GRB 070518: A Gamma-ray Burst with Optically Dim Luminosity

TL;DR

This paper reports optical observations of GRB 070518, revealing it as an optically dim, possibly dark burst with properties consistent with external shock models and known gamma-ray burst relations.

Contribution

It provides detailed optical and X-ray analysis of GRB 070518, confirming its optical dimness and consistency with classical external shock models and gamma-ray burst relations.

Findings

GRB 070518 is an optically dim burst with a redshift upper limit of 0.7.

The optical and X-ray afterglows are consistent with the external shock model.

GRB 070518 fits the Amati and Ghirlanda relations under certain assumptions.

Abstract

We present our optical observations of {\em Swift} GRB 070518 afterglow obtained at the 0.8-m Tsinghua University-National Astronomical Observatory of China telescope (TNT) at Xinglong Observatory. Our follow-up observations were performed from 512 sec after the burst trigger. With the upper limit of redshift $\sim$0.7, GRB 070518 is found to be an optically dim burst. The spectra indices $\beta_{ox}$ of optical to X-ray are slightly larger than 0.5, which implies the burst might be a dark burst. The extinction $A_{V}$ of the host galaxy is 3.2 mag inferred from the X-ray hydrogen column density with Galactic extinction law, and 0.3 mag with SMC extinction law. Also, it is similar to three other low-redshift optically dim bursts, which belong to XRR or XRF, and mid-term duration($T_{90}<10$, except for GRB 070419A, $T_{90}$=116s). Moreover, its $R$ band afterglow flux is well fitted by a single power-law with an index of 0.87. The optical afterglow and the X-ray afterglow in the normal segment might have the same mechanism, as they are consistent with the prediction of the classical external shock model. Besides, GRB 070518 agrees with Amati relation under reasonable assumptions. The Ghirlanda relation is also tested with the burst.