Lemoine-Goumard, M

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29th International Cosmic Ray Conference Pune (2005) 4, 283286Very high energy observations of the starburst galaxy NGC 253 withH.E.S.S.M. Lemoine-Goumard , J.A. Hinton, H.J. Vo¨lkfor the H.E.S.S. Collaboration(a) Laboratoire Leprince-Ringuet, IN2P3/CNRS, Ecole Polytechnique, F-91128 Palaiseau, France(b) Max-Planck-Institut f¤ur Kernphysik, P.O. Box 103980, D 69029 Heidelberg, GermanyPresenter: Jim Hinton (lemoine@poly.in2p3.fr), fra-lemoine-goumard-M-abs1-og23-oralWe present the result of 28 hours of observations of the nearby starburst galaxy NGC 253 with the H.E.S.S.Cherenkov telescopes in 2003. Gamma-ray emission above 400 GeV from NGC 253 was reported by theCANGAROO collaboration in 2002. We find no evidence for very high energy  -ray emission from this objectand we derive upper limits on the flux above 300 GeV of  	 photons cm  s  for a point-likesource and   photons cm  s  for a source of size 0.5  as reported by CANGAROO, both at aconfidence level of 99%. These upper limits are inconsistent with the spectrum reported by CANGAROO. Theexpected very high energy  -ray emission from this object is discussed in the framework of a galactic windpropagation model.1. IntroductionNGC 253 is the nearest spiral galaxy outside the Local Group (  = 2.5 Mpc) and is considered as a pro-totypical starburst galaxy. The cosmic-ray energy densities are expected to be ﬀﬂﬁﬃ !"#ﬀ%$'&)(ﬁ(whereﬀ%$'&)(ﬁ+*-,/.103245 is the value in our galaxy) (Vo¨lk et al. [14]) due to the high rates of massive star formationand supernova explosions in the so called ’starburst regions’, making them interesting candidates for  -raysources. For NGC 253 Blom et al. [2] obtained an upper limit of  6789: cm  s - above 100 MeV fromEGRET data.At VHE (E ; 100 GeV), the CANGAROO Collaboration claimed an extended gamma-ray signal with an rmsangular size estimated as 0.3-0.6  (Itoh et al. [7]), somewhat larger than the optical size of the galaxy (28 <=7 < ). The differential energy spectrum (Itoh et al. [8]) was described by: !>@?A!ﬀCBEDGF HJILKMNPOQRSC"DGﬀT?UWVX,.YRZ5/[ :'\Q]_^/[ `ba cm  s  TeV  , with a corresponding flux above 400 GeV of c ! 6Md9)cm  s  , or c eI % of the flux of the Crab Nebula above the same threshold. This detection was in-terpreted as inverse Compton emission, implying the existence of an extended halo of multi-TeV CR elec-trons around NGC 253 (Itoh et al. [9]). However, 32.5 hours of observations were also made on NGC253 by the HEGRA Collaboration and no signal was detected. They could derive a flux upper limit of>WfDg;IQPF=VX,.ﬂRih jSQ5 photons cm  s  at 99% confidence (Go¨tting [5]). The detection of NGC 253by CANGAROO, along with the theoretical expectation of very high energy  -rays from starburst galaxies,motivated the observation of this object with the High Energy Stereoscopic System (H.E.S.S. ).2. H.E.S.S. resultsH.E.S.S. is an array of four 13 m diameter telescopes, each equipped with 960 pixel (5  field-of-view) cam-eras (Hinton [6]). Commissioning of the array in Namibia (at 1800 m above sea level) began in 2002 and thearray was completed in December 2003. Observations of NGC 253 were made during the construction of theH.E.S.S. array. After run quality selection and dead time correction, 28 hours remain (24.6 hours with 2 tele-scopes and 3.4 hours with 3 telescopes). The mean zenith angle of observations was 14  . Only events where atleast two telescopes provided shower images were used in the analysis to enable stereoscopic reconstruction.284 M. Lemoine-Goumard et al.)2 (degrees2θ0 0.05 0.1 0.15 0.2 0.25 0.3Events050100150200250300350400On SourceOff SourcePoint Source Cut  Cuto0.5)2 (degrees2θ0 0.05 0.1 0.15 0.2 0.25 0.3Events050100150200250300350400On SourceOff SourcePoint Source Cut  Cuto0.5Figure 1. Left: Angular distribution of k -like images relative to the centre of NGC 253 (On) and relative to the back-ground control region (Off) for Analysis 1. Events are plotted versus the squared angular distance to give equal solidangle in each bin. Background curves (histograms) are determined relative to points lm away from the source position.Right: as for upper gure but using Analysis 2.Two independent methods have been applied to search for  -ray emission in this dataset.The first analysis (referred to as Analysis 1) is a “HEGRA”-like analysis (Aharonian et al. [1]) based on theHillas parameters. Since the observations were taken in wobble-mode (with the target source offset by 0.5 from the tracking position of the telescopes), the events pointing to the symmetrical position with respect tothe centre of the camera were used for background measurement.The second analysis (referred to as Analysis 2) employs a different reconstruction method based on a simple3D-modelling of an electromagnetic shower assuming rotational symmetry (Lemoine-Goumard [10]). For eachevent, eight parameters of the shower are reconstructed including the transverse standard deviation referred toas “3D-width”. The background subtraction is then performed independently in each bin of the FOV using amaximum likelihood method based on the knowledge of the 3D-width distributions of gamma-rays and back-ground events (Lemoine-Goumard [11]). For these two analyses, the standard calibration and image cleaninghave been used.Figure 1 shows the distribution of the squared angular distance ( nU ) of  -ray candidates from the centre ofNGC 253 for the whole data set. The distributions for the on-source and background measurements are con-sistent; no evidence for a  -ray signal is seen. The vertical dashed lines in this figure indicate the positions ofthe standard H.E.S.S.  -ray selection cut for point-like sources ( nohpq/6N ) and a much wider cut (at 0.5  ) atthe approximate extent reported by the CANGAROO collaboration.Figure 2 shows integral flux upper limits calculated assuming a spectrum of photon index -3.85 (as reported byCANGAROO) and following the method of Feldman & Cousins ([4]). Flux limits are shown for point-like andextended emission ( N IJ radius) for Analysis 2. Also shown are CANGAROO-II integral data points derivedfrom the differential spectrum given by Itoh et al. ([8]). Above 300 GeV we derive 99% confidence flux upperlimits of ! rQ-g photons cm  s - for a point-like source and  rN photons cm  s  for asource of radius 0.5  . The upper limit curves derived from Analysis 1 are similar in shape with somewhathigher values at 300 GeV: FQPFs-g photons cm  s  (point source) and  1 photons cm  s -(0.5  ).Very high energy observations.... 285Energy (TeV)10-1 1 )-1 s-2Flux > E (photons cm10-1310-1210-11CANGAROO-IIHEGRAH.E.S.S. point sourceH.E.S.S. extendedFigure 2. Upper limits from H.E.S.S. on the integral ux of k -rays from NGC 253 (99 % condence level) from Analysis 2.Curves for point like emission (solid lines) and for a source of 0.5mradius (dashed lines) are shown. CANGAROO integraldata points (derived from the differential spectrum given by Itoh et al. ([8])) and an upper limit from HEGRA (G¤otting [5])are shown for comparison.3. DiscussionFollowing Weaver et al. [16] we assume the central starburst (SB) region to be a cylinder of radius tdBueIA pcand height vwBx! pc in the plane of the galaxy, with an outflow that is collimated by a massive moleculartorus into the vertical direction, perpendicular to the disk. The outflow velocity yz{ |/} is not very well known,with “minimum reasonable values between 300 and 600 km s  ” (Strickland et al. [13]). For the purposes ofour estimates we assume here yNz9{ |/}~BIA km s  . The volume of the SB region is then y3"c69 F@7a pc 5 .The SN rate in this small nuclear volume is estimated as !3B  yr  , about as high as that of ourentire Galaxy (cf. Engelbracht et al. [3]). The total mass  in the nuclear region is rather uncertain. Weassume B#!Y#Ł which corresponds to the preferred value of Engelbracht et al. ([3]) and gives adensity of 580 protons cm 5 in the nuclear region. To estimate the spatial extent and the magnitude of thegamma-ray emission as a result of the SB, we first have to consider the time scales for particle transport (  ﬁ'|bfor convection and  }{  for diffusion) and energy losses ( () ). Assuming that the propagation properties ofenergetic particles in our own galaxy and its halo (Ptuskin et al. [12]) can be scaled to the SB parameters ofNGC 253, one calculates ''A4cPFo#!\ yr and g}/{ ﬃ	 o\ yr for a 1 TeV particle. Both exceed()YBrH o#A` yr. This means that CR protons with energies up to about 1 TeV behave calorimetrically:they lose all their energy by inelastic collisions before being able to leave the SB region. However, the increaseof the diffusion coefficient with particle energy makes the particles with energies above several TeV diffusiondominated, which implies that they escape the SB region before interacting. The turnover point is at  -rayenergies of a few hundred GeV.For TeV electrons the loss time is estimated as follows: the enormous far infrared (FIR) SB-luminosity /B!%Jg^3Ł deduced by Engelbracht et al. ([3]) implies a radiation energy density  3 &}cIA! eV cm 5 . Thisleads to an inverse Compton (IC) loss time ¡£¢()c¤HjDGﬀT?e6JVX,._Rb- yr for the electrons of energy ﬀEc6 TeVthat produce 1 TeV  -rays. Therefore, comparing with the proton emission, the SB region acts even morecalorimetrically for the emission of TeV  -rays via IC scattering. NGC 253 can therefore only be a TeV point286 M. Lemoine-Goumard et al.source.The hadronic  -ray emission in this scenario simply corresponds to the total amount of CR energy created. Inthis picture diffusion is irrelevant below a few TeV, and the total CR energy ﬀ¢N¥in the SB region amounts toﬀ¢N¥cEPI~1\'5,/¦)§¨©/ ªJ¦r«ﬀj/\'^,/¦)§¬­I®1`ªJ¦!¯_°(1)where  -¬­B±()²bﬁ'|b²-}{ , and«hx is the CR energy production efficiency for an average SNevent with a mechanical energy release ﬀ_3 . The  -ray energy spectrum corresponds to that of the SNRsources, assumed here to be J³¢¥?A!ﬀµ´ ﬀ¶/[  . Assuming the radiating TeV particles to freely penetratethe gas in and around their sources, the energy spectrum of the hadronic  -ray flux can be calculated on thebasis of eq. (13) of Vo¨lk et al. [15] by substituting our preferred values for  ,  , and ﬀﬂﬁ . We thus obtain>WfDg;ﬀ~R·BN H@S5¶8¸DGﬀT?UVX,.ﬂRb-b[  photons cm  s - up to a few hundred GeV, and significantlyfalling off at higher  -ray energies. The factor ¸ denotes the uncertainty of the assumed parameters, mainly 3 ,  , and«. Assuming that«c¹N and the average ﬀ 3 = \Z erg, we obtain PI#h¸ºh	I . Thisflux, with ¸ =1, is similar to – in fact somewhat larger than – the H.E.S.S. upper limit for the VHE  -ray flux,demonstrating the physical relevance of the H.E.S.S. result.The CR electron energy produced in the SNR sources of the SB can be assumed to amount to no more than afraction* of the energy generated in nuclear CR particles, as is the case in our galaxy.4. ConclusionThe recently completed H.E.S.S. instrument is currently the most sensitive detector of  -rays at VHE ener-gies. The non-detection of NGC 253 by H.E.S.S. is surprising given the rather high flux previously claimedby Itoh et al. ([7]). From theoretical considerations of  -ray production in starburst galaxies it appears that theH.E.S.S. limit is close to the expected flux from the NGC 253 starburst, already excluding extreme parametervalues ( ¸¶;E ). However, a diffusive transport away from the starburst region that is faster than assumed herecannot be excluded experimentally. The possible  -ray flux would then be correspondingly reduced.References[1] Aharonian et al. (H.E.S.S. Collaboration), 2005, A&A, 430, 865[2] Blom, J.J., Paglione, T.A.D., Carraninana, A., 1999, ApJ, 516, 744[3] Engelbracht, C.W., Rieke, M.J., Rieke, G.H. et al., 1998, ApJ, 505, 639[4] Feldman, G.J. & Cousins, R.D., 1998, Phys. Rev. D, 57, 7[5] Go¨tting, N., 2005, PhD thesis, in preparation[6] Hinton, J.A. (H.E.S.S. Collaboration), 2004, New Astron. Rev., 48, 331[7] Itoh, C., Enomoto, R., Yanagita, S. et al., 2002, A&A, 396, L1[8] Itoh, C., Enomoto, R., Yanagita, S. et al., 2003b, A&A, 402, 443[9] Itoh, C., Enomoto, R., Yanagita S., Yoshida, T., Tsuru, T. G., 2003, ApJ, 584, L65[10] Lemoine-Goumard, M., et al, 2004a, Proc. HDGS, Heidelberg[11] Lemoine-Goumard, M., et al, 2004b, Proc. HDGS, Heidelberg[12] Ptuskin, V.S. et al, 1997, A&A, 321, 434[13] Strickland, D.K., Heckman, T.M., Weaver, K.A., et al., 2002a, ApJ, 568, 689[14] Vo¨lk, H.J., Klein, U., Wielebinski, R., 1989, Astron. Astrophys., 213, L12[15] Vo¨lk, H.J., Aharonian, F.A., Breitschwerdt, D., 1996, Space Sci. Rev., 75, 279[16] Weaver, K.A., Heckman, T.M., Strickland, D.K., Dahlem, M., 2002, ApJ, 576, L 19

Very high energy observations of the starbust galaxy NGC 253 with H.E.S.S.

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