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Status of Identification of VHE gamma-ray sources

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Astrophysics and Space Science manuscript No.(will be inserted by the editor)

1Introduction

A systematic survey of the inner part of the Galaxy performed by the H.E.S.S.Cherenkov telescope system has revealed a number of previously unknown sources of VHE gamma-rays above 100GeV [1][2].While in terms of a population approach the sources can be de-scribed by common properties like generally rather hard energy spectra (photon index ～2.3)or a rather narrow distribution in Galactic latitude (rms of ～0.3?)the counterpart identi?cation calls for an individual study of these objects.An unambiguous counterpart identi?ca-morphology mechanism MWL picture yes yes yes

B yes yes no D tion of these (initially)unidenti?ed H.E.S.S.sources re-quires (i)spatial and ideally also morphological co-incidence ,(ii)a viable gamma-ray emission mech-anism for the object,and (iii)a consistent multi-wavelength behaviour matching the suggested identi-?cation and the particle distribution within the source.The H.E.S.S.sources can be classi?ed according to their con?dence in identi?cation with known astrophysical ob-jects following the three requirements given above.Ta-ble 1summarises the categories.Category A comprises sources for which the positional and/or morphological match (in case of an extended source)with a counter-part source is excellent and the emission processes can be modelled to provide a consistent picture describing the multi-frequency data.For these sources the association is beyond doubt.For Category B sources the emission mechanisms can be consistently modelled,however these sources show a less convincing positional and/or mor-phological match with the potential counterpart.Cate-gory

C sources on the other hand have a good positional counterpart,they show however a non-consistent multi-wavelength picture,being it because of insu?cient data

at other wavebands,being it because of a not fully un-derstood emission mechanism.For Category D sources no counterpart candidate exists,these are the classi-cal unidenti?ed sources .In the following I will describe examples for sources belonging to each of the 4cate-gories.The description will focus on Galactic gamma-ray sources,since for extragalactic objects the counterpart

2Category A-Sources with an established counterpart

Two classes of sources can be distinguished for which a counterpart to the VHE gamma-ray source has been es-tablished:a)point sources with a convincing posi-tional match and b)extended sources with a con-vincing positional and morphological match.For these objects with a?rm counterpart,having established the positional coincidence,the aim for these objects is to fully understand the details of the multi-frequency photon spectrum and to investigate the emission mech-anisms generating this photon spectrum.One important question in the VHE gamma-ray regime is for example whether the gamma-ray emission is generated by In-verse Compton scattering of ultra-relativistic electrons on photon?elds like the Cosmic microwave background (CMBR)or by pion-decay produced in proton-proton interactions,that is whether the gamma-ray emission has leptonic or hadronic origin.These two scenarios can not be directly distinguished from the gamma-ray data alone,but have to be separated by modelling the par-ent population of particles responsible for the emission. For any source identi?cation it should be mentioned that the good angular resolution of VHE Cherenkov instru-ments(typically of the order of0.1?per event)as well as the very low level of the di?use gamma-ray background at energies above100GeV helps against source

confu-sion.Source confusion was a problem that EGRET[3] strongly had to face,especially in observations in the Galactic plane where both the density of sources and the level of the di?use gamma-ray background was higher. The upcoming GLAST satellite measuring in the regime between100MeV to several hundreds of GeV will have the advantage of an improved angular resolution(～0.4?at1GeV)over EGRET but will also be susceptible to any systematic uncertainties in modelling the di?use gamma-ray?ux from cosmic ray interactions with in-terstellar material.The best-established example for a point-source with a convincing positional match is the Crab Nebula[4],which is frequently used as a cal-ibration source in VHEγ-ray astronomy.In order to es-tablish a positional correlation with a gamma-ray point-source,it has to be tested whether the nominal position of the counterpart candidate is within the statistical and systematic error limits of the reconstructed position of the gamma-ray emission region(for the Crab Nebula, the statistical error on the reconstructed position of the gamma-ray emission is5”,the systematic error of the order of20”).Other objects of this class,where a po-sitional counterpart to a point-like gamma-ray emission has been established are the newly discovered gamma-ray microquasars LS5039[5][6]and LS I+61303[7]or Fig.1ASCA x-ray image of RX J1713.7–3946,overlaid with smoothed and acceptance-corrected H.E.S.S.gamma-ray image contours linearly spaced at the level of30,60and 90counts.The ASCA image was smoothed to match the H.E.S.S.point-spread function for ease of comparison.

the composite PWN G0.9+0.1[8].A further strength-ening of the proposed association can be established if additionally(as in the case of LS5039)a correlated peri-odicity or variability between the gamma-ray source and the counterpart source can be detected(LS5039shows a modulation in the gamma-ray data that matches the orbital frequency of the binary system as discussed by deNaurois et al.in this proceedings).

The best established example for an extended source with a convincing morphological match is the Su-pernova remnant(SNR)RX J1713.7–3946[9][10](and also see Lemoine-Gourmard et al.in this proceedings) showing a striking correlation of the gamma-ray emis-sion to X-rays as e.g.measured by the ASCA satellite (correlation coe?cient:～80%).From the morphological match,the association of the gamma-ray source with the Supernova remnant is beyond doubt and can be regarded as a?rm association.Another object of this class is the Supernova remnant RX J0852.0–4622(Vela Jr.)showing also a high degree of correlation between the gamma-ray and X-ray emission[11].Other objects of this class are the PWNe MSH–15–52[12]and Vela X[13].

In terms of modelling the multi-frequency emission from these objects where a?rm counterpart has been established,the Crab Nebula can again serve as an ex-cellent example how the outstanding coverage in wave-bands from radio to VHEγ-rays can help to consis-tently describe the emission mechanism in this object and to derive important parameters like the strength of the magnetic?eld responsible for the synchrotron emis-sion.For the microquasar LS5039as well as for the ex-tended gamma-ray emission from the Supernova rem-

3Category B-Sources with a less convincing positional or morphological match

The best example for members of this class are the newly discovered gamma-ray sources,which seem to belong to the so-called

o?set Pulsar wind nebulae.These objects, for which Vela X[13]is the archetypal example show an extended emission around an energetic pulsar.The o?-set morphology is thought to arise from an anisotropy in the interstellar material surrounding the pulsar,that prevents the symmetric expansion of the PWN in one direction and shifts the emission to the other direction (see e.g.[14]for a hydro-dynamical simulation and dis-cussion of this e?ect).The gamma-ray emission in these objects is generated by Inverse Compton scattering of relativistic electrons accelerated in the termination shock of the PWN.The gamma-ray sources that could possibly

be explained in this framework are typically extended and their emission region overlaps with energetic pul-sars(energetic enough to explain the gamma-ray?ux by their spindown power)and that very importantly also show evidence for an X-ray PWN.Apart from Vela X and MSH–15–52,the gamma-ray emission of the PWN HESS J1825–137[15][16],possibly powered by the ener-getic pulsar PSR J1826–1334can be used to illustrate the properties of this class of objects.This object has been observed by H.E.S.S.in a very deep observation of～67 hours,due to its proximity to the microquasar LS5039 (at a distance of～1?).It was known to be a PWN can-didate also in VHE gamma-rays since through XMM-Newton X-ray observations of PSR J1826–1334[17]es-tablished an o?set X-ray PWN extending asymmetri-cally～5′′to the south of the pulsar.The asymmetric nature of the PWN as well as CO data that show a dense molecular cloud to the north of PSR J1826–1334[18]sup-port the picture described above in which dense material to the north shifts the PWN emission to the south.

The H.E.S.S.detected gamma-ray emission similarly shows an asymmetric emission extending to the south of the pulsar,however on a completely di?erent scale than the X-ray emission(the X-ray emission extends5”, whereas the gamma-ray emission extends～1?to the south).This at?rst glance prevents a direct identi?-cation as a counterpart,since the morphology can not be matched between X-rays and gamma-rays.However, modelling the emission mechanism and taking into ac-count the di?erent loss timescales of the gamma-ray and X-ray emitting electrons the di?erent scale of the emis-Fig.2a)X MM-Newton X-ray image in the energy range between2and12keV of the small region(7’×7’)surround-ing PSR J1826–1334.It can be seen,that the X-ray emission shows an asymmetric di?use emission extending to the south of the pulsar.b)H.E.S.S.gamma-ray excess image of the region surrounding HESS J1825–137and the energetic pul-sar PSR J1826–1334(white triangle).The image has been smoothed with a Gaussian of radius2.5’and has been cor-rected for the changing acceptance across the?eld.The in-set in the bottom left corner shows the PSF of the data set (smoothed in the same way as the excess map).The dashed black and white contours are linearly spaced and denote the 5σ,10σand15σsigni?cance levels.The best?t position of HESS J1825–137is marked with a black square,the best ex-tension and position angle by a black ellipse.Also shown (dotted white)is the95%positional con?dence contour of the unidenti?ed EGRET source3EG J1826–1302.The bright point-source to the south of HESS J1825–137is the micro-quasar LS5039.c)Three-colour image,showing the gamma-ray emission as shown in panel b),with di?erent colours, denoting the gamma-ray emission in di?erent energy bands, symbolising the changing morphology even in the gamma-ray band alone.More details can be found in the text and in[16].

sion regions can be plausibly explained in the following way:for a typical magnetic?eld of10μG(as also sug-gested from the X-ray data),1keV X-rays are generated by～50TeV electrons,whereas100GeV gamma-rays are generated by～1TeV electrons.The gamma-rays are therefore generated by lower energy electrons than the X-rays and the di?erent scales of the emission re-gions could be due to faster loss times of the higher en-ergetic synchrotron emitting electrons.This picture is

4Category C -Sources for which the

multi-frequency data does not (yet)provide a consistent picture

The sources belonging to this class of objects have a good positional counterpart,but the multi-frequency data does not (yet)provide a consistent picture of the emission mechanism.One object of this class is HESS J1813–178.This slightly extended gamma-ray source was originally discovered in the H.E.S.S.Galactic plane survey and originally described as unidenti?ed.Shortly afterwards several papers were published,describing the positional coincidence with an ASCA X-ray source (2-8keV)[19],an Integral hard X-ray source (20-100keV)[20],both having the same angular resolution and therefore like H.E.S.S.unable to resolve the object.

Finally VLA archival radio data (90cm)were re-ported [19]showing a 2.5’diameter shell-like object co-incident with the X-ray sources and with HESS J1813–178(see Figure 3).This observation led to the conclu-sion that the positionally coincident object was a Su-pernova remnant and that the gamma-ray emission was either caused by the shell or by a centrally located PWN.However,the multi-frequency data does not distinguish between the two scenarios,due to the lack of spatial res-olution of the X-ray and gamma-ray instruments.A pre-liminary analysis of a recent 30ks XMM-Newton obser-vation of the region point to a PWN origin of the emis-sion,which would in turn allow to model the radio to

Fig.3Gamma-ray image of HESS J1813–178overlaid with VLA 20cm radio data in which the shell-type structure of the positional counterpart to the gamma-ray source can be seen.The best ?t position of the gamma-ray excess is marked with a magenta cross.gamma-ray data in the picture of a composite SNR and therefore ?nally possibly identifying HESS J1813–178as a gamma-ray PWN.The example of HESS J1813–178shows that high-quality multi-frequency are a prereq-uisite in the identi?cation of a gamma-ray source and an ongoing programme is connected to the study of the unidenti?ed gamma-ray sources with high-resolution X-ray detectors like Chandra,XMM-Newton and Suzaku.Other objects for which the multi-frequency data at this moment do not allow ?rm conclusions about potential counterparts are e.g.HESS J1640–465and HESS J1834–087.

Source Comment

A large fraction of the new H.E.S.S.sources can be categorised into this class.Examples are:HESS J1702–420,HESS J1708–410or HESS J1745–303.As previously mentioned,there is an ongoing e?ort to investigate these sources with various instruments from radio to gamma-rays.References

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