The Astrophysical Journal, 808:74 (9pp), 2015 July 20 doi:10.1088/0004-637X/808/1/74 © 2015. The American Astronomical Society. All rights reserved. DISCOVERY OF γ-RAY EMISSION FROM THE STRONGLY LOBE-DOMINATED QUASAR 3C 275.1 Neng-Hui Liao1, Yu-Liang Xin1,2, Shang Li1,2, Wei Jiang1,2, Yun-Feng Liang1,2, Xiang Li1,2, Peng-Fei Zhang1,3, Liang Chen4, Jin-Ming Bai5, and Yi-Zhong Fan1 1 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China; [email protected], [email protected] 2 University of Chinese Academy of Sciences, Yuquan Road 19, Beijing 100049, China 3 Department of Physics, Yunnan University, Kunming 650091, China 4 Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China 5 Key Laboratory for the Structure and Evolution of Celestial Objects, Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China Received 2015 January 2; accepted 2015 June 7; published 2015 July 17 ABSTRACT We systematically analyze the 6 year Fermi/Large Area Telescope (LAT) data on lobe-dominated quasars (LDQs) in the complete LDQ sample from the Revised third Cambridge Catalogue of Radio Sources (3CRR) survey and report the discovery of high-energy γ-ray emission from 3C 275.1. The γ-ray emission of 3C 207 is confirmed and significant variability of the light curve is identified. We do not find statistically significant γ-ray emission from other LDQs. 3C 275.1 is the known γ-ray quasar with the lowest core dominance parameter (i.e., R = 0.11). We also show that both the northern radio hotspot and parsec jet models can reasonably reproduce the γ-ray data. The parsec jet model, however, is favored by the potential γ-ray variability on a timescale of months. We suggest that some dimmer γ-ray LDQs will be detected in the future and LDQs could contribute non-ignorably to the extragalactic γ-ray background. Key words: galaxies: active – galaxies: jets – quasars: individual (3C 275.1) – radiation mechanisms: non-thermal 1. INTRODUCTION consistent with the expectation that L is foreshortened at small viewing angles due to the geometrical projection effect (Hough Active galactic nuclei (AGNs) powered by accretion of & Readhead 1989). Observation of the parsec-scale morphol- material into super-massive black holes (SMBHs) are the most ogy of LDQs suggests that no counter jets are seen and the luminous and persistent sources of electromagnetic radiation in fi ( structural variations and the flux variability are mild (Hough the universe. In the orientation-based uni ed models Anto- ) nucci 1993; Urry & Padovani 1995), the observed diversity of et al. 2002 . These phenomena can be well understood via the AGNs has been ascribed to a few physical parameters such as orientation-dependent relativistic beaming scheme if such the orientation of the accretion disk/torus and jet to the objects are viewed at intermediate angles. This argument is supported by the anti-correlation between R and the width of observer, the accretion rate, and the mass and angular ( ) momentum of SMBHs (e.g., Antonucci 1984; Osterbrock & the broad line emissions e.g., Baker & Hunstead 1995 . Thus, Pogge 1985; Meier 1999; Ghisellini et al. 2009). the orientation angles of LDQs can be restricted by these ° °( ) The simple relativistic jet model (Blandford & Rees 1978) approaches, ranging from 10 to 40 Aars et al. 2005 . has thus far been reasonably successful in accounting for the Since the successful launch of the Fermi Gamma-ray Space ( ) primary properties of radio-loud AGNs, which constitute about Telescope Atwood et al. 2009 , our understanding of radio- 10% of all AGNs. In the unified scheme of radio-loud AGNs, it loud AGNs has been revolutionized. In the second Fermi/Large ( ) ( is postulated that blazars are viewed pole-on (Blandford & Area Telescope LAT source catalog 2FGL, Nolan ) γ Königl 1979; Antonucci & Ulvestad 1985) and hence the et al. 2012 , the extragalactic -ray sky is dominated by radiation of blazars is overwhelmed by the luminous and radio-loud AGNs. The vast majority of these sources are rapidly variable Doppler-boosted jet emission (Ulrich blazars (Ackermann et al. 2011). By comparison, MAGNs are et al. 1997). By comparison, radio-loud AGNs with misdir- not expected to be strong GeV sources (Abdo et al. 2010a). ected jets (i.e., the misaligned AGNs (MAGNs) including Recently, an analysis of 4 year LAT data shows that the radio galaxies and steep spectrum radio quasars, or SSRQs) number of significantly detected γ-ray MAGNs is still only a exhibit steep radio spectra and bipolar or quasi-symmetrical handful (Di Mauro et al. 2014). Nevertheless, detecting the γ- radio structures. Deboosted radio emissions from mildly ray emissions from MAGNs is attractive because they offer a relativistic outflows and/or extended radio lobes are significant different perspective than blazars for approaching high-energy for MAGNs, while relativistic core radio emissions are phenomena (Georganopoulos & Kazanas 2003a). In the dominant for blazars. The ratio of these two components, 2FGL, most γ-ray-detected MAGNs are Fanaroff–Riley type ( ) γ R º-SScore[] total S core (which is also called the core dom- FR, Fanaroff & Riley 1974 I radio galaxies whose -ray inance parameter, where S represents the observed flux density), emissions can be naturally explained by structured jet radiation is routinely used for classification (Orr & Browne 1982;Hough models (Georganopoulos & Kazanas 2003b; Ghisellini et al. & Readhead 1989; Yuan & Wang 2012).Lobe-dominated 2005). Since they are relatively nearby, γ-ray emissions have quasars (LDQs) are a special type of SSRQs with R <1. been well detected in these sources despite their large jet Since lobe emission should be orientation-unbiased, LDQs inclination angles and small R values. On the other hand, no FR are widely used to test the unified scheme of radio-loud AGNs. II sources at relatively high redshifts viewed at large angles, The anti-correlation between R and the projected linear size of such as strongly lobe-dominated quasars (R ; 0.1), have been a jet L in the complete sample of double-lobed radio quasars is reported in the 2FGL. The successful detection of γ-ray 1 The Astrophysical Journal, 808:74 (9pp), 2015 July 20 Liao et al. emissions from LDQs is valuable because such data may It has a large peak effective area (∼8000 cm2 for 1 GeV provide us with structural information about jets. Not only are photons) monitoring ;2.4 sr of the full sky with an angular the lobes of LDQs capable of generating strong radio resolution (68% containment radius) better than 1° at 1 GeV. In emissions, but also hotspots in the radio lobes are significant its routine survey mode, LAT performs a complete and uniform X-ray emitters (e.g., Massaro et al. 2011). Moreover, the coverage of the sky every 3 hr. modeling of multi-wavelength spectral energy distributions The latest Pass 7 reprocessed data used in our analysis were (SEDs) of these hotspots suggests that they could be potential collected in the energy range of 0.1–100 GeV during the first γ-ray emitters (e.g., Zhang et al. 2010). The discovery of 6 years of operation (i.e., from 2008 August 4 to 2014 August extended γ-ray emission from the nearby radio galaxy 4). Photon events belonging to the SOURCE class have been Centaurus A is in support of such a hypothesis (Abdo taken into account. The updated standard ScienceTools et al. 2010b).Ifγ-ray emissions from the hotspots of some software package version v9r33p0 together with the instrument LDQs have been detected, they may consist of a non-ignorable response functions of P7REP_V15 are adopted throughout the part of the extragalactic γ-ray background (EGB) and may be data analysis. The galactic diffuse model we use is accelerators of ultra-high-energy comic rays (Rachen & gll_iem_v05_rev1.fit and the isotropic diffuse emission Biermann 1993; Massaro & Ajello 2011). template is taken as iso_source_v05_rev1.txt.7 The entire In this work, we systematically analyze the 6 year Fermi/ data set is filtered with gtselect and gtmktime tasks following LAT data of the complete LDQ sample from the Revised third the standard analysis threads.8 Cambridge Catalogue of Radio Sources (3CRR) survey The binned likelihood algorithm implemented in the gtlike (Hough & Readhead 1989) to search for γ-ray counterparts. task has been adopted to extract the flux and spectrum. The This work is organized as follows. In Section 2 the LDQ region-of-interest (ROI) of each LDQ is taken to be a 20° ×20° sample and routines of Fermi/LAT and Chandra data analysis box centered at its radio position. Such a size is to match the are introduced. Results of γ-ray characteristics of the LDQs are broad point-spread function (PSF) of 100 MeV photons. The reported in Section 3. Finally, in Section 4 we summarize our data are binned into 30 logarithmically distributed energy bins results with some discussions. and 100 × 100 spatial bins with a size of 0 ◦.2. All sources from the 2FGL (Nolan et al. 2012) within 20° of the radio position 2. THE SAMPLE AND DATA ANALYSES are included. The flux and spectral parameters of sources within the ROI together with normalization factors of the two 2.1. The Sample diffuse backgrounds are set free, while parameters of other We analyze the complete LDQ sample (Hough & Read- sources are fixed to be those reported in the 2FGL. First, we ) ( ) add a presumed γ-ray source corresponding to the LDQ into the head 1989 from the 3CRR survey Laing et al.