Centaurus A: Multiwavelength observations of the nearest active galaxy from radio to gamma-rays

Adv. Space Res. Vol. 23, No. S/6, pp. 911-914, 1999 Q 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/99 $20.00 + 0.00 PII: SO273-1177(99)00239-2

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CENTAURUS A: MULTIWAVELENGTH OBSERVATIONS OF THE NEAREST ACTIVE GALAXY FROM RADIO TO GAMMA-RAYS H. Steinlel,

J. Bonnell’,

W. Hermsen7,

’ 2 s 4 5 6 ’ a

R.L. Kinzer3,

A. Tzioumis4,

H. Bignall’,

V. Schonfelderl,

K. Bennett’,

J. Ryan’

Max-Planck-Institut fur extraterrestrische Physik, Postfach 1603, 85740 Garching, NASA/GSFC, Greenbelt, MD 20771, U.S.A. NRL, 4555 Overlook Av., SW, Washington DC 20374-5352, U.S.A. ATNF, P. 0. Box 76, Epping, NSW 2121, Australia University of Adelaide, Adelaide, SA 5005, Australia Astrophysics Division, ESTEC, 2200 AG Noordwijk, The Netherlands SRON-Utrecht, Sorbonnelaan 2, 358.4 CA Utrecht, The Netherlands Space Science Center, University of New Hampshire, Durham, NH 03824, U.S.A.

Germany

ABSTRACT Centaurus A (Cen A, NGC 5128) is the nearest active galaxy and, notably, the viewing angle with respect to the jet axis is very large (> 70”). A first contemporaneous OSSE, COMPTEL, and EGRET spectrum obtained in October 1991 covers an energy range from 50 keV up to 1 GeV. This y-ray broad-band spectrum was taken when Cen A was in an intermediate emission state as defined by the BATSE X-ray light-curve. The first simultaneous multiwavelength spectrum from radio to y-rays was measured in July 1995 when Cen A was in a low emission state (the prevailing state for the last 7 years). The different spatial and temporal resolution in the different frequency regimes produces problems in the construction and interpretation of the multiwavelength spectra. These are addressed in this paper. The detection of emission > 1 MeV makes the inclusion of such high-energy emission into models for the spectral energy distribution mandatory. 01999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION With a distance of less than 4 Mpc (Hui et al., 1993) Cen A is the nearest active galaxy. The viewing angle with respect to the jet axis is greater than 70” (Jones et al., 1996)). This enables observers to investigate an Active Galactic Nucleus in great detail. However, this causes problems in the analysis and interpretation of multiwavelength data. In the optical, the central part of Cen A is obscured by a dust lane (Schreier et al., 1996), which, among others, is supposed to indicate, that the complex system is the result of a recent merger of an elliptical and spiral galaxy (Hui et ul., 1995). The active radio galaxy Cen A was one of the few known MeV y-ray sources when the Compton Gamma Ray Observatory (CGRO) was launched 1991 (Gehrels and Cheung (1992)). Since then, it has been observed frequently by all CGRO instruments. 911

H.Steinle er al.

912 Centaurus

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Fig. 1 (left). BATSE light-curve covering a time-span including the two observations (indicated by the filled triangles) reported in this paper. The data have been combined into lo-day averages. Fig. 2 (right). Broad-band y-ray spectrum of Centaurus A in an intermediate emission state. It is obvious, that the energy flux per logarithmic frequency interval peaks around 10” Hz (0.1 - 1 MeV). The detected emission extends up to 2.4 x 1O23 Hz (1 GeV). PROBLEMS

ASSOCIATED

WITH CENTAURUS

A MULTIWAVELENGTH

SPECTRA

The low resolution and low sensitivity y-ray measurements must be compared to time-resolved and Great care must be taken when such global spatially-resolved measurements at lower frequencies. multiwavelength spectra are compiled. Arc-second spatial resolution from the radio and optical frequency range up to X-ray energies is readily available, whereas the spatial resolution in the y-ray range is arc-minutes or degrees at best. Time variability, as observed in all AGN, makes multiwavelength spectra over such a wide frequency range difficult to obtain and to interpret. Usually detectors for low frequencies have a much higher sensitivity than detectors in the y-ray regime. Thus, the integration time in y-rays is much longer than in the other spectral bands, making the construction of “simultaneous” measurements difficult. Based on theoretical models and clues in the high-energy data, we assume that the high-energy emission observed from Cen A originates from the active nucleus. We have monitored the flux of the Cen A nucleus at the radio and X-ray wavelengths over the It has been verified, that no flux-variation in this time-interval 14 days of the y-ray observations. was present. Thus it is justified to average each of those data sets, so that the radio and X-ray data points shown in Figure 3 represent the average during this time interval. OBSERVATIONS The BATSE instrument on board CGRO continuously monitors the hard X-ray (20 - 200 keV) emission of Cen A and is used to define the global state of the emission. The light-curve covering the time interval of the two reported simultaneous observations is shown in Figure 1. October

1991 Observations

The first simultaneous observations of Cen A using all instruments on board CGRO were made from October 17 to October 31, 1991 (TJD 8546 - 8560). OSSE, COMPTEL and EGRET observed Cen

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Radio to Gamma-Rays SED of Cen A Centaurus

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Fig. 3. Spectral energy distribution of Cen A from radio to y-rays in a low emission state. Also in this state, the energy flux per logarithmic frequency interval peaks around 102’ Hz (0.1 - 1 MeV). A continuously during this 14day period. The data from BATSE, derived with the earth occultation technique (Paciesas et al., 1993), were used to monitor the global emission state on a daily basis to make comparisons with the other observations possible. Cen A was found to be in an intermediatehigh emission state. July 1995 Observations In July 1995, observations with various satellites and instruments on the ground had been organized to simultaneously observe Cen A from radio to y-ray frequencies. The “core” observations took place from July 10 to July 25, 1995 (TJD 9908 - 9922). The observatories used were ATNF (radio), ES0 La Silla (near infrared and optical), IUE (UV), ROSAT HRI (X-ray), and all CGRO instruments (y-ray). BASTE monitoring showed Cen A to be in a low emission state. SIMULTANEOUS

BROAD-BAND

r-RAY SPECTRUM

Figure 2 shows the contemporaneous broad-band y-ray spectrum obtained with the CGRO instruments OSSE, COMPTEL, and EGRET in October 1991. It covers 4 decades in the energy range from 50 keV up to 1 GeV (Steinle et al., 1998). SIMULTANEOUS

MULTIWAVELENGTH

SPECTRUM

FROM RADIO TO y-RAYS

A simultaneous multiwavelength spectrum from radio to y-rays was measured during July 1995 when Cen A was in a low (X-ray) emission state (the prevailing state for the last 7 years). Again, as in the intermediate emission state, the high energy flux per logarithmic frequency interval peaks around 102’ Hz. The absorption-corrected X-ray flux as measured with ROSAT HRI and the upper-limit derived from the EGRET data show that in this whole frequency interval, the energy output is

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H. Steinleet al.

dominated by the hard X-ray / soft y-ray emission. Due to the large uncertainties in the absorption in this region (e.g. Storchi-Bergmann et al. (1997) and Schreier et al. (1998)), IR, optical and UV data have not yet been fully analyzed. Usually AGN show another peak in the energy flux per logarithmic frequency interval in this frequency region and it will be interesting to compare Cen A with other AGN. CONCLUSIONS The spectral shape and the emission detected above 1 MeV (> 2.4 x 102’ Hz) puts hard constraints on models for the y-ray emission of Cen A. The large viewing angle between the jet axis and the observer requires that the energetic photons - usually assumed to be produced by relativistic boosting along the jet (e.g. in blazars) and beamed into a small cone around the jet axis - are scattered towards the observer. Alternatively, it requires the production of high-energy photons in the jet or the nucleus, which are emitted isotropically and thus directly observable. Although there are models which can produce sufficient high-energy emission visible from the side of the jet (e.g. Marcowith et al., 1995), they may not be able to fully account for the observations. It is obvious, that the inclusion of highenergy (> 1 MeV) emission into the models for the spectral energy distribution of Centaurus A and AGN in general is very important. ACKNOWLEDGMENTS This research was supported by the Bundesministerium fiir Forschung und Technologie under the grant 50 QV 9096, by NASA contract NAS5-26645 and by The Netherland’s Organization for Scientific Research (NWO). REFERENCES Gehrels N., C. Cheung, High Energy Continuum Measurements of Active Galactic Nuclei, in: Testing the AGN Paradigm, edited by S.S. Holt, S.G. Neff and C.M. Urry C.M., AIP Conference Proceedings 254, 348 (1992). Hui X., H.C. Ford, R. Ciardullo, and G.H. Jacobi, The Planetary Nebula System and Dynamics of NGC 5128. I. Planetary Nebulae as Standard Candles, Astrophys. J., 414, 463-473 (1993) Hui X., H.C. Ford, R. K.C. Freeman, and M.A. Dopita, The Planetary Nebula System and Dynamics of NGC 5128. III. Kinematics and Halo Mass Distributions, Astrophys. J., 449, 592-615 (1995) Jones D.L., S.J. Tingay, D.W. Murphy, D.L. Meier, D.L. Jauncey et al., Discovery of a Subparsec Radio Counterjet in the Nucleus of Centaurus A, Astrophys. J., 466, L63 (1996) Marcowith A., G. Henri and G. Pelletier, Gamma-ray emission of blazars by a relativistic electronpositron beam, Mon. Not. R. A&on. Sot., 277, 681-699 (1995) Paciesas W.S., B.A. Harmon, G.N. Pendleton, M.H. Finger, G. J. Fishman, C.A. Meegan, B.C. Rubin, and R.B. Wilson, Studies of hard X-ray source variability using BATSE, Ash-on. ~3 Astrophys. Suppl. 97, 253 (1993). Schreier E.J., A. Capetti, F. Macchetto, W.B. Sparks, and H. J. Ford, Hz&le Space Telescope Imaging and Polarimetry of NGC 5128 (Centaurus A), Astrophys. J., 459, 535-541, (1996) Schreier E.J., A. Marconi, D.J. Axon, N. Caon, D, Macchetto et al., Evidence for a 20 parsec disk at the nucleus of Centaurus A, Astrophys. J., 499, L143-L147, (1998) Steinle H., K. Bennett, H. Bloemen, W. Collmar, R. Diehl et al., COMPTEL observations of Centaurus A at MeV energies in the years 1991 to 1995, Ash-on. & Astrophys., 330, 97 (1998) Storchi-Bergmann Th., E. Bica, A.L. Kinney and Ch. Bonatto, The nature of the optical continuum in NGC 5128 (Centaurus A), Mon. Not. R. Ash-on. Sot., 290, 231-244 (1997)