GRB 170519A: Thermal Radiation in an X-Ray Flare and Decaying Magnetic Fields for the Early-time Afterglow

Abstract 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 Γ BB ∝ L 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 Γ ˆ = 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 .