X-Ray and Radio Follow-up Observations of High-redshift Blazar Candidates in the Fermi-LAT Unassociated Source Population

Abstract

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 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 _{2-8 keV} < 1.37 × 10^-14 erg cm^-2 s^-1. Radio observations at 6.7 GHz also result in an upper limit of S _{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 γ = 1.8^{+0.3}_{-0.2} and the unabsorbed flux is F _{2-8 keV} = 4.3^{+1.1}_{-1.0} × 10^-14 erg cm^-2 s^-1. Moreover, we detect unresolved radio emission at 6.7 GHz with flux S _{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, then we find that its multiwavelength spectrum is consistent with a blazar at redshift z ~ 3-4.