X-ray and radio follow-up observations of high-redshift blazar candidates in the Fermi-LAT unassociated source population

Y. Takahashi, J. Kataoka, K. Niinuma, M. Honma, Y. Inoue, T. Totani, S. Inoue, T. Nakamori, K. Maeda

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4 Citations (Scopus)


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 and the unabsorbed flux is F 2-8 keV = 4.3 × 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.

Original languageEnglish
Article number36
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2013 Aug 10


  • X-rays: general
  • galaxies: active
  • gamma rays: general
  • radiation mechanisms: non-thermal

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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