Fermi Large Area Telescope View of the Core of the Radio Galaxy Centaurus A

We present gamma-ray observations with the LAT on board the Fermi Gamma-Ray Telescope of the nearby radio galaxy Centaurus~A. The previous EGRET detection is confirmed, and the localization is improved using data from the first 10 months of Fermi science operation. In previous work, we presented the detection of the lobes by the LAT; in this work, we concentrate on the gamma-ray core of Cen~A. Flux levels as seen by the LAT are not significantly different from that found by EGRET, nor is the extremely soft LAT spectrum ($\G=2.67\pm0.10_{stat}\pm0.08_{sys}$ where the photon flux is $\Phi\propto E^{-\G}$). The LAT core spectrum, extrapolated to higher energies, is marginally consistent with the non-simultaneous HESS spectrum of the source. The LAT observations are complemented by simultaneous observations from Suzaku, the Swift Burst Alert Telescope and X-ray Telescope, and radio observations with the Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry (TANAMI) program, along with a variety of non-simultaneous archival data from a variety of instruments and wavelengths to produce a spectral energy distribution (SED). We fit this broadband data set with a single-zone synchrotron/synchrotron self-Compton model, which describes the radio through GeV emission well, but fails to account for the non-simultaneous higher energy TeV emission observed by HESS from 2004-2008. The fit requires a low Doppler factor, in contrast to BL Lacs which generally require larger values to fit their broadband SEDs. This indicates the $\g$-ray emission originates from a slower region than that from BL Lacs, consistent with previous modeling results from Cen~A. This slower region could be a slower moving layer around a fast spine, or a slower region farther out from the black hole in a decelerating flow.


Introduction
Since blazars are strong sources of beamed γ-rays, it is natural to think that radio galaxies may 62 be also. Several radio galaxies were detected by EGRET: 3C 111 (Hartman et al. 2008), NGC 6251 63 (Mukherjee et al. 2002), and Centaurus (Cen) A (Sreekumar et al. 1999;Hartman et al. 1999). The 64 identifications were rather uncertain, due to the large EGRET error circles. Only two radio galaxies have been detected so far with the latest generation of TeV atmospheric Cherenkov telescopes, HESS, providing a detailed look at the γ-ray spectrum essential for addressing emission models. In 83 addition to the LAT γ-ray source in the central few kpc (hereafter the γ-ray "core"), γ-rays from the 84 giant lobes of Cen A have also been seen with Fermi, with the origin likely to be Compton scattering which is a study of γ-ray emission of the core, the lobes are essentially background sources. 90 We present a summary of Cen A and observations of this object in § 2. The observations of 91 the core of Cen A with the LAT over the first 10 months of Fermi operation are presented in § 3. 92 We also present simultaneous Cen A core observations from Suzaku and Swift, and radio data from 93 the TANAMI program in § 4. In § 5 we combine these with archival data and model its SED of the 94 Cen A core. In § 6 we discuss the implications in detail, and we conclude with a brief summary ( § 95 7 and Chandra can resolve X-ray emission from it, which is likely caused by synchrotron emission 120 (Kraft et al. 2002;Hardcastle et al. 2003). The innermost region of Cen A has been resolved with 121 VLBI, and shown to have a size of ∼ 3 × 10 16 cm (Kellermann et al. 1997;Horiuchi et al. 2006).

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Observations at shorter wavelengths also reveal a small core, namely VLT infrared interferometry 123 which resolves the core size to ∼ 6×10 17 cm (Meisenheimer et al. 2007). VLBI images reveal a weak 124 counter jet on the milli-arcsecond scale (Jones et al. 1996). Based on the motion of the VLBI blobs, 125 and assuming the brightness differences of the different jets are due to Doppler effects, Tingay et al.    Figure 1 shows the the 0.2−30 GeV LAT image centered on Cen A, which is clearly detected.

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Also prominent is the Galactic emission toward the south, and several faint sources in the field. We Cen A was outside of the95 localization circle, so that there was some ambiguity as to whether 195 EGRET was actually detecting Cen A, but the new LAT position confirms the earlier 3EG result.

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The Suzaku data were fit with a single absorbed power-law, which was found to have a spectral   Event files were calibrated and cleaned with standard filtering criteria with the xrtpipeline task 305 using the latest calibration files available in the Swift CALDB.

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The XRT dataset was taken entirely in Windowed Timing mode. For the spectral analysis we  The combined January X-ray spectrum is highly absorbed. Hence it was fitted with an ab- -13 - The XRT spectrum included in the broadband SED was binned to ensure a minimum of 2500 318 counts per bin and was de-absorbed by forcing the absorption column density to zero in XSPEC, 319 and applying a correction factor to the original spectrum equal to the ratio of the de-absorbed 320 spectral model over the absorbed model. LAT emission originate from the same region, which is explored below ( § 5.2).

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Since the cores of many blazars have been shown to be γ-ray loud it is plausible to assume that 339 the radio core is the source of the central γ-rays from Cen A. However, one should keep in mind that where the Doppler factor is δ D = [Γ j (1 − β j µ)] −1 , the bulk Lorentz factor of the jet is Γ j =

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(1 − β 2 j ) −1/2 , β j c is the speed of the jet, and θ = cos −1 µ is the angle of the jet with respect to our 369 line of sight. Solving for Γ j in terms of δ D , . (2) In order for Γ j to be real, the quantity under the radical must be positive, which implies consistent with these observations.

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Finally, we note that the SED presented here is constructed from non-simultaneous data.

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Although Fermi and HESS γ-rays do not show appreciable variability, they could still be vari-

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The maximum energy to which cosmic rays can be accelerated is limited by the size scale of 474 the emitting region and the highest energy they can reach before they are cooled. The former 475 constraint implies that the highest energy a cosmic ray can reach is . For Cen A, D = 3.7 Mpc = 1.1 × 10 25 cm, and z ≈ 0. The spectral parameters can be obtained from the SED of the core of Cen A (see Fig. 5): ǫ syn pk = 1.6 × 10 −7 , ǫ SSC pk = 0.3, f syn pk = 3 × 10 −10 erg s −1 cm −2 , and f SSC pk = 9 × 10 −10 erg s −1 cm −2 . Note that here we assume that the X-ray data is from the jet; see above. Below the break in the synchrotron spectrum, A ≈ 0.5, and above A ≈ −1. The highest energy photon bin in the HESS spectrum is ǫ γ = 8 × 10 6 , so that f syn      ). Curves are model fits to nuclear region of Cen A. The green curve is a synchrotron/SSC fit to the entire data set. The dashed green curve shows this model without γγ attenuation. The violet curve is a similar fit but is designed to under fit the X-ray data, and the brown curve is designed to fit the HESS data while not over-producing the other data in the SED. The blue curve is the decelerating jet model fit (Georganopoulos & Kazanas 2003). See Table 2 for the parameters of these model curves.