2025ApJ...984...81Z

GRB 170519A was discovered by the Swift Burst Alert Telescope (BAT) and then observed by the Swift X-Ray Telescope, Swift UltraViolet and Optical Telescope, and ground-based telescopes. We report Lick Katzman Automatic Imaging Telescope observations of GRB 170519A and make temporal analysis and spectral joint fits of its multiwavelength light curves. The observations present a relatively complete afterglow structure, including two X-ray flares (Flares I and II), optical onset (Slice 1), normal decay (Slices 2 and 3), and a possible jet break. The spectrum of the bright X-ray flare (Flare II) indicates that a thermal component exists at t = 190-240 s. The blackbody (BB) emits in the photospheric radius R_ph ∼ 10¹¹ cm, and its temperature (kT) decreases with time from 1.08 to 0.37 keV, while its Lorentz factor of BB (Γ_BB) decreases with time from 67.71 to 46.70. The luminosity of the BB (L_BB), kT, and Γ_BB follows the relations L_BB ∝ kT^2.49±0.03 and ${{\rm{\Gamma }}}_{{\rm{BB}}}\propto {L}_{{\rm{BB}}}^{0.27}$. In the optical light curves, there is an onset bump in the early-time afterglow, rising with an index α_O,1 ≈ -0.43 and peaking ∼1174.9 s since the BAT trigger. The bump then decays with α_O,2 ≈ 0.88 in the normal decay phase, and the X-ray flux decays with a similar index of α_X,1 ≈ 0.95. There is no obvious spectral evolution in the normal decay phases, with photon index $\hat{{\rm{\Gamma }}}=1.86$ and 1.92 in Slices 2 and 3, respectively. We find that the multiwavelength light curves of the GRB 170519A afterglow can be well fitted by an external shock with time-dependent ϵ_B. In the early afterglow, the value of ϵ_B decays rapidly from 4.29 × 10^–2 to 8.23 × 10^–3.