Testing the distance duality relation using type Ia supernovae and
  ultracompact radio sources
  We test the possible deviation of the cosmic distance duality relation
$D_A(z)(1+z)^2/D_L(z)\equiv 1$ using the standard candles/rulers in a fully
model-independent manner. Type-Ia supernovae are used as the standard candles
to derive the luminosity distance $D_L(z)$, and ultra-compact radio sources are
used as the standard rulers to obtain the angular diameter distance $D_A(z)$.
We write the deviation of distance duality relation as
$D_A(z)(1+z)^2/D_L(z)=\eta(z)$. Specifically, we use two parameterizations of
$\eta(z)$, i.e. $\eta_1(z)=1+\eta_0 z$ and $\eta_2(z)=1+\eta_0 z/(1+z)$. The
parameter $\eta_0$ is obtained using the Markov chain Monte Carlo methods by
comparing $D_L(z)$ and $D_A(z)$ at the same redshift. The best-fitting results
are $\eta_0=-0.06\pm 0.05$ and $-0.18\pm 0.16$ for the first and second
parameterizations, respectively. Our results depend on neither the cosmological
models, nor the matter contents or the curvature of the universe.
