The wide field optical camera with high temporal resolution for the
continuous monitoring of the sky in order to catch the initial stages of GRBs
is described.Comment: 4 pages, 4 figures. To be published in "Il Nuovo Cimento",
  Proceedings of the 4th Rome Workshop on Gamma-Ray Bursts in the Afterglow
  Era, eds. L. Piro, L. Amati, S. Covino, B. Gendr

Beskin, G.

Biryukov, A.

Bondar, S.

Hurley, K.

Ivanov, E.

Karpov, S.

Katkova, E.

Pozanenko, A.

Zolotukhin, I.

English

arXiv

The wide-field optical camera with high temporal resolution for the continuous monitoring of the sky in order to catch the initial stages of GRBs is described

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10145-xIL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005Optical camera with high temporal resolution to search fortransients in the wide field(∗)S. Karpov(1), G. Beskin(1), A. Biryukov(2), S. Bondar(3), K. Hurley(4)E. Ivanov(3), E. Katkova(3), A. Pozanenko(5) and I. Zolotukhin(2)(1) SAO RAS, Nizhnij Arkhyz - Karachai-Cherkessia, Russia, 369167(2) Sternberg Astronomical Institute of Moscow State UniversityUniversitetskij pr. 13, Moscow, Russia, 117415(3) Research Institute for Precision Instrumentation - Moscow, Russia(4) Space Science Laboratory, University of California - Berkeley, USA(5) Space Research Institute - Profsojuznaja 84/32, Moscow, Russia, 117997(ricevuto il 23 Maggio 2005; pubblicato online il 12 Ottobre 2005)Summary. — The wide-field optical camera with high temporal resolution for thecontinuous monitoring of the sky in order to catch the initial stages of GRBs isdescribed.PACS 95.55.Cs – Ground-based ultraviolet, optical and infrared telescopes.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.Gamma-ray bursts (GRBs) are one of the most powerful transient events in the Uni-verse which are probably related to the compact relativistic objects. The fine timestructure of the GRB emission is defined by the properties of their central engine.At the same time a number of models predict generation of considerable optical fluxsynchronous with GRB event which can achieve 10–12m for 0.2 s [1] or even 8–9m for0.1–10 s [2]. In the model of early afterglow in the wind shell the optical flash is expectedto be as bright as 9–10m in 0.2–0.5 s with 0.5–60 s lag from proper GRB event [3].Thus, search and study with high temporal resolution of optical transients (OTs) ac-companying GRBs, can provide statistically reliable information about the nature of thesephenomena. To be successful such observations have to be carried out independently ofalerts receiving from space-borne gamma-ray telescopes and use optical instruments witha wide field of view. As a side effect such observations also give the possibility to detectand investigate short stochastic flares of different variable objects—SNs, flare stars, CVs,X-ray binaries and NEOs, natural and artificial.(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 747748 S. KARPOV, G. BESKIN, A. BIRYUKOV, ETC.Table I. – Main parameters of the camera.Main objective Intensifier CCDDiameter 150 mm Photocathode S 25 Dimensions 1280× 1024 pixFocus 180 mm Photocathode diameter 90 mm Image scale 56′′ / pixD/F 1/1.2 Gain 150 Exposures 0.13–10 sField of view 21× 16 deg Scaling coefficient 7.6 / 1 Pixel size 6.45 micronQuantum efficiency 10% Data stream 400 Gb / nightSimple analysis of the technical parameters of a camera for such a task may be per-formed as follows.It is well known that the detection limit in the observations with CCD is to determinenoise of the detector and the sky background.For wide-field instruments the last case is realized. In the V band, for the atmosphericand optics transparencies of 0.5 and 0.7 correspondingly, and for the sky background of20 mag/✷′′the detection limit is S = 4.9 aKlDF√etphotons/cm2/s, or, in magnitudes,m = 13 − 2.5 lg[aKlDF√et]. We used flux calibration from [4] and the following notation:a—detection threshold in 3σ, l—pixel size in 6.45 microns, D—telescope diameter in10 cm, F—focal length in 20 cm, e—detector quantum efficiency in 0.6, t—exposuretime in 0.13 s. K is the scaling coefficient in units of 4, which characterizes the reductionof the focal image linear size by the special unit—the taper, or the image intensifier, touse the single CCD to cover the whole original telescope field of view. For example, inour camera the reduction of the FOV of 80 mm size by the image intensifer and specialobjective by 7.6 times gives the possibility to use standard 2/3 inch TV-CCD. Certainly,in any case the spatial resolution of the main objective, reduction unit and the CCDhave to be in agreement.The rate N of GRBs to be detected by the wide-field camera may be easily estimatedfor standard slope of −3/2 of the “logNS- logS” relation. The number of events Nis then proportional to NS ∝ S−3/2, where S is the detection limit, and the camerafield of view Ω = 4.2 · 104P 2l2K2/F 2 arcsec2, where P is the CCD size in pixels. So,N ∝ NSΩ ∝ S−3/2P 2l2K2/F 2 ∝ a−3/2e3/4t3/4A1/2K1/2DP 2l1/2.Here A = D/F is the aperture of the system, which should be as high as possible,but can hardly exceed 1 for the acceptable aberrations in the wide field.The important consequence of these formulae is that the GRB rate depends on D lin-early and quadratically on P (thus, the field of view size). It means that for higher GRBcount rate the wider field of view is more important than the larger objective diameter.Also it is clear that the wide field may be achieved only by means of special methods offocal scale reduction (tapers, image intensifiers) which enlarges K (for the fixed maximalA). Certainly, the main objective has to give good quality FOV Ω corresponding to thedetector size P . That condition is much easier to achieve for smaller D.The camera (see fig. 1) has been created as a realization of ideas formulated above.Its parameters are given in table I. The camera has A = D/F = 1/1.2 = 0.83 anduses the image intensifier to lower the focal scale (K = 1.9, e = 0.17), so the theoreticalestimation of limiting magnitude for it is determined by the sky background noise andis equal to 11.6m for the 0.13 s exposure. The TV-CCD used (VS-CTT285-2001) is ableto operate in 7.5 Hz frame rate mode with such an exposure.The observational data from the TV-CCD are transmitted (see fig. 2) to the local PCwhich brodcasts it through the LAN to the storage—RAID array with 480 Gb capacity—OPTICAL CAMERA WITH HIGH TEMPORAL RESOLUTION 749Fig. 1. – Left panel: photo of a camera installed in Northern Caucasus near the 6-m telescope.Right panel: typical detection efficiency (the probability for the object of a given magnitude tobe detected) for the objects in dependence on magnitude.and to the real-time processing box. The data flow rate for the system is about 13 Mb/s.In order to process such a data stream in real time, no standard reduction routinesapplied usually for field photometry and source extraction may be used. For this reasonspecial software complex for detection and investigation of OTs has been created.The software is installed at the three PCs and operated by WINDOWS and LINUXOSes. Incoming information is a sample of 1280×1024 pixels CCD frames with exposuretime of 0.13 s. The software performs the data reduction in real time—detection andclassification of OTs, determination of their equatorial coordinates and magnitudes, theirpossible identification with known objects and transfer of information about OTs (alerts)to the local and global networks.The OT detection algorithm is based on the comparison of the current frame with theone averaged over 100 previous frames and is able to detect and classify any transientFig. 2. – Left panel: global scheme of the camera hardware and software complex and data flow.Right panel: example of a short flare detected by the camera. Total length of the event is 0.4 s(seen on 3 successive frames).750 S. KARPOV, G. BESKIN, A. BIRYUKOV, ETC.that is seen on three successive frames (i.e. with duration of 0.4 s), determine its shape,trajectory and light curve and cross-correlate it with catalogues of known transient ob-jects such as stars and satellites. When transient does not match the catalogues and doesnot look like a meteor the system is able to send its information to robotic telescopesand/or global networks.The example of a short (0.4 s) transient detected by the system is shown on fig. 2.The transient shown is due to geostationary satellite.As has been noted above, the faintest detectable object has 11.5m in a band close toV. This limit may be increased up to 12.5–14m by simultaneous analysis of sums of largenumber of frames (10–100) by the cost of temporal resolution loss.Nearly all instruments for gamma-ray bursts related OTs (prompt emission and af-terglow) search currently in operation are divided into two classes—the trigger basedrelatively large (25–100 cm) telescopes with up to 10 deg2 FOV and the monitoring cam-eras, consisting of 1–4 small objectives (5–10 cm) with 200-1500 deg2 FOV. Both classesare equipped with standart CCDs with exposures larger than 10 s (in rare cases—ROTSEIII—5 s). The instruments of the first class need 30–60 s for the pointing and thus cannotbegin the observations until the GRB fade off, but they are essential for the detectionand study of early afterglows (1–10 min after the trigger) due to ability to perform accu-rate photometry of 17–20m objects on the 5–60 s timescale [5, 6]. At the same time themonitoring systems are able to detect 10-12m OTs only for the 10–60 s one [7, 8]. Thecamera described has the 2m better sensitivity on the same time scale and even with 3–5smaller FOV is able to detect 3–5 times larger amount of OTs with similar duration (for−3/2 distribution law), but also it is able to detect the flares with 0.13–5 s time scale,undetectable by the all other systems.The camera is placed in the Northern Caucasus (near the 6-m optical telescope BTA)at the height of 2030 m above the sea level (see fig. 1). Since 2003 it has been monitoringon the regular basis the part of HETE-2 gamma-ray satellite field of view. For thewhole period of observations (approximately 150 good nights) no GRB triggers hit themonitored field. The camera detects the large number of meteors (approximately 300)and satellites (approximately 150) each night. The work is in progress.∗ ∗ ∗This work was supported by grants of CRDF (No. RP1-2394-MO-02), RFBR (No.04-02-17555), and INTAS (04-78-7366). SK thanks Russian Science Support Foundationfor support. GB thanks Cariplo Foundation for the scholarship and Merate Observatoryfor hospitality.REFERENCES[1] Eichler D. and Beskin G., Phys. Rev. Lett., 85 (2000) 13.[2] Liang et al., ApJL, 519 (1999) L21.[3] Wu et al., MNRAS, 342 (2003) 1131.[4] Fukujita M. et al., PASP, 107 (1995) 945.[5] Rykoff E. S. et al., AAS, 199 (2001) 1205.[6] Boe¨r M. et al., AAS, 138 (1999) 579.[7] Burd A. et al., astro-ph/0411456.[8] Vestrand W. T. et al., SPIE, 4845 (2002) 126.

Optical camera with high temporal resolution to search for

transients in the wide field

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Optical Camera with high temporal resolution to search for transients in the wide field

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