A synchrotron self-Compton model for the VHE gamma-ray emission from Cen A
J.-P. Lenain, M. C. Medina, C. Boisson, G. E. Romero, H. Sol, A. Zech
aa r X i v : . [ a s t r o - ph . H E ] J u l PROCEEDINGS OF THE 31 st ICRC, Ł ´OD ´Z 2009 1
A synchrotron self-Compton model for the VHE gamma-rayemission from Cen A
J.-P. Lenain ∗ , M. C. Medina ∗ , C. Boisson ∗ , G. E. Romero † , H. Sol ∗ and A. Zech ∗∗ LUTH, Observatoire de Paris, CNRS, Universit´e Paris Diderot; 5 Place Jules Janssen, 92190 Meudon, FranceEmail: [email protected], [email protected] † Instituto Argentino de Radioastronom´ıa (CCT La Plata-CONICET), C.C.5, (1984) Villa Elisa,Buenos Aires, Argentina
Abstract . The H.E.S.S. experiment has reported thedetection of very high energy (VHE:
E >
GeV) γ -ray emission from the well-known radio-galaxyCen A. Following this discovery, we refine the pre-vious multi-blob synchrotron self-Compton (SSC)model applied to the multi-wavelength emission fromthis source using the new H.E.S.S. data. The predic-tion of the VHE γ -ray level of emission for Cen Apresented in Lenain et al. paper agrees very well inthe view of this recent data. Further VHE observa-tions of Cen A might detect variability, which wouldcomfort our inner jet modeling. The next generationof imaging atmospheric ˇCerenkov telescopes, willhelp to reveal possible multiple sources of VHEemission in the complex structure of Cen A. Keywords : active galaxies, galaxies: individual:Cen A, non-thermal radiation.
I. I
NTRODUCTION
The H.E.S.S. experiment has recently reported thedetection of VHE γ -ray emission from the most nearbyradio-galaxy: Cen A [1]. This elliptical galaxy presentsstrong activity in a wide spectral range from radio tohigh energy γ -rays. It is a typical radio-loud AGNviewed with a jet at large angle from the line of sight.The reported detection is of high importance as itestablishes radio-galaxies as VHE γ -ray emitters. Cen Ais the second non-blazar AGN discovered at VHE,after the HEGRA detection of VHE γ -rays from M 87between 1998 and 1999 [2] [3], confirmed by H.E.S.S.observations in 2006 [4].The best-fit position obtained by H.E.S.S does notsupport the giant radio lobes as VHE γ -ray emittingzone. The emission direction seems to be compatiblewith the radio core and the inner jet region. The differen-tial photon spectrum follows a power law with a photonindex of . ± . stat ± . sys and no flux variabilitywas found in the period of observation.In a previous publication [5], a new synchrotron self-Compton (SSC) radiative scenario was developed toexplain the multi-wavelength emission of misalignedblazar-like objects, such as M 87 and FR I radio-galaxies.In this framework, three objects were investigated,namely Cen A, PKS 0521 −
36 and 3C 273, with predic-tion of their VHE γ -ray emission. In this paper we present a more detailed update onthe results of this model for Cen A with the addition ofthe new data from H.E.S.S. and Fermi /LAT. Section IIcontents a brief compilation of observational data onCen A. A short description of the multi-blob SSC sce-nario developed in [5] is given in Section III. Following,the implications of the model results on the descriptionof this source are summarized in Section IV. Finally,some discussions about the model parameters, limits andfuture developments are given in Section V.II. C EN ACen A is the closest FR I radio-galaxy ( . Mpc, [6])and its proximity makes it uniquely observable amongsuch objects, even though its bolometric luminosity isnot large by AGN standards. It is very active at radiowavelength, presenting an extremely rich jet structure.We can distinguish in this structure two components:inner jets at a kpc scale and giant lobes which covers ∼ ◦ in the sky. A detailed description of its radiomorphology can be found in [7]. The inner kpc jet hasalso been detected in X-rays [8] with a structure of knotssummed to diffuse emission. Recently, [9] reported thedetection of non-thermal X-ray emission from the shockof the southwest inner radio lobe from deep Chandra observations.The supermassive black hole at the center of the activegalaxy has an estimated mass of about . × M ⊙ [10]to M ⊙ [11]. The stellar body associated with theradio source is an elliptical galaxy (NGC 5128) with atwisted disk of dust, gas and young stars which obscuresthe central engine at optical wavelengths. This dustlane and the extended shell structures detected in longexposure optical images are thought to be the remnantof a recent merging process between a massive ellipticalgalaxy and a spiral one [12].The inner part of Cen A shows a circumnuclear diskof about 400 pc diameter and a central cavity of about90 pc. This disk is emitting in the (sub) millimeter range.Against this emission, a variety of molecular absorptionlines are seen [13]. In addition, some evidence for a40 parsec diameter disk (2.5 arcsec) of thin ionized gascentered on the nucleus of Centaurus A has been foundin the light of Pa α using the NICMOS instrument onboard HST [14]. This disk is not perpendicular to the jet
J.-P. LENAIN et al.
AN SSC MODEL FOR CEN A and thus it is not directly associated with the accretiondisk of the black hole.After the early detection in X-ray was made duringa rocket flight [15], between 1991 and 1995, Cen Awas observed by the Compton Gamma Ray Observatory(
CGRO ) and all its instruments from MeV to GeVenergies ([16], [17], [18], [19]). These observationsrevealed a peak in the spectral energy distribution (SED)in νF ν representation at ∼ . MeV with a maximumflux of about erg cm − s − [18]. At this time it hasexhibited large X-ray variability [20]. Some soft gamma-ray variability was also reported ([18], [20]) but [21]found that EGRET flux was stable during the wholeperiod of CGRO observations.In 1999 the new
Chandra
X-ray Observatory took im-ages of Cen A with unprecedented resolution. More than200 point X-ray sources with an integrated luminosityof L X > erg s − can be identified in those images[22].Before the discovery reported by H.E.S.S., a tentativedetection of Cen A at VHE during a giant X-ray outburstin the 1970’s was reported by [23]. Subsequent VHEobservations made with the Mark III [24], JANZOS[25], CANGAROO [26], and H.E.S.S. [27] experimentsresulted in upper limits.Cen A was early proposed by [28] as a possiblesource of UHE cosmic rays. Recently, the Pierre AugerCollaboration reported the existence of anisotropy on thearrival directions of UHE cosmic rays [29], remarkingthat at least 2 of this events can be correlated with theCen A position ( ◦ circle). Further works have claimedthat there are several events that can be associatedwith Cen A and its big radio lobes ([30], [31]) but thiscorrelation is until now statistically weak.Finally, the last news about Cen A are coming from Fermi /LAT which has detected it in the three first monthsof survey with a signal significance above 10 σ [32].III. T HE MULTI - BLOB
SSC
MODEL FOR MISALIGNEDBLAZAR - LIKE OBJECTS
We summarize here the principles of the multi-blobSSC model developed by [5]. The VHE γ -ray emittingregion is assumed to consist in several small blobstraveling through the extended jet. The radiation occursat ∼ r g (where r g is the gravitational radius of thecentral black hole) from the central engine, just beyondthe Alfv´en surface as found in magnetohydrodynamicmodels [33], in the broadened jet formation region inthe innermost part of the jet. The cap of small blobs inthe inner jet introduced in [5] should be seen as a sketchto describe an inhomogeneous turbulent flow crossing astationary shock. In this case, even for large viewingangles between the jet axis and the line of sight, as it isthe case in Cen A ( ◦ - ◦ ) ([34], [35]), some materialcould move closely aligned to the line of sight, and thusbenefit from strong Doppler boosted emission.In another scenario presented in [36], the authorsdevelop a model for the VHE γ -ray emission of BL Lac objects and radio-galaxies based on an structured jetcomposed by a fast spine surrounded by a slower layer.Depending on the viewing angle between the jet and theline of sight, these components contribute differently tothe spectral energy distribution. In the case of M 87 [37],this model can also reproduce the data correctly.IV. R ESULTS
We present here an update of the prediction for theVHE flux expected with H.E.S.S. made by [5] (see theirFig. 7). The data sample chosen here is almost the sameas in the earlier publication, but it now includes VHEpositive detection and
Fermi data. The strongly con-straining
CGRO /COMPTEL γ -ray data are taken from[18]. The RXTE and
INTEGRAL data were providedby [38]. From [39] we obtained the
HST /NICMOS andWFPC2 data, which was carefully dereddened. SCUBAdata at 800 µ m was taken from [40] while ISO andSCUBA (450 µ m and 850 µ m) data are from [41].The VLA data were acquired from [42]. Evans et al.in [43] report X-ray observations by XMM -Newton onFebruary 2 and 6, 2001 and by Chandra on May 9 and21, 2001 with a photon index Γ = 2 for the parsec-scale jet component. Data from the Nasa ExtragalacticDatabase are also shown as non-constraining points ( ingray ) for comparison.The Fermi /LAT collaboration recently published asource list of the bright AGNs [32] giving a simplepower-law reconstruction of the spectrum. Even thoughthe high energy spectrum of Cen A might be morecomplex, we consider this simple power-law assumptionas a first clue of its real spectrum in
Fermi energy range.Furthermore, the
Fermi /LAT analysis revealed thatthe high energy spectrum seems constant over the firstthree months of scientific operation of the instrument.This constant behavior is also supported by the fact that
CGRO also reported a constant flux from Cen A. Wethus model together both
CGRO and
Fermi /LAT data.Figure 1 gives an update of our multi-blob SSC modelto the SED of Cen A, slightly modifying the parametersas compared to [5] to account for the recent
Fermi /LATand H.E.S.S. spectra. We note that a discrepancy ap-pears between the flux normalizations of
Fermi /LAT andH.E.S.S. This difference might be resolved when thespectral points from
Fermi /LAT will become available,however it might also be due to variability in the flux,since the data were not taken simultaneously. The thickblack line represents the average result of the SSC, withthe parameters given in Table I. This average is betweentwo extreme scenarios: “on-blob” (thin and solid line)and “inter-blob”(dashed line) (see [5] for more details).Considering these recent high energy and VHE re-sults, we favor the second hypothesis presented in [5],that is the soft γ -ray data observed by CGRO and now
Fermi are more likely of synchrotron nature rather thaninverse Compton. In Table I we give both the parametersfor this second hypothesis from [5] and from the updatedmodel presented here.
ROCEEDINGS OF THE 31 st ICRC, Ł ´OD ´Z 2009 3
Fig. 1: SED of Cen A, along with the H.E.S.S. spectrum in the VHE range (filled triangles). There are also shown thedata from
CGRO /COMPTEL [18],
RXTE and
INTEGRAL [38],
HST /NICMOS and WFPC2 [39], SCUBA at 800 µ mfrom [40], ISO and SCUBA (450 µ m and 850 µ m)[41], the XMM -Newton and
Chandra spectrum, the
Fermi /LATspectrum (shaded bow-tie) considering a simple power law and finally, data from the NASA Extragalactic Database(NED) (grey filled circles). The thick black line represents the average result of the multi-blob SSC model, obtainedwith the parameters given in Table I. The thin and solid line corresponds to the “on-blob” case and the dashed lineto the “inter-blob” case described in [5]. In the radio and optical ranges, fluxes of the central region of Cen A canbe reproduced on the average by an extended synchrotron emitting jet.TABLE I: Parameters used in the multi-blob SSC model for Cen A.
Parameter [5] this work Γ b
20 15 θ ◦ ◦ R cap r g r g B
10 G 10 G r b . × cm . × cm K . × cm − . × cm − n n γ min γ b . × . × γ c . × . × V. D
ISCUSSION AND PERSPECTIVES
The multi-blob SSC model describes well the ob-served spectral energy distribution with only a smallset of free parameters. The only parameter that differsnotably from the standard SSC results for blazars is therelatively high magnetic field of 10 G, which is neededto arrive at the observed flux level. Other solutionsare possible due to the inherent degeneracy betweenthe parameters. Even though the muti-blob SSC modelachieves a good description for the two misalignedblazars with VHE γ -rays emission, there is also anotherleptonic description based on the particle accelerationalong rotating magnetic field lines in the magnetosphere of the supermassive black hole, which has been appliedsuccessfully to M 87 and could possibly also work forCen A [44].A more complete model would need to take into ac-count the existence of hadrons in the jet and their contri-bution to the observed emission. Current hadronic mod-els investigate the case where emission from hadronicprocesses dominates the spectral energy distribution athigh energies. One possibility is given by the syn-chrotron proton blazar model [45], in which the VHEemission is dominated by synchrotron radiation fromprotons in magnetic fields of several tens of Gauss incases such as M 87. J.-P. LENAIN et al.
AN SSC MODEL FOR CEN A
In a different scenario, far larger values of the mag-netic field are estimated when assuming the emissionregion to be close to the jet apex [46]. In such a highlymagnetized context, in the innermost part of the jet,synchrotron losses become catastrophic for electrons,and IC interactions lose importance. In this scenario,the resulting photon field becomes a suitable target forphoto-pion production by relativistic protons acceleratedby shocks in the innermost part of the jet as in modelsfor galactic jets [47], and protons can achieve energiesup to ∼ eV.With its intended high resolution and sensitivity, thefuture Cherenkov Telescope Array (CTA) Observatorywill be an ideal instrument to ponder the role played byrelativistic protons in the inner part of Cen A.A CKNOWLEDGMENT
We would like to acknowledge the support from theGDR-PCHE and LEA-ELGA.R
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