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Not to appear in Nonlearned J.,45.Early (<0.3day)R-band light curve of the optical afterglow of GRB030329Yuji Urata 1,2,Takashi Miyata 3,Shingo Nishiura 3,Toru Tamagawa 1,R.A.Burenin 4Tomohiko Sekiguchi 5,Seidai Miyasaka 6,Chiaki Yoshizumi 7,Junzi Suzuki 8Hiroyuki Mito 3,Yoshikazu Nakada 3,Tsutomu Aoki 3,Takao Soyano 3,Kenichi Tarusawa 3,Shigetomo Shiki 1,and Kazuo Makishima 1,9urata@crab.riken.go.jp ABSTRACT We observed the optical afterglow of the bright gamma-ray burst GRB030329on the nights of 2003March 29,using the Kiso observatory (the University of Tokyo)1.05m Schmidt telescope.Data were taken from March 2913:21:26UT to 17:43:16(0.072to 0.253days after the burst),using an Rc -band ?lter.The obtained Rc -band light curve has been ?tted successfully by a single power law function with decay index of 0.891±0.004.These results remain unchanged when incorporating two early photometric data points at 0.065and 0.073days,reported by Price et al.(2003)using the SSO 40inch telescope,and further including
RTT150data(Burenin et al.2003)covering at about0.3days.Over the period
of0.065-0.285days after the burst,any deviation from the power-law decay is
smaller than±0.007mag.The temporal structure reported by Uemura et al.
(2003)does not show up in our R-band light curve.
Subject headings:Gamma-ray Bursts:afterglow
1.Introduction
The bright and long gamma-ray burst(GRB),GRB030329,was detected on2003March 2911:37:14.67UT,with the HETE-2spacecraft(Ricker et al.2003).The burst lasted for more than25s in the30-400keV band.The?uence of the burst was1x10?4ergs cm?2and the peak?ux over1.2s was>7×10?6ergs cm?2s?1(i.e.,>100×Crab?ux)in the same energy band.The subsequent ground analysis of the HETE-2data produced an accurate location for the burst,which was reported in a GCN Position Notice at12:50:24UT,73 minutes after the burst.The location is centered atα2000=10h44m49s,δ2000=+21?28′44′′, with the90%-con?dence error radius of2′(Vanderspek et al.2003).
The optical afterglow was found within the2′-radius error circle at0.077days after the burst,at the coordinates ofα2000=10h44m49.s5,δ2000=+21?31′23′′.1(Peterson and Price2003;Torii2003).The redshift of GRB030329was determined as0.168±0.001by numerous absorption and emission lines in the optical spectra of the afterglow(Greiner et al.2003;Price et al.2003).In the afterglow spectrum,a bright Type Ic supernova feature appeared at～7.6days;the associated supernova has been named SN2003dh(e.g., Stanek et al.2003;Hjorth et al.2003;Chornock et al.2003;Zaritsky et al.2003). This detection strongly suggests a physical link between GRBs and supernovae explosion. Uemura et al.(2003)claim that the optical light curve of the afterglow,produced by their un?ltered observation,deviates from a single power-law decay,exhibiting temporal breaks at0.085±0.028,0.163±0.060,and0.227±0.043days.
We have established a GRB follow-up observation system at Kiso observatory(Urata et al.2003).Because the Japan area had been blank for the GRB follow-up observational network,this observational system is very important in studying the temporal and spectral evolution of early(<1day)afterglows.Another merit of the system is its multi-color ca-pability,using two instruments(2k×2k CCD and KONIC)and B-,V-,Rc-,Ic-,J-,H-, and K-bandpass?lters(Urata et al.2003).We have already performed early optical color measurements of a number of GRB afterglows.In the present burst,we have also performed an early Rc-band follow-up observation of the afterglow,from0.072to0.253days.Here
we study the temporal evolution of the optical afterglow over a very early phase(0.065to 0.286days),by combining our Kiso measurements with the published photometric points from the Siding Spring Observatory(SSO040-inch telescope(Price et al.2003),and the1.5 m Russian-Turkish Telescope(RTT150)by Burenin et al.(2003).
2.Observations
We carried out Rc-band follow-up observations of the optical afterglow of GRB030329, using the system described in section1.Speci?cally,we used the1.05m Schmidt telescope and a2k×2k CCD Camera at Kiso observatory,the University of Tokyo.The?eld of view was51.′2×51.′2and the pixel size was1′′.5square.The observation started at2003March 13:21:26UT(0.075days),and ended at17:43:16(0.253days).Meanwhile,we acquired36 Rc-band images in total,mostly with60-s exposure.An example of the Rc-band image we obtained is shown in?gure1,where the afterglow is extremely bright.
3.Analysis
The data reduction was carried out by a standard method using the NOAO IRAF. We performed the dark-subtraction and?at-?elding correction using appropriate calibration data.The photometric calibrations were done for all frames using APPHOT package in IRAF,with7standard stars around the afterglow suggested by Henden(2003).We indicate these standard stars in?gure1.For each data,we set the one-dimensional aperture size to 4times as large as the full-width at half maximum of the objects.The magnitude error in each optical images is estimated asσ2e=σ2ph+σ2sys,whereσph is the photometric errors of GRB030329afterglow,estimated from the output of IRAF PHOT,andσsys is the photomet-ric calibration error estimated by comparing our instrumental magnitude of the7standard stars over the36frames.The typical errors are0.001～0.003mag.for photometric,and ～0.007mag.for systematic.
4.Result
In?gure2,we plot the Rc-band light curve of the afterglow of GRB030329based on our photometry.The light curve is well?tted with a single power law function of the form ∝t?α,where t is the time after the burst andαis the decay index;we have obtained α=0.891±0.004with reduced chi-squared(χ2/ν)of0.819forν=34.In order to better
constrain the early-time(<0.25day)behavior of the light curve,we combined our data with the two Rc-band photometric points reported by Price et al.(2003)using the SSO40-inch telescope;R=12.6±0.015mag.at0.065day,and R=12.786±0.017mag.at0.073day.These are among the earliest?ltered observations of this afterglow.We have successfully?tted the combined Rc-band light curve again with a single power law,of which the decay index is 0.891±0.003withχ2/ν=0.817forν=36.We have further included13Rc-band photometric data points obtained at about0.3days after the burst by Burenin et al.(2003)using the Russian-Turkish telescope(RTT150),but the results did not change either;the decay index is0.890±0.03,withχ2/ν=1.03forν=49.
5.Discussion
We observed the optical afterglow associated with GRB030329from0.072to0.253days after the burst.The light curve based on our photometry is well?tted by a single power law function with a decay index of0.891±0.004.The result remains essentially unchanged when we include the published two SSO data points(Price et al.2003)covering an even earlier phase(up to0.065days),and the published13RTT150data points(Burenin et al. 2003)taken at about0.3days.Thus,our single power-law representation of the Rc-band decay light curve is valid altogether over a period of0.065–0.285days after the burst.
Uemura et al.(2003)reported that their un?ltered light curve has three temporal breaks,at0.085±0.028,0.163±0.060,and0.227±0.043days,across which the power-law decay index changes by0.2–0.5.In?gure2,we have plotted their best-?t model function, together with the±0.04magnitude constant error suggested by them.Because the model normalization is not speci?ed by Uemura et al(2003),we estimated it using0.2day data points of their?gure1.Although their measurements are thus consistent with ours within their measurement errors,the reality of the suggested breaks remain an important issue to be clari?ed.
For a more quantitative examination of the issue,we split our data into three phases, namely0.065 We also tried to?t our light curve with the smoothly broken power-law model used by Uemura et al.(2003),which is expressed as f(t)={f1(t)?n+f2(t)?n}?1/n with f i=k i t?αi (i=1,2),where t is the time after the burst onset,f(t)is the R c-band?ux,α1andα2are the decay indices at early and late times respectively,k i is a normalization constant,and n indicates a smoothness parameter.The break times were?xed to the values reported by Uemura et al.(2003).Furthermore,we?xed n at unity after Uemura et al(2003)when ?tting the0.085 We further tried to?t the present light curve(Kiso,SSO,and RTT150)with the best-?t model function reported by Uemura et al.(2003),in which the over all normalization alone is allowed to vary.The?tting has yielded withχ2/ν=6.91forν=50.Based on this large value ofχ2/ν,we can rule out the model of Uemura et al.(2003)at more than99.99% con?dence level.A discrepancy between the?ltered and un?ltered light curves could arise if there were signi?cant color changes during the decay.In this case,a similar deviation from a single power-law decay should be observed in un?ltered light curves by other observers. However,there is no such structures in the un?ltered light curve obtained by Torii et al. (2003);it is?tted successfully by a single power law with a decay index of0.891±0.016,in a good agreement with the present result.Furthermore Uemura et al.(2003)report that their un?ltered CCD has response close to that of the Rc-system.Therefore,the origin of the discrepancy must be found somewhere else. We thank many high school students who participated in the education program at Kiso observatory in the night to collaborate with us.We also thank RTT150collaboration for their observations as well as for making their data publicly available.Y.Urata acknowledges support from the Japan Society for the Promotion of Science(JSPS)through JSPS Research Fellowships for Young Scientists. REFERENCES Burenin,R.,et al.2003,Astronomy Letters,29,573 Chornock,R.,Foley,R.J.,Filippenko,A.V.,Papenkova,M.,&Weisz,D.2003,IAU Circ., 8114 Greiner,J.,et al.,2003,GCN Circ.,2020 Henden,A.,2003,GCN Circ.,2023 Hjorth,J.,et al.,2003,Nature,423,847 Matheson,T.,et al.,2003,ApJ,582,L5 Matheson,T.,et al., Peterson and Price,2003,GCN Circ.,1985 Price,P.,A.,et al.,2003,Nature,423,844 Ricker,G.,R.,et al.,2003,AIP conf.proce.,662,3 Stanek,K.,Z.,et al.,2003,ApJ,591,L17 Torii,K.,2003,GCN Circ.,1986 Torii,K.,et al.,2003,ApJ,597,L101 Uemura,M.,Kato,T.,Ishioka,R.,Yamaoka,H.,Monard,B.,Nogami,D.,Maehara,H., Sugle,A.,&Takahashi,S.,2003,Nature,423,843 Urata Y.,2003,Rome2002procceings Vanderspek,R.,et al.,2003,GCN Circ.,1997 Zaritsky,D.,Bolte,M.,Garnavich,P.M.,Bonanos,A.Z.,&Stanek,K.Z.2003,GCN Circ., 2169 Fig.1.—An Rc-band image of the GRB030329?eld obtained at the Kiso observatory,with a60-s exposure starting at2003March2914:23:50UT(0.116days after the burst).The afterglow is indicated by an arrow near the image center.Circles indicate the standard stars used in the photometric calibration. Fig.2.—The Rc-band light curve based on the photometry at Kiso,shown together with the SSO(Price et al.2003)and the RTT150(Burenin et al.2003)data points.The solid line indicates the best?t power-law to the Kiso and SSO points.Dashed lines indicate±0.04 mag error band around the un?ltered light curve reported by Uemura et al(2003). Table1:Results of?tting the present Rc-band light curve. 0.065 —0.085d0.906±0.0010.895±0.0010.69c11 0.085 —0.163d0.949±0.0500.844±0.0530.4925 0.163 —0.227d0.881±0.0010.873±0.001 1.2033
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