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BRIGHT OPTICAL FLASH FROM GRB 060117 - AN EXAMPLE OF EVENT DETECTED WITH OMC CLASS INSTRUMENT
Д Д Petr Kubanek1,2 , Martin JelД inek3 , Michael Prouza4,5,6 , Rene Hudec1 , and Martin Nekola1 1 Astronomicky ustav Akademie ved Ceske republiky, Ondrejov, Czech Republic ДД Д 2 INTEGRAL Science Data Center, Chemin d'Ecogia 16, Versoix, Switzerland 3 Instituto de AstrofД isica de AndalucД (IAA-CSIC), Granada, Spain ia 4 Columbia University, New York, USA 5 FyzikalnД ustav Akademie ved Ceske Republiky, Praha, Czech Republic Д iД Д 6 Pierre Auger Collaboration

ABSTRACT We present observations of a very bright (magnitude 10) optical transient associated with GRB 060117. These observations were obtained with an OMC-class ground based device - wide-field CCD camera atop the FRAM robotic telescope, operated by the Pierre Auger collabo? ration at Los Leones site, Malargue, Argentina. The detected optical counterpart of GRB is presented and discussed, together with consequences for probability of detection of analogous event by INTEGRAL OMC. Key words: gamma ray bursts; robotics telescopes; INTEGRAL.

The optical Monitoring Camera, located on board INTEGRAL satellite, has similar parameters as FRAM widefield camera - it is 50 mm lens system, with 1024з1024 CCD, which is capable to reach 15 mag in 100 sec exposure. Similar to FRAM's capabilities, the OMC is able to work in fast follow-up mode. When a GRB is detected at the INTEGRAL Science Data Center (ISDC) by the INTEGRAL Burst Alert System (IBAS)[7] and its coordinates lie inside the OMC FOV, IBAS commands OMC to start taking exposures centred on the interesting region. Those frames are then downlinked to ground stations and stored at ISDC for future processing. So far, the fast follow-up mode was activated only once in now almost three years long INTEGRAL mission, for GRB 050626. Unfortunately, the location of GRB was very close to bright star, which saturated obtained frames.

1. THE INSTRUMENT The FRAM telescope is a part of the Pierre Auger cosmic-ray observatory [9]. Its main purpose is an immediate monitoring of the atmospheric transmission. FRAM works as an independent, RTS2 driven [5], fully robotic system. It performs a photometric calibration of the sky on various UV to optical wavelengths using a 0.2 m telescope and a photoelectric photomultiplier. As a primary objective FRAM observes a set of chosen standard stars and a terrestrial light source. From these observations are calculated instant extinction coefficients and the wavelength dependence of extinction. As an additional activity FRAM is able to follow GCN alerts, using its wide-field camera with a fixed R-band filter. The wide-field camera consists of a Carl Zeiss Sonnar 200 mm f /2.8 telephoto lens, SBIG ST7 imager and Bessel R-band filter. The ST7 camera has a 768з512 Kodak KAF-0402E CCD which covers a field of view (FOV) of 120 з 80 with a scale of 9.6. /pixel. The effective diameter of the lens is 71 mm and the 3 limiting magnitude under optimum conditions reaches R 15.0 for 120 s exposure.

2.

GRB 060117

A bright long-soft GRB 060117 was detected by Swift satellite on January 17, 2006, at 6:50:01.6 UT. Coordinates computed by Swift were available within 19 s and immediately distributed by GCN. Swift itself could not observe the GRB with its X-ray and optical instruments, because of the Sun observing constraint [2]. It was the most intense (in terms of peak flux) GRB ever detected by SwiftBAT[1], and had one of the few brightest optical transients ever observed. It was GRB with brightest peak flux detected by BAT, with one of the brightest optical transient. FRAM started to observe the GRB location 128s after the burst. It detected a rapidly decaying transient, which had the R-band magnitude 10.2 at the first image. The decay is studied in detail in [4]. as a combination of reverse and forward shock. For the lightcurve plot see figure 1.


10

GRB GRB GRB GRB GRB GRB

990123 021211 050502a 050525a 050801 060117

12

observed magnitude

14

Observed magnitude 9 11 13 14 15 16 17 18 18+

Number of GRBs 1 1 1 2 6 8 10 10 183

Sum 1 2 3 5 11 19 29 39 222

0.45% 0.90% 1.35% 1.96% 4.95% 8.55% 13.06% 17.57% 100.00%

Table 1. Distribution of peak brightness of GRBs followed within 0.1 day.
16

18

100 time in restframe [seconds]

200

300

Figure 1. Light curves of bright optical transients of GRBs Observed R magnitude are shown, except for GRB 050525a, where the V-band values are plotted. 3. PROBABILITY OF DETECTION OF SIMILAR EVENT BY OMC IN GRB FOLLOWUP MODE GRB optical brightness depends on various factors such as GRB distance, released energy, GRB environment and absorptions systems between GRB and the observer. The true observed GRB rate depends on instrumentation and detection thresholds. INTEGRAL was designed to resolve faint point sources in the -ray background and to provide additional information about their properties. INTEGRAL GRBs are detected during -ray observations of knows sources. Swift is a satellite designed primary for GRBs observations. The INTEGRAL ISGRI[12] detector has a (half-coded) FOV 12 times smaller then BAT. ISGRI is the primary source of events that trigger OMC fast follow-up mode. Precisely localised GRBs are usually used for fast followup observations. INTEGRAL detects on average 12 GRBs per year with few arc minutes localisation. Swift detects about 100 GRBs good localised GRBs per year. That is in good agreement with the smaller INTEGRAL FOV weighted by higher sensitivity. INTEGRAL GRBs detections are biased towards the Galactic core and the Galactic plane, as those are the areas of the sky where INTEGRAL spends most of its observing time. Due to this observational strategy, roughly 3 out of 4 of the INTEGRAL GRBs will lie in areas of the sky with high Galactic extinction and hence there is a low chance to see bright optical counterpart. As can be deduced from figure 1, in the last 7 years there were only two GRBs which had an observed brightness

above 11th magnitude. Given the understanding of the GRB optical transients nowadays ([8], [6]), we may estimate that the GRB 030329 optical counterpart, for which first images were obtained 75 minutes after the trigger and showed 12.0m source [11], could have reached above 11th magnitude at the time comparable with observation of GRB060117. However, in order to avoid confusion with different models, we will investigate only observed peak brightness, e.g. brightness measured on acquired frames. We searched GRBlog[10] for the peak optical observed brightness of GRBs which were followed less than 0.1 day after trigger. The results binned by 1 magnitude are presented in table 1. Out of 222 GRBs, 19 had have observed counterparts brighter than 16 magnitude. If we take 16 as a conservative magnitude limit for prompt GRB detection by the OMC, then 8% of GRBs have a counterpart detectable by the OMC. IBAS detects roughly 10 times fewer GRBs than Swift, at an average rate of one GRB per month. Out of 12 GRBs detected by ISGRI per year we can expect one to have optical counterpart brighter than 16th mag nitude. If we take ISGRI half-response FOV (19. x19. ), the area is more than 14 times bigger than the OMC FOV (5. x5. ). Based on that numbers, we can conclude that the expected number of GRBs detected by OMC is less then one in 10 years. Those numbers are only a very rough estimate. There are a lot of unknowns in them - not all GRBs have observations during first few tens of seconds after trigger, which the OMC can provide. If the counterpart brightness decays with a power law, the OMC can observe a GRB during a phase when it could be brighter than the above data suggests. Different filters were used in observations of sample GRBs and magnitudes were not corrected for Galactic extinction, which lowers the probability of detecting bright counterpart by INTEGRAL.

4.

CONCLUSION

Based on the presented data, we concluded that the probability of detecting GRB optical counterpart by the OMC


is very low, but worth trying. All the above is only calculation of probability - with our FRAM WF camera, we obtained two GRBs detections - GRB 060117[4] and GRB 060418[3], after less than one year of observation, which is well above the rate we expect.

[11] K. Torii et al. The Earliest Optical Observations of GRB 030329. The Astrophysical Journal, 597:L101-L105, November 2003. [12] P. Ubertini et al. IBIS: The Imager on-board INTEGRAL. Astronomy and Astrophysics, 411:L131- L139, November 2003.

5. ACKNOWLEDGEMENTS The FRAM telescope was built and is operated under the support of the Czech Ministry of Education, Youth and Sports via its grant programs LA134 and LC527. MJ would like to thank to the Spanish Ministry of Education and Science for the support via FPU grant AP2003 1407. MP was supported by the GA AV CR via grant B300100502. PK, MN and RH would like to thank to GA AV CR for the support via grant A3003206 and ESA PECS Project 98023.

REFERENCES [1] S. D. Barthelmy et al. The Burst Alert Telescope (BAT) on the SWIFT Midex Mission. Space Science Reviews, 120:143-164, October 2005. [2] S. Campana et al. GRB 060117: Swift-BAT detection of a bright burst. GCN 4533, 2006. [3] M. Jelinek et al. GRB 060418: FRAM early afterglow observation. GRB Coordinates Network, 4976:1-+, 2006. [4] M. JelД inek et al. The bright optical flash from GRB 060117. Astronomy and Astrophysics, 454:L119- L122, August 2006. Д [5] P. Kubanek et al. RTS2 - Remote Telescope System, 2nd Version. In AIP Conf. Proc. 727: GammaRay Bursts: 30 Years of Discovery, September 2004. [6] E. Liang and B. Zhang. Identification of Two Categories of Optically Bright Gamma-Ray Bursts. The Astrophysical Journal, 638:L67-L70, February 2006. [7] S. Mereghetti et al. The INTEGRAL Burst Alert System. Astronomy and Astrophysics, 411:L291- L297, November 2003. [8] M. Nardini et al. Clustering of the optical-afterglow luminosities of long gamma-ray bursts. Astronomy and Astrophysics, 451:821-833, June 2006. [9] Pierre Auger Collaboration. The Pierre Auger Observatory progress and first results. In Proceedings of the 29th International Cosmic Ray Conference, Pune, India, also the FERMILAB preprint FERMILAB-PUB-05-469-A-TD, August 2005. [10] R. Quimby et al. GRBlog: A Database for GammaRay Bursts. In E. Fenimore and M. Galassi, editors, AIP Conf. Proc. 727: Gamma-Ray Bursts: 30 Years of Discovery, pages 529-532, September 2004.