A search for Very High Energy gamma-ray emission from Passive Super-massive Black Holes
aa r X i v : . [ a s t r o - ph ] S e p TH I NTERNATIONAL C OSMIC R AY C ONFERENCE
A search for Very High Energy γ -ray emission from Passive Super-massive BlackHoles G.P
EDALETTI , , S. W AGNER , W. B ENBOW , FOR THE
H.E.S.S.
COLLABORATION Landessternwarte, Universit¨at Heidelberg, K¨onigstuhl, 69117 Heidelberg, Germany Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl, 69117 Heidelberg, Germany Max-Planck-Institut f¨ur Kernphysik, PO Box 103980, 69029 Heidelberg, Germany [email protected]
Abstract:
Jets of Active Galactic Nuclei (AGN) are established emitters of very high energy (VHE; >
100 GeV) γ -rays. VHE radiation is also expected to be emitted from the vicinity of super-massiveblack holes (SMBH), irrespective of their activity state. Accreting SMBH rotate and generate a dipolarmagnetic field. In the magnetosphere of the spinning black hole, acceleration of particles can take placein the field gaps. VHE emission from these particles is feasible via leptonic or hadronic processes. There-fore quiescent systems, where the lack of a strong photon field allows the VHE emission to escape, arecandidates for emission. The H.E.S.S. experiment has observed the passive SMBH in the nearby galaxyNGC 1399. No VHE γ -ray signal is observed from the galactic nucleus. Constraints set by the NGC1399 observations are discussed in the context of different mechanisms for the production of VHE γ -rayemission. Introduction
Spheroidal systems (such as elliptical galaxies,lenticular galaxies and early-type spiral galaxieswith bulges) are commonly believed to host inthe central region super-massive black holes withmasses in the range M BH = 10 − M ⊙ [8].During the early stages of galaxy evolution theseSMBH accrete matter at high rates and are ob-served as bright QSOs. The radiative output atlow energy (e.g. optical) decays from redshiftz > γ -rays to escape from the nu-clear region without suffering from strong absorp-tion via γ -photon pair absorption. Several mod-els [1, 12, 6, 14] are proposed for the productionof VHE γ -rays emission from these passive AGN.In all cases a large mass of the central object isthe most important characteristic for generating ahigh VHE flux. H.E.S.S. has already observed ninenearby galaxies whose black hole mass is mea-sured [5, 13]. Only the case of NGC 1399 is con-sidered here. Constraints on the physical parame- ters of the system (e.g. the magnetic field B ) arederived using several of the aforementioned mod-els. Acceleration Mechanism
If the central black hole is accreting matter from adisk that also carries magnetic flux, it will developa magnetosphere similar to those surrounding neu-tron stars. If the charge density is not too high inthe magnetosphere of the spinning black hole, it ispossible to have a non-zero component of the elec-tric field E parallel to the magnetic field B . In thisconfiguration field gaps are created, where acceler-ation of particles can take place [14].Various methods can be used to estimate the mag-netic field B. For example, B is estimated: • assuming equipartition B π = 12 ρ ( r ) v r ( r ) , (1)where ρ is the mass density and v r is theradial infall velocity of the accreting matter SEARCH FOR
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SMBH (both being a function of r , the distance tothe inner edge of the disk); • from the angular momentum as in [11] B = 3 . × ˙ m / M / (cid:18) rr g (cid:19) − / Gauss,where ˙ m is the mass accretion rate in unitsof the Eddington mass accretion rate, r g isthe gravitational radius of the black hole and M = ( M BH / M ⊙ ) .In the model of [14] protons accelerated in theouter part of the black hole magnetosphere willcollide with other protons present in the accretiondisk producing pions some of which decay intoVHE γ -rays . The available power is W max ∼ ( M BH ) ( B ) ergs s − , (2)where B = ( B/ Gauss ) . Here it is assumedthat the magnetic energy density is in equiparti-tion with the accretion energy density, which de-pends on various properties of the accretion disk(see Eq. 1).In other models [1, 12, 6] VHE γ -rays origi-nate from electromagnetic processes such as syn-chrotron or curvature emission. Following theanalogous arguments given in [1] for the Galac-tic Center, synchrotron emission is not feasible dueto a cut-off for protons and electrons at ǫ γ, max ≃ . TeV and ǫ γ, max ≃ . GeV respectively.These cut-offs are independent of the magneticfield strength. The energy of curvature photons(when curvature losses are the dominant ones) doesnot depend on the mass of the particle, so it isthe same for electron or proton originated photons.The emission spectrum from curvature radiationcan extend up to VHE energies, with a cut-off at: E max ≃
14 ( M ) / ( B ) / TeV. (3)
VHE Observation of NGC 1399
The giant elliptical galaxy NGC 1399 is located inthe central region of the Fornax cluster at a distanceof . Mpc. An SMBH of M BH = 1 . × M ⊙ resides in the central region. The nucleus of thisgalaxy is well known for its low emissivity at all wavelengths [7]. Considering also the visibility ofcandidate sources for H.E.S.S., NGC 1399 there-fore emerged as the best candidate for this study.NGC 1399 was observed with the H.E.S.S. arrayof imaging atmospheric-Cherenkov telescopes fora total of 22.4 h (53 runs of ∼
28 min each). Afterapplying the standard H.E.S.S. data-quality selec-tion criteria a total of 13.9 hours live time remain.The mean zenith angle is Z mean = 22 ◦ . The datawere reduced using the standard analysis tools andselection cuts [2] and the Reflected-Region method[3] for the estimation of the background. This leadsto a post-analysis threshold of 200 GeV at Z mean .No significant excess (-29 events, -1 σ ) is detectedfrom NGC 1399 (see Fig. 1 and Fig. 2). Results areconsistent with independent analysis in the collab-oration.Assuming a photon index of Γ =2.6, the upper limit(99 % confidence level; [10]) on the integral fluxabove 200 GeV is: I ( > GeV ) < . × − cm − s − , or 1% of the Crab Nebula flux. ] [deg q E ve n t s [deg q E ve n t s Figure 1:
Distribution of squared angular distance fromNGC 1399 for gamma-ray-like events in the ON region(dots) and in the OFF region (filled area, normalized).The dotted line represents the cut for point-like sources.Preliminary.
Constraints from NGC 1399 Observa-tions
As can be seen from the spectral energy distribu-tion (SED) of NGC 1399 in Fig. 3, the VHE frac- TH I NTERNATIONAL C OSMIC R AY C ONFERENCE
Right Ascension D ec li n a t i on (cid:176) -37 00’ (cid:176) -37 30’ (cid:176) -36 00’ (cid:176) -36 30’ (cid:176) -35 00’ (cid:176) -35 30’ (cid:176) -34 00’ (cid:176) -34 30’ (cid:176) -33 -60-40-200204060 m h m h m h m h Figure 2:
The smoothed (smoothing radius r=0.09)VHE excess in the region centered on NGC 1399. Theyellow star indicates the position of the optical centre ofNGC 1399. Preliminary. tion of its total energy budget is potentially not-negligible. The H.E.S.S. limit on the isotropicVHE γ -ray luminosity is: L γ < . × erg s − . Here it is assumed that the γ -ray emission orig-inates solely from the nucleus, even though theentire galaxy is point-like considering the angularresolution of H.E.S.S.In the case of NGC 1399 photon-photon pair ab-sorption would not hide any possible VHE emis-sion. The cross section σ γγ of this process dependson the product of the energies of the colliding pho-tons. In the case of VHE photons, the most ef-fective interaction is with background photons ofenergy: ǫ IR ≈ ( E/ − eV.The optical depth resulting from this absorption, ina source of luminosity L and radius R , reads: τ ( E, R IR ) = L IR σ γγ πR IR ǫ IR ≃ (cid:20) L IR ( ǫ )10 − L Edd (cid:21) (cid:20) R S R IR (cid:21) (cid:20) E (cid:21) ,
10 15 20 25 30VLA HST CHANDRA H.E.S.S. log frequency [HZ]
Figure 3:
The SED of NGC 1399. All the data arefor the core region. The archival points are VLA radiodata (red triangles; [9]), HST optical data (green squares;[7]), and Chandra X-ray upper limits (solid line; [4]).The blue dot is the H.E.S.S. upper limit derived from the2005 observations. Preliminary. where R S is the Schwarzschild radius of the blackhole and L Edd is the Eddington luminosity. In thesystem here presented, the visibility of a 200 GeVphoton requires L IR < . × ergs s − , a con-dition that seems to be satisfied.In the p-p interaction scenario, assuming that allthe available power (Eq. 2) will be radiated in theVHE domain, the following limit for the magneticfield is obtained from the H.E.S.S. result: B < . Gauss.In order to maintain gaps in the magnetosphere, asis essential for particle acceleration, pair produc-tion should be avoided. Translating this conditioninto an upper limit for the magnetic field yields:
B < . × ( M ) − / = 6 . × Gauss.Therefore the H.E.S.S. NGC 1399 data allow plau-sible values of the magnetic field. Considering theproduction of a 1 TeV photon via curvature emis-sion (Eq. 3) requires in the case of NGC 1399: B = 1 . × Gauss.
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The non-detection of NGC 1399 does not con-strain the magnetic field. In all the aforementionedscenarios, hadronic and/or leptonic, no clear con-straints on the magnetic field are derived.
Conclusions
VHE emission from passive SMBH is plausibleeither via leptonic or hadronic processes. In or-der to detect this emission the giant ellipticalgalaxy NGC 1399 was observed by H.E.S.S. in2005. NGC 1399 is not detected in these obser-vations. The corresponding upper limit does notallow a firm estimation of the circumnuclear mag-netic field.
Acknowledgments
The support of the Namibian authorities and ofthe University of Namibia in facilitating the con-struction and operation of H.E.S.S. is gratefully ac-knowledged, as is the support by the German Min-istry for Education and Research (BMBF), the MaxPlanck Society, the French Ministry for Research,the CNRS-IN2P3 and the Astroparticle Interdisci-plinary Programme of the CNRS, the U.K. Sci-ence and Technology Facilities Council (STFC),the IPNP of the Charles University, the Polish Min-istry of Science and Higher Education, the SouthAfrican Department of Science and Technologyand National Research Foundation, and by theUniversity of Namibia. We appreciate the excel-lent work of the technical support staff in Berlin,Durham, Hamburg, Heidelberg, Palaiseau, Paris,Saclay, and in Namibia in the construction and op-eration of the equipment.This work has been supported by the InternationalMax Planck Research School (IMPRS) for Astron-omy & Cosmic Physics at the University of Heidel-berg.
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