The existence of intrinsic obscuration of Fanaroff-Riley I objects is a
controversial topic. M87, the nearest such object, is puzzling in that although
it has very massive central black hole it has a relatively low luminosity,
suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it
is not known with certainty whether or not M87 has a dusty torus.
  Infrared observations indicate that if a torus exists in M87 it must have a
rather low infrared luminosity. Using arguments from unification theory of
active galactic nuclei, we have earlier suggested that the inner parsec-scale
region of M87 could still harbour a small torus sufficiently cold such that its
infrared emission is dwarfed by the jet emission. The infrared emission from
even a small cold torus could affect through photon-photon pair production
interactions the escape of 100 GeV to TeV energy gamma rays from the central
parsec of M87.
  The TeV gamma-ray flux from the inner jet of M87 has recently been predicted
in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at
least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of
gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported.
We discuss the interactions of gamma-rays produced in the inner jet of M87 with
the weak infrared radiation expected from a possible dusty small-scale torus,
and show that the HEGRA detection shows that the temperature of any torus
surrounding the gamma-ray emission region must be cooler than about 250 K. We
suggest that if no gamma-rays are in future detected during extreme flaring
activity in M87 at other wavelength, this may be expected because of torus
heating.Comment: 7 pages, submitted to Prog. Theor. Phys. Suppl., ps fil

Donea, A. -C.

Protheroe, R. J.

English

arXiv

Copyright © 2003. Progress of Theoretical Physics 2003 All rights reserved. Printed in U.S.A. Submitted to Cornell University’s online archive www.arXiv.org in 2003 by Donea A.-C. Post-print sourced from www.arxiv.org.The existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic. M87, the nearest such object, is puzzling in that although it has very massive central black hole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have a rather low infrared luminosity. Using arguments from unification theory of active galactic nuclei, we have earlier suggested that the inner parsec-scale region of M87 could still harbour a small torus sufficiently cold such that its infrared emission is dwarfed by the jet emission. The infrared emission from even a small cold torus could affect through photon-photon pair production interactions the escape of 100 GeV to TeV energy gamma rays from the central parsec of M87. /par The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactions of gamma-rays produced in the inner jet of M87 with the weak infrared radiation expected from a possible dusty small-scale torus, and show that the HEGRA detection shows that the temperature of any torus surrounding the gamma-ray emission region must be cooler than about 250 K. We suggest that if no gamma-rays are in future detected during extreme flaring activity in M87 at other wavelength, this may be expected because of torus heating.Alina C. Donea and Raymond J. Prothero

Donea, A.

Protheroe, R.

Adelaide Research & Scholarship

arXiv:astro-ph/0303522v1  24 Mar 20031Gamma ray and infrared emission from the M87 jet and torusAlina C. Donea and Raymond J. ProtheroeDepartment of Physics and Mathematical Physics, The University of Adelaide,Adelaide, SA 5005, AustraliaThe existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic.M87, the nearest such object, is puzzling in that although it has very massive central blackhole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of itsproximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dustytorus.Infrared observations indicate that if a torus exists in M87 it must have a rather lowinfrared luminosity. Using arguments from unification theory of active galactic nuclei, wehave earlier suggested that the inner parsec-scale region of M87 could still harbour a smalltorus sufficiently cold such that its infrared emission is dwarfed by the jet emission. Theinfrared emission from even a small cold torus could affect through photon-photon pairproduction interactions the escape of 100 GeV to TeV energy gamma rays from the centralparsec of M87.The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in thecontext of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV(Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactionsof gamma-rays produced in the inner jet of M87 with the weak infrared radiation expectedfrom a possible dusty small-scale torus, and show that the HEGRA detection shows thatthe temperature of any torus surrounding the gamma-ray emission region must be coolerthan about 250 K. We suggest that if no gamma-rays are in future detected during extremeflaring activity in M87 at other wavelength, this may be expected because of torus heating.§1. IntroductionM87 is usually classified as a Fanaroff-Riley Class I (FRI) radio galaxy havinga relativistic jet1) pointing at ∼ 30◦ to the line of sight.2) Despite of its proximityto us (∼16 Mpc3)) it is not known with certainty whether or not M87 has a dustytorus. However, it does appear to have a deficiency of dust at pc scales compared toother galaxies of its class,4) and this may suggest that the supply of dust in M87 isscarce. Non-observations of polarized emission from the core5) suggests that we seethe nucleus through a depolarizing layer which may be ionized gas filling the torus.Chandra X-ray observations of the nucleus of M87 show that the accretion flowhas low radiative efficiency6) which would not appear to favour a standard accretiondisk (α-disk), and therefore it is likely that the inflowing gas settles into an advection-dominated accretion flow (ADAF).7), 6), 8) As a consequence, the heating of any dustytorus would be currently inefficient. Strong mid-infrared emission was not detectedfrom the centre of M87,9), 10) suggesting either that there is no torus, or that thedust cannot settle easily into a ’standard’ well-defined large-scale torus configuration.Corbin et al.,11) have obtained HST WFPC2 images of M87, and from these theydid not exclude the possibility of an outer radius of a torus, if present, being smallerthat 50 pc.typeset using PTPTEX.cls 〈Ver.0.89〉2 A.-C. Donea and R.J. ProtheroeThe existence of the intrinsic obscuration of FRIs is a controversial topic (seeWhysong & Antonucci12) and references therein). Chiaberge et al.13) suggest thatFRIs (including M87) with detected optical fluxes could lack tori, or have geometri-cally very thin tori which allow the optical emission to be seen. On the other hand,Gambill et al.14) suggested that at least some FRIs show excess X-ray absorption,possibly a signature of a molecular torus obscuring the core. In addition, severalFRI objects show incontestable proof of having toroidal dust structures: CentaurusA,15), 12) 3C270,16) 3C218.17), 18)The presence of a broad line region in M87 is undeniable as the Lyα line hasobserved19) a width of 3000 km s−1, providing direct evidence that BLR clouds areactive in M87. Either the NLR or BLR may contribute to the observed depolarizationof the radio flux.5) Donea & Protheroe20) have discussed the coexistence of BLR andtori in active galactic nuclei (AGNs) in the context of unification theories of AGNand their implications for TeV emission from jets. In this context, it is hard to acceptthat M87 completely lacks a dusty component. M87 might still have sufficient dustaround the nucleus such that its infrared emission may affect gamma-ray escape.21)The putative torus of M87 probably differs from a standard quasar torus by beingsmaller, probably cooler, having a wide opening angle (φ ≥ 30◦) and a large densitygradient in the dust. Such a torus would be quite different from the large tori withexternal radii of ≈ 100 − 200 pc assumed to populate quasars. The limits imposedin modelling this torus come from the fact that the infrared flux must not exceedthe detected mid-IR flux detected. In Section 3 we will discuss the geometry of thetorus in details.Since M87 has a large central mass, and by considering aging arguments8), 22)it could harbour a nucleus comprising of two or more black holes. For a binaryblack hole system a dusty torus could form with a clumpy structure with ratherdiluted polar dust caps filling the space between the jet and the equatorial belt ofthe torus.23), 24)We shall discuss the implications of the existence of such a patchy, or clumpy,small-scale torus in M87 for observing gamma-rays from the inner jet. At the distanceof M87 absorption by photon-photon pair production on the intergalactic infraredbackground can be neglected below 1 TeV. We suggest that the detection or non-detection of gamma-ray fluxes from M87 can provide useful information about toriin FRIs. However, if an ADAF is present in place of a standard accretion disk, asseems likely in M87, its radiation field may also contribute to the optical depth forescape of gamma-rays from regions of the jet close to the black hole.§2. Modeling the spectral energy distribution of M87In this section we summarize the main results from modeling the high energygamma-ray emission from the inner jet of M87. The origin of the high-energy compo-nent of the SEDs of AGNs, starting at X-ray or γ-ray energies and extending in somecases to TeV-energies, is uncertain. Using a leptonic model, Bai & Lee2) suggestedthat M87 could emit detectable gamma rays with an inverse-Compton emission peakat 0.1 TeV. In the context of hadronic models, the spectrum of gamma-ray emissionGamma ray and infrared emission from the M87 jet and torus 3Fig. 1. Predicted high energy component of the spectral energy distribution30) (SED) of the nucleusof M87 assuming it to be a mis-aligned HBL (solid curves) or LBL (dashed curve). The brokenpower-laws on the left show the electron synchrotron radiation which provides the target photonsfor proton interactions; the curves on the right show the predicted X-ray to gamma-ray flux.Error bars attached to solid curves correspond to uncertainty due to the Doppler factor of M87which we taken to be in the range 0.67 ≤ δ ≤ 1.5. The sensitivities of the EGRET (E), GLAST(G), MAGIC (M) and VERITAS (V) gamma ray telescopes are indicated, as well as the HEGRAobservation.26) Non-simultaneous data from M87 unresolved nuclear jet emission refs.27), 10), 32)Vertical lines show the cut-off energies for photon-photon pair production for γ-rays from theinner jet interacting with IR photons from the assumed torus with the temperatures indicated.from the nucleus of M87 has been predicted using the SPB model by Protheroe etal.30) by treating M87 as a mis-aligned BL Lac object. The SPB model25) employshadronic interactions in relativistic jets and the high-energy radiation is producedthrough photomeson production, proton and muon synchrotron radiation, and sub-sequent pair-synchrotron cascading in the highly magnetized environment. In thecase of BL Lacs internal photon fields (i.e. produced by synchrotron radiation fromthe co-accelerated electrons at the radio to UV or X-ray frequencies) serve as thetarget for pion photoproduction. Protheroe et al.30) have shown that, M87 couldbe either a high-frequency peaked (HBL) or a low-frequency peaked (LBL) BL Lacobject, and predicted the gamma-ray emission (see Fig 1) from the nuclear jet toextend up to at least 100 GeV. Subsequently the HEGRA Collaboration reportedthe detection of gamma rays above 730 GeV with luminosity ∼ 1041 erg/s,26) con-sistent with that predicted.30) Future telescopes with higher sensitivity (see Fig. 1)such as GLAST and MAGIC, and possibly VERITAS, should be able to measurethe spectrum in detail.The difference in the shapes of the high energy hump of the SED when M874 A.-C. Donea and R.J. Protheroeis modelled as a mis-aligned LBL or HBL is due the nature of the proton interac-tions and interactions of particles and radiation produced as secondaries. If M87 ismodelled as a mis-aligned LBL (dashed curve in Fig 1) pion photoproduction lossesdetermine the maximum proton energy, and hence the maximum gamma-ray energywhich results from the cascades initiated by proton synchrotron radiation and piondecay (including pion and muon synchrotron radiation). In the case of mis-alignedHBL, because of the lower energies of target photons, synchrotron losses determinethe maximum proton energy, and synchrotron radiation by muons can become im-portant (see Protheroe et al.30) for details).The uncertainties in the Doppler factor of M87 (range considered 0.66 – 1.6)give rise to a much larger uncertainty in the bolometric luminosity (indicated in thetheoretical SED by the slanted error-bars). In both cases the predicted neutrino fluxis well below detection levels of future neutrino telescopes, however it is possiblythat the flux will increases during an extreme flare in the LBL case.§3. Gamma ray absorption in torus and ADAF fieldsFig. 2. Optical depth for the absorption of GeV–TeV pho-tons travelling along the jet from distance z from theblack hole to infinity: in the infrared field of a compacttorus with temperature T = (100, 250, 1000) K (thickcurves from bottom to top, z ≪ 1 pc); in the radia-tion field of an ADAF for z = (0.001, 0.01, 0.1) pc (thincurves from top to bottom).We calculate the photon-photon pair-production op-tical depth τγγ of GeV-TeVphotons emitted by the jetof M87 interacting with IRphotons from the torus as inDonea & Protheroe.20) InSection 1 we mentioned thatM87 could have an ADAF,and with the jet contribut-ing little to the heating thetorus, a compact cold toruswould result. We take theinner radius of the torus tobe Rin ≈ 1 pc. For an open-ing angle of at least φ ≈ 30◦,such that the inner jet couldbe seen from the observer weinfer a maximum height of1.7 pc of the inner wall of thetorus. If the torus cross sec-tion is a rectangle with theinner edges cut way at theangle φ ≥ 30◦, then the toruscould be higher at radii larger than 2.5 pc (such that the nucleus remains unob-scured). Since the γ-ray emission is assumed to come mainly from the inner jet(distances z ≪ 1 pc), then it is completely imbedded in the infrared radiation of thetorus. In this case the outer radius of the torus is irrelevant, and only the temperatureGamma ray and infrared emission from the M87 jet and torus 5of its inner wall is important. However, if the γ-rays are produced somewhere abovethe torus, then the radial extent of the dust should be considered. We take a mini-mum value for the outer radius of the torus Rout = 2.5 pc, and from Rout ≈ 2.5 pc to50 pc11) the dust is considered to have a very patchy configuration. The calculationof the torus emission follows Barvainis29) where the ultraviolet absorption efficiencyof the grains is taken to be unity. A torus heated up to T ∼ 250 K could accountfor the observed infrared flux.Fig 2 shows the γ-γ opacity for γ-rays travelling along the jet axis from z ≪1 pc to infinity in the infrared radiation of a compact torus for three temperaturesT = (100, 250, 1000) K. It can be seen that γ-rays with energies above 10 TeV couldstill be absorbed even when the torus is cold (T = 100 K) provided the emissionregion lies within the torus.Radiation emitted by an ADAF may also provide target photons for photon-photon pair production. We have taken the geometry of the ADAF to be a disk ofinner radius 6 gravitational radii and the outer radius at 1000 gravitational radii,and the ADAF spectrum is taken from Di Matteo et al.6) The γ-γ opacity for γ-rays interacting with the anisotropic ADAF radiation field is also shown in Fig 2 forthree different positions of the emitting region along the jet. As one can see, theTeV emission could be attenuated if the gamma-ray emission region is sufficientlyclose to the black hole.§4. DiscussionThe gamma-ray flux observed fromM87 by HEGRA is for energies above 730 GeV,the flux level is such that with the sensitivity of the HEGRA telescopes it is impossi-ble to determine to what energies the spectrum continues (the predicted fluxes alsonot extend TeV energies). Nevertheless, given the HEGRA data it is possible tosay something about the nature of any torus present. With the emission region wellinside the putative torus, one expects a cut-off in the gamma rays spectrum at anenergy determined by the torus temperature (vertical lines in Fig 1). The fact thatHEGRA has detected gamma-ray emission at 730 GeV confirms that the torus (ifit exists) should be cold, with a temperature less than ∼ 250 K, consistent with theinterpretation of the directly observed infrared flux.10)M87 appears to be in a dormant state, and the situation could change drasticallyduring extreme flaring activity, possibly related to spin flip during binary black holemerger,24) turbulences or a sudden increase in the accretion mass rate. Owen et al.22)pointed out that M87 could display on-off activity cycles, assuming that there couldbe a phenomenon which turns off/on the central engine. In this case the small-scaletorus could undergo periodic heating during which the torus temperature could riseup to the sublimation temperature ∼ 1500 K. This would present any TeV photonsproduced within the reactivated torus escaping (see the top solid curve of Fig 2corresponding to T = 1000 K). Such a reactivation of the nucleus of M87 wouldaffect other radiation fields making them relevant target fields for interaction ofparticles and radiation within the jet. In this case the SPB model would need tobe replaced with a different model, perhaps a proton quasar model31) in which pion6 A.-C. Donea and R.J. Protheroephotoproduction and Bethe-Heitler pair production dominate for energetic protons.In this case, a reduction in the TeV gamma ray flux would be accompanied by anincrease in the flux at other wavelength, and also to enhanced neutrino emission,perhaps at a level observable by planned km3 extensions to the AMANDA neutrinodetector. Regular monitoring of M87 in several wavelength bands would be helpfulto detect the onset of extreme flaring activity.AcknowledgmentsThis work is supported by a grant from the Australian Research Council. A.Donea would like to acknowledge the hospitality offered by the University of Tokioin January 2003.References1) J.A. Biretta, in The Radio Galaxy M87, ed. H.-J. Ro¨ser & K. Meisenheimer (Springer:Berlin), (1999) 159.2) J.M. Bai and M.G. Lee, Astrophys. J. 549 (2001) L1733) J.C. Cohen et al. , Astron. J. 119 (2000), 162.4) W.B. Sparks, J.A. Biretta, F. Macchetto, Astrophys. J. 473 (1996), 2545) R.T. Zavala ,G.B. Taylor, Astrophys. J. 566 (2002), 96) T. Di Matteo, S.W. Allen, A.C. Fabian, A.S. Wilson, Andrew J. Young, Astron. J. 582(2003), 1337) R. Narayan, I Yi and R Mahadevan, Nature 375 (1995), 6238) C.S. Reynolds ,T. Di Matteo, A. C., Fabian , U. Hwang and C.R. Canizares , MNRAS283 ( 1996), L1119) D. Whysong, R. Antonucci, astro-ph/010638110) E.S. Perlman, W.B. Sparks, J. 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Gamma ray and infrared emission from the M87 jet and torus

Alina C. Donea

Raymond J. Protheroe

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Gamma Ray and Infrared Emission from the M87 Jet and Torus

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Gamma ray and infrared emission from the M87 jet and torus

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