Abstract
We report the first detection of an optical afterglow of a GRB (060108) that would have been classified as 'dark' in the absence of deep, rapid ground-based optical imaging with the 2-m robotic Faulkes Telesscope (FTN). Our multiwavelength analysis reveals an X-ray light curve typical of many Swift long GRBs (3-segments plus flare). Its optical afterglow, however, was already fainter than the detection limit of the UVOT within 100s of the burst. Optical imaging in BVRi' filters with the FTN began 2.75 minutes after the burst and resulted in the detection of the optical afterglow at 5.3 minutes, with a UKIRT K-band identification at ~45 mins. R and i'-band light curves are consistent with a single power law decay in flux, F(t) prop t^-a where a=0.43+/-0.08, or a 2-segment light curve with a steep decay a_1 <0.88, flattening to a_2 ~ 0.31, with evidence for rebrightening at i' band. Deep VLT R-band imaging at ~12 days reveals a faint, extended object (R ~23.5 mag) at the location of the afterglow. Although the brightness is compatible with the extrapolation of the a_2 slow decay, significant flux is likely due to a host galaxy. This implies that the optical light curve had a break before 12 days, akin to what observed in the X-rays. We derive a maximum photometric redshift z<3.2 for GRB 060108 and a best-fitting optical-to-X-ray SED at 1000 s after the burst consistent with a power law with index beta_OX = 0.54 and a small amount of extinction. The unambiguous detection at B-band and the derived photometric redshift rule out a high redshift as the reason for the optical faintness of GRB 060108. Instead, the hard opt/X-ray spectral index confirms it as one of the optically-darkest bursts detected and with modest host extinction explains the UVOT non-detection (abridged).
Highlights
Gamma ray bursts (GRBs) are brief, intense and totally unpredictable flashes of gamma rays on the sky that are thought to be produced during the core collapse of massive stars or the merger of two compact objects such as two neutron stars or a neutron star and stellar-mass black hole
We report the discovery of a faint optical afterglow detected in deep BV Ri′ band imaging obtained with the Faulkes Telescope North (FTN) beginning 2.75 minutes after the burst
The afterglow is below the detection limit of the Ultraviolet Optical Telescope (UVOT) within 100s of the burst, while is evident in K-band images taken with the United Kingdom Infrared Telescope (UKIRT) 45 minutes after the burst
Summary
Gamma ray bursts (GRBs) are brief, intense and totally unpredictable flashes of gamma rays on the sky that are thought to be produced during the core collapse of massive stars (long-duration bursts) or the merger of two compact objects such as two neutron stars or a neutron star and stellar-mass black hole. Until the recent launch of the Swift satellite in November 2004, it was notoriously difficult to observe GRBs at other wavelengths within seconds or minutes after the burst. Similar breakthroughs for short bursts have recently occurred, showing them to be extragalactic, but less luminous and less distant than long bursts (Gehrels et al 2005; Villasenor et al 2005; Fox et al 2005; Covino et al 2006; Barthelmy et al 2005). With the availability of Swift’s promptly-disseminated arcsec localizations and the on-board rapid-slew X-ray and ultraviolet/optical telescopes (XRT, UVOT; Gehrels et al 2004), multi-wavelength monitoring of GRBs from the earliest possible times is being performed for a significant number of bursts. Large aperture ground-based robotic telescopes such as the 2-m Liverpool (Steele 2004) and Faulkes telescopes respond rapidly to GRB alerts and begin automatically imaging the target field within minutes of receipt of an alert, providing early deep upper limits or multi-colour follow-ups of optical counterparts as faint as R ∼ 18 - 22 mag (e.g. Guidorzi et al 2005a, 2006b; Monfardini et al 2006b)
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