GeV emission from the nearby radio galaxy Centaurus A P. Sreekumara,b D.L. Bertscha R.C. Hartmana P.L. Nolanc D.J. Thompsona 9 9 aCode 661, NASA/Goddard Space Flight Center 9 b 1 Universities Space Research Association n cW.W. Hansen Laboratory, Stanford University a J 0 2 Abstract 1 v EGRET has detected 67 sources associated with active galactic nuclei. With the 7 exception of radio galaxy Cen A, all are classified as belonging to the blazar class of 7 2 active galactic nuclei. The cumulative exposurefrom multiple EGRET observations 1 has provided the first clear detection of Centaurus A. Unlike the γ-ray blazars seen 0 9 by EGRET which are believed to exhibit near-alignment of the central jet along 9 the line-of-sight, Cen A provides the first evidence for >100 MeV emission from / h a source with a confirmed large-inclination jet. Although the high-energy emission p represents a lower luminosity than most EGRET blazars, with the advent of new - o more sensitive instruments such as GLAST and VERITAS, the detection of off- r axis high-energy emission from more distant radio galaxies (space density of radio t s galaxies being ∼103 times the blazar density) is an exciting possibility. a : v i X Key words: radio galaxy; Centaurus A; gamma rays r a 1 Introduction Observations in the 30–10000 MeV energy range by the high-energy γ-ray telescope EGRET on board the Compton Observatory (CGRO) has shown the presence of a class of γ-ray bright blazars. Blazars are in general charac- terized by flat radio spectra, rapid time variability at most wavelengths and typically emit the bulk of their bolometric luminosity at γ-ray energies. The recently released 3rd EGRET catalog (1) contains 271 γ-ray sources of which 68 are known to be extragalactic (66 blazars, 1 radio galaxy (Cen A) and 1 2rd normal galaxy (LMC)). Thus almost of the known γ-ray sources remain 3 unidentified. Preprint submitted to Elsevier Preprint 5 February 2008 We report here the results from the analysis of all available EGRET data ∼ ( 10 weeks of on-axis exposure) on the nearby radio galaxy Centaurus A. ∼ Centaurus A, at a distance of 3.5 Mpc (2), is the closest active galactic nucleus. The proximity of Cen A has made it the subject of numerous studies at many wavelengths from radio to TeV energies. Radio studies have shown the presence of a double-lobed radio morphology (3). In the past, the presence of an obscuring dust lane has prevented detailed optical studies of the central nucleus andthe inner regionsof thejet. However recent NICMOS observations (4) show extended emission and a bright unresolved central nucleus which ∼ may have associated with it a small ( 40 pc diameter) inclined disk. A one- sided X-ray jet is visible (5) and is collimated in the direction of the giant radio lobes. At TeV energies Grindlay et al.(6) used an early non-imaging Cherenkov system to report the discovery of emission from Cen A during a period of overall high activity at lower frequencies. More recent observations by more sensitive instruments (CANGAROO) provide only upper limits (7). Earlier attempts to detect Cen A from individual EGRET observations (typ- ically 2 weeks long) were hampered by weak detection significance which re- sultedinlargepositionaluncertainty.Thepossibilityofassociationoftheγ-ray ◦ excess with another likely candidate, BL Lac object 1312-423 (1.95 away from Cen A), could not be ruled out . The strong Galactic diffuse emission and the larger uncertainties associated with the diffuse model (8; 9), also contributed to the difficulties in confirming the detection early on in the mission (10; 11). 2 Results A likelihood analysis (12) shows a 6.5σ detection of a point source type ex- cess, positionally coincident with Centaurus A. The average >100 MeV flux ± × −8 −2 −1 is (13.6 2.5) 10 photons cm s . The nearby BL Lac object 1312-423 is well outside the 95% confidence contour. Unlike the variability seen at lower energies(13),theemissionabove100MeVappearssteady.Thelackofvariabil- itycouldarisefromthenear thresholddetectionassociatedwiththeindividual observations. The 30-10000 MeV photon spectrum is well characterized by a ± single power law of index 2.40 0.28. This is steeper than the average power law spectrum from γ-ray blazars (2.15±0.04) (14) and also steeper than the observed extragalactic γ-ray background (2.10±0.03) (9). A comparison of the EGRET measurements with OSSE (15) and COMPTEL (16) data at keV andMeVenergiesyieldsasmooth,continuousspectrumthatappearstoevolve from a power law above 200 keV (index=1.97) and steepen gradually above 1 MeV. The inclusion of significant systematic errors in comparing results from different instruments is small, as evidenced by the good cross-comparison of the single power-law Crab pulsar spectrum across these energy bands (17). A systematic search forγ-ray emission fromother nearby radio galaxies/Seyferts 2 yielded no significant detection. 3 Conclusions EGRET localization and spectral measurements provide unique confirmation that the source detected by OSSE and COMPTEL is Cen A. Unlike the er- ror regions defined by OSSE and COMPTEL, the nearby XBL 1312-4221 ∼ ◦ ( 2 away from Cen A) lies well outside the EGRET 99% confidence contour. The consistency of the spectrum going from 50 keV to 1 GeV argues favorably for emission from a single source coincident with Cen A. A detailed analysis of EGRET archival data shows >100 MeV emission from only the nearest radio galaxy (Cen A). The low γ-ray luminosity of Cen A (∼1041 ergs/s, about 105 times smaller than the typical γ-ray blazar) if typical of this source class, provides the most likely explanation for the non-detection of more distant members of this source class. If Cen A is indeed a misaligned blazar (18) this provides new evidence for >100 MeV emission from radio-loud AGN with jets at large inclination angles (>60◦). This is contrary to the model of Skibo, Dermer & Kinzer (19) which suggests a significant cut off in the spectrum around a few MeV. Assuming a unification model for AGN, and increasing high-energy emission with decreas- ing inclination angles (20), the detection of more distant radio-loud AGN with intermediate jet inclination angles can be expected. This provides a new ex- tragalactic source class for future high-energy experiments such as GLAST and VERITAS. 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