A new TeV source, HESS J1641-463, has been serendipitously discovered in the
Galactic plane by the High Energy Stereoscopic System (H.E.S.S.) at a
significance level of 8.6 standard deviations. The observations of HESS
J1641-463 were performed between 2004 and 2011 and the source has a moderate
flux level of 1.7% of the Crab Nebula flux at E > 1 TeV. HESS J1641-463 has a
rather hard photon index of 1.99 +- 0.13_stat +- 0.20_sys. HESS J1641-463 is
positionally coincident with the radio supernova remnant SNR G338.5+0.1, but no
clear X-ray counterpart has been found in archival Chandra observations of the
region. Different possible VHE production scenarios will be discussed in this
contribution.Comment: 5 pages, 5 figures, 2012 Fermi Symposium proceedings - eConf C12102

Behera, B.

Bordas, P.

Collaboration, H. E. S. S.

Dalton, M.

Djannati-Ataï, A.

Hahn, J.

Marandon, V.

Oya, I.

Schwanke, U.

Spengler, G.

English

arXiv

A new TeV source, HESS J1641-463, has been serendipitously discovered in the Galactic plane by the High Energy Stereoscopic System (H.E.S.S.) at a significance level of 8.6 standard deviations. The observations of HESS J1641-463 were performed between 2004 and 2011 and the source has a moderate flux level of 1.7% of the Crab Nebula flux at E > 1 TeV. HESS J1641-463 has a rather hard photon index of 1.99 +- 0.13_stat +- 0.20_sys. HESS J1641-463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1, but no clear X-ray counterpart has been found in archival Chandra observations of the region. Different possible VHE production scenarios will be discussed in this contribution

Collaboration, H.

MPG.PuRe

arXiv:1303.0979v2  [astro-ph.HE]  6 Mar 20134th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 2012 1Discovery of the VHE gamma-ray source HESS J1641−463I. Oya, U. Schwanke, G. SpenglerInstitut fu¨r Physik, Humboldt-Universita¨t zu Berlin, Newtonstrasse 15, D-12489 Berlin, GermanyM. DaltonCentre d’ ´Etudes Nucle´aires de Bordeaux Gradignan, Universite´ Bordeaux 1, CNRS/IN2P3 33175 Gradignan,FranceB. BeheraDeutsches Elektronen-Synchotron, DESY, Platanenallee 6, D-15738 Zeuthen, GermanyP. BordasInstitut fu¨r Astronomie und Astrophysik, Universita¨t Tu¨bingen, Sand 1, D 72076 Tu¨bingen, GermanyA. Djannati-Ataı¨APC, AstroParticule et Cosmologie, Universite´ Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris,Sorbonne Paris Cite´, 10, rue Alice Domon et Le´onie Duquet, 75205 Paris Cedex 13, France.J. Hahn, V. MarandonMax-Planck-Institut fu¨r Kernphysik, P.O. Box 103980, D-69029 Heidelberg, GermanyFor the H.E.S.S. CollaborationA new TeV source, HESS J1641−463, has been serendipitously discovered in the Galactic plane by the HighEnergy Stereoscopic System (H.E.S.S.) at a significance level of 8.6 standard deviations. The observations ofHESS J1641−463 were performed between 2004 and 2011 and the source has a moderate flux level of 1.7% ofthe Crab Nebula flux at E >1 TeV. HESS J1641−463 has a rather hard photon index of 1.99 ± 0.13stat ±0.20sys. HESS J1641−463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1, but noclear X-ray counterpart has been found in archival Chandra observations of the region. Different possible VHEproduction scenarios will be discussed in this contribution.1. H.E.S.S. analysis and resultsH.E.S.S. is an array of five imaging Cherenkov tele-scopes situated in the Khomas Highland in Namibiaat an altitude of 1800 m above sea level (see e.g.Bernlo¨hr et al. [2003], Funk et al. [2004]). In the ini-tial phase of the H.E.S.S. project (Phase I), the ar-ray was composed of four 13 m diameter Cherenkovtelescopes, whereas in Phase II a single huge dishwith about 600 m2 mirror area was added at thecenter of the array, increasing the energy coverage,sensitivity and angular resolution of the instrument[Giebels 2012]. Due to the accumulation of expo-sure with H.E.S.S. in the Galactic plane, complexVHE (Very High Energy; E > 100TeV) gamma-raysources are better resolved and frequently new struc-tures appear. This is the case for a new source,dubbed HESS J1641−463, that has been discoverednear the strong gamma-ray source HESS J1640-465[Aharonian et al. 2005]. Observations have been per-formed between 2004 and 2011 with H.E.S.S. PhaseI for a total acceptance-corrected livetime of 72 h.The standard H.E.S.S. run selection procedure hasbeen used to select observations taken under goodweather conditions, and data have been analyzed withthe Hillas analysis technique [Aharonian et al. 2006],resulting in the detection of a new source with a sig-nificance of 8.6σ above 4 TeV (see Fig. 1), [Li & Ma1983]. The significance of the detection correspondsto an excess of 68 counts at the source best fit posi-tion, the total number of ON- and OFF-source eventsbeing NON = 107 and NOFF = 757, respectively, withan on/off sampling area factor of α = 0.05.The emission of HESS J1640−465/HESSJ1641−463has been modeled with a double Gaussian functionconvolved with the instrument point spread function(PSF), showing a clear increase of the second sourcewith increasing energy (see Fig. 2). The preliminarybest-fit position of HESS J1641−463 is found to beat RA: 16h 41m 1.7s ± 3.1sstat ± 1.9ssys, DEC =46◦18′ 11′′ ± 35′′stat ± 20′′sys (J2000).The time-averaged differential VHE γ-ray spectrumof the source, has been derived using the forward-folding technique described in Piron et al. [2001].The spectrum is well fitted by a power-law functionwith a photon index of 1.99 ± 0.13stat ± 0.20sys for theenergy range from 0.64 to 30TeV (χ/d.o.f. equivalentvalue of 9.08/7 corresponding to a p-value of 25%).The flux density of HESS J1641−463 is 1.9 ± 0.2stat× 10−12 erg cm−2 s−1 above 1 TeV, corresponding toa 1.7% of the Crab nebula flux at this energy. Thespectral index of HESS J1641−463 is one of the hard-ests found so far in a VHE γ-ray source.No significant variability in the VHE γ-ray emissioncould be established for HESS J1641−463. The inte-gral flux is found to be constant within errors overthe H.E.S.S. dataset. A fit of the period-by-periodlight curve for energies above threshold with a con-stant value yields a χ2/dof = 11.7/14, with a p-valueof 67%. No variability can be seen either in other timebinnings tested (from year-by-year to run-by-run test-ing).eConf C1210282 4th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 201230°00°020406080100       HESS J1641-463            HESS J1640-465 Right Ascension-46 contour levelsσ5, 6, 7, 8m38h16m40h16m42h16m44h16-46DeclinationH.E.S.S. PRELIMINARYFigure 1: Smoothed map (smoothing radius 0.05◦) of excess events with energies E > 4 TeV for the region aroundHESSJ1641−463. The black solid contours indicate the significance of the emission at the 5σ, 6σ, 7σ and 8σ levels. thebox and arrow indicate the area and direction for the extraction of the profiles shown in Fig. 2. The inset illustratesthe PSF of the instrument.2. Searching for a MWL counterpartHESS J1641−463 is found within the bounds ofthe Supernova Remnant (SNR) G338.5+0.1 [Green2009], see Fig. 3. According to the Green Cata-logue of SNR [Green 2009], SNR G338.5+0.1 has aroughly circular morphology, and shows a flux den-sity at 1 GHz of 12 Jy and an angular size of 0.15◦.The distance to the SNR is estimated to be ∼11kpc [Kothes & Dougherty 2007], indicating that it hasa physical size of about 30 pc. No X-ray sourceor massive star counterpart is found in the differ-ent catalogues within 0.03◦ of the best fit positionof HESS J1641−463. No Fermi LAT source is locatedin the area, besides the nearby 2FGLJ1640.5-4633,associated with HESS J1640−465 [Slane 2010].Two archival Chandra observations, both ob-tained with the Advanced CCD Imaging Spectrome-ter (ACIS) detector, cover the region of interest (ROI)(Tab I). These archival data have been re-calibratedand analyzed with CIAO 4.4 and CALDB 4.4.7 ver-sions. The area at a distance smaller than 0.1◦ tothe best fit position of HESS J1641−463 has beenscanned with the CIAO wavdetect, celldetect andvtpdetect tools, revealing the presence of 25 faintX-ray sources in the region. In order to reduce thesample of revealed sources, a filter has been appliedby requesting a S/N >3, and a positive hardness ratioof counts with energies in the range of 2−10 keV overcounts with 0.3−2 keV. After these selection criteria,the sample is reduced to to 12 sources, and from these,two are located within 0.03◦ of the best-fit position ofHESSJ1641−463 (see Fig. 4 and tab 2.). The fluxdensities of these faint sources have been calculatedwith the CIAO command calc energy flux for the0.3−10 keV energy range (see Tab. II).In order to investigate a possible variability in theemission of the found X-ray sources, a one-sampleKolmogorov-Smirnov (KS) test has been used to cal-culate the probability, PKS , for the null hypothesis ofa uniform flux for each found X-ray source (see Fig. 4).For all the sources, PKS >0.1 and therefore no vari-ability was found in the emission of any of them.3. DiscussionThe fact that HESS J1641−463 is found within thebounds of a radio SNR immediately suggests the pos-sibility of the detection of a new VHE SNR. However,the existing X-ray observations do not provide addi-tional support to this scenario due to the lack of de-tection of an extended X-ray feature at the position ofHESSJ1641−463. In addition, the larger extension ofSNR G338.5+0.1 as compared to HESS J1641−463,and the relatively old age of the SNR inferred fromeConf C1210284th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 2012 3# Excess020406080100# Excess020406080Projected distance [º]-0.5# Excess-1001020304050SNR G338.5+00.1HESS J1640-465# Excess020406080100120140# Excess050100150200250# Excess050100150200250300350SNR G338.3-00.0SNR G338.5+00.1HESS J1640-465HESS J1641-463SNR G338.3-00.0HESS J1641-463E > 5 TeVE > 4 TeVE > 3 TeVE > 2 TeVE > 1 TeVAll events-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5PRELIMINARYPRELIMINARYFigure 2: Distribution of gamma-ray flux along the slice indicated in Fig. 1. Full lines model a double Gaussianconvolved with the instrument PSF, (compared with a single Gaussian indicated with the dashed line). Vertical linesshow the position of SNR 338.0+0.0, SNR G338.5+0.1, HESSJ1641−463 and HESSJ1640−465.its physical size of ∼30 pc suggests that the emissionmight not be necessarily connected with the SNR butrather with a Pulsar Wind Nebula (PWN) at its cen-ter, driven by an yet undetected pulsar. From en-ergy considerations, any of the faint X-ray sources ofthe area near HESSJ1641−463 can be its counterpartin the PWN scenario. In particular, the flux den-sity level of the nearest found X-ray source, (sourceL) is a factor ∼ 15 less energetic when comparedwith the energy flux density of HESS J1641−463 atthe VHE energies. Yet, several associations of “dark”TeV sources and weak X-ray synchrotron PWN havebeen established, were ratios of TeV/keV energy fluxmuch larger have been observed (e.g. HESS J1303-631[Abramowski et al 2012]).Another possibility could be that HESS J1641−463is a binary binary system, similar to HESS J0632+057[Hinton et al. 2009]. Hard spectral indexes (Γ ∼1.2 − 2) are found in those systems, similar to thosemeasured in HESS J1641−463. In a γ-ray binary sce-eConf C1210284 4th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 2012Table I X-ray observations.Obs Original Exposure Mode ChipsID target time with ROI11008 Mercer 81 40.0 ks VFaint I2 and I312508 Norma 18.8 ks VFaint S3Table II X-ray sources within the possible extension ofHESSJ1641−463.Source RA Dec Net Flux densityID (J2000) (J2000) counts erg cm−2 s−1L 16:40:58.9 -46:17:02.8 15.2±5.1 1.5 10−13G 16:40:57.2 -46:19:30.7 20.7±5.0 4.2 10−14nario, however, a certain degree of flux variability inboth X-rays and TeV γ-rays is expected, although thisvariability could appear in a relatively large range oftime-scales. In this regard, the lack of such variabilityfeatures does not permit to further constrain the γ-raybinary scenario based on the temporal distribution ofthe TeV datasets. A flux of ∼10−14 erg cm−2 s−1 maybe expected from an X-ray faint binary system sim-ilar to HESS J0632+057 if located at the estimateddistance of SNR G338.5+0.1 (11 kpc). Massive starsthat could be tracers of the location of a γ-ray bi-nary position were investigated, but none were found.However, the lack of an obvious optical counterpartcould be explained due to the high optical extinctionexpected given both the large distance and the posi-tion of the source, close to the Galactic plane.4. ConclusionsThe excellent capabilities of H.E.S.S. have enabledthe serendipitous discover a new TeV source showingone of the hardest spectral indices found so far. Theinspection of astronomical catalogues and archival X-ray observations do not provide an obvious counter-part to the VHE source but reveal two potential, butweak, X-ray counterparts. The spectral and morpho-logical characteristics of HESS J1641−463, as well asits location induce to think it could be most likely aSNR or a PWN, but other source types like a binarycannot be excluded with the existing data and there-fore the source remains unidentified. Further observa-tions, in particular with high resolution X-ray instru-ments, are encouraged for the proper identification ofthis new VHE source.AcknowledgmentsThe support of the Namibian authorities and of theUniversity of Namibia in facilitating the constructionand operation of H.E.S.S. is gratefully acknowledged,as is the support by the German Ministry for Educa-tion and Research (BMBF), the Max Planck Society,the French Ministry for Research, the CNRS-IN2P3and the Astroparticle Interdisciplinary Programme ofthe CNRS, the U.K. Science and Technology FacilitiesCouncil (STFC), the IPNP of the Charles University,the Polish Ministry of Science and Higher Education,the South African Department of Science and Tech-nology and National Research Foundation, and by theUniversity of Namibia. We appreciate the excellentwork of the technical support staff in Berlin, Durham,Hamburg, Heidelberg, Palaiseau, Paris, Saclay, andin Namibia in the construction and operation of theequipment.This research has made use of Chandra Archivaldata, as well as the Chandra Source Catalog, pro-vided by the Chandra X-ray Center (CXC) as part ofthe Chandra Data Archive. This research has madeuse of software provided by the Chandra X-ray Center(CXC) in the application packages CIAO, ChIPS, andSherpa. This research has made use of the SIMBADdatabase, operated at CDS, Strasbourg, France.ReferencesAbramowski, A., et al. (H.E.S.S. Collaboration), 2012,A&A, 548, A46Aharonian, F. et al. (H.E.S.S. Collaboration) 2006,A&A, 457, 899Aharonian, F. et al. (H.E.S.S. Collaboration) 2005,Science, 307, 1839.Bernlo¨hr, K., et al., Astropart. Phys. 20, 111 (2003).Green D. A., 2009, Bulletin of the Astronomical Soci-ety of India, 37, 45.Hinton, J.A., et al. 2009, ApJ, 690 L101.Funk, S., et al., Astropart. Phys. 22, 285 (2004)).Giebels, B. (H.E.S.S. Collaboration), Status and Re-cent Results from H.E.S.S. 2012 Fermi Symposium,eConf Proceedings eConf C121028Kothes, R. & Dougherty, S. M. 2007, A&A, 468, 993.Li, T. P., & Ma, Y. Q. 1983, ApJ, 272, 317Piron, F., Djannati-Ata¨ı, A., Punch, M., et al. 2001,A&A, 374, 895Slane, P., et al. 2010, ApJ, 720, 266.eConf C1210284th Fermi Symposium : Monterey, CA : 28 Oct-2 Nov 2012 5G338.5+00.1SNR G338.3-0.0HESS J1640-465G338.39+0.16HII RegionG338.45+0.06HII Region2FGL J1640.5-4633PRELIMINARYFigure 3: Radio view (MOST 843 MHz) of the region of interest. The catalog position and extension of the SNR isindicated as a dashed white circle. The solid contours indicate the significance of the emission at E > 4 TeV at the 5σ,6σ, 7σ and 8σ levels. The black cross indicates the best fit position of HESSJ1641−463, and the black triangle thelocation of 2FGL J1640.5-4633.Mercer 81ABCDEFGHIJKFGJKLPRELIMINARY PRELIMINARYFigure 4: Exposure corrected, smoothed (10 arcsec) image from Chandra Obs 11008 (left) and 12508 (right) of thearea near HESSJ1641−463 (the black cross indicating the best fit, the black circle a region of 0.03◦ radius around it,and the dashed contours the significance of the emission at E >4 TeV at the 5σ to 8σ levels). The small circles indicatethe detected faint X-ray sources.eConf C121028

Discovery of the VHE gamma-ray source HESS J1641-463

Discovery of the VHE gamma-ray source HESS J1641-463

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