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DISCOVERY of GEV EMISSION from the CIRCINUS GALAXY with the FERMI -LAT Masaaki Hayashida1, 2, 3,Lukaszstawarz 4, 5, Chi C

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DISCOVERY of GEV EMISSION from the CIRCINUS GALAXY with the FERMI -LAT Masaaki Hayashida1, 2, 3,Lukaszstawarz 4, 5, Chi C Accepted to the Astrophysical Journal SLAC-PUB-153836 Preprint typeset using LATEX style emulateapj v. 04/17/13 DISCOVERY OF GEV EMISSION FROM THE CIRCINUS GALAXY WITH THE FERMI -LAT Masaaki Hayashida1, 2, 3,LukaszStawarz 4, 5, Chi C. Cheung6, Keith Bechtol7, Greg M. Madejski2, Marco Ajello8, Francesco Massaro2, Igor V. Moskalenko2, Andrew Strong9, and Luigi Tibaldo2 Accepted to the Astrophysical Journal ABSTRACT We report the discovery of γ-ray emission from the Circinus galaxy using the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Circinus is a nearby (∼ 4 Mpc) starburst with a heavily obscured Seyfert-type active nucleus, bipolar radio lobes perpendicular to the spiral disk, and kpc-scale jet-like structures. Our analysis of 0.1{100 GeV events collected during 4 years of LAT observations reveals a significant (' 7:3 σ) excess above the background. We find no indications of variability or spatial extension beyond the LAT point-spread function. A power-law model used to describe the 0:1 − 100 GeV γ-ray spectrum yields a flux of (18:8 5:8) × 10−9 ph cm−2 s−1 and photon index 2:190:12, corresponding to an isotropic γ-ray luminosity of 3×1040 erg s−1. This observed γ-ray luminosity exceeds the luminosity expected from cosmic-ray interactions in the interstellar medium and inverse Compton radiation from the radio lobes. Thus the origin of the GeV excess requires further investigation. Subject headings: radiation mechanisms: non-thermal | galaxies: active | galaxies: individual (Circi- nus) | galaxies: jets | galaxies: Seyfert | gamma rays: galaxies 1. INTRODUCTION non-relativistic and comparatively low-powered (e.g., Lal Active galactic nuclei (AGN) often possess fast nu- et al. 2011), so that AGN and starburst activities may clear outflows and collimated radio-emitting jets pow- both be equally important factors in forming of kpc-scale ered by accretion onto supermassive black holes (e.g., outflows and lobes (e.g., Gallimore et al. 2006). Seyfert Krolik 1999). Relativistic jets produced in radio-loud galaxies seem γ-ray quiet as a class (Teng et al. 2011; AGN (such as blazars and radio galaxies), which are typ- Ackermann et al. 2012a). ically hosted by early-type galaxies, are well-established Besides radio-loud AGN, another class of established sources of Doppler-boosted γ-ray emission, dominating extragalactic γ-ray sources consists of galaxies lacking the extragalactic source population in the GeV range any pronounced nuclear activity, but experiencing a (Nolan et al. 2012). The bulk of the observed γ-ray jet burst of vigorous star formation. The observed GeV emission in such systems is believed to originate within emission from these sources is most readily explained by a few parsecs of the central black hole (e.g., Abdo et al. interactions of galactic cosmic rays (CRs) with ambient 2010a). However, Fermi-LAT observations of nearby ra- matter and radiation fields of the interstellar medium dio galaxies indicate that large-scale structures | here- (ISM; Ackermann et al. 2012b, and references therein). after `lobes' or `bubbles' | formed by jets outside their Starburst-driven outflows routinely found in such sys- hosts due to interactions with the intergalactic medium, tems may lead to the formation of large-scale bipolar can also be bright GeV emitters (Abdo et al. 2010b; see structures extending perpendicular to the galactic disks, also Katsuta et al. 2013). and somewhat resembling jet-driven AGN lobes (Veilleux et al. 2005). Radio-quiet AGN, the most luminous of which are clas- sified as Seyferts by their optical spectra, are typically Circinus is one of the nearest (distance D = 4:2 hosted by late-type galaxies. The `radio-quiet' label does 0:7 Mpc; Tully et al. 2009) and most extensively stud- not necessarily mean these sources are completely‘radio- ied composite starburst/AGN systems, but its location silent', and the nuclear jets found in these systems are behind the intense foreground of the Milky Way disk, (l, b) = (311:3◦, −3:8◦), resulted in exclusion from sev- 1 Institute for Cosmic Ray Research, University of Tokyo, 5- eral previous population studies using LAT data (Acker- 1-5 Kashiwanoha, Kashiwa, Chiba, 277-8582, Japan 2 mann et al. 2012a,b). Circinus exhibits a polarized broad Kavli Institute for Particle Astrophysics and Cosmology, ∼ −1 SLAC National Accelerator Laboratory, Stanford University, Hα line with FWHM 3000 km s (Oliva et al. 1998), 2575 Sand Hill Road M/S 29, Menlo Park, CA 94025, USA evidencing a heavily obscured Seyfert 2 nucleus which 3 email: [email protected] ranks as the third brightest Compton-thick AGN (Yang 4 Institute of Space and Astronautical Science, JAXA, 3-1-1 et al. 2009). Radio observations have mapped bipolar Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan 5 Astronomical Observatory, Jagiellonian University, ul. Orla bubbles extending orthogonal to the spiral host, as well 171, 30-244 Krak´ow,Poland as kpc-scale jet-like structures likely responsible for the 6 Space Science Division, Naval Research Laboratory, Wash- formation of the lobes (Elmouttie et al. 1998). The ex- ington, DC 20375-5352, USA tended morphology of the system including the lobes has 7 Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 been recently resolved and studied in X-rays (Mingo et 8 Space Sciences Laboratory, 7 Gauss Way, University of Cal- al. 2012). ifornia, Berkeley, CA 94720-7450, USA Here, we report the discovery of a significant γ-ray 9 Max-Planck Institut f¨ur extraterrestrische Physik, 85748 excess positionally coinciding with the Circinus galaxy Garching, Germany Published in arXiv:1310.1913. Work supported in part by US Department of Energy under contract DE-AC02-76SF00515. 2 Hayashida et al. Fig. 1.| The left color map represents the 1◦ × 1◦ (0◦:02 pixels) spatial TS variation of the > 100 MeV γ-ray signal excess centered on Circinus. The green lines denote positional errors of the γ-ray excess at 68 % and 95 % confidence levels for inner and outer lines, respectively. The magenta cross indicates the position of the galaxy core while the cyan ellipse corresponds to the positional error at 95% confidence of 2FGL J1415.7−6520. The black square corresponds to the area of the right panel, which shows the Australia Telescope Compact Array (ATCA) 1.4 GHz radio contours at ' 2000 resolution (Elmouttie et al. 1998) superposed with the 2MASS H-band color image (Jarrett et al. 2003); here the galactic disk extends in the NE{SW direction, while the radio lobes in the SE{NW direction. using 4 years of Fermi-LAT data and discuss possible origins for this emission. 2. LAT DATA ANALYSIS AND RESULTS The LAT is a pair-production telescope onboard the Fermi satellite with large effective area (6500 cm2 on axis for > 1 GeV photons) and large field of view (2.4 sr), sen- sitive from 20 MeV to > 300 GeV (Atwood et al. 2009). Here, we analyzed LAT data for the Circinus region col- lected between 2008 August 5 and 2012 August 5, fol- lowing the standard procedure10, using the LAT analysis software ScienceTools v9r29v0 with the P7SOURCE V6 instrument response functions. Events in the energy range 0.1{100 GeV were extracted within a 17◦ × 17◦ re- gion of interest (RoI) centered on the galaxy core position (RA = 213◦:2913, Dec = −65◦:3390: J2000). γ-ray fluxes and spectra were determined by a maximum likelihood fit with gtlike for events divided into 0◦:1-sized pixels and Fig. 2.| GeV spectrum of Circinus as measured by LAT. The 30 uniformly-spaced log-energy bins. The background black lines represent the best-fit PL model (1 σ confidence band). model included all known γ-ray sources within the RoI The vertical bars denote 1 σ statistical errors for the flux estimates from the 2nd LAT catalog (2FGL: Nolan et al. 2012), within energy bins given by horizontal bars. The arrow denotes a except for 2FGL J1415.7−6520, located 0◦:266 away from 95% confidence level upper limit. Circinus. Additionally, the model included the isotropic and Galactic diffuse emission components11; flux normal- izations for the diffuse and background sources were left of the system. free in the fitting procedure. Assuming 2FGL J1415.7−6520 is an additional γ-ray Our analysis yields a test statistic (TS) of 58 for the source distinct from the Circinus galaxy, we repeat maximum likelihood fit when placing a new candidate the analysis including 2FGL J1415.7−6520 in the back- source at the core position of Circinus, corresponding to ground model. The γ-ray excess at the position of Circi- a formal detection significance of ' 7:3 σ. Figure 1 shows nus yields in such a case a TS of 52, while no sig- the spatial variation in the TS value for the candidate nificant excess can be seen at the formal position of source when evaluated over a grid of positions around 2FGL J1415.7−6520 (TS ' 5.6). Our analysis confirms the galaxy (TS map). Circinus is located within the 68% therefore a single point-like γ-ray excess positionally co- confidence region for the direction of the γ-ray excess. inciding with Circinus, which may be identified with The distribution of the γ-ray excess is consistent with 2FGL J1415.7−6520 after the localization of the 2FGL a point-like feature, but we note that the LAT does not source is refined using the 4 year-accumulation of LAT have sufficient angular resolution (see e.g., Ackermann et data. The 2FGL analysis flagged 2FGL J1415.7−6520 al. 2013b) to spatially resolve the core, the radio lobes, or as a source with a low signal-to-background ratio (2FGL the galactic disk (all shown in the right panel of Figure 1) flag was set to `4 ’), meaning its position can be rel- atively strongly affected by systematic uncertainties in 10 http://fermi.gsfc.nasa.gov/ssc/data/analysis/ the Galactic diffuse emission model (see also Acker- 11 iso p7v6source.txt and gal 2yearp7v6 v0.fits mann et al.
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