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