REM observations of GRB 060418: the fireball Lorentz factor determination

0.1 MB·English

by  S. D. Vergani

IL NUOVOCIMENTO Vol. ?, N. ? ?
REM observations of GRB060418: the fireball Lorentz factor de-
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termination
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2 S. D. Vergani(1) on behalf of the REM collaboration

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( ) DIAS - DCU, Ireland
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Summary.—WemeasuredthefireballLorentzfactorofGRB060418bythedirect
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observation of the onset of the NIR afterglow carried with the REM telescope. We
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8 found Γ0 ∼400.
6 PACS 98.70.Rz – gamma-ray bursts..
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1. – Data
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r REM(RapidEyeMount;http://www.rem.inaf.it/)isa60cmdiameterfastreact-
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s ing robotic telescope located at the ESO-La Silla in Chile, primarily designed to follow
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the early phases of the afterglow of GRBs detected by spaceborne γ-alert systems such
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v as Swift [1, 2]. The telescope hosts two instruments: REMIR, an infrared (z′,J,H,K′)
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X imaging camera, and ROSS, a visibile imager (V, R, I filters) and slitless spectrograph.
GRB060418 was detected by Swift at 03:06:08UT [3], with T = 52 ± 1s (90%
r 90
a error). The REM telescope beganobserving the field of GRB06041864s after the burst
(39s after the reception of the alert). A bright NIR source was identified [4]. REM
′ ′
followedthe eventdowntothesensitivitylimits inz JHK -bands. LaterTNGandVLT
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observationswereobtainedbyourteam. Thecompletelight-curves,includingalsothez
point reported by [5], are shown in Fig.1 together with the Swift-XRT light-curve. The
light-curveinthe NIRbandshowsarapidincreasefollowedbyamaximumandthenthe
beginning of the regular power-law decay. As the decay after the peak is not different
fromthelaterafterglowpowerlaw,weinterprettheriseasthebeginningoftheafterglow
[6]. In the X-ray band a prominent flare overimposed to the decaying X-ray afterglow,
also visible in the BAT data, was observed by XRT at about 128s after the trigger [7].
We interpret this component as some late activity of the inner engine, not correlated to
the standard afterglow emission.
2. – Determination of the Lorentz factor Γ
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The afterglow NIR light curve of the burst peak at a time t of 153s, therefore
peak
t > T as expected in an impulsive regime outflow ’thin shell’ case [8, 9]. The
peak 90
(cid:13)c Societa`ItalianadiFisica 1
2 S.D.VERGANIETC.
Fig. 1. – NIR,optical and X-ray afterglow light-curvesof GRB060418.
peak of the afterglow represents the time at which the dissipated power of the fireball
is maximum and it can be considered coincident at first approximationwith the fireball
deceleration timescale t =r /(2cΓ2), where r is the deceleration radius given by
dec dec dec
1/3
3E
(1a) r =
dec (cid:18)4π n m c2Γ2(cid:19)
0 p
foranoutflowtotalenergyE andanexternalmediumparticledensityn . Itistherefore
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possibleusingthetworelationstoestimatethefireballLorentzfactorΓatt /(1+z)≃
peak
t [9] which is expected to be Γ ∼ (1/2)Γ [8] where Γ is the initial fireball Lorentz
dec 0 0
factor. Using E =9×1052erg [10] andz =1.489[11], we obtain Γ∼200and Γ ∼400
γ 0
[6]. Thisresultisratherinsensitivetothe totalenergyandtothe density ofthe external
medium and it is in agreement with the fireball model prediction of Γ ∼> 100. This is
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the largest bulk Lorentz factor ever measured for any astrophysicalaccreting object.
REFERENCES
[1] Zerbi, R. M. et al., Astronomische Nachrichten, 322 (2001) 275
[2] Chincarini, G. et al., The Messenger, 113 (2003) 40
[3] Falcone, A. D. et al., GCN 4966 (2006).
[4] Covino, S. et al., GCN 4967 (2006).
[5] Nysewander, M. et al., GCN 4971 (2006).
[6] Molinari, E. et al., astro-ph/0612607 (2007).
[7] Falcone, A. D. et al., GCN 4973 (2006).
[8] M´esza´ros, P., Reports of Progress in Physics 69 (2006) 2259.
[9] Sari, R. et al., ApJ520 (1999) 641.
[10] Golenetskii, S. et al., GCN 4989 (2006).
REMOBSERVATIONSOFGRB060418: THEFIREBALLLORENTZFACTORDETERMINATION 3
[11] Dupree, A. K. et al., GCN 4969 (2006).