| الملخص: | 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 ∼ 10 ^11 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 .
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