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

GRB 170519A was discovered by \emph{Swift}/BAT, and then observed by \emph{Swift}/XRT, \emph{Swift}/UVOT, and ground-based telescopes. We report Lick/KAIT 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. $\mathbf{\text{The}}\mathbf{\text{blackbody}}\mathbf{\text{emits}}\mathbf{\text{in}}\mathbf{\text{the}}\mathbf{\text{photospheric}}\mathbf{\text{radius}}{R}_{\mathrm{p}\mathrm{h}}\sim {10}^{11}\mathbf{\text{cm,}}$ and its temperature ($kT$) decreases with time from $\mathbf{\text{1.08}}\mathbf{\text{to}}\mathbf{\text{0.37}}\mathbf{\text{keV,}}\mathbf{\text{its}}\mathbf{\text{Lorentz}}\mathbf{\text{factor}}\mathbf{\text{of}}\mathbf{\text{blackbody}}\mathbf{\text{(}}{\mathrm{\Gamma }}_{\mathrm{B}\mathrm{B}}\mathbf{\text{)}}\mathbf{\text{decreases}}\mathbf{\text{with}}\mathbf{\text{time}}\mathbf{\text{from}}\mathbf{\text{67.71}}\mathbf{\text{to}}\mathbf{\text{46.70.}}\mathbf{\text{The}}\mathbf{\text{luminosity}}\mathbf{\text{of}}\mathbf{\text{the}}\mathbf{\text{blackbody}}\mathbf{\text{(}}{L}_{\mathrm{B}\mathrm{B}}\mathbf{\text{),}}kT\mathbf{\text{and}}{\mathrm{\Gamma }}_{\mathrm{B}\mathrm{B}}\mathbf{\text{follow}}\mathbf{\text{the}}\mathbf{\text{relations}}{\mathbf{L}}_{\mathrm{B}\mathrm{B}}\propto \mathbf{k}{\mathbf{T}}^{\mathbf{2.49}\pm \mathbf{0.03}}\mathbf{\text{and}}{\mathrm{\Gamma }}_{\mathrm{B}\mathrm{B}}\propto L_{\mathrm{B}\mathrm{B}}^{0.27}\mathbf{\text{(estimated}}\mathbf{\text{from}}\mathbf{\text{cite}}\mathbf{\text{fan2012).}}$ In the optical light curves, there is an onset bump in the early-time afterglow, rising with an index ${\alpha }_{O,1}\approx -0.43$ and peaking $\sim 1174.9$ s since the BAT trigger. The bump then decays with ${\mathbf{\alpha }}_{\mathbf{O},\mathbf{2}}\approx \mathbf{0.88}$ in the normal decay phase, and the X-ray flux decays with a similar index of ${\mathbf{\alpha }}_{\mathbf{X},\mathbf{1}}\approx \mathbf{0.95}$. There is no obvious spectral evolution in the normal decay phases, with photon index $\widehat{\mathrm{\Gamma }}=1.86$ 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 ${\epsilon }_B$. In the early afterglow, the value of ${\epsilon }_B$ decays rapidly from $\mathbf{4.29}\times {\mathbf{10}}^{-\mathbf{2}}$ to $\mathbf{8.23}\times {\mathbf{10}}^{-\mathbf{3}}$.