Discovery of very high energy gamma-ray emission coincident with molecular clouds in the W28 (G6.4-0.1) field

0.45 MB·English

by  HESS Collaboration

Astronomy&Astrophysicsmanuscriptno.w28˙aa c ESO20081
(cid:13)
February2,2008
Discovery of very high energy gamma-ray emission coincident with
−
molecular clouds in the W 28 (G6.4 0.1) field
F.Aharonian1,13,A.G.Akhperjanian2,A.R.Bazer-Bachi3,B.Behera14,M.Beilicke4,W.Benbow1,D.Berge1 ⋆,
K.Bernlo¨hr1,5,C.Boisson6,O.Bolz1,V.Borrel3,I.Braun1,E.Brion7,A.M.Brown8,R.Bu¨hler1,T.Bulik24,
I.Bu¨sching9,T.Boutelier17,S.Carrigan1,P.M.Chadwick8,L.-M.Chounet10,A.C.Clapson1,G.Coignet11,
8 R.Cornils4,L.Costamante1,25,B.Degrange10,H.J.Dickinson8,A.Djannati-Ata¨ı12,W.Domainko1,L.O’C.Drury13,
0 G.Dubus10,J.Dyks24,K.Egberts1,D.Emmanoulopoulos14,P.Espigat12,C.Farnier15,F.Feinstein15,A.Fiasson15,
0
A.Fo¨rster1,G.Fontaine10,Y.Fukui26,Seb.Funk5,S.Funk1,M.Fu¨ßling5,Y.A.Gallant15,B.Giebels10,
2
J.F.Glicenstein7,B.Glu¨ck16,P.Goret7,C.Hadjichristidis8,D.Hauser1,M.Hauser14,G.Heinzelmann4,G.Henri17,
n
a G.Hermann1,J.A.Hinton1,14 ⋆⋆,A.Hoffmann18,W.Hofmann1,M.Holleran9,S.Hoppe1,D.Horns18,
J A.Jacholkowska15,O.C.deJager9,E.Kendziorra18,M.Kerschhaggl5,B.Khe´lifi10,1,Nu.Komin15,K.Kosack1,

8 G.Lamanna11,I.J.Latham8,R.LeGallou8,A.Lemie`re12,M.Lemoine-Goumard10,J.-P.Lenain6,T.Lohse5,
2 J.M.Martin6,O.Martineau-Huynh19,A.Marcowith15,C.Masterson13,G.Maurin12,T.J.L.McComb8,R.Moderski24,

] Y.Moriguchi26,E.Moulin15,7,M.deNaurois19,D.Nedbal20,S.J.Nolan8,J-P.Olive3,K.J.Orford8,J.L.Osborne8,
h M.Ostrowski23,M.Panter1,G.Pedaletti14,G.Pelletier17,P.-O.Petrucci17,S.Pita12,G.Pu¨hlhofer14,M.Punch12,
p
S.Ranchon11,B.C.Raubenheimer9,M.Raue4,S.M.Rayner8,O.Reimer⋆⋆⋆,M.Renaud1,J.Ripken4,L.Rob20,
-
o L.Rolland7,S.Rosier-Lees11,G.Rowell1 ,B.Rudak24,J.Ruppel21,V.Sahakian2,A.Santangelo18,L.Sauge´17,
r †
t S.Schlenker5,R.Schlickeiser21,R.Schro¨der21,U.Schwanke5,S.Schwarzburg18,S.Schwemmer14,A.Shalchi21,
s
a H.Sol6,D.Spangler8,Ł.Stawarz23,R.Steenkamp22,C.Stegmann16,G.Superina10,T.Takeuchi26,P.H.Tam14,
[ J.-P.Tavernet19,R.Terrier12,C.vanEldik1,G.Vasileiadis15,C.Venter9,J.P.Vialle11,P.Vincent19,M.Vivier7,

3 H.J.Vo¨lk1,F.Volpe10,S.J.Wagner14,andM.Ward8
v
5 Correspondence to:[email protected], [email protected]
5
(Affiliationscanbefoundafterthereferences)
5
3
Received/Accepted
.
1
0
8 ABSTRACT
0
: Aims. Observations of shell-type supernova remnants (SNRs) in the GeV to multi-TeV γ-ray band, coupled with those at millimetre radio
v
wavelengths,aremotivatedbythesearchforcosmic-rayacceleratorsinourGalaxy.Theold-agemixed-morphologySNRW28(distance 2kpc)
i ∼
X isaprimetargetduetoitsinteractionwithmolecularcloudsalongitsnortheasternboundaryandothercloudssituatednearby.
Methods.WeobservedtheW28field(for 40h)atveryhighenergy(VHE)γ-rayenergies(E>0.1TeV)withtheH.E.S.S.Cherenkovtelescopes.
r ∼
AreanalysisofEGRETE >100MeVdatawasalsoundertaken. ResultsfromtheNANTEN4mtelescopeGalacticplanesurveyandotherCO
a
observationswereusedtostudymolecularclouds.
Results. We have discovered VHE γ-ray emission (HESS J1801 233) coincident with the northeastern boundary of W 28 and a complex
−
of sources (HESS J1800 240A, B and C) 0.5 south of W 28 in the Galactic disc. The EGRET source (GRO J1801 2320) is centred on
◦
HESSJ1801 233butma−yalsoberelatedtoH∼ESSJ1800 240giventhelargeEGRETpointspreadfunction.TheVHEdiffe−rentialphotonspectra
arewellfitb−ypurepowerlawswithindicesΓ 2.3to2.−7.ThespectralindicesofHESSJ1800 240A,B,andCareconsistentwithinstatistical
errors.AllVHE sourcesare 10 inintrinsicr∼adiusexcept forHESSJ1800 240C, whichapp−ears pointlike. TheNANTEN12CO(J=1-0)data
′
revealmolecularcloudspositi∼onallyassociatingwiththeVHEemission,span−ninga 15kms 1rangeinlocalstandardofrestvelocity.
−
Conclusions. TheVHE/molecularcloudassociationcouldindicateahadronicorig∼inforHESSJ1801 233andHESSJ1800 240,andseveral
− −
cloudcomponentsinprojectionmaycontributetotheVHEemission.Thecloudshavecomponentscoveringabroadvelocityrangeencompassing
thedistanceestimatesforW28( 2kpc)andextendingupto 4kpc.Assuminghadronicoriginanddistancesof2and4kpcforcloudcomponents,
∼ ∼
therequiredcosmic-raydensityenhancement factors(withrespecttothesolarvalue)areintherange 10to 30.Ifsituatedat2kpcdistance,
suchcosmic-raydensitiesmaybesuppliedbySNRslikeW28. Additionallyand/or alternatively, part∼icleacc∼elerationmaycomefromseveral
catalogued SNRsand SNR candidates, the energetic ultra compact HII region W 28A2, and the HII regions M 8 and M 20, along with their
associatedopenclusters.Furthersub-mmobservationswouldberecommendedtoprobeindetailthedynamicsofthemolecularcloudsatvelocites
>10kms 1andtheirpossibleconnectiontoW28.
−
Keywords.gammarays:observations–stars:supernova-remnants–interstellarmedium:HIIregions–individualobjects:W28
1. Introduction:W28andsurroundings at the northern and northeastern boundaries where interaction
withthemolecularcloudisestablished.Furtherindicationofthe
Thestudyofshell-typesupernovaremnants(SNRs)atγ-rayen-
influenceofW28onitssurroundingsistheexpandingHIvoid
ergiesismotivatedbythelong-heldideathattheyarethedom-
atadistance 1.9kpc(Vela´zquezetal.2002).TheX-rayemis-
inant sites of hadronic Galactic cosmic-ray (CR) acceleration ∼
sion,whichoveralliswell-explainedbyathermalmodel,peaks
to energies approaching the knee ( 1015 eV) (e.g. Ginzburg
∼ intheSNRcentrebuthaslocalenhancementsinaregionover-
& Syrovatskii1964, Blandford& Eichler1987). CRs (hadrons
lappingthenortheasternSNR/molecularcloudinteraction(Rho
and electrons) are injected into the SNR shock front, and are
&Borkowski2002).
thenacceleratedviathediffusiveshockacceleration(DSA)pro-
In the neighbourhoodof W 28 are the radio-brightHII re-
cess(forareviewseeDrury1983).Subsequentγ-rayproduction
gions M 20 (Trifid Nebula at d 1.7 kpc Lynds etal. 1985
from the interaction of these CRs with ambient matter and/or ∼
– with open cluster NGC 6514), M 8 (Lagoon Nebula at
electromagnetic fields is a tracer of such non-thermal particle
d 2 kpc Tothill etal. 2002 — containing the open clus-
acceleration, and establishing the hadronicor electronic nature ∼
ters NGC 6523 and NGC 6530) and the ultra-compact HII
oftheparentCRsinanyγ-raysourceremainsakeyissue.Two
region W 28A2, all of which are representative of the mas-
SNRs,RXJ1713.7 3946andRXJ0852.0 4622,havesofares-
− − sive star formation taking place in the region. Further discus-
tablishedshell-likemorphologyinVHEγ-rays(Aharonianetal.
sion concerningthe active star formationin thisregionmay be
2004a,2005c,2006b, 2007b,2007a),withspectraextendingto
found in van den Ancker etal. (1997) and references therein.
20 TeV and beyond. In particular for RX J1713.7 3946, par-
− Additional SNRs in the vicinity of W 28 have also been iden-
ticle acceleration up to at least 100 TeV is inferred from the
tified: G6.67 0.42 and G7.06 0.12 (Yusef-Zadeh etal. 2000),
H.E.S.S.observations.AlthoughahadronicoriginoftheVHEγ- − −
G5.55+0.32,G6.10+0.53andG7.20+0.20(Broganetal.2006).
rayemissionishighlylikelyintheabovecases(Aharonianetal. The pulsar PSR J1801 23 with spin-down luminosity E˙
2006b, 2007b, Berezhko&Vo¨lk 2006, Berezhko,Pu¨hlhofer& 6.2 1034ergs 1anddis−tanced =13.5kpc(basedonitsdisper∼-
Vo¨lk2007),anelectronicoriginisnotruledout. × −
sionmeasure)isatthenorthernradioedge(Kaspi1993).More
Disentangling the electronic and hadronic components in
recentdiscussion(Claussenetal.2002)assignsalowerlimitof
TeVSNRsmaybemadeeasierbystudying:(1)SNRγ-rayspec-
9.4 2.4kpcforthepulsardistance.
trawellbeyond 10TeV,anenergyregimewhereelectronssuf- ±
∼ W 28 has also been linked to γ-ray emission detected at
fer strong radiative energy losses and due to Klein-Nishina ef-
E > 300MeVbyCOS-B(Pollock1985)andE > 100MeVby
fectstheresultinginverse-Comptonspectratendtoshowacut-
EGRET (Sturner & Dermer 1995, Esposito etal. 1996, Zhang
off; (2) older SNRs (age approaching 105 yr) in which accel-
etal. 1998). The EGRET source, listed in the 3rd catalogue
erated electrons have lost much of their energy through radia-
(Hartmanetal. 1999) as 3EGJ1800 2338,ispositionedatthe
tivecoolinganddonotreachmulti-TeVenergies;(3)SNRsin- −
southernedgeoftheradioshell.Wehavealsoperformedananal-
teractingwith adjacentmolecularcloudsof veryhigh densities
ysisofEGRETdata,withadditionaldatanotincludedinthe3rd
n > 103 cm 3.Itisthelatterregardespecially(andtoacertain
− catalogue,andresultsarediscussedlaterinthispaper.
degreethe second)whichmakesthe SNR W28(G6.4 0.1)an
− Previous observations of the W 28 region at VHE ener-
attractivetargetforVHEγ-raystudies.Inthispaperweoutline
gies by the CANGAROO-I telescope revealed no evidence for
the discovery of VHE γ-ray emission from several sites in the
such emission (Rowell etal. 2000) and upper limits at the
W 28 field and briefly discuss their relationship with molecu-
0.2 to 0.5 Crab-flux level for energies E > 1.5 TeV (1.1 to
larclouds,W28,andotherpotentialparticleacceleratorsinthe 2∼.9 10 11ergcm 2s 1)weresetforvariousregions.
region. × − − −
W28(G6.4 0.1)isamixed-morphologySNR,withdimen-
−
sions50′x45′andanestimateddistancebetween1.8and3.3kpc 2. ResultsatVHEand E >100MeVγ-rayenergies
(eg.Goudis1976,Lozinskaya1981).Itisanold-ageSNR(age
35000to150000yr;eg.Kaspietal.1993),thoughttohaveen- 2.1.H.E.S.S.VHEanalysisandresults
tered its radiative phase of evolution (eg. Lozinskaya 1981) in
The High Energy Stereoscopic System (H.E.S.S.) was used to
whichmuchofitsCRshaveescapedintothesurroundinginter-
observetheW28region.OperatingintheSouthernHemisphere,
stellarmedium(ISM).Wenotealsothattheevolutionarystatus
H.E.S.S. consists of four identical 13 m diameter Cherenkov
(Sedovand/orradiative)ofshell-typeSNRsmaydependonthe
telescopes (Bernlohr etal. 2003). H.E.S.S. employs the stereo-
densityoftheirsurroundings(seeeg.Blondinetal.1998).
scopicimagingatmosphericCherenkovtechnique,andissensi-
W 28 is distinguished by its interaction with a molecular
tivetoγ-raysaboveanenergythresholdof 0.1TeV(Funketal.
cloud (Wootten 1981) along its north and northeastern bound- ∼
2004).Anangularresolutionof5 to6 (Gaussianstandarddevi-
aries. This interaction is traced by the high concentration of ′ ′
ation)onanevent-by-eventbasisisachieved,andthelargefield
1720 MHz OH masers (Frail etal. 1994, Claussen etal. 1997,
ofview(FoV)withfullwidthathalfmaximumFWHM 3.5
1999),andalsothelocationofveryhigh-density(n>103cm−3) permits survey coverage in a single pointing. A point ∼source◦
shockedgas(Arikawaetal.1999,Reachetal.2005).Theshell-
sensitivity approaching 0.01 Crab flux ( 10 13 erg cm 2s 1
likeradioemission(Longetal.1991,Dubneretal.2000)peaks ∼ − − −
at 1 TeV) is achieved for a 5σ detection after 25 hr observa-
∼
tion.FurtherdetailsconcerningH.E.S.S.canbefoundinHinton
Send offprint requests to: [email protected],
(2004)andreferencestherein.
[email protected]
⋆ nowatCERN,Geneva,Switzerland ThetotalobservationtimecoveringtheW28regionamounts
⋆⋆ nowatSchoolofPhysics&Astronomy,UniversityofLeeds,Leeds to 42 hr in a series of runs (with typical duration 28 min)
∼ ∼
LS29JT,UK spread over the 2004, 2005 and 2006 seasons. Runs were ac-
⋆⋆⋆ nowatStanfordUniversity,HEPL&KIPAC,Stanford,CA94305- ceptedforanalysisif theymetqualitycontrolcriteriabasedon
4085,USA therecordedrateofisotropicCRbackgroundevents,thenumber
now at School of Chemistry & Physics, University of Adelaide, ofmalfunctioningpixelsineachcamera,thecalibrationandthe
†
Adelaide5005,Australia trackingperformance(seeAharonianetal.2004bfordetails).
Aharonianetal.:AssociatedVHEandCOemissionintheW28field 3
Data were analysed using the moment-based Hillas anal- 2.2.EGRETE >100MeVanalysisandresults
ysis procedure, the same used in the analysis of the inner
WehavealsoanalysedEGRETdatafortheW28region,using
Galactic Plane Scan datasets (Aharonian etal. 2005a, 2006a).
CGRO observation cycles (OC) 1 to 6. This slightly expands
Observationscoveredarangeofzenithanglesleadingtoenergy
on the dataset of the 3rd EGRET catalogue (using OCs 1 to
thresholdsof 320GeVwithhardcuts(Cherenkovimageinte-
∼ 4; Hartman etal. 1999), which revealed the pointlike source,
gratedintensityorsize>200photoelectrons)and 150GeVfor
∼ 3EG J1800 2338 (E > 100 MeV). Our analysis confirms
standardcuts(size>80photoelectrons).Hardcutswereusedin −
the presence of a pointlike E > 100 MeV source in this re-
VHE γ-rayimages, sourcelocationstudiesand energyspectra.
gion, here labeled GRO J1801 2320 (for E > 100 MeV).
Inaddition,Standardcutswereusedinenergyspectrainorder −
GRO J1801 2320appearsslightlyshifted ( 0.2 ) with respect
to increase the energycoverageof extractedspectra. Generally − ∼ ◦
to the 3EG position. The 3EG position refers to a E >100
consistent results were obtained using an alternative analysis
MeV determination based on the diffuse model as of Hunter
based on a modelof Cherenkovimage parameters(de Naurois
etal. (1997). Our dedicated analysis of archival EGRET data
2006),whichalsoutilisesanindependentcalibrationandlower
comprises different analysis compared to the 3EG catalogue.
cut on image size of >60 photoelectrons.A forthcomingpaper
WefirstemployedthefinalisedEGRETinstrumentalresponses,
willhighlightresultsindetailfromthisanalysis,whichachieves
whichweremadeavailableby2001andareconsideredmanda-
improvedsensitivitiesatlowerthresholdscomparedtothepure
tory for investigating an EGRET source under conditions ap-
Hillas-basedanalysis.
plicable from the end of OC 4 (narrow field of view modus;
The VHEγ-rayimage(Fig.1)revealstwo sites ofVHE γ- rapidlydeterioratingsparkchamberefficiency;andotherissues).
ray emission in the direction of the northeastern and southern Second, we restricted the analysis both in narrowing the data
boundaries of the W 28 SNR. The colour scale in this figure selection to pointingangleswith respectto ourregionof inter-
depictstheGaussian-smoothedVHEexcesscountsaboveaCR est, whichavoids the need to invokea wide-anglepointspread
background estimate according to the template model (Rowell function(PSF).Thirdly,theimprecisionoftheinterstellaremis-
2003), alongwith significancecontoursobtainedafterintegrat- sion modelwas counteredvia adjustmentsonanalysisparame-
ing eventswithin a radiusof 0.1◦ from eachbin centre (appro- ters gmult and gbiasto accountfor localdeviationsfrom the
priate for pointlikesource searching).Similar imageswere ob- large-scalediffuseemissionmodelintheregionofinterest.The
tained using alternative CR background models. A smoothing 68%and95%locationcontoursof GRO J1801 2320are plot-
radiusof 4.2′ was usedto sufficientlysmoothoutrandomfluc- tedinFig.1,andmatchwellthelocationofHE−SSJ1801 233.
tuationsintheimage.AnassessmentoftheVHEpost-trialsig- Since however the EGRET degree-scale PSF easily en−com-
nificanceswasmadefromouroriginalsearchformarginallyex- passes both of the VHE sources, we cannot rule out a rela-
tended sources, which employed an a priori integration radius tionship with HESS J1800 240. For the energy spectrum of
θ=0.2◦.Underthisschemeweapplied 2.2 105trials(avery GRO J1801 2320, we have−used the flux points extracted at
conservativevalueappliedtothesedata)∼accum×ulatedinsearch- theposition−of3EGJ1800 2338asnegligibledifferenceswere
ing for sources in the inner Galactic Plane (as in Aharonian found between ours and th−at obtained at the nominal 3EG po-
2005a).Thepre-trialsignificanceoftheVHEsources,at +7σ, sition.Fittingapurepowerlawweobtainedaspectralindexof
≥
isthereforeconvertedtoapost-trialsignificanceof +5σ. Γ = 2.16 0.10,quiteconsistentwiththepublishedvaluefrom
≥ ±
Based on the significance contours in Fig. 1, we assign la- Hartman etal. (1999). Comparisons are made with the VHE
bels to the northeastern source, HESS J1801 233, and to the spectrumofHESSJ1801 233andHESSJ1800 240in 5.
− − − §
complex of sources to the south, HESS J1800 240, according
−
to their best fit positions (fitting a 2D Gaussian and ellipse re-
3. NANTENandotherobservationsofMolecular
spectively to the unsmoothed excess map). Three components
of HESS J1800 240 are identified, labeled here A, B and C Clouds
−
from East to West. These components represent local peaks
In searching for molecular cloud counterparts to the VHE
2σabovetheirsurrounds.Althoughnotconvincinglyresolved sources, we analysed 12CO (J=1–0) molecular line observa-
∼
under this analysis these components may comprise separate
tions taken by the 4-meter mm/sub-mm NANTEN telescope,
sources(oratleastinpart)duetotheirpossiblerelationshipwith
at Las Campanas Observatory, Chile (Mizuno & Fukui 2004).
distinctmultiwavelengthcounterparts(discussedlater).
TheNANTENGalacticPlaneSurveydataof1999to2003(see
Differential photon energy spectra were extracted Matsunagaetal.(2001)andreferencesthereinfordetails)were
from HESS J1801 233 and all three components of used,andfortheW28region,thesurveygridspacingwas4.
′
HESS J1800 240. −Spectra were well-fit by pure power Figure 2 (upper left panel) shows the 12CO (J=1–0)image
laws (dN/dE−= k(E/1TeV) Γ) with photon indices Γ 2.5 integratedovertheLocalStandardofRestvelocity(V )range
− LSR
to 2.7 in the energy range 0.3 to 5 TeV (see Table∼1 for 0to10kms 1,whiletherightpanelshowstheimageintegrated
−
results). Spectral fits were o∼btained u∼sing fluxes from a com- over the range V =10 to 20 km s 1. Two prominent 12CO
LSR −
bination of hard and standard cuts to maximise the energy features representing molecular clouds centred at (l,b)=(6.7 ,
◦
coverage.Spectral analysis employed the reflected background 0.3 ) and (l,b)=(5.9 , 0.4 ) spatially correspond with the
◦ ◦ ◦
− −
model (Berge etal. 2007), in which control regions reflected VHEγ-rayemission.AsshowninFig.2,thesemolecularclouds
through each tracking position (taking care to avoid known spanbothV ranges.AccordingtotheGalacticrotationmodel
LSR
VHEγ-raysources)wereusedtoestimatetheCRbackground. ofBrand&Blitz(1993),theseV rangesformallycorrespond
LSR
Within the statistical and systematic errors, the photon indices to kinematic distances of approximately 0 to 2.5 kpc (over-
∼
appear consistent throughout HESS J1800 240. Except for lapping the Sagittarius arm), and 2.5 to 4 kpc (reaching the
− ∼
HESS J1800 240C, all of the VHE sources appear extended Scutum-Cruxarm)respectively.Giventheuncertaintiesinrota-
−
with intrinsic radii of 10. At a distance of 2 kpc, the VHE tionmodelsclosetotheGalacticcentre,suchV rangeswould
′ LSR
∼
source luminositiesin the energyrange0.3 to 3 TeV would be coverthedistanceestimatesforW28,themostprominentSNR
ontheorderof1033ergs 1. in theregion.Muchdiscussionhascentredonthe systemic ve-
−
4 Aharonianetal.:AssociatedVHEandCOemissionintheW28field
eg) H.E.S.S. 80 Fthieg.1V.HIEmaγg-eray(1.5ex◦c×ess1.5co◦)unotsf
d
( (events), corrected for exposure
o3 0 andsmoothedwithaGaussianof
-2 00. PSR J1801-23 radius 4.2′ (standard deviation).
0 Overlaidaresolidgreencontours
2 W 28 (Radio Boundary)
J HESS J1801-233 ofVHEexcess(pre-trial)signif-
c icancelevelsof4,5,and6σ,af-
e
D 60 ter integrating events within an
oversampling radius θ=0.1 ap-
◦
propriate for pointlike sources.
The thin-dashed circle depicts
the approximate radio boundary
of the SNR W 28 guided pre-
´
0 dominantly by the bright north-
o3 40 ern emission (see Dubner etal.
3 2000 & Brogan etal. 2006).
2
- GRO J1801-2320 Identified here are VHE source
regions HESS J1801 233 to
−
the northeast, and a complex
of sources HESS J1800-240
(A, B & C) to the south of
20
W 28. Also indicated are: HII
6.225-0.569 regions (black stars); W 28A2
(see text), G6.1 0.6 (Kuchar
−
& Clark 1997), 6.225 0.569
G6.1-0.6 −
o (Lockman 1989); The 68% and
4 95% location contours (thick-
2
- dashed yellow lines) of the
0
E > 100 MeV EGRET source
GRO J1801 2320; the pulsar
−
C PSR J1801 23 (white triangle).
W 28A2 −
A The inset to the bottom left
B depicts a pointlike source for
HESS J1800-240 RA J2000.0 (hrs) this analysis after the Gaussian
smoothing applied to the main
18h03m 18h image.
Table1.Numericalsummary fortheVHEandE>100MeVsourcesintheW28regionincludingpositionalandspectralinformation.
†‡
Bestfitposition(J2000.0) Spectralanalysis
Name R.A.[deg] Dec[deg] 1σ [deg] 2S [σ](evts) 3k 4Γ 5L
src
HESSJ1801 233 270.426 0.031 23.335 0.032 0.17 0.03 +7.9(281) 7.50 1.11 0.30 2.66 0.27 1.5
− ± − ± ± ± ± ±
HESSJ1800 240A 270.491 0.001 23.962 0.001 0.15 +6.0(180) 7.65 1.01 0.50 2.55 0.18 1.5
§
− ± − ± ± ± ±
HESSJ1800 240B 270.110 0.002 24.039 0.009 0.15 +7.8(236) 7.58 0.90 0.15 2.50 0.17 1.4
§
HESSJ1800−240C 269.715±0.014 −24.052±0.006 0.02 0.15 +4.5(71) 4.59±0.89±0.20 2.31±0.35 0.8
HESSJ1800−240 270.156±0.044 −23.996±0.022 0.32R±A 0.05 +10.3(652) 18.63± 1.85± 1.20 2.49±0.14 3.6
§§
− ± − ± 0.17Dec± 0.03 ± ± ±
±
GROJ1801 2320 270.360 0.150 23.340 0.150 – +13.2 3.35 0.52 2.16 0.10 480.0
− ± − ± ± ±
†VHEphotonspectraarederivedfromaregionofradiusθ= p0.12+σ2srccenteredoneachsource’spositionunlessotherwiseindicated.
‡Inspectra,afunctiondN/dE=kE−Γphcm−2s−1TeV−1isfitted.EisinTeVunits(H.E.S.S.data);GeVunits(EGRETdata).
1.Fittedintrinsicsourcesize(Gaussianstd.dev.)
2.Statisticalsignificanceandexcesseventsinbrackets;forH.E.S.S.sourcesusingLi&Ma(1983);forEGRETsourcesgivenbyS = √TsforTsdefinedbyMattoxetal.(1996)
3.ForH.E.S.S.sources:×10−13phcm−2s−1TeV−1at1TeV(withstatisticalandsystematicerrors);ForEGRETsources:×10−2phcm−2s−1GeV−1at1GeV(withstatisticalerrors)
4.Onlystatisticalerrorsindicated.Systematicerrorisestimatedat 0.2
±
5.Luminosity×1033ergs−1at2kpc(0.3to3TeVforH.E.S.S.;0.04to6GeVforEGRET)
§DuetocrosscontaminationbetweencomponentsA&B,afixedvalueofσsrc=0.15◦estimatedvisuallyfromFig.1wasused.
§§Spectrumextractedfroma0.8◦×0.6◦ellipticalregionencompassingallcomponentsA,B,C,andmatchingthesizeofthecorrespondingmolecularcloud.
locity (SV) of W 28 (and hence its distance), and how much mayalsoextendovertheV = 25to+38kms 1 range,giv-
LSR −
W 28 has influenced matter in the region.Hα (Radhakrishman ingrisetoashockspeedof 30k−ms 1.Torresetal.(2003)and
−
etal.1972)andHIabsorptionfeatures(Lozinskayaetal.1981) Reachetal.(2005)haveals∼ostudiedthelarge-scale12CO(J=1-
have suggested SV 18 km s 1. Claussen etal. (1997) have 0)emissionforthisregionusingthesurveydataofDameetal.
−
pointedtoSV 17km∼ s 1.MorerecentHIstudiesbyVela´zquez (2001), suggesting that the parent molecular cloud under the
−
etal.(2002)su∼ggestSV=+7kms 1(whichleadstothedistance influence of W 28 is presently centred at V 19 km s 1.
− LSR −
∼
estimate for W 28 at 1.9 kpc). They also suggest a HI shell TheGalacticlongitude-velocity(l-v)diagram(bottompanelsof
∼
Aharonianetal.:AssociatedVHEandCOemissionintheW28field 5
Fig.2)fromourNANTENdataintegratedovertheGalacticlat- material along this line of sight is physically connected at the
itude ranges b = 0.125 to 0.5 and b = 0.125 to 0.7 same distance (for example d 2 kpc) and possibly distrupted
◦ ◦ ◦ ◦
− − − − ∼
shows the distribution of molecular material in relation to the or shocked by a local energy source. Systematic effects in the
SV of W 28 from the HI studies of Vela´zquez. The wider, lat- mass estimates arise from the velocity crowdingin this part of
ter b range shows the effect of including the cloud component theGalactic plane,andalsothe broadvelocityrangeforwhich
overlappingHESSJ1800 240A.AvoidordipinCOemission X-factorusedabovemaynotnecessaryapply.Inthelattercase,
−
appears at a similar V range as found in the HI data, with theX-factormayunderestimatethecloudmasssinceanappre-
LSR
muchof the molecularmaterialappearingto surroundthe void ciable fraction of gas may be heated under the assumption of
inpositiveV valueswithrespecttotheSVofW28.Asimilar distruptedand/orshock-heatedgas.Onemustallowfor 4kpc
LSR
longitude-velocity picture was revealed by Torres etal. (2003) distances for some or even all of the V >10 km s 1∼cloud
LSR −
(seetheirFig.22). components, and therefore the conclusion that they are not re-
The V =0 to 10 km s 1 component of the northeast lated toW 28andotherinterestingobjectsatd 2 kpc.If the
LSR − ∼
cloudoverlappingHESSJ1801 233isalreadywellstudied(see cloudsarerelated,W28couldplayadisruptingrole.Thelevel
Reach etal. 2005andreference−stherein).Shocked12CO(J=3– ofthisdisruptionishoweverunclearsinceseveralotherplausible
2) moleculargasas indicatedby a broadwing-likeline disper- candidatesrelatedto the star formation(discussedlater)in this
sion (Arikawa etal. 1999 — hereafter A99; using the James regioncouldalsocontribute.Someothermolecularcloudcom-
ClerkMaxwellTelescope(JCMT);in15 gridsteps)andahigh plexes have also been discussed as possibly disrupted by adja-
′′
concentrationofOHmasers(Claussenetal.1997),suggestsma- centSNRsand/orenergeticsources(eg.Yamaguchietal.1999,
terialherehasbeencompressedbytheSNRshockinW28.The Moriguchi etal. 2000). In table 2, we present a full summary
line dispersion, ∆V 70 km s 1, is an indicator of the SNR ofcloudmassesanddensities(forregionscenteredontheVHE
−
≤
shock speed in this particular region. The unshocked gas was source coordinates as in table 1) for various combiniations of
also mapped by A99 via 12CO(J=1–0) observations with the cloudcomponentsanddistancesof2and4kpc.Velocitysepara-
Nobeyama 45 m telescope (in 34 grid steps for V = +4 tionofcloudcomponentsarebasedontheirapparentdistribution
′′ LSR
to +9 km s 1). The shocked and unshocked gas extends to the inFig.2(bottompanels).
−
northeastandnorthernboundariesofW28(seeFig.3ofA99),
and it appears just their northeastern componentsare position-
4. RadiotoX-rayviews
ally coincident with the VHE emission of HESS J1801 233.
−
A99 estimate the mass and average density of the shocked gas Figure 3 compares the radio (left panel), infrared and X-ray
at M 2 103 M andn 104 cm−3 respectively.Fortheun- views(rightpanel)oftheW28regionwiththeVHEsignificance
shock∼edg×as,A99o⊙btained∼M 4 103 M andn 103 cm−3 contours.TheVeryLargeArray(VLA)90cmcontinuumradio
respectively.TheVLSR=10to2∼0km×s−1 ran⊙geinou∼rNANTEN image from Brogan etal. (2006) illustrates the shell-like SNR
data also reveals additional molecular clouds along the line of morphology peaking strongly along the northern and eastern
sightthatcouldcontributetotheVHEemission. boundaries.HESSJ1801 233canbeseentooverlapthenorth-
−
The southern cloud overlaps all components of eastern shell of the SNR, coinciding with a strong peak in the
HESS J1800 240, with a dominant fraction of the cloud 90cmcontinuumemission.Wenotethatathermalcomponentis
−
overlappingcomponentsAandB.Thecomponentofthiscloud likelypresentinthispeak,givenitsspectralindexα 0.2(for
visible in the V = 0 to 10 km s 1 range coincideswell with S να) between 90 and 20 cm (Dubner etal. 2000∼).−Outlines
LSR −
∝
HESSJ1800 240Bandthe HII regionW 28A2.The strongest oftheSNRstracedbynon-thermalradioemission,G6.67 0.42
CO temperat−ure peak of this component at (l,b)=(5.9 , -0.4 ) and G7.06 0.12(Yusef-Zadehetal. 2000, Helfand etal.−2006,
◦ ◦
is within 0.02 of W 28A2, and is likely the dense material labelled as−G6.51 0.48 and G7.0 0.1 by Brogan etal. 2006)
◦
− −
surrounding this HII region. Moreover the peak’s velocity at are also indicated. In addition, Brogan etal. notes that the
V 9–10 km s 1 (with dispersion of 15 km s 1), suggests non-thermal radio arc G5.71 0.08, which overlaps well with
LSR − −
∼ −
a distance ( 2.4 kpc) similar to that of W 28A2 ( 2 kpc; HESS J1800 240C, could be a partial shell and therefore an
Acordetal. 1∼997), and also W 28. In the V = 10 km∼ s 1 to SNR candida−te. The distances to G6.67 0.42 and G5.71 0.08
LSR −
20 km s 1 range, molecular material appears to coincide with are presently unknown. Directly south o−f W 28, the ultra−com-
−
all three VHE components of HESS J1800 240. In particular, pact HII region W 28A2 is a prominent radio source, and is
−
HESS J1800 240Aand C have molecularcloud overlapsonly positioned within 0.1 of the centroid of HESS J1800 240B.
inthislatterV− range. The other HII region◦s G6.1 0.6 (Kuchar & Clark 199−7) and
LSR
−
Using the relation between the hydrogen column density 6.225 0.569 (Lockman 1989) are also associated with radio
N(H ) and the 12CO(J=1–0)intensity (the X-factor)W(12CO), emissi−on.
2
N(H ) = 1.5 1020 [W(12CO)/(Kkm/s)] (cm 2) (Strongetal. TheX-raymorphologyasshown (Fig.3 rightpanel)in the
2 −
×
2004),weestimateatotalmassforthenortheasterncloudfrom ROSAT PSPC (0.5 to 2.4 keV) image from Rho & Borkowski
our NANTEN data at 5 104 M for d =2 kpc within an (2002) reveals the central concentration of X-ray emission,
elliptical region of diam∼eter×0.2 0⊙.4 (7 14 pc; centred on which is predominantly thermal in nature with characteristic
◦ ◦
HESS J1801 233)for the velocity×range 0–×25 km s 1. An av- temperaturesin the rangekT 0.4 to 2 keV. An X-raypeak or
−
erage density−(for neutral hydrogen)of 1.4 103 cm 3 is also Ear lies at the northeastern b∼oundary and just outside the 4σ
−
∼ ×
derived. Similarly the total mass of the southern cloud is es- significancecontourofHESSJ1801 233.Anon-thermalcom-
timated at 1.0 105 M for d=2 kpc and combining clouds ponenttotheearemission(3 1.5)−(2.1 10 14 ergcm 2 s 1 at
′ − − −
fromacircu∼lara×reaofra⊙dius0.15 (5pc)forthevelocityrange 1 keV) with a power-law ind×ex Γ=1.3 h×as been suggested by
◦
12–20 km s 1, and area 0.3 0.6 (10.5 21 pc) in diame- Uenoetal.(2003a)basedonXMM-Newtonobservationsinthe
− ◦ ◦
ter for the velocity range 0–12×km s 1 (bot×h regions are cen- 0.5to10keVenergyrange.ThetotalkineticenergyoftheSNR
−
tredonHESSJ1800 240B).Thecorrespondingaveragedensity isestimatedat 4 1050erg,whichcouldbealowerlimitdue
is 1.0 103 cm 3. −By integrating over the rather broad 0–20 to the possible∼brea×k-out of the SNR along the southern edge
−
km∼s 1×and 0–25 km s 1 ranges we assume that the molecular away from the molecular cloud to the north and east (Rho &
− −
6 Aharonianetal.:AssociatedVHEandCOemissionintheW28field
eg) NANTEN 12CO(J=1-0) 0-10 km/s eg) NANTEN 12CO(J=1-0) 10-20 km/s
d d
o23 0.0 ( o23 0.0 (
- 00 100 - 00 100
2 W 28 (Radio Boundary) 2 W 28 (Radio Boundary)
c JHESS J1801-233 c JHESS J1801-233
e e
D D
80 80
l=+7.0o l=+7.0o
0´ 0´
3 3
o o
3 3
2 60 2 60
- GRO J1801-2320 - GRO J1801-2320
l=+6.0o l=+6.0o
6.225-0.569 40 6.225-0.569 40
G6.1-0.6 G6.1-0.6
o o
4 4
-2 b=0.0o -2 b=0.0o
20 20
W 28A2 C W 28A2 C
b=-1.0o A B b=-1.0o A B
HESS J1800-240 RA J2000.0 (hrs) HESS J1800-240 RA J2000.0 (hrs)
0 0
18h03m 18h 18h03m 18h
Fig.2. Upper Left: NANTEN 12CO(J=1-0) image of the W 28 region (linear scale in K km s 1) for V =0 to 10 km s 1 with VHE γ-ray
− LSR −
significance contours overlaid (green) — levels 4,5,6σ as in Fig. 1. The radio boundary of W 28, The 68% and 95% location contours of
GRO J1801 2320 and the location of the HII region W 28A2 (white stars) are indicated. Upper Right: NANTEN 12CO(J=1-0) image for
V =10to−20kms 1 (linearscaleand samemaximaasfor upper leftpanel). Bottompanels:Distributionof COemissionover theGalactic
LSR −
longitudeandV planeintegratedoverGalacticlatitudebranges 0.125 to 0.5 (left)and 0.125 to 0.7 (right).Thelatterrangeisusedto
showtheeffectoLSfRextendingthelatituderangetoencompasscompo−nentA◦ofH−ESS◦J1800 24−0.The◦bold−circ◦leindicatesthesuggestedsystemic
velocity(7kms 1)ofW28fromtheHIstudiesofVela´zquezetal.(2002). −
−
Borkowski2002). The HII regions,W 28A2 and G6.1 0.6are 5. Discussion
−
prominent in the 8.28 µm image (Fig. 3 right panel) from the
MidcourseSpaceExperiment(MSX),showingthatahighcon-
Our discovery of VHE γ-ray emission associated with dense
centrationofheatedduststillsurroundstheseveryyoungstellar (n 103cm 3)molecularcloudsintheW28fieldaddstothelist
−
objects. ≥
ofsuchassociationsafterthedetectionofdiffuseγ-rayemission
from the Galactic Ridge (Aharonian etal. 2006c), the associa-
tionofHESSJ1834 087withtheold-ageSNRW41(Lemie´re
−
etal. 2005, Albert etal. 2006) and VHE emission discovered
Aharonianetal.:AssociatedVHEandCOemissionintheW28field 7
Fig.3. Left: VLA 90cm radio image from Brogan etal. (2006) in Jy beam 1 (rebinned by a factor 1.2 compared to the original). The VHE
−
significance contours (green) from Fig. 1 are overlaid along with the HII regions (white stars) and the additional SNRs and SNR candidates
(withyellowcirclesindicatingtheirlocationandapproximatedimensions)discussedinthetext.Right:ROSATPSPCimage—0.5to2.4keV
(smoothedcountsperbinfromRho&Borkowskietal.2002).Overlaidarecontours(cyan—10linearlevelsupto5 10 4Wm 2sr 1)fromthe
− − −
×
MSX8.28µmimage.Othercontoursandobjectsareasfortheleftpanel.TheX-rayEarrepresentingapeakatthenortheasternedgeisindicated.
fromIC443(Albertetal.2007).TheVHE/molecularcloudas- Table2.DetailsforthemolecularcloudstowardstheVHEsourcesin
sociation could indicate a hadronic origin for the parentmulti- theW28field,assumingadistanced.
TeVparticleswherethe γ-rayemission(multi-GeVto TeVen-
ergies)arisesfromthedecayofneutralpionsresultingfromthe
interactionofacceleratedprotons(andhigherZnuclei)witham- VHESource VLSR d †M ‡n §kCR
(kms 1) (kpc)
bientmatterofdensityn.Inthiscasetheγ-rayfluxwouldscale −
with cloud mass or density, and the total energyin accelerated HESSJ1801 233 0-25 2.0 0.5 1.4 13
−
particlesorCRspenetratingthecloud(s).Wenotethataperfect HESSJ1801 233 0-12 2.0 0.2 2.3 32
−
correlationbetweentheVHEandmolecularcloudmorphologies HESSJ1801 233 13-25 4.0 1.1 0.6 23
−
isnotexpectedduetocomplextimeandenergy-dependentprop- HESSJ1800 240 0-20 2.0 1.0 1.0 18
−
HESSJ1800 240A 12-20 4.0 1.0 0.7 28
agationofCRtoandwithinthecloud(seeGabicietal.2006for −
HESSJ1800 240B 0-12 2.0 0.4 2.3 18
a discussion). Projection effects are also likely to be important −
HESSJ1800 240B 12-20 4.0 1.5 1.2 19
forthe examplesdiscussed heresince the VHE emission could −
ehsasvaericlyonptrhiybsuitciaolnlys fcroonmneccltoeuddstoatondeiffaenroenthtevr.elFoocritieexsa,mnoptlentehce- ‡†CClloouudddmeansssit×y1×0150M3⊙cm−3
relationship between HESS J1801 233 and the W 28/molecu- §Cosmic-raydensityenhancement,kCRabovethelocalvaluerequiredto
− producetheE>1TeVVHEγ-rayemission(usingEq.10ofAharonian(1991)).
larcloudinteractionisnotentirelyclearduetotheoverlapping
molecularcloudcomponentsatV >10kms 1.
LSR −
Oneshouldalsoconsideracceleratedelectronsasthesource
of γ-ray emission, via inverse-Compton (IC) scattering of am- dicatedfromZeemansplittingmeasurementsinthecompactar-
bient soft photon fields and/or non-thermal Bremsstrahlung eas(arcsecondscale)surroundingthe1720MHzOHmasersof
from the interaction of electrons with dense ambient matter. the northeastern interaction region (Hoffman etal. 2005), co-
Maximumelectronenergieshowevermaybeconsiderablylower inciding with HESS J1801 233. To the north of W 28, an-
−
(factor 10 or more than that of protons) due to synchrotron otherpotentialsourceofparticleaccelerationisPSRJ1801 23,
∼ −
cooling in magnetic fields and low shock speeds, in the ab- where the VHE emission may arise in an asymmetric pulsar-
sence of strong electron replenishment. An assessment of the wind-nebula (PWN) scenario (a primarily leptonic scenario),
role of accelerated electrons requiresconsiderationof the non- similartoHESSJ1825 137(Aharonianetal.2006d).However
thermal radio and X-ray emission (where a convincing mea- with a spin-down powe−r of E˙ 6.2 1034 erg s 1 at distance
−
∼ ×
surement of the latter is so far lacking), and also magnetic d > 9.4 kpc, this pulsar appears unlikely to power any of the
fields in this region. Such observations will also provide con- γ-ray sources observed in the region. A PWN scenario would
straintsonsynchrotronemissionexpectedfromsecondaryelec- thereforerequireasofarundetectedenergeticpulsar.
trons resulting from primary hadron interactions with ambient In the case of a hadronic origin and following Eq.10 of
matter (as discussed above). Relatively high magnetic fields Aharonian(1991),wecanestimatetheCRdensityenhancement
B 100(n/104cm 3)0.5 µG are inferred in dense molecular factor k in units of the local CR density required to explain
− CR
∼
clouds(Crutcheretal. 1999). In addition,higher valuesare in- the VHE emission, given an estimate for the cloudmasses and
8 Aharonianetal.:AssociatedVHEandCOemissionintheW28field
assumptionsondistance.ConvertingtheVHEenergyspectrain
Table1to anintegralvaluefor E > 1TeV, assumingdistances
of 2 and 4 kpc for the various cloud components, and that all
the VHE emission in each source is associated with the cloud -1s ) GROJ1801-2320
2
component under consideration, we arrive at values for kCR in -m -10
therange13to32(Table2). g c10
Overall,theselevelsofCRenhancementfactorwouldbeex- er
pectedintheneighbourhoodofCRacceleratorssuchasSNRs.If 2F ( 10-11 HESSJ1800-240
thecloudswereallat 2kpc,anobviouscandidateforsuchpar- E
∼
ticleaccelerationistheSNRW28,themostprominentSNRin
-12
the region. Despite its old age, multi-TeV particle acceleration 10
may still occur in W 28 (Yamazaki etal. 2006), with protons
reaching energiesof several 10’s of TeV dependingon various -13
HESSJ1801-233
10
SNR shock parameters such as speed, size and ambient mat-
ter density. In addition, CRs produced at earlier epochs have
likely escaped and diffused throughout the region, a situation -5 -4 -3 -2 -1
10 10 10 10 10 1
discussedatlengthinAharonian&Atoyan(1996).Aharonian&
Energy ( TeV )
Atoyanshowforslowdiffusion(diffusioncoefficientat10GeV
D 1026cm2s 1asmightbeexpectedindenseenvironments) Fig.4.Energyfluxesof HESSJ1801 233 andHESSJ1800 240 (for
CR10e∼nhancemen−t factors in the required range could be found regionsdefinedinTab.1)comparedt−otheE > 100MeVco−unterpart
in the vicinity (within 30 pc – note that if at 2 kpc distance, GROJ1801 2320.Thepowerlawfitsanddatapoints(summarisedin
−
Tab.1)arealsoindicated:HESSJ1801 233(solidbluelineandpoints);
HESS J1800 240would lie 10 pc fromthe southern circular
−
− ∼ HESS J1800 240 (open red points and solid line); GRO J1801 232
boundaryofW28)ofacanonicalSNRasanimpulsiveaccelera- − −
(solidblackpointsandgrey1σconfidenceband).
torupto 105yraftertheSNexplosion(seetheirFig.1).Inthis
∼
sense,W28asasourceofCRsintheregioncouldbeplausible
scenario.
duction. Finally, if the VHE emission is associated with truly
TheW28fieldhoweverisarichstarformationregion,and
distantcloudcomponentsapproachingtheScutum-Cruxarmat
several additional/alternative sources of CR acceleration may
4kpc,undetectedbackgroundparticleacceleratorswouldthen
be active. The SNR G6.67 0.42 is positioned directly to the ∼
playarole.
−
southeastofHESSJ1801 233(Fig.3leftpanel)whiletheSNR
− Fig. 4 also compares the EGRET and VHE spectra. Given
G7.06 0.12is situated 0.25 northof HESSJ1801 233and
− ∼ ◦ − thedegree-scaleEGRETPSF,GROJ1801 2320remainsunre-
on the west side of the HII region M 20. M 20 itself may also −
solved at scales of the VHE sources. Although the peak of the
beanenergysourceforthemolecularcloudsinthisregion.The
EGRET emission coincides with HESS J1801 233, we there-
SNR candidate G5.71 0.08 (Brogan etal. 2006) may also be −
− fore cannot rule out unresolved MeV/GeV components from
responsiblein some way forHESS J1800 24Cgiventhe good
− HESS J1800 240.Observationswith GLAST will be required
positionaloverlapbetweenthetwo.TheseradioSNR/SNRcan- −
todeterminetheMeV/GeVcomponentsoftheVHEsources.
didates are without a distance estimate making it unclear as
to how they relate to the molecular clouds in the region. The
morphology of HESS J1800 240 displays several peaks, per-
6. Conclusions
−
haps resulting from changes in cloud density and/or the pres-
enceofadditionalparticleacceleratorsandlocalconditions.For In conclusion, our observations with the H.E.S.S. γ-ray tele-
HESSJ1800 24B,apotentialenergysourceistheunusualultra- scopes have revealed VHE γ-ray sources in the field of
−
compactHIIregionW28A2(G5.89 0.39),representingamas- W 28 which positionally coincide well with molecular clouds.
−
sive star in a very young phase of evolution. W 28A2 exhibits HESS J1801 233 is seen toward the northeast boundary of
−
very energetic bipolar molecular outflows (Harvey & Forveille W 28, while HESS J1800 240situated just beyond the south-
−
1988, Acord etal. 1997, Sollins etal. 2004) which may arise ernboundaryofW28comprisesthreecomponents.Ourstudies
fromtheaccretionofmatterbytheprogenitorstar.Theoutflow withNANTEN12CO(J=1-0)datashowmolecularcloudsspan-
agesareestimatedatbetween 103 to104 yr.Recentobserva- ningabroadrangeinlocalstandardofrestvelocityV =5to
LSR
tions(Klaassenetal.2006)sug∼gestbothoutflowsextendovera 20kms 1,encompassingthedistanceestimatesforW28and
−
∼
combineddistanceof 2 (or 1.2pcatd = 2kpc),withtotal variousstarformationsitesintheregion.Ifconnected,andata
′
kinetic energyof 3.5 ∼1046 er∼g. Surroundingthe outflows is a distance 2kpc,thecloudsmaybepartofalargerparentcloud
verydense(>104cm×3)molecularenvelopeofdiameter0.5 to possibly∼disruptedbyW 28and/oradditionalobjectsrelatedto
− ′
1.Despitethe lackofanymodeltoexplainmulti-TeVparticle theactivestarformationintheregion.Cloudcomponentsupto
′
acceleration in such HII regions, its kinetic energy budget and 4kpcdistance(V >10kms 1)however,remainapossibil-
LSR −
∼
itsspatialoverlapwithaVHEsourcemakesW28A2atempting ity.
candidateforsuchacceleration.Already,therearetwoexamples The VHE/molecular cloud association could indicate a
of VHE emission possibly related to the environments of hot, hadronicoriginfortheVHEsourcesintheW28field.Underas-
young stars — TeV J2032+4130 (Aharonian etal. 2005b) and sumptionsofconnectedcloudcomponentsatacommondistance
HESSJ1023 575(Aharonianetal. 2007c).Inthiscontext,the of 2 kpc, or, alternatively, separate cloud componentsat 2 and
−
HII regions G6.1 0.6 and 6.225 0.569 may also play a simi- 4kpc,ahadronicoriginfortheVHEemissionimpliescosmic-
− −
larroleinHESSJ1800 24A.Amongtheprominentopenclus- ray densities 10 to 30 times the local value. W 28 could
− ∼ ∼
tersin thearea, NGC6523andNGC 6530 0.5 southeastof providesuch densitiesin thecase ofslow diffusion.Additional
◦
∼
HESSJ1800 240,andNGC6514associatedwithM20 0.7 and/oralternativeparticle acceleratorssuch as HII regionsrep-
◦
− ∼
northofHESSJ1801 233mayalsoprovideenergyforCRpro- resentingvery youngstars, otherSNRs/SNR candidatesand/or
−
Aharonianetal.:AssociatedVHEandCOemissionintheW28field 9
several open clusters in the region may also be contributors. GinzburgV.L.&Syrovatskii,S.I.1964,TheOriginofCosmicRays(NewYork:
Alternatively, if cloud components at V >10 km s 1 are at Macmillan)
LSR −
distances d 4 kpc, as-yet undetected particle accelerators in Gabici S.,Aharonian F.A.,BlasiP.2006InProc. ’Multi-messenger approach
∼ to high energy gamma-rays” Barcelona June2006 (Ap.& SS arXiv:astro-
the Scutum-Crux arm may be responsible. Detailed modeling
ph/0610032)
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13 DublinInstituteforAdvancedStudies,5MerrionSquare,Dublin2,
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15 Laboratoire de Physique The´orique et Astroparticules,
IN2P3/CNRS, Universite´ Montpellier II, CC 70, Place Euge`ne
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23 ObserwatoriumAstronomiczne,UniwersytetJagiellon´ski,Krako´w,
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24 NicolausCopernicusAstronomicalCenter,Warsaw,Poland
25 European Associated Laboratory for Gamma-Ray Astronomy,
jointlysupportedbyCNRSandMPG
26 Department of Astrophysics, Nagoya University, Chikusa-ku,
Nagoya464-8602,Japan
ListofObjects
‘W28’onpage1
‘W28A2’onpage4
‘G6.1 0.6’onpage4
−
‘6.225 0.569’onpage4
−
‘GROJ1801 2320’onpage4
−
‘PSRJ1801 23’onpage4
−
‘G7.06 0.12’onpage5
−
‘G5.71 0.08’onpage5
−