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 δ I = 0.88 − 7.49. The average Doppler factor (〈δ I〉 = 2.56 ± 0.30) of the 30 FR-Is is smaller than that (〈δ BL〉 = 10.28 ± 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 γ-ray luminosity against the photon spectral index ( log L γ – α 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.

Why Are Some Radio Galaxies Detected by Fermi, but Others Not?

“…Ye et al [25] have proposed that FR Is can be regarded as reliable standard candles; see also [29]. However, this observational fact is not valid for FR IIs; therefore, we only derive the Doppler factor for FR Is sources and perform the discussion on those detected by Fermi-LAT or not.…”

“…We adopt log L in 1.4GHz = 5 × 10 39 erg s −1 [25,30], and finally, Equation ( 7) yields log δ = (log…”

“…Ye et al [25] proposed an effective method to estimate the Doppler factors for FR Is via the standard cradles. We also derive the Doppler factors for Fermi-detected FR Is (FFR Is) and non-Fermi-detected FR Is (NFFR Is) using the method presented in Ye et al [25]. However, FR IIs cannot be seen as reliable standard cradles.…”

“…A unified model of AGNs proposes that blazars and radio galaxies are the same sources but observed from different angles. If blazars have discrepancies between Fermi-detected and non-Fermidetected sources due to a beaming effect, see e.g., [23][24][25], it is reasonable to expect that radio galaxies may exhibit similar variations. In order to answer the question of why some radio sources can be detected by Fermi but others cannot, we compile a sample of Fermi-detected radio galaxies and non-Fermi-detected radio galaxies as large as possible to compare them via several parameters, e.g., optical absolute magnitude (M r ), the ratio of flux density radio loudness (B), the Doppler factor of FR Is (δ), core dominance parameters (R) and the mass of the central black hole (M BH ).…”