Abstract
GRB 060614 is a remarkable gamma-ray burst (GRB) observed by Swift with puzzling properties, which challenge current progenitor models. In particular, the lack of any bright supernova (SN) down to very strict limits and the vanishing spectral lags during the whole burst are typical of short GRBs, strikingly at odds with the long (102 s) duration of this event. Here we present detailed spectral and temporal analysis of the Swift observations of GRB 060614. We show that the burst presents standard optical, ultraviolet and X-ray afterglows, detected beginning 4 ks after the trigger. An achromatic break is observed simultaneously in the optical and X-ray bands, at a time consistent with the break in the R-band light curve measured by the VLT. The achromatic behaviour and the consistent postbreak decay slopes make GRB 060614 one of the best examples of a jet break for a Swift burst. The optical and ultraviolet afterglow light curves have also an earlier break at 29.7 ± 4.4 ks, marginally consistent with a corresponding break at 36.6 ± 2.4 ks observed in the X-rays. In the optical, there is strong spectral evolution around this break, suggesting the passage of a break frequency through the optical/ultraviolet band. The very blue spectrum at early times suggests this may be the injection frequency, as also supported by the trend in the light curves: rising at low frequencies, and decaying at higher energies. The early X-ray light curve (from 97 to 480 s) is well interpreted as the X-ray counterpart of the burst extended emission. Spectral analysis of the BAT and XRT data in the ∼80 s overlap time interval show that the peak energy of the burst has decreased to as low as 8 keV at the beginning of the XRT observation. Spectral analysis of following XRT data shows that the peak energy of the burst continues to decrease through the XRT energy band and exits it at about 500 s after the trigger. The average peak energy Ep of the burst is likely below the BAT energy band (<24 keV at the 90% confidence level) but larger than 8 keV. The initial group of peaks observed by BAT (∼5 s) is however distinctly harder than the rest of the prompt emission, with a peak energy of about 300 keV as measured by Konus Wind. Considering the time-averaged spectral properties, GRB 060614 is consistent with the Eiso − E rest , Eγ − E rest ,a ndLp,iso − E rest correlations.
Highlights
The Swift Gamma-Ray Burst Explorer (Gehrels et al 2004), successfully launched on 2004 Nov. 20, is a multi-wavelength space observatory with a payload that includes one wide-field instrument, the Burst Alert Telescope (BAT, 15−350 keV energy band; Barthelmy et al 2005b), and two narrow-field instruments (NFIs), the X-Ray Telescope (XRT, 0.2−10 keV; Tables 1 and 2 are only available in electronic form at http://www.aanda.orgBurrows et al 2005) and the Ultraviolet/Optical Telescope (UVOT, 1700−6500 Å; Roming et al 2005)
Duration, Ep ∼ 300 keV) followed by a longer (∼170 s), softer and highly variable bump which ends in a tail that smoothly matches the early and partly simultaneous X-ray light curve observed by XRT
The measured Ep variation strongly supports the interpretation of the early XRT light curve of gamma-ray burst (GRB) 060614 as the low energy counterpart of the fading and softening tail seen by BAT
Summary
The Swift Gamma-Ray Burst Explorer (Gehrels et al 2004), successfully launched on 2004 Nov. 20, is a multi-wavelength space observatory with a payload that includes one wide-field instrument, the Burst Alert Telescope (BAT, 15−350 keV energy band; Barthelmy et al 2005b), and two narrow-field instruments (NFIs), the X-Ray Telescope Gehrels et al (2006) noted that the first 5 s of the prompt emission, including the brightest peaks of the BAT light curve, show many sub-pulses with time lags consistent with zero, like short bursts and unlike long GRBs, which usually have positive lags (Norris 2002). In the peak luminosity-time lag plane, the brightest peak of GRB 060614 lies in the region occupied by short bursts (Gehrels et al 2006) Another similarity with short bursts is the structure of the BAT light curve, starting with a series of bright, hard peaks, followed by a group of lower luminosity, softer peaks and a smooth tail. The list of all BAT, XRT and UVOT observations of GRB 060614 used for the present analysis is shown in Tables 1 and 2.
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