A multiwavelength study of Swift GRB 060111B constraining the origin of its prompt optical emission

Abstract

Context. The detection of bright optical emission measured with good temporal resolution during the prompt phase of GRB 060111B makes this GRB a rare event that is especially useful for constraining theories of the prompt emission. Aims. For this reason an extended multi-wavelength campaign was performed to further constrain the physical interpretation of the observations. Methods. In this work, we present the results obtained from our multi-wavelength campaign, as well as from the public Swift/BAT, XRT, and UVOT data. Results. We identified the host galaxy at R ∼ 25 mag from deep R-band exposures taken 5 months after the trigger. Its featureless spectrum and brightness, as well as the non-detection of any associated supernova 16 days after the trigger, enabled us to constrain the distance scale of GRB 060111B within 0.4 ≤ z ≤ 3 in the most conservative case. The host galaxy spectral continuum is best fit with a redshift of z ∼ 2, and other independent estimates converge to z ∼ 1-2. From the analysis of the early afterglow SED, we find that non-negligible host galaxy dust extinction, in addition to the Galactic one, affects the observed flux in the optical regime. The extinction-corrected optical-to-gamma-ray SED during the prompt emission shows a flux density ratio F γ /F opt = 10 −2 -10 −4 with spectral index β γ,opt > β γ , strongly suggesting a separate origin of the optical and gamma-ray components. This result is supported by the lack of correlated behavior in the prompt emission light curves observed in the two energy domains. The temporal properties of the prompt optical emission observed during GRB 060111B and their similarities to other rapidly-observed events favor interpretation of this optical light as radiation from the reverse shock. Observations are in good agreement with theoretical expectations for a thick shell limit in slow cooling regime. The expected peak flux is consistent with the observed one corrected for the host extinction, likely indicating that the starting time of the TAROT observations is very near to or coincident with the peak time. The estimated fireball initial Lorentz factor is Γ ≥ 260-360 at z = 1-2, similar to the Lorentz factors obtained from other GRBs. GRB 060111B is a rare case of a GRB with both a bright, well-observed optical counterpart and a "canonical" early X-ray light curve, thus providing a good test case of the reverse shock emission mechanism in both energy ranges.

THE AFTERGLOWS OFSWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFTANDSWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

“…This has been interpreted as the rise of the forward-shock afterglow at deceleration time, with classical examples being GRBs 060418 and 060607A (Molinari et al 2007;Nysewander 70 Racusin et al (2008) interpret the intermediately rapid decay in the early light curve of this afterglow as a reverse-shock flash component that becomes dominant over the very rapidly fading prompt optical emission. 71 A reverse-shock origin has also been implied for the very early, steeply decaying emission of GRB 060117 (mentioned before, Jelínek et al 2006b) and GRB 060111B (Stratta et al 2009), but the latter GRB, which incidentally shows evidence for very high host extinction as well, has no redshift beyond estimates and is also missing from our sample.…”

“…• Initial fast decay: These afterglows show a similar steep-to-shallow transition as described above, but the observations did not begin until after the peak, implying it must be very early. An example is GRB 090102 (Gendre et al 2010, see also Stratta et al 2009). To our knowledge, an early steep decay for GRB 090424 is reported here for the first time.…”

Gamma-Ray Bursts Polarization

“…The latter measure allowed to point out the presence of a magnetized reverse shock with an ordered magnetic field, confirming the presence of high magnetic fields in the GRB ejecta, and indicating that the multi-wavelength approach could be fruitful, even if there is currently no consensus on the common origin of the γ-ray and optical emission in the Gamma-Ray Bursts Polarization 61 prompt phase of GRBs (e.g. Vestrand et al 2005, Stratta et al 2009, Götz et al 2011, Guidorzi et al 2011, Kopač et al 2013.…”

A faint optical flash in dust-obscured GRB 080603A: implications for GRB prompt emission mechanisms

“…The variety of observed behaviours in other GRBs is rich: the prompt optical was observed to be uncorrelated with the ongoing high‐energy emission for GRB 990123 (Akerlof et al 1999; e.g. see also GRB 060111B, Klotz et al 2006; Stratta et al 2009; GRB 080607, Perley et al 2011), whereas a strong correlation was observed, for example, for GRB 050820A (Vestrand et al 2006), superposed on the onset of the afterglow (Cenko et al 2006). Similar cases of some degree of correlation between the γ‐ray prompt and optical emissions are GRB 041219A (Blake et al 2005; Vestrand et al 2005), GRB 060526 (Thöne et al 2010) and GRB 080319B (Racusin et al 2008; Beskin et al 2010).…”