tokens: for the jet structure finding almost identical results. Figure 6. GRB 061121 light curves as observed by XRT (energy flux in the 0.5–10 keV band, blue error bars) and UVOT (mean flux density in the white filter, red error bars, de-reddened assuming E ( B − V ) = 0.055) compared to the predictions of our model (solid lines), where both the high-latitude emission (dotted lines) and forward shock (dashed lines) emission are taken into account. The adopted parameters are reported in the text. The w filter model light curve accounts for the transmission curve of the filter, retrieved from the HEASARC website ( https://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/docs/uvot/ ). 3.2.2. GRB 100906A We collected multi-filter optical data from Gorbovskoy et al. ( 2012 ) and A. Melandri (2020, private communication). The observed magnitudes are corrected for galactic extinction (Schlafly Finkbeiner 2011 ) and for extinction in the host galaxy. Figure 7 shows the comparison between the observed light curves and our high-latitude and forward shock emission models, obtained assuming a Gaussian jet structure with Γ c = 160, θ c = 42, θ Γ = 33, E c = 5 × 10 53 erg, n = 15 cm −3 , p = 2.1, e = 0.03, and B = 2 × 10 −2 . For the prompt emission, we assume again a 2BPL spectrum as before, with parameters taken from spectral fitting of the brightest BAT pulse. Taking the fitting parameters from Oganesyan et al. ( 2017 ), we obtain , , and α 1 = −2/3, α 2 = −1/2 as before. The high-energy photon index is set to β = −3.2, and we assume a prompt emission radius R 0 = 3 × 10 15 cm. A second, later emission event with an almost comparable peak flux is present in the XRT light curve at around 100 s post-trigger. The spectrum of this pulse can be fitted by the 2SBPL with E peak ∼ 5 keV and E break ≲ 1 keV, with a high-energy spectral slope β ∼ −3.7. Although it is subdominant, given the softer spectrum, we included the high-latitude emission from this pulse for completeness. For this pulse, we adopt the same jet structure of the first one, and we assume a starting time t 0 = 85 s in the observer frame. Also in this case, the combination of the high-latitude and forward shock emission is able to explain the features of the multiwavelength light curves (see Figure 7 ), with the high-latitude emission dominating over the forward shock emission during the plateau phase in the X-rays, while it is negligible in the optical range. Note that we find equally satisfying results with a power-law structure, setting θ c = θ Γ = 2° and using slopes k E = 2 and k Γ = 3.1. In this latter case, the structure would be very similar to the one we find for GRB 061121, the differences being almost entirely in the distance and ISM density, pointing to the quasi-universal jet structure hypothesis (Salafia et al. 2015 , 2019a , 2019b ). Figure 7. Same as Figure 6 but for GRB 100906A light curves as observed by XRT (energy flux in the 0.5–10 keV band, cyan error bars) and in optical as
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