Domingo-Santamaria, E

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29th International Cosmic Ray Conference Pune (2005) 4, 5154A model of the diffuse gamma-ray emission from the central region ofNGC 253E. Domingo-Santamarı´a and Diego F. Torres(a) Institut de F·sica d’Altes Energies, Edici C-n, Campus UAB, 08193 Bellaterra, Spain(b) Lawrence Livermore National Laboratory, 7000 East Ave. L-413, Livermore, CA 94550, USAPresenter: E. Domingo-Santamarı´a (domingo@ifae.es), spa-domingo-santamaria-E-abs1-og21-oralThis paper discusses a model for the diffuse emission of gamma-rays (produced as secondaries of cosmic rayinteractions, inverse Compton, and Bremsstrahlung, subject to losses) from the central -starburst- region ofNGC 253. It is a multifrequency model, so that radio and IR data are also described. The resulting predictionsfor the very high energy regime (hundreds of GeV and TeV energies) are used to analyze whether one shouldexpect NGC 253 to appear as a source for ground based Cherenkov telescopes.1. Phenomenology of the central region of NGC 253NGC 253 is located at a distance of  Mpc and it is a nearly edge-on (inclination 78 	 ) barred Sc galaxy.The continuum spectrum of NGC 253 has a luminosity of  L  (Melo et al. 2002). The FIR luminosityis at least a factor of 2 larger than that of our own Galaxy, and it mainly comes from the central nucleus. IRemission can be understood as cold (  ) dust reprocessing of stellar photon fields.When observed at 1 pc resolution, at least 64 individual compact radio sources have been detected within thecentral 200 pc of the galaxy (Ulvestad & Antonucci 1997), and roughly 15 of them are within the central arcsecof the strongest radio source, considered to be either a buried active nucleus or a very compact SNR. Of thestrongest 17 sources, about half have flat spectra and half have steep spectra. This indicates that perhaps halfof the individual radio sources are dominated by thermal emission from H II regions, and half are opticallythin synchrotron sources, presumably SNRs. There is no compelling evidence for any sort of variability in anyof the compact sources over an 8 yr time baseline. The region surrounding the central 200 pc has also beenobserved with subarcsec resolution and 22 additional radio sources stronger than 0.4 mJy were detected within2kpc of the galaxy nucleus (Ulvestad 2000). The region outside the central starburst may account for about20% of the star formation of NGC 253. It is subject to a supernova explosion rate well below ﬀ yr ﬁﬂ , andhas an average gas density in the range 20–200 cm ﬁﬃ , much less than the most active nuclear region (Ulvestad2000).Carilli (1996) presented low frequency radio continuum observations of the nucleus at high spatial resolution.Free-free absorption was claimed to be the mechanism producing a flattening of the synchrotron curve at lowenergies, with a turnover frequency located between 10  "! # and 10 $ Hz. The emission measures needed for thisturnover to happen, for temperatures in the order of 10 % K, is at least 10#pc cm ﬁ'& . As shown by infrared,millimeter, and centimeter observations, the 200 pc central region dominates the current star formation inNGC 253, and is considered as the starburst central nucleus (e.g., Ulvestad and Antonucci 1997, Ulvestad2000). Centimeter imaging of this inner starburst, and the limits on variability of radio sources, indicates asupernova rate less than 0.3 yr ﬁﬂ (Ulvestad & Antonucci 1997), which is consistent with results ranging from0.1 to 0.3 yr ﬁﬂ inferred from models of the infrared emission of the entire galaxy (Rieke et al. 1980; Rieke,Lebofsky & Walker 1988, Forbes et al. 1993). When compared with Local Group Galaxies, the supernova ratein NGC 253 is one order of magnitude larger (Pavlidou and Fields 2001).Current estimates of the gas mass in the central 20” – 50” ( (*)+ pc) region range from 2.5 ,- M (Harrison, Henkel & Russell 1999) to 4.8 ,  M  (Houghton et al. 1997), see Bradford et al. (2003), Sorai52 E. Domingo-Santamar·a and D.F. Torreset al. (2000), and Engelbracht et al. (1998) for discussions. For example, using the standard CO to gas massconversion, the central molecular mass was estimated as 1.8 ,.  M  (Mauersberger et al. 1996). It wouldbe factor of 0/ lower if such is the correction to the conversion factor in starburst regions which are betterdescribed as a filled intercloud medium, as in the case of ULIRGs, instead of a collection of separate largemolecular clouds, see Solomon et al. (1997), Downes & Solomon (1998), and Bryant & Scoville (1999) fordiscussions. Thus we will assume in agreement with the mentioned measurements that within the central 200pc, a disk of 70 pc height has  2 ,1- M  uniformly distributed, with a density of 2)+ cm ﬁﬃ . Additionaltarget gas mass with an average density of  50 cm ﬁ'ﬃ is assumed to populate the central kpc outside theinnermost region, but subject to a smaller supernova explosion rate   yr ﬁ3 , 10% of that found in the mostpowerful nucleus (Ulvestad 2000).2. Diffuse modellingThe approach to compute the steady multiwavelength emission from NGC 253 follows that implemented in4-DIFFUSE, which we have used with a few further improvements (Torres 2004). We refer the reader to Torres(2004) and Domingo-Santamarı´a & Torres (2005) for details. The steady state particle distribution is computedas the result of an injection distribution being subject to all losses and secondary production in the interstellarmedium. The normalization of the spectrum is obtained from the total power transferred by supernovae intoCRs kinetic energy within a given volume. For electrons, the total rate of energy loss considered is the sumof ionization, inverse Compton scattering, bremsstrahlung, adiabatic losses, and synchrotron radiation. Forthe production of secondary electrons,4-DIFFUSE computes knock-on electrons and charged pion processesproducing both electrons and positrons. In the case of 5 -ray photons, we compute bremsstrahlung, inverseCompton and neutral pion decay processes. For radio photons, we compute, using the steady distribution ofelectrons, the synchrotron, and free-free emission. Free-free absorption is also considered in order to reproducethe radio data at low frequencies. The FIR emission is modelled with a dust emissivity law given by 678:9<;.=>@? ,where ACBD and 8 is the Planck function. The computed FIR photon density is used as a target for inverseCompton process as well as to give account of losses in the 5 -ray scape. The latter basically comes from theopacity to 55 pair production with the photon field of the galaxy nucleus. The opacity to pair production fromthe interaction of a 5 -ray photon in the presence of a nucleus of charge E is considered too. For further detailsand relevant formulae see Domingo-Santamarı´a & Torres (2005).3. ResultsThe numerical solution of the diffusion-loss equation for protons and electrons is shown in Figure 1a and 1b.We have adopted a diffusive residence timescale of 10 Myr, a convective timescale of 1 Myr, and a density of)+ cm ﬁﬃ . In the case of electrons, the magnetic field with which synchrotron losses are computed in Figure1b is 300 F G. The latter is fixed requiring that the steady electron population produces a flux level of radioemission matching observations. An injection electron spectrum is considered –in addition to the secondaries–in generating the steady electron distribution. From about GIHKJ,LMHNO.ﬁﬂ to  GeV, the secondary populationof electrons dominates, in any case. The IR continuum emission is modelled with a spectrum having a diluteblackbody (graybody) emissivity law, proportional to 6P78Q9R;=ST? , where 8 is the Planck function. Figure 1cshows the result of this modelling and its agreement with observational data when the dust emissivity indexAUBV+  and the dust temperature ﬂWX"Y[ZNB\]+ . Whereas free-free emission is subdominant when comparedwith the synchrotron flux density, free-free absorption plays a key role at low frequencies, determining theopacity. We have found a reasonable agreement (see Figure 1d) with all observational data for a magneticfield in the innermost region of 300 F G, an ionized gas temperature of about 10 % K, and an emission measureA model of ^ -ray emission from NGC 253 53Figure 1. From top to bottom and left to right: a) Steady proton distributions in the innermost region of NGC 253. b) Idemfor the steady electron distribution. c) IR ux from NGC 253 assuming a dilute blackbody with temperature _'`badcﬀe3fhgi Kand different total luminosities. d) Multifrequency spectrum of NGC 253 from radio to IR: comparison of experimental datapoints with the result of our modelling. e) Differential ^ -ray uxes from the central region of NGC 253. Total contributionof the surrounding disk is separately shown, as are the EGRET upper limits. Also shown are the relative contributions ofbremsstrahlung, inverse Compton, and neutral pion decay to the ^ -ray ux. f) Integral ^ -ray uxes. The EGRET upperlimit (for energies above 100 MeV), the CANGAROO (Itoh et al. 2002) integral ux as estimated from their t, the (4telescopes) HESS sensitivity (for a 5 j detection in 50 hours), and the recently released (2 telescopes) HESS upper limitcurve on NGC 253 (Aharonian et al. 2005) are given. Absorption effects are already taken into account.of k#pc cm ﬁ& . Figure 1e shows bremsstrahlung, inverse Compton, and pion decay 5 -ray fluxes fromthe central nucleus of NGC 253. These results are obtained with the model in agreement with radio and IRobservations. Our predictions, while complying with EGRET upper limits, are barely below them. If thismodel is correct, NGC 253 is bound to be a bright 5 -ray source for GLAST. The integral fluxes are shown inFigure 1f. Our model predicts that, given enough observation time, NGC 253 is also to appear as a point-like54 E. Domingo-Santamar·a and D.F. Torressource in an instrument like HESS.1 Our fluxes are only a few percent of those reported by the CANGAROOcollaboration. The effect of the opacity on the integral 5 -ray fluxes only plays a role above 3 TeV.4. ConclusionsThe ease of all the assumptions made in our model, its concurrence with all observational constraints, andthe unavoidability of the processes analyzed, lead us to conclude that 1) GLAST will detect NGC 253, beingour predicted luminosity (  /Qlmﬁ  photons cm ﬁ'n s ﬁ3 above 100 MeV) well above its 1 yr all sky surveysensitivity ; 2) that our predicted TeV fluxes are about one order of magnitude smaller than what was claimedto be detected by CANGAROO; and 3) that HESS should detect the galaxy as a point like source provided itis observed long enough ( o] hours, for a detection between 300 and 1000 GeV.)5. AcknowledgementsThe work of ED-S was done under a FPI grant of the Ministry of Science and Technology of Spain. Thework of DFT was performed under the auspices of the U.S. D.O.E. (NNSA), by the University of CaliforniaLawrence Livermore National Laboratory under contract No. W-7405-Eng-48.References[1] Aharonian F. A. et al. 2005, A&A, in press., astro-ph/0507370[2] Bradford C. M., et al. 2003, ApJ 586, 891[3] Bryant P. M., & Scoville N. Z. 1999, ApJ 117, 2632[4] Carilli C. L. 1996, 305, 402[5] Domingo-Santamarı´a E., & Torres D.F. 2005, astro-ph/0506240[6] Downes D., & Solomon P.M. 1998, ApJ 507, 615[7] Engelbracht C. W., Rieke M. J., Rieke G. H., Kelly D. M. & Achterman J. M. 1998, ApJ 505, 639[8] Forbes D.A., et al. 1993, ApJ 406, L11[9] Harrison A., Henkel C. & Russel A. 1999, MNRAS 303, 157[10] Houghton S., et al 1997, A&A 325, 923[11] Itoh C., et al. 2002, A&A 396, L1[12] Mauersberger R., Henkel C., Wiebelinski R., Wiklind T. & Reuter H.-P. 1996, A&A 305, 421[13] Melo V. P., et al. 2002, ApJ 574, 709[14] Pavlidou, V., & Fields, B. 2001, ApJ, 558, 63[15] Rieke G. H., Lebofsky M. J., Thompson R. I., Low F. J. & Tokunga A. T. 1980, Ap. J. 238, 24[16] Rieke G. H., Lebofsky M. J. & Walker C. E. 1988, ApJ 325, 679[17] Solomon P. M., Downes, D., Radford, S. J. E. & Barrett, J.W. 1997, ApJ, 478, 144[18] Sorai K., Nakai N., Nishiyama K. & Hasegawa T. 2000, Publ. Astron. Soc. Japan 52, 785[19] Torres, D. F. 2004, ApJ 617, 966[20] Ulvestad J. S. 2000, ApJ 120, 278[21] Ulvestad J. S. & Antonucci R. R. J. 1997, ApJ 488, 621pThe HESS array has just released (Aharonian et al. 2005) their results for NGC 253, based on a total of 28 hours taken during theconstruction of the array with 2 and 3 telescopes operating. The energy threshold for this dataset was 190 GeV. Upper limits from HESSon the integral (99 % confidence level) are shown in Figure 1f. As an example, above 300 GeV, the upper limit is qsr tvuwq>x+ypRzphotonscm yzs yp. Our predictions are below these upper limits at all energies but still above HESS sensitivity for reasonable observation times.

A model of the diffuse gamma-ray emission from the central region of NGC 253

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