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

Abstract

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. \textbf{The blackbody emits in the photospheric radius $R_{\mathrm{ph}}\sim 10^{11}$ cm,} and its temperature ($kT$) decreases with time from \textbf{1.08 to 0.37 keV, its Lorentz factor of blackbody (${\Gamma }_{\mathrm{BB}}$) decreases with time from 67.71 to 46.70. The luminosity of the blackbody ($L_{\mathrm{BB}}$), $kT$ and ${\Gamma }_{\mathrm{BB}}$ follow the relations ${\mathbf{L}}_{\mathrm{BB}}\propto \mathbf{k}{\mathbf{T}}^{2.49\pm 0.03}$ and ${\Gamma }_{\mathrm{BB}}\propto L_{\mathrm{BB}}^{0.27}$ (estimated from \cite{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 ${\alpha }_{\mathbf{O},2}\approx 0.88$ in the normal decay phase, and the X-ray flux decays with a similar index of ${\alpha }_{\mathbf{X},1}\approx 0.95$. There is no obvious spectral evolution in the normal decay phases, with photon index $\hat{\Gamma }=1.86$ 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 $4.29\times 10^{-2}$ to $8.23\times 10^{-3}$.