X-ray and Radio Follow-up Observations of High-Redshift Blazar
  Candidates in the Fermi-LAT Unassociated Source Population
  We report on the results of X-ray and radio follow-up observations of two GeV
gamma-ray sources 2FGL J0923.5+1508 and 2FGL J1502.1+5548, selected as
candidates for high-redshift blazars from unassociated sources in the {\it
Fermi} Large Area Telescope Second Source Catalog. We utilize the Suzaku
satellite and the VLBI Exploration of Radio Astrometry (VERA) telescopes for
X-ray and radio observations, respectively. For 2FGL J0923.5+1508, a possible
radio counterpart NVSS J092357+150518 is found at 1.4 GHz from an existing
catalog, but we do not detect any X-ray emission from it and derive a flux
upper limit $F_{\rm 2-8 keV} <$ 1.37 $\times$ 10$^{-14}$ erg cm$^{-2}$
s$^{-1}$. Radio observations at 6.7 GHz also result in an upper limit of
$S_{\rm 6.7 GHz}$ $<$ 19 mJy, implying a steep radio spectrum that is not
expected for a blazar. On the other hand, we detect X-rays from NVSS
J150229+555204, the potential 1.4 GHz radio counterpart of 2FGL J1502.1+5548.
The X-ray spectrum can be fitted with an absorbed power-law model with a photon
index $\gamma$=1.8$^{+0.3}_{-0.2}$ and the unabsorbed flux is $F_{\rm 2-8
keV}$=4.3$^{+1.1}_{-1.0}$ $\times$ 10$^{-14}$ erg cm$^{-2}$ s$^{-1}$. Moreover,
we detect unresolved radio emission at 6.7 GHz with flux $S_{\rm 6.7 GHz}$=30.1
mJy, indicating a compact, flat-spectrum radio source. If NVSS J150229+555204
is indeed associated with 2FGL J1502.1+5548, we find that its multiwavelength
spectrum is consistent with a blazar at redshift $z \sim 3-4$.
