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 {\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 $F_{2-8keV}<$ 1.37 $\times$ 10${}^{-14}$ erg cm${}^{-2}$ s${}^{-1}$. Radio observations at 6.7 GHz also result in an upper limit of $S_{6.7GHz}$ $<$ 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${}_{-0.2}^{+0.3}$ and the unabsorbed flux is $F_{2-8keV}$=4.3${}_{-1.0}^{+1.1}$ $\times$ 10${}^{-14}$ erg cm${}^{-2}$ s${}^{-1}$. Moreover, we detect unresolved radio emission at 6.7 GHz with flux $S_{6.7GHz}$=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 $z\sim 3-4$.

24 pages, 7 figures, 6 tables, accepted for publication in ApJ

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• The VLA Low-frequency Sky Survey
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