The H.E.S.S. experiment (High Energy Stereoscopic System) is an array of imaging Cherenkov telescopes for the detection of γ-rays in the energy domain above 100 GeV. Its improved sensitivity and angular resolution in comparison to previous instruments and the large field of view of 5 degrees makes H.E.S.S. perfectly suited for a survey of the galactic plane. We report on a scan of the inner part of the Galactic plane in very high energy γ-rays with the H.E.S.S telescope system. The Galactic plane between longitude 330 deg and 30 deg and Galactic latitude -3 deg to +3 deg was observed for a total of 230 hours, reaching an average flux sensitivity of 3% of the Crab Nebula at energies above 200 GeV. Several unknown sources of very high energy γ-ray emission were found at a high statistical significance. We will present results for these new sources, along with a discussion on possible counterparts in other wavelength bands

Funk, S

Funk, S.

MPG.PuRe

Results from the H.E.S.S. Galactic Plane survey

Very high energy γ-rays are a precious tool to probe the long-standing mystery of the origin of cosmic rays. Produced in the interactions of accelerated particles in astrophysical objects, they can be used as a tracer of the high energy universe. Until now, our knowledge of the very high energy γ-ray sky was limited to only a handful of objects. A survey of the inner part of the Milky-Way with the High Energy Stereoscopic System reveals a population of eight firmly detected sources, thereby almost doubling the number of very high energy γ-ray sources known

First results from the H.E.S.S. Galactic plane survey

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29th International Cosmic Ray Conference Pune (2005) 4, 123126Results from the H.E.S.S. Galactic Plane surveyS.Funk for the H.E.S.S. Collaboration,(a) Max-Planck Institut f¤ur Kernphysik, Postfach 103980, D-69120 Heidelberg, GermanyPresenter: S. Funk (Stefan.Funk@mpi-hd.mpg.de), ger-funk-S-abs1-og22-oralThe H.E.S.S. experiment (High Energy Stereoscopic System) is an array of imaging Cherenkov telescopes forthe detection of  -rays in the energy domain above 100 GeV. Its improved sensitivity and angular resolution incomparison to previous instruments and the large field of view of 5 degrees makes H.E.S.S. perfectly suited fora survey of the galactic plane. We report on a scan of the inner part of the Galactic plane in very high energy -rays with the H.E.S.S telescope system. The Galactic plane between longitude 330 deg and 30 deg andGalactic latitude -3 deg to +3 deg was observed for a total of 230 hours, reaching an average flux sensitivityof 3% of the Crab Nebula at energies above 200 GeV. Several unknown sources of very high energy  -rayemission were found at a high statistical significance. We will present results for these new sources, along witha discussion on possible counterparts in other wavelength bands.1. IntroductionVery high energy (VHE, defined here as 		 eV)  -rays provide the most direct view currently availableof extreme environments in the local universe. As  -rays are produced by interactions of relativistic particlesthey represent a good probe of non-thermal astrophysical processes. Relativistic electrons can produce  -rays by non-thermal bremsstrahlung or by inverse Compton scattering on background radiation fields, whereasprotons (and atomic nuclei) can generate  -rays via the decay of  s produced in hadronic interactions withambient material. VHE  -radiation is therefore expected from the acceleration sites of particles with energiesabove 1 TeV ( 	 eV). In particular,  -rays can be seen as a probe of sources of nucleonic cosmic rays in ourGalaxy and may thus provide part of the solution to the century-old puzzle of the origin of this radiation.Theoretically predicted particle accelerators in the Galaxy include pulsars and their pulsar wind nebulae(PWN), supernova remnants (SNR), microquasars, and massive star forming regions. Detected sources ofvery high energy  -rays in the Galaxy include PWN, SNRs and objects with no identified counterpart in otherwavebands. Most potential galactic  -ray sources are associated with massive star formation and thereforecluster along the Galactic Plane, concentrated in the direction towards the centre of the Galaxy. From cata-logues, it is known that 91 SNRs and 389 pulsars are located within the inner 60  in galactic longitude and6  in galactic latitude [2, 3]. A systematic survey of the Galactic plane thus provides a promising way toinvestigate properties of source classes and to search for as-yet unknown types of Galactic VHE  -ray emit-ters. Until the completion of the High Energy Stereoscopic System (H.E.S.S.) [4] in early 2004, no systematicVHE  -ray survey of the southern sky, or of the central region of the Galaxy had been performed. Here wereport on a survey of this region conducted with H.E.S.S. in 2004, at a flux sensitivity of 3% of the Crab flux.H.E.S.S. is an array of telescopes using the stereoscopic imaging Cherenkov technique. H.E.S.S. consists offour atmospheric Cherenkov telescopes and has the largest field of view (  diameter) of all presently operatingimaging Cherenkov telescopes, which yields a considerable advantage for surveys. The angular resolution forindividual  -rays is better than 0.1  , allowing a source position error of   to be achieved, even for relativelyfaint sources.124 S.FunkFigure 1. Signicance (pre-trial) map of the H.E.S.S. 2004 Galactic plane scan. Re-observations of candidates from theinitial scan are included here. The background level was estimated using a ring around the test source position. On-sourcecounts are summed over a circle of radius  , where ﬁﬀﬃﬂ ﬂ! degrees  , a cut appropriate for point-like sources.2. The H.E.S.S. Galactic plane surveyThe inner part of the Galactic plane (Galactic longitude -30 " l " 30  and latitude -3 #" b " 3  ) was scannedwith H.E.S.S. from May to July 2004 in observations of 28 minute duration. Individual observations werespaced by %$'& in longitude and ! in latitude for a total observation time of 112 hours. An average 5 standarddeviation detection sensitivity of )(+*,		 photons cm *- s *.	 above 100 GeV ( / 3% of the Crab Flux) wasreached for points on the Galactic plane. Seven promising candidate sources from the survey were re-observedfrom July to September for typically 3.5 hours per source. Eight unknown VHE sources were detected at thelevel of larger than 6 standard deviations after accounting for all trial factors (see Fig. 1). The new GalacticVHE  -ray sources cluster along the Galactic plane (with a mean 0 of -0.25  and an rms of 0.25  in the latitudedistribution). This is a clear indication that we see a population of Galactic (rather than extragalactic) sources.All of the sources are significantly extended beyond the size of the H.E.S.S. point spread function. A searchfor counterparts for the new  -ray sources has been conducted in other wavelength bands. Fig. 2 shows the -ray emission from the region around each of the new sources together with potential counterparts.Results from the H.E.S.S. Galactic Plane survey 125Figure 2. Smoothed excess maps in units of counts of the regions around each of the eight new sources in Galacticcoordinates in degrees. A Gaussian of rms 0.05 1 is used for smoothing to reduce the impact of statistical uctuations.The best-t centroids for the 2 -ray emission are shown as crosses, and the best-t rms size as a black circle. Possiblecounterparts are marked by white triangles, with circles indicating the nominal source radius (or the position error in thecase of EGRET sources). The lower right panel indicates the simulated point spread function of the instrument for thesedata, smoothed in the same way as the other panels.3. Discussion of possible counterpartsSNR candidates: The chance probability for coincidences with SNR in the whole scan region is 6%. Fourplausible associations exist with an SNR, for HESS J1640-465, HESS J1834-087 and HESS J1804-216 and asrecently discovered also for HESS J1813-178. HESS J1640-465 is spatially coincident with the SNR G338.3-0.0, which is a broken shell SNR lying on the edge of a bright ultra-compact HII region. The unidentifiedEGRET source 3EG J1639-4702 lies 35  away and could also be connected to HESS J1640-465, given theposition uncertainty of 34  of the EGRET source. HESS J1834-087 is spatially coincident with SNR G23.3-0.3(W 41), a 27  diameter shell-type SNR. HESS J1813-137 is spatially coincident with the recently publishedradio SNR candidate G12.8–0.0 [5], that was also detected in soft  -rays by the INTEGRAL satellite [6].HESS J1804-216 coincides with the southwestern rim of the shell-type SNR G8.7-0.1 (W30) of radius 26  .126 S.FunkPulsar wind nebula candidates: HESS J1804-216 is also one of three plausible associations with neb-ulae powered by sufficiently energetic young pulsars. The others are HESS J1825-137 and HESS J1616-508.HESS J1825-137 lies 13  from the pulsar PSR J1826-1334 which has been associated with the close-by uniden-tified EGRET source 3EG J1826-1302. A 5  diameter diffuse emission region extending asymmetrically to thesouth of the pulsar was detected using XMM [7]. This diffuse emission is interpreted as synchrotron emissionproduced by a pulsar wind nebula (PWN). The VHE emission is similarly located south of PSR J1826-1334,and could be coincident with the PWN. The conversion efficiency implied from spin down power to VHE -rays is less than 1%. HESS J1616-508 is located 9  from the young hard X-ray pulsar PSR J1617-5055. TheVHE  -ray flux is again less than 1% of the spin-down luminosity of the pulsar.Other sources: For HESS J1837-069 a potential counterpart is the diffuse hard X-ray source G25.5+0.0,which is 12  across and was detected in the ASCA Galactic plane survey [8]. The nature of this bright X-raysource is unclear but it may be an X-ray synchrotron emission dominated SNR such as SN 1006 or a PWN. Thebrightest feature in the ASCA map (AX J1838.0-0655), located south of G25.5+0.0, coincides with the VHEemission and is therefore another counterpart candidate. This still unidentified source was also serendipitouslydetected by BeppoSAX and also in the hard X-ray (20-100 keV) band in the Galactic plane survey of Integral.HESS J1614-518 so far has no plausible SNR or pulsar counterpart. This source is in the field of view of thenearby HESS J1616-508.4. SummaryFour of the eight newly discovered sources are potentially associated with supernova remnants, two withEGRET sources. In three cases an association with pulsar-powered nebulae is not excluded. In conclusion, wehave on the basis of the survey generated a first unbiased catalogue of very high energy  -ray sources in thecentral region of our Galaxy, extending our knowledge of the Milky Way into the VHE domain.References[1] Aharonian, F., et al. (H.E.S.S. Collaboration), Science, 307, 1938, 2005[2] D. A. Green, BASI, 32, 335, 2004[3] R. N. Manchester, G. B. Hobbs, A. Teoh, M. & Hobbs, AJ, 129, 1993-2006, 2005[4] J. A. Hinton, NewAR, 48, 331, 2004[5] C. L. Brogan et al. submitted to ApJ, 2005 (available at http://xxx.lanl.gov/abs/astro-ph/0505145)[6] P. Ubertini, et al. submitted to ApJL, 2005 (available at http://xxx.lanl.gov/abs/astro-ph/0505191)[7] B. M. Gaensler et al., ApJ, 588, 441, 2003[8] A. Bamba et al. ApJ, 589, 253, 2003

Results from the H.E.S.S. Galactic Plane survey

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