Analysis of the Prompt Optical Emission of the Naked-Eye GRB 080319B
C. Bartolini, G. Greco, A. Guarnieri, A. Piccioni, G. Beskin, S. Bondar, S. Karpov, E. Molinari
aa r X i v : . [ a s t r o - ph . H E ] J un Analysis of the Prompt Optical Emission of the Naked-EyeGRB 080319B
C. Bartolini ∗ , G. Greco ∗ , A. Guarnieri ∗ , A. Piccioni ∗ , G. Beskin † , S. Bondar ∗∗ , S.Karpov † and E. Molinari ‡ ∗ Astronomy Department of Bologna University, Bologna, Italy † Special Astrophysical Observatory of Russian Academy of Science, Nizhnij Arkhyz, Russia ∗∗ Institute for Precise Instrumentation, Nizhnij Arkhyz, Russia ‡ INAF-TNG, Canary Islands, Spain
Abstract.
We present the observed / intrinsic optical parameters and the variability analysis of the Naked-Eye Burst, GRB080319B, observed by the TORTORA wide-field optical monitoring system. The event is extreme not only in observed prop-erties but also intrinsically: it is the most luminous event ever recorded at optical wavelengths. The temporal properties suggestshort-lived periodic activities of the internal engine. This is the fastest optically variable source detected at cosmological dis-tances. Keywords: g -ray burst, ground-based observations, time variability. PACS:
INTRODUCTION TO THEPHENOMENON: PROMPT OPTICALEMISSION
Over the past nine years the prompt optical emissionscontemporaneous with the g -ray-active phase of a GRBhave became subject of exciting debate in the astronomi-cal community; the brevity of these phenomena and theirfleeting nature makes them elusive and difficult to study.Due to the relative short duration of the prompt g -rayemission (T ∼ g -ray telescope onboard of the Swift satellite (BAT: Burst Alert Telescope)has permitted to trace the optical burst time structure ofthe GRB 080319B with unprecedented level of accuracy.At the beginning of the May 2006 TORTORA was in-stalled in the dome of the REM telescope located at ESO-La Silla Observatory (Chile) and successfully achievedits First Light [6]. OBSERVATION AND DATA REDUCTION
Observation
On 19 March 2008 at 06:12:49 UT (hereafter t ) theSwift’s Burst Alert Telescope triggered and located GRB080319B (trigger = 306757; [7]) with a ∼
3’ radius errorbox. The bright burst was simultaneously detected by theKonus-Wind (KW) satellite [8] yielding a burst fluenceof S g = 6.23 ± × − erg cm − [20 keV to 7 MeV].Assuming z = 0.937 [9] and the standard cosmologymodel ( H = 71 Km s − M pc − , W M = 0.27 and W L =0.73) the isotropic energy release is E iso = 1 . × erg .The field of the GRB 080319B was imaged before theGRB event by three independently ground-based opticalsky monitoring. No optical precursors were detected inTORTORA, "Pi of the Sky" and RAPTOR surveys withobservations starting 26 minutes, 16 seconds and 30 min-utes before the Swift / BAT trigger, respectively. In RAP-TOR [10] and "Pi of the Sky" [11] the first image with de-tectable optical emission started at t = 1.87 ± t = 2.75 ± ≈ m . In the TORTORA high temporal resolution dataset(0.13 sec exposure time), the first frame in which we de-tected the optical flux started at t = 9.18 ± ≈ m [12]. The bright visual peaks which oc-curred during the prompt g -ray emission approximatelyreached V ≈ . m , this made it visible with naked eye inthe BOOTES constellation for ∼
40 seconds, assumingan observer in a dark location.Since 05:46:22 UT REM telescope had observed thebox of previous burst, GRB 080319A. At 06:12:49 UT,Naked-Eye Burst, GRB 080319B, flashed at ∼
10 degreesfrom the former, near the edge of TORTORA’s field ofview. At 06:13:13 UT, REM started automatic repoint-ing, and from 06:13:20 UT the burst location stayed atthe center of camera’s field of view. The observationalconditions at that time of g –trigger were suboptimal. Theburst occurred at a zenith distance ≈ ◦ , the sky wasbright due to a nearly full moon, and a large part of cam-era field of view had been covered by the REM dome.The Fig. 1 shows a summary of TORTORA, "Pi of theSky" and Swift BAT light curves. Data Reduction
The values of the effective air mass at middle time ex-posure and the seeing as measured by La Silla–MeteoMonitor were ∼ ∼ R v = 3.1) implies A B = 0.049, A V = 0.034, A R = 0.030. The TORTORA fast wide-field camera took the data on the field of GRB 080319Bfrom 05:46:22 UT ( t =
26 minutes before trigger) to06:15:41.00 ( t =
172 sec after trigger) collecting ∼ PHOT/DAOPHOT function in
IRAF . http://grb.sonoma.edu/. IRAF: Image Reduction and Analysis Facility, http://iraf.noao.edu/.
TABLE 1.
Prompt Optical Parametersof GRB 080319B: Peak Flux.
Peak Flux [ erg cm − s − ] F opt , (2 . ± . ) × − F opt , (2 . ± . ) × − F opt , (1 . ± . ) × − F opt , (1 . ± . ) × − For the REM repointing time interval fluxes have beenderived using custom elliptic aperture photometry codeafter summation of 10 consecutive frames with compen-sated motion of the stars. Unfortunately, it seems impos-sible to reconstruct the light curve of this interval withany better resolution due to massive blurring of star PSFcaused by their motion. For all other intervals, photome-try has been performed both with 10-frames (1.3 sec ef-fective exposure) binning, and with original (0.13 sec)time resolution. As a result, we have identified a newpeak in the prompt optical light curve. Finally the pho-tometry performed in instrumental system was calibratedtowards the V magnitudes of several nearby Tycho2stars. We have no data on prompt light curve color forGRB 080319B at early times ( t < t < 60 sec). Thus, noadditional color corrections have been applied to TOR-TORA’s unfiltered data. LIGHT CURVE STRUCTURE
TORTORA has tracked a fast rise of optical emissionfrom t +10 sec to t +15 sec, followed by a complex evo-lution until t +45 sec and a slow decay thereafter. Therise from V ≈ . m to V ≈ . m may be approximatedby a ∼ t power-law originated at t ≈
0; while g –rayemission started earlier, at t ≈ -4 sec. The decay since t +45 sec is a ∼ t − . power-law. Four peaks can clearlybe seen in optical data with an inter-peak separation of ∼ Observed and Intrinsic Optical Parameters
The fluxes of the four well-detected optical peaks F opt , , , , F opt = × ( . × − × − . × mag ) (1)were obtained by using the calibration of [14] andwere corrected for galactic extinction (Table 1). Hostgalaxy reddening correction A v was applied assumingthe mean value reported by [15], E (B–V) = 0.05. Theisotropic equivalent Luminosity L opt (Table 2) for the IGURE 1.
The light curve of GRB080319B acquired by TORTORA’s wide-field camera (upper curve) alongside withSwift / BAT g -ray one (lower curve, light gray color). Also, transient brightness measurements by "Pi of the Sky" optical cameraare shown. The Swift / BAT light curve is the sum of all four energy channels. TORTORA data points show both full resolution(original data frames, grey color) and low-resolution (10 images co-added, red color). Full-resolution data are unavailable for thetime interval of REM telescope repointing due to massive blurring of object image. well-detected optical peaks is related to the peak flux F opt , , , , using the equation L opt = pk opt ( z ) D l ( z ) F opt (2)where D l ( z ) is the luminosity distance for the cosmo-logical standard model and k opt ( z ) is the cosmological k -correction that accounts for the transformation of the V passband in the proper GRB frame: k opt = R n V ( + z ) n V ( + z ) n − b d n R n V n V n − b d n = ( + z ) − b (3)Here, n V and n V are the frequency boundaries of the V band and b is the power-law index in the optical spec-trum F n (cid:181) n − b . For k -correction we assume b = . ± .
07 as reported by [15]. The optical fluence S opt , V wasdetermined by numerically integrating the prompt lightcurve in the interval from the earliest observation to thelatest one with a power-law interpolation of the flux inthe segments between the experimental points yielding S opt , V = ( . ± . ) × − erg cm − (1 .
87 sec < t <86 sec). The isotropic equivalent total energy in V band E opt , V in the rest frame of the source was determinedfrom the optical fluence S opt , V using the relation: E opt , V = pk opt ( z ) D l ( z ) S opt , V ( + z ) (4) TABLE 2.
Prompt Optical Parameters ofGRB 080319B: Peak Luminosity.
Peak Luminosity [ erg s − ] L peak , ( . ± . ) × L peak , ( . ± . ) × L peak , ( . ± . ) × L peak , ( . ± . ) × yielding E opt , V = ( . ± . ) × erg . The com-plex evolution from t +15 sec to t +45 sec consists oftwo well-distinct regions of different peak intensity lev-els: F opt , , ≈ . × − erg cm − s − from t +15 secto t +30 sec and F opt , , ≈ . × − erg cm − s − from t +30 sec to t +45 sec which roughly correspondto two regions of g -ray light curve. The event is extremenot only in observed properties but also intrinsically: it isthe most luminous event ever recorded at optical wave-lengths and has an exceedingly high isotropic-equivalentenergy release in g -rays. The previous record was held inbrightness by GRB 990123 [1], GRB 050904 [16], GRB061007 [17]. In spite of its initial brightness, the behav-ior of the afterglow at middle/late time does not appear tobe peculiar. The extrapolated luminosity e.g. at 10 hourand at 13 hour after the trigger in the rest frame of thesource ( L opt @10 h ≈ × erg s − and L opt @13 h ≈ × erg s − , respectively) are comparable withthe average luminosity of the afterglow sample detectedover the past few years [18]. ariability Analysis As reported previously the light curve is approximatedby a four nearly equidistant flares with an inter-peakseparation of ∼ t +40 sec to t +50 sec aperiodicity at a frequency of ∼ CONCLUSIONS
The prompt optical emission of the GRB 080319B is pe-culiar for several reasons: it is the most luminous eventever recorded reaching a visual peak absolute magnitudeof M V , peak = -38.4 and is the variable object at cosmolog-ical distances with the shortest optical periodicity everdiscovered. The temporal structures reflect the behaviourof the internal engine (periodicity of ∼ ∼ ACKNOWLEDGMENTS
This work was supported by the University of BolognaProgetti Pluriennali 2003, by grants of CRDF (No.RP1-2394-MO-02), RFBR (No. 04-02-17555 and 06-02-08313), INTAS (04-78-7366), and by the Presidium ofthe Russian Academy of Sciences Program.
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