HESS J1632-478 is an extended and still unidentified TeV source in the galactic plane. In order to identify the source of the very high energy emission and to constrain its spectral energy distribution, we used a deep observation

of the field obtained with XMM-Newton together with data from Molonglo, Spitzer and Fermi to detect counterparts at other wavelengths. The spectral energy density features two large prominent bumps with the synchrotron emission peaking

in the ultraviolet and the external inverse Compton emission peaking in the TeV. HESS J1632-478 is an energetic pulsar wind nebula with an age of the order of 104 years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the current pulsar spin down power. The synchrotron nebula has a size of 1 pc and contains an unresolved point-like X-ray source, probably the pulsar with its wind termination

shock

Balbo, Matteo

Pavan, L.

Pohl, M.

Saouter, P.

Tramacere, A.

Walter, R.

Zurita-Heras, J.-A.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2011-10893-xColloquia: Scineghe2010IL NUOVO CIMENTO Vol. 34 C, N. 3 Maggio-Giugno 2011HESS J1632-478: An energetic relicM. Balbo(1)(2)(∗), P. Saouter(3), R. Walter(1)(2), L. Pavan(1)(2),A. Tramacere(1)(2), M. Pohl(3) and J.-A. Zurita-Heras(4)(1) INTEGRAL Science Data Centre, Universite´ de Gene`ve - Chemin d’Ecogia 16CH-1290 Versoix, Switzerland(2) Observatoire de Gene`ve, Universite´ de Gene`ve - Chemin des Maillettes 51CH-1290 Sauverny, Switzerland(3) De´partement de Physique Nucle´aire et Corpusculaire, Universite´ de Gene`veQuai Ernest-Ansermet 24, CH-1211 Gene`ve 4, Switzerland(4) AIM Paris Saclay, CEA/DSM-CNRS/INSU-Universite´ Paris DiderotIRFU/Service d’Astrophysique - 91191 Gif-sur-Yvette, France(ricevuto il 25 Febbraio 2011; pubblicato online l’1 Giugno 2011)Summary. — HESS J1632-478 is an extended and still unidentified TeV sourcein the galactic plane. In order to identify the source of the very high energy emis-sion and to constrain its spectral energy distribution, we used a deep observationof the field obtained with XMM-Newton together with data from Molonglo, Spitzerand Fermi to detect counterparts at other wavelengths. The spectral energy den-sity features two large prominent bumps with the synchrotron emission peakingin the ultraviolet and the external inverse Compton emission peaking in the TeV.HESS J1632-478 is an energetic pulsar wind nebula with an age of the order of104 years. Its bolometric (mostly GeV-TeV) luminosity reaches 10% of the currentpulsar spin down power. The synchrotron nebula has a size of 1 pc and contains anunresolved point-like X-ray source, probably the pulsar with its wind terminationshock.PACS 98.38.-j – Interstellar medium (ISM) and nebulae in Milky Way.PACS 97.60.Gb – Pulsars.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.1. – IntroductionHESS J1632-478 has been discovered as a diffuse very high energy (VHE) γ-ray sourceduring the 2004-2006 survey of the inner Galaxy [1] performed with the High EnergyStereoscopic System Cherenkov Telescope Array [2] H.E.S.S.(∗) E-mail: Matteo.Balbo@unige.chc© Societa` Italiana di Fisica 197198 M. BALBO, P. SAOUTER, R. WALTER, ETC.Fig. 1. – a) XMM-Newton MOS mosaic image in the energy band 0.2–12 keV. Ellipses indicatethe source (solid) and the background (dashed) extraction region for the extended source. b) Theenlarged image extracted from the MOS1 camera shows the extraction region for the point-like(black) and extended (white) sources, respectively.Though many galactic VHE sources are recognized to be supernova remnants, pul-sars and pulsar wind nebulae, a small number of them still lack a clear identification.HESS J1632-478 is one of them, even if tentatives to explain its nature have already beenpresented [1,3]. A detailed VHE spectral and positional analysis of HESS J1632-478 hasbeen reported in [1], based on 4.5 hours of H.E.S.S. observations. The source best-fitposition is RA=16h 32m09s DEC=–47h 49m12s (J2000), placing it in the direction ofthe near 3 kpc arm tangent in the galactic plane. The TeV source is extended with asemi-major axis of (12 ± 3)′, forming an angle of 21 ± 13◦ with respect to the positivegalactic longitude axis.To understand HESS J1632-478, we collected multi-wavelength data from Fermi,XMM-Newton, Spitzer and Molonglo, to construct its spectral energy distribution anddiscuss the emission mechanisms.2. – ObservationsIn the period from August to September 2008, XMM-Newton performed 9 observa-tions of a field centered close to HESS J1632-478, collecting data for a total of 92 ks.Standard data reduction procedures were applied using XMM Science Analysis Software(SAS v.9) and selected for the energy ranges 0.2–12 keV for MOS and 0.2–15 keV forpn. Periods with enhanced background due to soft proton flares were disregarded inthe analysis, resulting in a total filtered exposures of 87 ks and 62 ks for the MOS andpn cameras, respectively. Merging together all the observations we obtained the mosaicimage in fig. 1a, where the extended source is highlighted with the white continuousellipse.Zooming inside this ellipse (fig. 1b) it is shown an unresolved point-like componentwith a black continuous line. The point and extended sources have a significances of 15and 18σ, respectively. We fitted both the point-like and the extended source spectrawith an absorbed power-law model with Γpoint = 2.6+1.3−0.8 and Γext = 3.4+0.6−0.8 [4].We analysed also the radio energy band using the 2nd Molonglo Galactic Plane Sur-vey (MGPS-2) at 843MHz [5]. Integrating the excess counts based on a conservativedetection threshold of the MGPS-2 survey yields to an upper limit for the flux densityof 25mJy. This radio excess corresponds well to the position and size of the extendedHESS J1632-478: AN ENERGETIC RELIC 199X-ray source. In the infrared energy band between 3.6 and 8μm we have taken intoaccount the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire [6] GLIMPSEand we did not find any sign for diffuse infrared emission corresponding to the X-ray andradio counterparts. We extracted a flux density upper limit of ∼ 25mJy at 3.6μm. Welooked for possible high energy counterpart in the Fermi-LAT first year Catalogue [7],and found two unidentified GeV sources in the neighborhood of HESS J1632-478. Boththese sources are flagged with the letter “c”, indicating that they are to be consideredas potentially confused with galactic diffuse background or perhaps spurious detection.It is therefore very unclear if any of these Fermi sources could be a counterpart of HESSJ1632-478. The HESS ellipse is extended with a semi-major axis of (12±3)′ and it is par-allel to the line joining the two Fermi sources, lying close to the extremity of that ellipse.If the two Fermi sources are real and radiate in the TeVs, the extension and inclinationof HESS J1632-478 could be significantly affected by confusion. The VHE spectrum,between 0.2 and 4.5TeV, can be fitted with a power-law model, yielding a photon indexΓ = 2.12± 0.20 and a flux above 200GeV of (28.7± 5.3)× 1012 ph cm2 s1 [1].3. – ModellingThe XMM-Newton and H.E.S.S. spectra indicate the presence of two spectral bumpsmatching the expected synchrotron and inverse Compton emission of a Pulsar WindNebula. We used a phenomenological description of the electron energy distribution [8,9]used to model the SED of a sample of PWNs:(1) n(γ) =⎧⎪⎨⎪⎩K(γγc)exp[γγc]= n0(γ) γ ≤ γ1,n0(γ) + K1(γγc)−αexp[γγc1]γ > γ1,where γc is the Lorentz factor of the electrons at the termination shock and we fixed [8]γ1 = 7γc. The resulting SED was obtained using a well-tested code [10] and is reportedin fig. 2. We reproduced both the synchrotron self-Compton and the external Comptonemission, assuming a one-zone (post terminal shock) homogeneous emitting region [4].4. – ConclusionWe observed the unidentified TeV source HESS J1632-478 with XMM-Newton andlooked for counterparts in the GeV, infrared and radio bands. An extended faint X-raysource is detected close to the centroid of the H.E.S.S. ellipse. A radio excess corre-sponding to the X-ray source is found in the Molonglo sky survey. Upper limits havebeen derived from Spitzer and Parkes data. The GeV image obtained by Fermi showstwo close-by sources, but none of them corresponds to the X-ray source. The flux den-sity emitted by HESS J1632-478 at very high energies is at least 20 times larger thanobserved at the other wavebands probed. The source shape and spectral energy distri-bution suggests a pulsar wind nebula and can be used to constrain a one-zone model forthe post terminal shock region of the pulsar wind nebula. The assumed relativistic elec-tron distribution is Maxwellian (γ ∼ 105) with a non-thermal tail extending to γ ∼ 108.The synchrotron nebula is faint because of the low magnetic field (3μG). The point-likeX-ray source, detected in the synchrotron nebula, is probably the signature of the pulsarand of the termination shock. The ratio of the γ-ray and X-ray fluxes suggests a pulsarage of ∼ 104 years [11].200 M. BALBO, P. SAOUTER, R. WALTER, ETC.Fig. 2. – The continuous line indicates the prediction of the model used to represent the emission.At low energy the dotted line represents the synchrotron emission from the electron distributiondescribed by eq. (1), the dot-dashed and the dashed lines show the CMB and IR dust photonscomponents, respectively. At high energy, the dotted line represents the SSC component, thedot-dashed and the dashed lines show the external Compton emission on the CMB and dustphotons components, respectively [4]. Errors on the VHE spectrum are only statistical.∗ ∗ ∗Based on observations obtained with XMM-Newton, an ESA science mission withinstruments and contributions directly funded by ESA Member States and NASA. TheMolonglo survey is supported by the Australian Research Council. One of us (JAZH)acknowledges the Swiss National Science Foundation for financial support.REFERENCES[1] Aharonian F. et al., Astrophys. J., 636 (2006) 777.[2] Hinton J. A., New Astron. Rev., 48 (2004) 331.[3] Walter R., Astrophys. Space Sci., 309 (2007) 5.[4] Balbo M. et al., Astron. Astrophys., 520 (2010) A111.[5] Murphy T. et al., Mon. Not. R. Astron. Soc., 382 (2007) 382.[6] Benjamin R. A. et al., Publ. Astron. Soc. Pac., 115 (2003) 953.[7] Abdo A. A. et al., Astrophys. J. Suppl., 188 (2010) 405.[8] Spitkovsky A. Astrophys. J., 682 (2008) L5.[9] Fang J. and Zhang L., Astron. Astrophys., 515 (2010) A20.[10] Tramacere A. et al., Astron. Astrophys., 501 (2009) 879.[11] Mattana F. et al., Astrophys. J., 694 (2009) 12.

HESS J1632-478: An energetic relic

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HESS J1632-478: An energetic relic

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