GRB 070518: a gamma-ray burst with optically dim luminosity

We present our optical observations of the 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 s after the burst trigger. With the upper limit of redshift ∼0.7, GRB 070518 is found to be an optically dim burst. The spectra indices β<inf>ox</inf> of optical to X-ray are slightly larger than 0.5, which implies that the burst might be a dark burst. The extinction <it>A<inf>V</inf></it> 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 the Small Magellanic Cloud extinction law. Also, it is similar to three other low-redshift optically dim bursts, which belong to X-ray rich gamma-ray bursts or X-ray flashes and have mid-term duration (<it>T</it><inf>90</inf> < 10, except for GRB 070419A, <it>T</it><inf>90</inf>= 116 s). Moreover, its <it>R</it>-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 the Amati relation under reasonable assumptions. The Ghirlanda relation is also tested with the burst.

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