The beaming effect for Fermi-LAT-detected FR-I radio galaxies

Abstract

Our knowledge of Giga-electron volt (GeV) radio galaxies has been revolutionized by the Fermi-LAT Telescope, which provides an excellent opportunity to study the physical properties of GeV radio galaxies. According to the radio power and morphology, radio galaxies can be separated into Fanaroff-Riley Type I radio galaxies (FR-Is) and Type II radio galaxies (FR-IIs). In this paper, we consider the unification of FR-Is and BL Lacertae objects (BL Lacs), and assume FR-Is to be a standard candle to discuss the beaming effect for Fermi-LAT-detected FR-Is. Our main conclusions are as follows: (1) The estimated Doppler factors for 30 Fermi-LAT-detected FR-Is are in a range of ${\delta }_{\mathrm{I}}=0.88-7.49$. The average Doppler factor ($<{\delta }_{\mathrm{I}}>=2.56\pm 0.30$) of the 30 FR-Is is smaller than that ($<{\delta }_{\mathrm{BL}}>=10.28\pm 2.03$) of the 126 Fermi-LAT-detected BL Lacs, supporting the unification model that FR-Is are regarded as the misaligned BL Lacs with smaller Doppler factors; (2) We propose that different regions of FR-Is in the plot of the $\gamma$-ray luminosity against the photon spectral index $(\log L_{\gamma }-{\alpha }_{\mathrm{ph}})$ may indicate the different beaming effects; (3) The average Doppler factor of the 6 TeV FR-Is is similar to that of the 24 non-TeV FR-Is, which implies that the difference between the TeV and GeV emissions is not driven by the beaming effect in the Fermi-LAT-detected FR-I samples.

arXiv comment15 pages, 6 figures, 2 tables. publication in PASP

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