Lamanna, G.

Hal - Université Grenoble Alpes

Observations of very high energy gamma-ray galactic sources with H.E.S.S.

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Observations of very high energy gamma-ray galacticsources with H.E.S.S.G. LamannaTo cite this version:G. Lamanna. Observations of very high energy gamma-ray galactic sources with H.E.S.S..5th Workshop on Science with the New Generation of High Energy Gamma-ray Experiments(SciNeGHE07), Jun 2007, Frascati, Italy. Infn Frascati, A paraˆıtre, 2007, Frascati PhysicsSeries. <in2p3-00175511>HAL Id: in2p3-00175511http://hal.in2p3.fr/in2p3-00175511Submitted on 22 Nov 2007HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Frascati Physics Series Vol. VVV (2007) pp.1-9Science with the New Generation of High Energy ExperimentsFrascati, 18-20 June, 2007OBSERVATIONS OF VERY HIGH ENERGYGAMMA-RAY GALACTIC SOURCES WITH H.E.S.S.Giovanni Lamannaafor the H.E.S.S. collaborationa LAPP - Laboratoire d’Annecy-le-Vieux de Physique des Particules IN2P3/CNRS,9 Chemin de Bellevue, 74941 Annecy-le-Vieux, FranceAbstractThe H.E.S.S array of imaging Cerenkov telescopes has discovered a num-ber of previously unknown gamma-ray sources at very high energy (VHE)and has provided exciting results from the Galactic plane survey. In thiscommunication a selected sample of highlights are presented.1 The H.E.S.S. telescope systemThe H.E.S.S. array, a system of four large (13 m diameter) imaging atmo-spheric Cerenkov telescopes, is operated since December 2003 by an interna-tional collaboration of about 100 physicists. Located in the Khomas highlandof Namibia, the H.E.S.S. system covers a 5o field of view, with a sensitivity12 Frontier Science 2004, Physics and Astrophysics in Spacewhich allows to detect sources with a flux of 1% of the Crab Nebula in 25 hof observation and an energy threshold between 100 and 700 GeV increasingwith the observation zenith angle. The four telescopes provide multiple im-ages of gamma-ray induced air showers in the Cerenkov light emitted by theshower particles, enabling the stereoscopic reconstruction of the shower geom-etry and the shower energy. The estimated energy resolution is 15% and 0.1ois the angular resolution for individual gamma-ray corresponding to 1′ locationposition of a VHE gamma-ray source. A personal selection of the highlightsfrom H.E.S.S. is imposed by the lack of space: most recent published resultsfrom the galactic plane survey and dedicated source observations together witha summary on the studies of shell-type supernova remnants and pulsar windnebulae will be the main topics of this letter.2 The H.E.S.S. galactic plane surveyThe Galactic plane survey was conducted in the summer of 2004 covering theregion of -30o to 30o galactic longitude and -2.5o to 2.5o in galactic latitude,resulting in 15 new VHE gamma ray sources plus three previously known.Searching for counterparts in radio- and X-ray catalogs they resulted to be re-lated to SNR, a significant fraction to PWNe and at least three “Dark accelera-tors” without counterpart known. A new observation campaign was conductedduring the years 2005-2007 with the scanned region now reaching from -80o to60o galactic longitude. The number of new sources is more than 15 with 6 new“Dark accelerators”, others sources incrementing the known classes and somenew results. Details on published results on the two campaigns can be foundin [1] and [2] respectively.2.1 HESS J1023-575The discovery [3] of the source HESS J1023-575 is one of the most relevanthighlight of the 2006 data taking: a clue to the investigation on the cosmic-rays origin. The detection of VHE gamma-ray emission associated with theyoung stellar cluster Westerlund 2 in the HII complex RCW 49 provides ev-idence that particle acceleration to extreme energies is associated with thisregion, a luminous massive star formation region already well studied at var-ious wavelengths. The source (Fig. 1) has been observed for a total 14 h ofdata for a corresponding statistical significance of more than 9 σ and clearlyextended beyond the nominal PSF. The differential energy spectrum, extendedabout two order of magnitude in energy and with a minimum threshold of 380GeV, can be described by a power law with index 2.53 ± 0.16 and an integralflux of 1.3 ± 0.3 × 10−11 cm−2 s−1. A variety of potential emission scenariosare suggested [4] for the interpretation of HESS J1023-575, a new type of as-tronomical object, profoundly distinguished from other source findings madeG. Lamanna for the H.E.S.S. collaboration H.E.S.S. Galactic Sources 3Figure 1: Left: H.E.S.S. gamma-ray sky map of the Westerlund 2 region. TheWR stars WR20a and WR20b are marked as filled triangles, while the dashedcircle is the extension of the luminous stellar cluster Westerlund2. Right: HESSJ1023 5, 7, and 9 σ significance contours overlaid on a radio image.during Galactic Plane Scan observations. Further investigation with H.E.S.S.will allow to discriminate among alternative interpretations.3 Study of the shell-type supernova remnants (SNRs)Two supernova shells already detected as gamma sources by CANGAROO,RX J1713.7-3946 [5] and RX J0852.0-4622 (“Vela Junior”) [6](Fig. 2), are nowfirmly established VHE gamma-ray emitters and morphologically resolved byH.E.S.S.. The energy spectra follow a power law with index of about 2.3,constant across the entire remnants. For both sources, the gamma-ray shellintensity observed with H.E.S.S. is highly correlated with the X-rays one. Thiscorrelation would be natural if a common population of primary electrons wereresponsible for both emission regimes. Assuming that X-rays represents syn-chrotron radiation and that the gamma rays are generated in Inverse Comptonscattering, as it is shown in Fig. 3, simple electronic models assuming an elec-tron injection index of 2.5 and with a local magnetic field of B ∼ 10 µG,which accommodates both levels of spectra, fail to consistently fit the multi-wavelength data (e.g.: over shooting the radio flux [8]). In contrast models as-suming higher magnetic field and adding gamma-rays from proton-interactions,achieve a good description of wide-band spectra [7]. This interpretation wouldsupport the hadronic origin of gamma rays even if a conclusive evidence isstill lacking. The currently modest number of shell SNR resolved in VHE hasbeen increased by the recent observation of RCW 86, a supernova remnantwith a barrel-shaped shell, visible in X-rays, radio and optical waves. Hintsfor gamma-ray emission were seen with CANGAROO-II instrument, but nofirm detection was claimed. A clear gamma-ray signal with more than 9 σ has4 Frontier Science 2004, Physics and Astrophysics in SpaceFigure 2: Shell supernova remnants seen in gamma rays by H.E.S.S.: RXJ1713.7-3946 (left) and RX J0852.0-4622 (right).Figure 3: RX J1713.7-3946: energy spectrum (left); wide band spectra (to-gether with X-rays, radio data and H.E.S.S. gamma rays measurements) fora magnetic field ∼ 10 µG and an electron injection index of 2.0, 2.25 and 2.5(right).G. Lamanna for the H.E.S.S. collaboration H.E.S.S. Galactic Sources 5Figure 4: W 28 VHE gamma-rays excess map with 4, 5 and 6σ contour levelsfor HESS J1801-233 and HESS J1801-240 A, B and C sources.been detected by H.E.S.S.. A detailed analysis is in progress and preliminaryresults [9] have shown: a flux 5-10% of the Crab nebula, a 2.3-2.5 spectral indexand a shell type morphology.3.1 HESS J1800-240 and HESS J1801-233An other composite or mixed-morphology SNR, which is an ideal target forVHE observations, is W 28 (G6.4-0.1). The old-age W 28 SNR is thought tohave entered its radiative phase of evolution. The shell-like radio emission peaksat the northern and northeastern boundaries where interaction with molecularcloud is established. The X-ray emission, which overall is well-explained bya thermal model, peaks in the SNR center but has local enhancements in thenortheastern SNR/molecular cloud interaction region. On the south bound-ary several HII regions, including ultra-compact HII region W 28A2 are found.H.E.S.S. observations of W 28 have revealed VHE gamma-ray emission situatedat its northeastern (HESS J1801-233) and southern boundaries (HESS J1801-240 with components A, B and C) (Fig. 4) [10]. A multi-wavelength analysisof W 28 has revealed a dense molecular cloud enveloping the southern region,and EGRET MeV/GeV emission centered on HESS J1801-233 and the north-eastern interaction region. Overall, these results suggest that old-age SNRsare capable of multi-TeV particle accelerators and candidate hadrons diffusiveshock accelerators.6 Frontier Science 2004, Physics and Astrophysics in SpaceFigure 5: HESS J1825-137, a PWN candidate manifesting an offset from theassociated pulsar position, a steepening of the power-law spectral index withincreasing distance from the pulsar and a larger extension than in X-ray.4 Study of the Pulsar Wind NebulaePulsar Wind Nebulae (PWN) are responsible for a significant fraction of thenew VHE Galactic sources observed by H.E.S.S.. The purpose of the PWNstudy is a diagnostic of the spatial and spectral distribution of the high en-ergy electrons responsible of the TeV gamma-ray production dominated by theInverse Compton scattering off the well-known cosmic microwave background.HESS J1825-137 is a particularly interesting PWN candidate: it is a strongsource extended over a fraction of degree [11]. It was detected during the firstGalactic plane survey and then further observed. It is located south of thepulsar PSR B1823-13 which exhibits an X-ray nebula trailing extended over∼5’ in the direction of the VHE source but then much smaller in size. A nat-ural explanation is that the X-ray generating electrons (via interaction withthe nebula magnetic field ∼10 µG) have higher energies than those responsiblevia Inverse Compton scattering for the VHE gamma rays. The higher energyX-ray electrons cool faster and have a shorted range. More importantly, forthe first time observations have revealed the energy dependent morphology ofthe source. This manifests itself as a steepening of the power-law spectral in-dex with increasing distance from the pulsar, as would be expected from theradiative losses of high-energy electrons injected by the pulsar (see Fig. 5).Among the number of PWNe detected by H.E.S.S. and apart from the al-most point-like Crab nebula, extensively studied through different consecutiveG. Lamanna for the H.E.S.S. collaboration H.E.S.S. Galactic Sources 7Figure 6: Smoothed gamma-ray excess map (from left to right) from Vela X,MSH 15-52 and the two sources in the Kookaburra region. White contours areX-ray corresponding to count rates contour lines.observation campaigns [12], Vela X associated with the Vela pulsar (Fig. 6)is likely the most extended one (about a degree south of the pulsar) and sig-nificantly old (age ∼11 kyr). The energy spectrum is very hard reaching 50TeV. The radio, X-rays and VHE gamma-rays emission regions of Vela X aremarkedly offset from the pulsar position. This may be due to the supernovaexplosion occurring in an inhomogeneous medium, and the resulting asymmet-ric reverse shock displacing the PWN in the direction away from the higherdensity medium. The displacement of the nebulae from the pulsar positions isa surprising constant of almost of extended PWN candidate sources. It is alsothe case of MSH 15-52, associated with the pulsar PSR B1509-58 inside the G320.4-1.0 / RCW 89 shell. This elongated and single-sided nebula was revealedby H.E.S.S. as aligned in the same direction of the jet-like high-resolution X-rayChandra image. More recently, studies of the “Kookaburra” region revealedtwo new TeV gamma-ray sources, one most likely associated with the pulsarPSR J1420-6048, the other one with the “Rabbit” feature presumably resultingfrom another pulsar. Both sources have relatively hard spectra index around2.2, both are extended on the scale of about 10 pc and both are displaced bya similar amount from their pulsars.Re-observations of the mentioned extended PWNe have been recently accom-plished and analysis are in progress to further infer about the spectral andspatial distribution of the energetic electrons within the leptonic interpretationof the VHE emission.A systematic search for gamma-ray counterparts of known pulsars is ad-dressed by the possibility that all pulsars have associated VHE gamma raysnebulae. Such a research has produced a bunch of new PWN candidates: e.g.:HESS J1718-385 and HESS J1809-193 (Fig. 7) [13]. They show that amongpulsars with a spin-down energy flux above 1035ergs/s/kpc2, a large fraction is8 Frontier Science 2004, Physics and Astrophysics in SpaceFigure 7: Two new PWNe candidates, HESS J1718-385 and HESS J1809-193,observed during the Galactic sky survey.visible as gamma-ray emitters, converting about 1% of their spin-down energyinto 1-10 TeV gamma rays. This implies that about 10% of pulsar spin-downenergy is fed into high-energy electrons. More observations of these sourcesand corresponding multi-wavelength investigations would provide importantprogress in the physics of PWNe.References[1] F. Aharonian et al, Astrophy. Jour. 777 (2006)[2] S. Hoppe et al., Proceedings ICRC07 (2007) and references therein.[3] F. Aharonian et al, Astron. Astrophy. 467, 1075 (2007)[4] O. Reimer et al., First GLAST Symposium Conference Proceedings (2007)[5] F. Aharonian et al, Astron. Astrophy. 449, 223 (2006)[6] F. Aharonian et al, Astrophy. Jour. 661, 236 (2007)[7] F. Aharonian et al, Science 307, 1938 (2005)[8] J.S. Lazendic et al, Astrophy. Jour. 602, 271 (2004)[9] S. Hoppe et al., Proceedings ICRC07 (2007)[10] F. Aharonian et al, Astron. Astrophy. in press (2007)[11] F. Aharonian et al, Astron. Astrophy. 460, 365 (2006)[12] Y. Gallant, In Proc. Barcelona Conf., Astr. and Space Sc. (2006) andreferences therein.[13] F. Aharonian et al, Astron. Astrophy. in press (2007)

Observations of very high energy gamma-ray galactic sources with H.E.S.S.

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