Observations of the Galactic Plane performed by the H.E.S.S. telescope array have revealed a significant excess at very-high-energies (VHE; E>0.1 TeV) from the direction of PSR J1459-60, a rather old gamma-ray pulsar (64 kyr) with a spindown energy of ~10^36 erg/s, discovered by the Fermi/LAT satellite in high-energy (HE) gamma-rays. The X-ray pulsar counterpart has been recently detected using the Suzaku satellite. In this contribution, we present the discovery of a new VHE gamma-ray source, including morphological and spectral analyses. Its association with the gamma-ray pulsar in a PWN scenario will be discussed

Chaves, R.

collaboration, f.

De los Reyes, R.

Zajczyk, A.

English

MPG.PuRe

arXiv:1205.0719v1  [astro-ph.HE]  3 May 201232ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011A newly discovered VHE γ-ray PWN candidate around PSR J1459-60R. DE LOS REYES1 , A. ZAJCZYK2, R.C.G. CHAVES1 ON BEHALF OF THE H.E.S.S. COLLABORATION1Max-Planck-Institut fu¨r Kernphysik, PO Box 103980, 69029 Heidelberg, Germany2Nicolaus Copernicus Astronomical Center, Warsaw, Polandreyes@mpi-hd.mpg.deAbstract: Observations of the Galactic Plane performed by the H.E.S.S. telescope array have revealed a significant excessat very-high-energies (VHE; E>0.1 TeV) from the direction of PSR J1459-60, a rather old gamma-ray pulsar (64 kyr)with a spindown energy of ∼1036 erg/s, discovered by the Fermi/LAT satellite in high-energy (HE) γ-rays. The X-raypulsar counterpart has been recently detected using the Suzaku satellite.In this contribution, we present the discovery of a new VHE γ-ray source, including morphological and spectral analyses.Its association with the γ-ray pulsar in a PWN scenario will be discussed.Keywords: gamma-rays, PWN, galactic source, pulsar, PSR J1459-601 IntroductionIn the last decades, the population of Galactic sources atVHE γ-rays has increased considerably proving the do-main of the Imaging Atmospheric Cherenkov Telescope(IACT) technique in this energy range. Among the TeVgalactic population, the majority of them are supernovaremnants (SNRs) and pulsar wind nebulae (PWNe), al-though a large number of Galactic sources is still unidenti-fied without counterpart at any other wavelength. The de-tection and study of SNRs and PWNe have revealed themas efficient accelerators of cosmic rays (up to ∼100 TeV)in our Galaxy. However, to understand the accelerationmechanism inside these sources observations at differentwavelengths, specially from X-rays to TeV γ-rays, are re-quired. In particular, gamma-rays above 100 MeV willbe a direct probe of the acceleration of the cosmic raysto ultra relativistic energies. These high energy particlesproduce γ-rays by two different mechanisms: through In-verse Compton (IC) scattering of the electrons with pho-ton fields (like cosmic microwave background radiation(CMBR), star light, infrared (IR) dust emission or pro-duced synchrotron radiation) or through pi0s decay fromthe interaction of protons with interstellar material or closemolecular clouds.In the VHE domain, the H.E.S.S. Galactic Scan has con-tributed significantly to increase the population of sources.Up to now 5 TeV shell type SNRs and more than 26 PWNehave been detected by H.E.S.S.In this contribution we present the discovery of HESSJ1458-608 at TeV energies, a PWN candidate around oneof the Fermi/LAT discovered γ-ray pulsars, PSR J1459-60.After the description of the observations and results of thedata analysis, arguments about a possible PWN scenariowill be discussed.2 Observation and data analysisThe source presented here was discovered in the first placein the Galactic Plane Survey, performed by H.E.S.S. since2004. After its first marginal detection (2σ post-trials) ded-icated observations were carried out in 2009 and 2010.Since the field of view (FoV) of the region around thissource (l=317.75◦, b=-1.7◦) is quite crowded at VHE, theanalysis data-set presented in this work includes an impor-tant fraction of observations of nearby sources.After the standard H.E.S.S. quality cuts on the observationruns [4], we get 96 hours of total live time. Since the dataset includes observations of nearby sources (at offset anglesfrom the source position >1.5◦), the mean offset is 1.8◦,while the dedicated observations are taken at typical offsetbetween 0.7◦-1.0◦. The mean zenith angle of this data setis 39◦, approximately the culmination zenith angle of thesource.The results were obtained using the standard H.E.S.S. anal-ysis [4] based on cuts on the Hillas second moment param-eters [8]. Hard cuts and TMVA cuts [13] have been appliedto remove hadron γ-like events from the γ-ray events can-didates. These analysis cuts include a cut in the numberof photo-electrons recorded from the Extended Air Shower(EAS) (> 160 p.e.) and several others that are optimized toget the best angular resolution (σ68%=0.09◦) [12] and sup-press the contamination from the nearby sources. A mostR. DE LOS REYES et al. VHE γ-RAY PWN CANDIDATE AROUND PSR J1459-60PSFFigure 1: VHE gamma-rays excess map of HESS J1458-608. The excess counts have been smoothed with a Gaus-sian of width 13.2’. The blue-red transition in the colorscale is set at approximately 3σ significance. The blackcontours correspond to 5 and 7σ significance. The positionof the pulsar PSR J1459-60 is marked by a green and redfilled square, which correspond to Fermi/LAT and Suzakubest fit position respectively. The best fit position of HESSJ1458-608 is marked by a black cross and the error barscorrespond to 1σ error in the position fit.sensitive analysis at high energies is also achieved despitea high energy threshold of 0.75 TeV, also due to the zenithangle range of the observations.To obtain the sky maps, the background [7] was estimatedfor each bin from a ring of inner radius of 0.9◦ coveringan area 10 times larger than the on region. For the spectralanalysis the Reflected background method was used. Thismethod integrates the signal from regions with the samesize as the on region but placed at an offset of 0.4◦ fromthe center of the FoV. All the presented results are consis-tent with a complete alternative analysis chain which in-cludes an independent calibration and γ/hadron separationmethod [12].3 ResultsAfter camera acceptance correction w.r.t. the γ-ray pulsarcoordinates (RA=14h59m29.8s, Dec=-60◦52’40”) [1] andthe γ/hadron separation cuts, we end up with 58 hours ofeffective time for the analysis.3.1 MorphologyA fit of a 2-D model of a symmetrical gaussian, con-volved with the telescope point spread function (PSF), tothe uncorrelated image gives the centroid of the sourceFigure 2: Energy differential spectrum of HESS J1458-608at the best fit position. The resulting power law fit, between0.750 and 60 TeV, is displayed as a solid red line. The plotat the bottom shows the residuals of the power-law fit to thedata points (black filled dots in the top panel).(black cross in figure 1) at αJ2000=14h58m9.6s (± 7s) andδJ2000=-60◦52’38” (± 4”). The 1σ extension of the fittedGaussian is 0.17◦ ± 0.07◦. The best fit position locatesHESS J1458-608 at ∼ 0.16◦ to the West of the Fermi pul-sar position. Figure1 shows the raw excess counts sky mapsmoothed with a symmetrical Gaussian with a standard de-viation of 13.2’. The best fit position is marked as a blackcross. A 2-D model of a combination of two symmetricalgaussians has been rejected at a level of < 3σ.An analysis at the best fit position of HESS J1458-608 witha cut θ = 0.33◦ (∼ 95% of source enclosure) gives a signif-icance of 8.1σ (∼ 6σ post-trials) and reveals a clear ex-tended emission around the pulsar position with an excessmaximum offset by ∼10 arcmin from the pulsar position(Fig. 1).3.2 SpectrumThe results of the spectral analysis for HESS J1458-608presented here were obtained using a circular extraction re-gion centered in the best fit position, with a radius corre-sponding to the 95% source enclosure ( θ=0.33◦).Figure 2 shows the photon spectrum, well fitted by a powerlaw (dN/dE = K·E−Γ), displayed as a red line in the samefigure. The spectral fit used a forward folding algorithmwith a minimum significance per bin of 2σ. The fit re-sults on a photon index of Γ= 2.8 ± 0.2 and differen-tial flux normalization at 1 TeV K = (2.1 ± 0.4)×10−12TeV−1cm−2s−1. The corresponding integral flux above 1TeV is (1.2 ± 0.4)×10−12 cm−2s−1, which corresponds to∼6% of the Crab Nebula flux at the same energies.32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 20114 Possible associationsAn extensive search on the archival data at other wave-lengths has not revealed any counterpart at the coordinatesof HESS J1458-608.The only counterpart in the vicinity of HESS J1458-608 isthe Fermi pulsar PSR J1459-60 located within the extendedemission at 9.6’ from the source fit position.The pulsar PSR J1459-60 (αJ2000=14h59m29.8s, δJ2000=-60◦52’40”) [1] was discovered in γ-rays, followed bythe detection of a faint counterpart in X-rays [10] [14].No radio counterpart has been reported up to now. Thepulsar has a period of 103 ms, a surface magnetic fieldB=1.64×1012G, a spin-down age of τc=64 kyr and a spin-down energy loss rate 9.2×1035 erg/s. The fitted spec-trum to the pulsed emission follows a power-law func-tion plus an exponential cut-off, represented by dN/dE =K · E−Γexp(−E/Ec) with Γ=1.83±0.24 and an expo-nential cut-off Ec=2.7±1.1 GeV. The integral energy fluxabove 100 MeV is F(>100 MeV) = 10.6±1.2×10−11 ergcm−2 s−1[1].To find the corresponding PWN contribution from theFermi pulsars, Fermi collaboration has carried out an anal-ysis in the off-pulsed emission region for all the γ-ray pul-sars. No significant emission has been found in the case ofPSR J1459-60. Only 4 PWNe have been detected with 1-yrsensitivity of Fermi [2], all of them associated with young(1-10 kyr) and bright (logE˙> 36.85 erg/s) pulsars. ClearlyPSR J1459-60 is beyond those limits.X-rays observations by Suzaku [10] have confirmed point-like non-thermal emission [10] at 0.8 arcmin from theγ-ray pulsar position. The emission was fitted to twomodels: an absorbed power-law dN/dE = exp[−NH ·σ(E)] ×KPL(−Γ + 2)E−Γ/(E−Γ+22 − E−Γ1 ) and an ab-sorbed power-law plus black-body. Black-body contribu-tion was ruled out from the fit and the absence of of emis-sion/absorption lines in the spectra. Therefore the best fitmodel to the X-ray emission is an absorbed power-law withΓ=1.54+0.39−0.30, KPL=(0.142±0.02)×10−12erg cm−2s−1. The estimated distance from this analysis is 4 kpc.Recently, pulsed emission in X-rays has been reported bySwift [14] at an offset of 9.8” from Fermi ephemeris.5 DiscussionThe lack of an enhancement of the molecular cloud con-tent and the location of the Fermi pulsar within the HESSJ1458-608 extended emission suggests a leptonic scenarioas the origin of the TeV emission.Despite the offset on the position of the pulsar and HESSJ1458-608, the hypothesis that HESS J1458-608 is the pul-sar wind nebula of the old γ-ray pulsar PSR J1459-60seems likely.From the morphological point of view and considering thethe pulsar and its PWN are at the same distance to the ob-Figure 3: Offset (in pc) between the X-ray and the TeVemission for a population of pulsar-PWN systems [11] asa function of the pulsar age (spin-down age). The blackpoints correspond to those sources with X-ray emission de-tected while the blue points to those which X-ray emissionis faint or nonexistent. HESS J1458-608 - PSR J1459-60system have an offset of 11 pc and a log(τc)=4.8.server, the fact that the pulsar is displaced from the bulk ofthe TeV emission can be explained by the kick-off momen-tum transmitted to the pulsar on its birth as a result of a nonsymmetric supernova explosion. Considering the age ofPSR J1459-60 (64 kyr), the estimated distance (4 kpc) andthe offset between the current pulsar position and the peakof TeV emission (9.6 arcmin) one can estimate the pulsarvelocity∼200 km/s. This velocity falls into the range of thetypical pulsar velocities [15]. Therefore, the pulsar couldhave moved from the birth location by ∼11 (d/4 kpc) pcduring its lifetime. In fact, evolved PWN systems typicalysurround old pulsars, displaced few arc minutes from thePWN center [9]. Figure 3 shows the offset between thepulsar and the TeV emission of the associated PWN as afunction of the pulsar characteristic age τc [11]. For oldpulsar-PWN systems (log(τc) (yr) > 4) the typical offsetvalues seem to be between ∼ 2-16 pc.This scenario of the evolved PSR-PWN system describedabove is consistent with the spectral results from X-raysand TeV γ-rays. The electrons producing the synchrotronemission (X-rays between 0.5-10 keV) at the pulsar posi-tion would require interstellar magnetic fields of the orderof 0.4-1.5µG, much lower than the typical minimum valueof ∼ 3-5µG [3], if they were produced 64 kyr ago. There-fore, the faint X-ray emission is produced by a recentlyemitted population of electrons. On the other hand, theelectrons responsible for the TeV might be much older, cor-responding to a population of electrons emitted by the pul-sar at its birth (64 kyr ago). The IC emission of such elec-trons interacting with the CMB would produce the VHEgamma-rays from HESS J1458-608 at energies below 30TeV [3] (presented in Fig. 2). This old electron population,R. DE LOS REYES et al. VHE γ-RAY PWN CANDIDATE AROUND PSR J1459-60below 100 TeV, would have a spectral index of 1.8, whichis typical of PWN scenarios.Assuming that HESS J1458-608 is the PWN of the pulsarPSR J1459-60 we can estimate the conversion efficiencybetween the spin-down energy (E˙) and the TeV luminos-ity above 1 TeV ηTeV =LTeV /E˙=2% while the conversionefficiency in X-rays is ηX=0.01%. These two values givea luminosity ratio TeV/keV (log LTeV /Lx) of the order of2, which is within the range between 1 and 3 typical of oldPWN systems [11].6 SummaryThe newly discovered source HESS J1458-608 has beendetected at VHE by H.E.S.S. and its extension includesthe energetic Fermi/LAT pulsar PSR J1459-60, which faintemission in X-rays have been recently confirmed by Suzakuand Swift. During the search of counterparts in the HESSJ1458-608 FoV no other significant counterpart at otherwavelengths has been found at the best fit position of theTeV source.A morphological study of the TeV emission and the pul-sar PSR J1459-60 seems to link the two objects into anevolved PSR-PWN scenario, where the pulsar has movedby ∼11 pc from its birth place, located at the best-fit posi-tion of HESS J1458-608. A leptonic model could explainthe synchrotron emission produced by a recent populationof electrons at the pulsar position while the IC from olderpopulation of electrons would be located at 9.6 arcmin tothe West of PSR J1459-60. Within this scenario, the ag-ing of the pulsar might also explain the lack (or faint) ofX-ray emission closer to the TeV emission, although dedi-cated observations at the HESS J1458-608 position wouldbe needed to confirm the first results’ hypothesis.7 AcknowledgmentsThe support of the Namibian authorities and of the Uni-versity of Namibia in facilitating the construction and op-eration of H.E.S.S. is gratefully acknowledged, as is thesupport by the German Ministry for Education and Re-search (BMBF), the Max Planck Society, the French Min-istry of Research, the CNRS-IN2P3 and the Astroparti-cle Interdisciplinary Programme of the CNRS, the U.K.Science and Technology Facilities Council (STFC), theIPNP of the Charles University, the Polish Ministry ofScience and Higher Education, the South African Depart-ment of Science and Technology and National ResearchFoundation, and by the University of Namibia. We ap-preciate the excellent work of the technical support staffin Berlin, Durham, Hamburg, Heidelberg, Palaiseau, Paris,Saclay, and in Namibia in the construction and operationof the equipment. AZ acknowledges partial support fromMNiSW grant N203 387737.References[1] Abdo a. et al, 2010, ApJS, 187, 460.[2] Ackermann M. et al, 2011, ApJ, 726, 35.[3] Aharonian F. et al., 1997, MNRAS, 291, 162.[4] Aharonian, F. et al, 2004, A&A, 457,899.[5] Aharonian F. et al., 2006, A&A, 457, 899.[6] Aharonian F. et al., 2006, ApJ, 636, 777.[7] Berge D. et al., 2007, A&A, 466, 1219.[8] Hillas, A., 1995, ICRC proceedings.[9] de Jager, O.C. & Djannati-Atai, A., 2008, arXiv0803.0116.[10] Kanai Y., 2010, PhD thesis, Tokyo Institute of Tech-nology.[11] Kargaltsev O. & Pavlov, G.G., 2010, AIPC, 1248, 25.[12] de Naurois, M. & Rolland, L., 2009,APh.,32,231.[13] Ohm S. et al., 2009, APh., 31, 383.[14] Ray, P.S., et al, 2011, ApJS, 194, 17.[15] Romani, R.W., 2005, ASP Conference Series, 328.

A newly discovered VHE gamma-ray PWN candidate around PSR J1459-60

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