]

562 s

16.8

3

GRB 060319

4892 [16] 4888 [17]

162 s

19.5

2

GRB 060213

4767 [18] 4765 [19]

55 h 17 m

17.5

2

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1008 Table 2. (Contd.) Publication in GCN circular 4741 [20] 4718 [21] 4572 [22] Time from detection of the GRB 282 s 235 s 96 m

LIPUNOV et al.

GRB

Limiting magnitude 14.5 15.8 14.4

First observation? 2 2 5

Comments

GRB 060211B GRB 060209A GRB 060124

Snow, gaps in the clouds. No optical candidate found. SWIFT GRB 060124. Information about the GRB did not arrive through the alert system. The weather was hazy and cloudy. HETE alert 4006 without coordinates. Observations before and after the alert with the wide-field camera. All exposures were 3 s. No object brighter than 8m detected. Observations began half an hour before dawn. No optical candidate detected. IPN triangulation GRB 051103 [27]. Observations began several minutes after receiving the alert. A total of 36 images with a total exposure of 1080 s were obtained. No optical candidate brighter than 18.5m found (the weather was hazy). Four galaxies (M81, M82, PGC 2719634, PGC 028505) are located near or in the triangulation region; it is possible the GRB arrived from a source in PGC 028505. HETE alert GRB 051028. No objects brighter than 17.9m (sum of nine frames taken between 17:32:40 and 18:03:03 UT) and 19.4m (8 h 26 m after the alert, total exposure of 1200 s) detected. Technical alert. Sunset, haze. No new object found based on comparison with USNO-A2. HETE alert 3947. Sunset, haze. No new object found based on comparison with USNO-A2. First image with exposure 5 s obtained 45 s after detection of the GRB. A secod image with exposure 30 s obtained 87 s after the GRB. No new objects detected in the error box. Optical candidate detected.

GRB 060118

4549 [23]

Synchronous

8

1

GRB 060111A

4485 [24]

130 s

16

4

GRB 051103

4198 [25] 4206 [26]

58 h 30 m

18.5

1

GRB 051028

4171 [28] 4173 [29] 4182 [30]

3h23 m

17?19.4

2

SWIFT trigger 160640 GRB 051021.6

4119 [31]

1h11 m

16.3

1

4118 [32]

1h29 m

14

4

GRB 051011

4082 [33] 4083 [34]

45 s

17.0

1

GRB 050824

3886 3883 3869 3870

[35] [36] [37] [38]

764 s

19.4

1

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2007


THE "MASTER" ROBOTIC TELESCOPE Table 2. (Contd.) Publication in GCN circular 3882 [39] 3769 [40] 3767 [41] 3755 [42] 3221 [43] 3188 [44] 3108 [45] Time from detection of GRB 94 s 74 s 113 s 198 s 5h 1h09 m 103 s synchronous Limiting magnitude 18.9 17.1 14.5 18.6 18.5 14.7 19.5 16.5 First observation? 1 2 1 2 3 7 1

1009

GRB GRB 050825 GRB 050805b GRB 050805a GRB 050803 GRB 050410 GRB 050408 GRB 050316

Comments SWIFT alert, no new objects found. SWIFT trigger 149131. No new objects found (Milky Way region). Weak object detected at the noise level. Subsequently not confirmed. SWIFT alert. No new objects found. SWIFT alert. No new objects found. Sunset, cloudy; no new objects found. No object was found on the summed frame (19.5m ) or the image obtained from the survey observations coincident with the GRB. This event was later classified by the HETE group as unconfirmed. Preliminary result; 50 frames with an exposure of 30 s. No new objected detected based on a comparison with USNO-B. A total of 50 spectra ? (50 A) of a 40 ' 30 region obtained. Object from GCN 2986 not detected. Optical transient at the noise level. Not confirmed. First observations of the region of this SWIFT GRB.

GRB 050316

3106 [46]

103 s

18

1

GRB 050126 GRB 050126 GRB 050117

2988 [47] 2986 [48] 2954 [49] 2953 [50]

2h48 m 2h48 m 2h

17 15 19 17

1 1 1

Table 3. Observations of GRB 050824 Time (UT) 23:25:00 23:25:00?23:47:55 23:49:00?00:09:03 Time from GRB 788 s 24 min 47 min Magnitude >17.8m 18.6 + 0.3 19.4 + 0.3 Exposure 45 s 15 ' 30 s 15 ' 30 s Comments Upper limit

Table 4. Bright galaxies near and inside the large error box for GRB 051103 Name M81 M82 PGC 2719634 PGC 028505 Type Sab Sb ? E Coordinates (2000) B 09h 55m 33.2s+69 03 55 09 55 52.2+69 40 47 09 51 32.3+68 31 24 09 53 10.2+69 00 02 7.8 9.3 17.8 17.0
m

Magnitude MASTER ? ? 16.7 14.8
m

Redshift 0.000376 0.000677 ? ?

Diameter of 25m isophote large large 16.9 6.0

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m 17 18 19 20 21 22 0.1 1 Time, hrs R

4. THE SHORT BURST GRB 051103--A SOFT GAMMA-RAY REPEATER IN THE GALAXY M81? GRB 051103 may be the first soft gamma-ray repeater (SGR) detected outside our Galaxy; the first image of the error-box region was obtained on the MASTER telescope. The bright, short (0.17 s) burst GRB 051103 was detected by Konus-Wind, as well as HETE-Fregate, Mars Odyssey (GRS and HEND), RHESSI, and SWIFT-BAT [27]. The MASTER telescope started to observe the error-box region for GRB 051103 [25] several minutes after receiving the alert telegram [27]. The first image was obtained at 19 : 55 : 47 UT on November 5, 2005, 2 d, 10 hr, and 30 min after the GRB. We obtained 36 images with a total exposure time of 1080 s between 19 : 55 : 47 and 21 : 45 : 17 UT. No optical transient was found in the error box to 18.5m (in the presence of a full Moon and light haze). Analysis of the frame showed the presence of four bright galaxies near or in the large error box (Fig. 4, Table 4). The most likely candidate host galaxy is M81 [27], and the burst itself has been interpreted as a SGR. The error box lies outside any spiral arms, where strongly magnetized neutron stars (magnetars, which are thought to be the sources of SGRs) would be likely to form. However, the structure of M81 is distorted by tidal interaction, and part of a disrupted spiral may fall in the error box. For example, the ultraluminous X-ray source (ULX) M81 X-9 [57] is located at a similar distance from the center of M81 (on the side opposite to the error box), and belongs to that galaxy's population of massive stars. It would be interesting to search for supernova remnants within the error box (unfortunately, the supernova-remnant survey [58] does not encompass the error box of the GRB). In our telegram [25], we also noted the elliptical galaxy PGC 028505, which is close to the center of the triangulation error box. Its distance is estimated to be 80 Mpc. If the GRB occurred in PGC 028505, the isotropic energy of the burst can be estimated to be 2 ' 1049 erg. This exceeds the energy of the short burst GRB 050509b, which is associated with an elliptical galaxy [59], by an order of magnitude, but remains fairly characteristic of long GRB energies. Based on our data, Holland et al. [60] carried out photometry of PGC 028505 the following night, without finding any optical transient brighter than 21m , although this work excluded the region of the galactic bulge. Nevertheless, the absence of an optical object represents an additional argument that GRB 051103 is the first SGR observed beyond our Galaxy,
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Magnitude

M2 V

U W1 B W2

8

Fig. 2. Upper limit and brightness estimate for the optical counterpart of GRB 050824 obtained in the first minutes after the burst. The hollow circles are ROTSE estimates [54], the dark circles MASTER estimates [36], the diamonds the R magnitudes from [53], and the triangles the SWIFT estimates in various photometric bands [55].

sum of 10?15 images lowers the limiting magnitude to 20m . The results of our observations are summarized in Table 2. 3. GRB 050824: EARLIEST IMAGE Information about the coordinates of GRB 050824 arrived at the MASTER observatory on August 24, 2005 after some delay due to the processing of the signal in the SWIFT data center [52]. The first image of the region was obtained 110 s seconds after obtaining the alert, i.e., 764 s after the SWIFT detection (trigger 151905), during a nearly fully Moon. The optical limit reached was 17.8m . The observations were continued [35?38] (Table 3), and, during the reduction of the summed frames and preparation of a telegram on the results of our observations, telegram GCN3865 [53] arrived with the coordinates of an 18m object found 37 min after the GRB in an R image. This object was present in our earlier summed images. The earliest available optical image of this object, obtained on our MASTER system, can be found at the address http://observ.pereplet.ru/images/ GRB050824/1.jpg. Figure 2 presents the upper imits and magnitudes obtained in the first minutes of observations. Figure 3 shows our data together with the R data of the MDM Observatory [56]. The MDM R observations obtained from 5.6 to 12.6 h after the GRB are consistent with a power-law decay in the flux (with index -0.55 + 0.05), and also with our data obtained 24 min and 47 min after the burst.


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1011

18

19 MASTER (GCN 3883) t?
0.55 + 0.05

20

21 t? 22 MDM (1.3-m telescope)
0.43 + 0.04

R

23 24 0.01

MDM (2.4-m telescope) 0.1 1 Time, days 10

Fig. 3. Observations of GRB 050824 from the MASTER telescope and the MDM Observatory (R) [56].

in the galaxy M81 [61]. Our full image of the errorgox region can be found at the address http://observ. pereplet.ru/images/GRB051103.4/sum36.jpg. 5. GRB 060926: EARLIEST IMAGE AND DETECTION OF AN OPTICAL BURST Our observations of GRB 060926, detected by the SWIFT gamma-ray observatory [62], were carried out in an automated regime under good weather conditions [63]. The earliest image was obtained on September 26, 2006 at 16 : 49 : 57 UT, 76 s after the detection of the burst. We found an optical transient in our first and subsequent summed frames, at the position RA = 17h 35m 43.66s + 0.05s , Dec = 13 02 18.3 + 0.7 , which coincides within the errors with the coordinates of the optical transient reported in [62]. Our photometry of the object provided the earliest points on the light curve (Table 5). Our preliminary reduction indicated a more gradual brightness decrease than the OPTIMA-Burst observations [64] (the power-law index for the brightness decrease in the first 10 min was 0.69). However, subjecting the data to finer time binning revealed an optical burst: after its initial decrease, the brightness began to increase beginning 300 s after the GRB, reaching a maximum 500?700 safter theGRB (Fig. 5a). Synchronous SWIFT-XRT measurements of the X-ray flux show similar behavior (Fig. 5b). Such an event had already been observed in at least two other cases: GRB 060218A (z = 0.03) 1000 s
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after the GRB [66], and GRB 060729 (z = 0.54) 450 s after the GRB [67, 68]. Note that the GRB considered here has a redshift of 3.208 [69]. The absorption indicated by the X-ray data corresponds to nH = 2.2 ' 1021 cm-2 , of which nH = 7 ' 1020 cm-2 occurs in the Galaxy [65]. Taking into account the redshift, the total absorption in our band should be 3m . Naturally, we assume here that the dust-to-hydrogen ratio is the same as it is in our Galaxy. A comparison of our optical data with the SWIFTXRT X-ray fluxes [65] can be used to determine the slope of the electromagnetic spectrum (FE E - ), which turned out to be constant within the errors and equal to 1.0 + 0.2 [5], which coincides with the corresponding value for the X-ray spectrum. The earliest image of the optical transient can be found at http://observ.pereplet.ru/images/GRB060926/ GRB060926_1.jpg, the sum of the five following frames at http://observ.pereplet.ru/images/GRB060926/ GRB060926_5.jpg, and the sum of the ten following frames at http://observ.pereplet.ru/images/GRB060926/ GRB060926_10.jpg.


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Fig. 4. Image of the region of the short burst GRB 051103 (the error box is shown) obtained from a sum of 30 MASTER frames. The limiting optical magnitude in a frame is 18.5m . An image of the error box can be found at http://observ.pereplet.ru/images/GRB051103.4/sum 36.jpg.

Images of this same region obtained during the previous set of survey observations can be found at http://observ.pereplet.ru/images/ GRB060926/ GRB060926_2005.jpg. 6. OPTICAL EMISSION OF GRBS IN THE FIRST HOUR AFTER THE BURST In this section, we consider the question of how universal the behavior of the optical emission of GRBs is with regard to their absolute luminosity and time behavior. We collected observations of the following GRBs with known redshifts in the first hour after their onset (Fig. 4): GRB 990123 GRB 021004 GRB 021211 [70] [71, 72] [73-75]

GRB 030723 GRB 040924 GRB 041006 GRB 050319 GRB 050401 GRB 050502 GRB 050525 GRB 050730 GRB 050820 GRB 050824 GRB 050904 GRB 050908 GRB 050922C
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[76] [77-79] [80, 81] [83, 84] [85, 86] [87, 88] [89-91] [92-95] [96-98] [33, 35, 36, 53] [99, 100] [101-106] [107-110]
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THE "MASTER" ROBOTIC TELESCOPE Table 5. Photometry of GRB 060926 obtained on the MASTER telescope Beginning of exposure, s 76 150 165 255 343 432 519 608 707 804 901 Middle of exposure, s 91 165 343 432 519 608 707 804 1001 1200 1298 Exposure, s 1 ' 30 1 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 5 ' 30 Magnitude 17.3m + 0.3 18.5 + 0.3 19.3 + 0.3 18.9 + 0.3 18.5 + 0.3 18.3 + 0.3 18.4 + 0.3 18.7 + 0.3 20.0 + 0.3 20.1 + 0.3 > 20.1 + 0.3
m

1013

Flux, erg cm-2 s

-1

eV

-1

Flux after correction for absorption, erg cm-2 s-1 eV-1 (4.3 + 1.0) ' 10 (1.4 + 0.4) ' 10 (6.9 + 1.7) ' 10 (9.9 + 2.4) ' 10 (1.4 + 0.3) ' 10 (1.7 + 0.4) ' 10 (1.6 + 0.4) ' 10 (1.2 + 0.3) ' 10 (3.6 + 0.9) ' 10 (3.3 + 0.8) ' 10 < (3.3 + 0.8) ' 10
-12 -12 -13 -13 -12 -12 -12 -12 -13 -13 -13

(1.4 + 0.3) ' 10 (4.6 + 1.1) ' 10 (2.2 + 0.5) ' 10 (3.2 + 0.8) ' 10 (4.6 + 1.1) ' 10 (5.8 + 1.3) ' 10 (5.1 + 1.2) ' 10 (3.9 + 0.9) ' 10 (1.2 + 0.3) ' 10 (1.1 + 0.3) ' 10 < (1.1 + 0.3) ' 10

-13 -14 -14 -14 -14 -14 -14 -14 -14 -14 -14

GRB 051109 GRB 051111 GRB 060926

[111-116] [117-122] [5-7].

where fi are the individual fluxes and ni is the number of GRBs with fluxes in the interval i ' 10-6 fi (i +1) ' 10-6 . (5) The normalization in redshift (z0 = 1) was carried out using the formula m
z0 opt

We then normalized the magnitudes to a single redshift (to take into account their different distances; Fig. 7) and gamma-ray flux (to take into account their directional beaming, assuming the beaming for the optical and gamma-ray emission is similar; Fig. 8). Since the largest number of afterglows have power-law light curves, we chose as boundaries in the synthetic light curves straight lines (in logarithmic coordinates) of the form f (t) = 2.5 ? 0.8 ? log t + c = 2 ? log t + c, (2)

=m

dl (z ) =m dl (z0 ) (1 + z )I (z ) , - 5log (1 + z0 )I (z0 )
z opt

- 5log

z opt

(6)

where
(1+z )d-
1/3

I (z ) =
1

dq q 3 +1

,

d = 0.3/0.7.

(7)

where c = const. The width of the band bounding the synthetic light curve is then m = cmax - cmin . (3)

We obtained synthetic curves in the optical and gamma-ray normalized to the same redshift and gamma-ray flux using the formula m = mz - 2.5log
z F 1+ z +2.5log z0 . 1+ z0 F0

It's obvious that, the smaller m, the better the model used to construct the synthetic light curve describes the real physical situation. We normalized the observed curves using the mean weighted gamma-ray flux for the entire sample: F0 =
n
i

(8)

fi n

i n
i

ni = 2.13 ' 10-5 erg/cm ,
2

(4)
ASTRONOMY REPORTS Vol. 51 No. 12 2007

Correcting for interstellar absorption does not appreciably affect the distribution. Applying the above normalization substantial narrowed the width of the synthetic light curve, although m remained fairly large. We assumed that the synchrotron mechanism operates in GRB sources, suggesting that the spectrum should have the form .


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LIPUNOV et al.

10?1 17 18 Magnitude 19 20 21 1 ' 10?1 22 100 1000 Time, s 5 ' 10?1 () F, erg, cm2 s eV 10?1

1

(b)
2

~ ~
5

2 ' 10?1

5

6

100 Time, s

1000

Fig. 5. Comparison of the light curve of GRB 060926 obtained using the MASTER telescope [63] (points) with the (a) OPTIMA Burst [64] (circles) and (b) SWIFT XRT (0.3?10 keV; diamonds) [65] light curves.

6 Magnitude (R or unfiltered) 8 10 12 14 16 18 20 22 0.01 0.1 Time after burst, hrs 1 MASTER GRB 050824 MASTER GRB 060926

Fig. 6. Master light curve for the GRB afterglows observed within an hour after the burst. The width of the band, shown by dashed lines, corresponds to m = 8.5m .

Let us try to predict the value of based on the tendency for the width of the synthetic curve normalized in this way to narrow. We must take into account the so-called K correction: (9) Kz = 2.5log (1 + z ) s()d . +2.5log I ()s()d I 1+ z Formula (8) then takes the form m = mz - 2.5 log
z F 1+ z +2.5log z0 . 1+ z0 F0

(10)

Figure 9 shows the dependence of the width of the

synthetic curve on the spectral index. We can see a weak minimum for = -1, with the corre