Unveiling the origin of HESS J1809-193

0.65 MB·

by  G. Castelletti

Astronomy&Astrophysicsmanuscriptno.astroph-castelletti (cid:13)cESO2016
January20,2016
−
Unveiling the origin of HESS J1809 193
G.Castelletti1,,E.Giacani1,2,andA.Petriella1,3
1 InstitutodeAstronomíayFísicadelEspacio(IAFE),UBA-CONICET,CC67,Suc.28,1428BuenosAires,Argentina
e-mail:[email protected]
2 FADU,UniversityofBuenosAires,BuenosAires,Argentina
3 CBC,UniversityofBuenosAires,BuenosAires,Argentina
Received<date>;Accepted<date>
6
1
0 ABSTRACT
2
Aims.ThemaingoalofthispaperistoprovidenewinsightsontheoriginoftheobservablefluxofγraysfromHESSJ1809−193

n usingnewhigh-qualityobservationsintheradiodomain.
a Methods.WeusedtheExpandedVeryLargeArray(nowknownastheKarlG.JanskyVerylargeArray,JVLA)toproduceadeep
J full-synthesisimagingat1.4GHzofthevicinityofPSRJ1809−1917.Thesedatawereusedinconjunctionwith12COobservations

9 fromtheJamesClerkMaxwellTelescopeinthetransitionlineJ=3−2andatomichydrogendatafromtheSouthernGalacticPlane
1 SurveytoinvestigatethepropertiesoftheinterstellarmediuminthedirectionofthesourceHESSJ1809−193.
Results.Thenewradiocontinuumimage,obtainedwithasynthesizedbeamof8′′×4′′andasensitivityof0.17mJybeam−1,reveals

] withunprecedented detailalltheintensitystructuresinthefield.NoradiocounterparttotheobservedX-rayemissionsupposedto
E beapulsarwindnebulapoweredbyPSRJ1809−1917isseeninthenewJVLAimage.Wediscoveredasystemofmolecularclouds
H ontheedgeofthesupernovaremnant(SNR)G11.0−0.0shockfront,whichispositionallycoincidentwiththebrightestpartofthe
TeVsource HESSJ1809−193. Wedetermine, onthebasisof kinematicandmorphological evidences, aphysical linkoftheSNR
.
h withthecloudsforwhichweestimatedatotal(molecularplusatomic)massof∼3×103 M andatotalprotondensityintherange
⊙
p 2-3×103cm−3.
- Conclusions.Weproposeasthemostlikelyoriginoftheveryhigh-energyγ-rayradiationfromHESSJ1809−193ahadronicmech-
o
anismthroughcollisionsofionsacceleratedattheSNRG11.0−0.0shockwiththemolecularmatterinthevicinityoftheremnant.
r
st Keywords. ISM:individualobjects:HESSJ1809−193-ISM:supernovaremnants:G11.0−0.0-ISM:cloudsISM-Radiocontinuum:
a general
[

1
v1. Introduction significanceconsiderablytoavaluewellabove10σ.Thedetec-
2 tionoftheTeVsourceoccursinanareaofradius0◦.25±0◦.02.
6Sincetheirdiscovery,Galacticγ-raysourceshavebeenlinkedto Fitting theexcessmap with a 2D symmetricGaussian, Renaud
9a variety of astrophysicalphenomena like X-ray binaries, star- etal.(2008)estimateditsbest-fitpositionatR.A.=18h09m52s,
4forming material, remnants of supernova explosions, and pul- Dec.=−19◦23′42′′(J2000.0).
0sarwindnebulae(PWNs),amongothers(deNaurois2015).The
.importanceofabroadbandapproachusingmultiwavelengthob- The peak of the VHE emission lies about 6′ to the south
1
0servations for understanding the γ-ray emission processes has of PSR J1809−1917, a middle-aged (τ =51 kyr) and energetic
6beenfirmlydemonstratedoverthecourseof thelast fewyears. (spin-downluminosityE˙=1.8×1036ergs−1)82.7msradiopul-
1Good examplesof this class of studies performedespecially in sar, discovered during the Parkes Multibeam Pulsar Survey of
:the radio and X-rays bands are those presented by Aharonian the Galactic Plane (Morris et al. 2002). Using dispersion mea-
v
etal.(2008),H.E.S.S.Collaborationetal.(2011),Gotthelfetal. suredata,distancesof∼ 3.5and∼ 3.7kpcwerederivedtothis
i
X(2014),ontheγ-raysourcesnamedHESSJ1713−381(linkedto pulsar by Cordes & Lazio (2002) and Morris et al. (2002), re-
rthesupernovaremnant(SNR)CTB37B),HESSJ1731−347(in spectively. In the X-ray domain, on the basis of Chandra ob-
aSNRG353.6−0.7),andHESSJ1640−465(inPSRJ1640−4631, servations, Kargaltsev & Pavlov (2007) reported the detection
SNRG338.3−0.0),respectively.Surprisingly,thereisstillasig- of emission from PSR J1809−1917 and synchrotron emission
nificantfractionof casesforwhich theproductionmechanisms fromtheassociatedpulsarwindnebula.TheoverallX-raymor-
ofγrayshasnotbeenestablished1. phology of the nebula is dominated by a bright inner compo-
In this paper we focus on the very high-energy (VHE) nent, 3′′ ×12′′ in diameter,elongatedin the north-southdirec-
sourceHESSJ1809−193.Itwasdiscoveredduringthesystem- tion(withthepulsarclosetoitssouthend),whichinturnisim-
atic search for VHE γ radiation from pulsars in the Galactic mersed in a halo of lower surface brightness ∼ 20′′ × 40′′ in
plane survey performedby the High Energy Stereoscopic Sys- size and elongatedin the same direction. The energyspectrum
tem (H.E.S.S.)Collaboration(Aharonianet al. 2007).Since its of the PWN is characterized by a power law index Γ = 1.4,
firstdetection,newdatawererecordedincreasingthe detection while its luminosity is L ∼ 4 × 1032 erg s−1 between 0.8 and
7 keV at a distance of 3.5 kpc. The X-ray nebula is also em-
Sendoffprintrequeststo:G.Castelletti beddedina fainterbutstill clearly detectableemission thatex-
1 For alist of the discovered VHE γ-raysources see the online TeV tends∼ 4′ southwardofthepulsar(Kargaltsev&Pavlov2007).
γ-raycatalogTeVCathttp://tevcat.uchicago.edu. Lateron,astudyperformedwiththeSuzakusatelliteconfirmed
Articlenumber,page1of7
A&Aproofs:manuscriptno.astroph-castelletti
theexistenceofdiffusenon-thermalX-rayemissionawayfrom Table1.SummaryofJVLAobservations
the pulsar up to ∼ 20′ (Anada et al. 2010) in the direction of
HESSJ1809−193.Onthebasisofthisdetectionalongwiththe
factthatthepulsarisenergeticenoughtopowerthebulkofthe Configuration Date Hours
observed γ-ray emission, PSR J1809−1917 and its nebula are C 2012Feb5 1.5
considered the most likely counterpartto HESS J1809−193 to C 2012Feb18 1.5
date. The offset of the VHE centroid from the pulsar position B 2012Jun20 1.5
isnotatypicalasis exemplifiedbyVelapulsarPSR B0833−45 B 2012Jun21 1.0
associated with HESS J0835−455 (Aharonian et al. 2006a)
and PSR B1823−13 with HESS J1825−137 (Aharonian et al.
2006b).ThisPWNeoffsetcanbeexplainedbyeitherahighspa-
tial velocity of the pulsar (van der Swaluw et al. 2004) or the
evolution of the SNR blastwave into an inhomogeneous inter- targetwas observedat intervals of ten minutes, each bracketed
stellarmedium(ISM).Thus,anasymmetricreverseSNRshock by the phase calibrator source J1834−1237.The standard cali-
reachesonesideofthePWNsoonerthantheotherside,crushing brator3C286wasusedforfluxandbandpass.Thefluxdensity
thePWN(Blondinetal.2001).Thisscenariohasbeenproposed scalewassetaccordingtothePerley-Butlercoefficientsderived
forthe PSRJ1809−1917/HESSJ1809−193system(Kargaltsev attheJVLAin2010.
&Pavlov2007)andcouldbetestedthroughthedetectionofra-
Data processing for each observing run of the JVLA was
dioemissionfromtherelicelectronsofthecrushedPWN.
fullyperformedwith the CommonAstronomySoftwareAppli-
Remarkably, the presence of other possible sites of ener-
cations(CASA)package.Foreachdataset,afterupdatingthean-
geticparticlessuchasradiosupernovaremnantsandHIIregions
tennapositionsandremovingobviouslycorrupteddatabyhand,
within the extension of HESS J1809−193 hampers a decisive
we determine appropriate complex antenna gains for the cali-
conclusionontheoriginoftheγraysandmakesitnecessaryto
brators, which were then applied to the target data. The final
revisethePWNscenarioconsideredfortheVHEsource.
calibratedvisibility data fromall the available observationsfor
Inthispaper,wereportonnewhigh-qualitysynthesisimag-
eachconfigurationwerecombinedintoasingleuv-dataset,be-
ing obtained from observations carried out with the Expanded
forestartingtheimagingprocess.Duringdeconvolutionweem-
Very Large Array (or the Karl G. Jansky Very Large Ar-
ployed the Multi-Scale Multi-Frequency Synthesis imaging al-
ray as it is now commonly known) of a large region toward
gorithm(Rau&Cornwell2011)tosimultaneouslydealwiththe
HESS J1809−193.The purposesof our research are the detec-
multiple scales and the variationsof the spectral indices of the
tion of the radio counterpartto the proposedX-ray PWN pow-
sourcesinthefield.Inaddition,weusedtheCASAimplementa-
ered by PSR B1809−1917and/or any trace of the alleged host
tionoftheW-Projectionalgorithmtocorrectfortheskycurva-
SNR of this pulsar,aswell as the identificationof otherpoten-
ture.TheimagingprocesswasperformedbyadoptingtheBriggs
tialparticleacceleratorsinthefieldasalternativecandidatesthat
robustparameterequalto 0,a compromisebetweenthe natural
produce the VHE γ-ray emission. Additionally, we outline the
anduniformweightingschemesofvisibilities.
discoveryofmolecularmaterialadjacenttotheSNRG11.0−0.0
AftercombiningthedatafromtheCandBconfigurationsthe
well within the perimeter of HESS J1809−193and analyze its
resultingFWHMofthesynthesizedbeamis8′′.3×4′′.4,P.A.=-
relationship with the production of the detected γ-ray photons
169◦.62, and the noise level is 0.17 mJy beam−1. The final ra-
throughahadronicmechanism.
dio map of the field is shown in Fig. 1. This is the first high-
resolution image of the region towards HESS J1809−193 that
2. Data providesunprecedentedsensitivitytotheradioemissionsurpass-
ingbymorethananorderofmagnitudethesensitivityofavail-
2.1.Newradioobservations able radio images at the same frequency.As can be seen from
thisfigure,thisisacomplexregionofourGalaxy,fullofSNRs
The radio continuum observations at L-band were performed
andHIIregions,allofthemindicatedinthefigure.
under the shared-riskcommissioning phase with the Expanded
Very Large Array (EVLA2) of the National Radio Astronomy
Observatory3initsBandCarrayconfigurationsassummarized
2.2. Surroundingmedium
inTable1(programcode12A-166).Thephasecenteroftheob-
servationswaslocatedatR.A.=18h09m43s,Dec.=−19◦17′38′′ The interstellar medium properties of the region around
(J2000.0).Thecorrelatorwasconfiguredtogivea1GHzwide- HESSJ1809−193wereinvestigatedusingtheneutralhydrogen
band in full polarization. The baseband was comprised of six- (HI) data from the Southern Galactic Plane Survey (McClure-
teen64MHzsub-bandsorspectralwindows(SPWs)containing Griffithsetal.2005)whichmapstheatomicgaswithanangular
64frequencychannels.BothinB-andC-arraydata,significant resolution of 2′, a velocity resolution of 0.82 km s−1, and an
interferencewasfoundespeciallyaffectingtheSPWsinthefre- rmssensitivityof1K.Wealsoexploredthe12COJ=3−2tran-
quency range ∼1.5-1.7 MHz, which was entirely removed. In sition in the region using observations extracted from the data
addition, ten channels at the edges of the SPWs were flagged archive of the James Clerk Maxwell Telescope (JCMT, Mauna
owingtotheroll-offofthedigitalfiltersinthesignalchain.Our Kea, Hawaii). The observations (project ID: M10AU20) were
obtained using the HARP-ACSIS instrument and have angular
2 Hereafter,weusetheacronymJVLAtorefertoeitherdataorimag- and spectral resolutions of 14′′ and 0.42 km s−1, respectively.
ing products obtained from the Expanded Very Large Array observa-
WeusedthereduceddataproducedbythestandardORAC-DR
tionscarriedoutunderourprogramcode12A-166.
3 TheNationalRadioAstronomyObservatory(NRAO)isafacilityof pipelines,whicharepresentedinunitsofcorrectedantennatem-
theNationalScienceFoundationoperatedbyAssociatedUniversities, peratureTA∗. To convertto main beam temperatureTmb, we ap-
Inc. under cooperative agreement with the National Science Founda- plied the relation Tmb = TA∗/ηmb, where ηmb is the main beam
tion. efficiency.FollowingBuckleetal.(2009),weusedηmb =0.6.
Articlenumber,page2of7
G.Castellettietal.: UnveilingtheoriginofHESSJ1809−193
0.38 0.39 0.42 0.47 0.54 0.64 0.75 0.88 1.03 1.21 1.40
0
0
0:
0
9:
1
-
SNR G11.4-0.1
0
0
5:
0
0
0
0:
1
SNR G11.1+0.1
0
0) 5:0
0. 1 PSR J1809-1917
0
0
2
J
n ( 00 HII G011.034+0.062
o 0: HHEESSSS JJ11880099--119933
ati 2
n HII G011.034+0.061
cli
De 00 SNR G11.0-0.0 HII G010.957+00.022
5:
2
0
0
0:
3 HII G010.877+00.084
0
0
5:
3
0
0
0:
4
11:00 30.0 18:10:00 30.0 09:00 08:30
Right Ascension (J2000.0)
Fig.1.Radiocontinuumimageat1.4GHzoftheregionsurroundingthepulsarPSRJ1809−1917constructedusingtheCandBconfigurations
oftheJVLA.TheintensityscaleisbasedonasquarerootrelationinunitsofmJybeam−1.Thefinalbeamsizeis8′′.3×4′′.4withaposition
angleof∼−170◦.Thenoiselevelis0.17mJybeam−1 withoutconsideringattenuationoftheprimarybeam.ThenamesofSNRsandHIIregions
lyingwithinthefieldareindicated.ThelocationofPSRJ1809−1917isshownwithayellowplussign,whilethebest-fitpositionofthecentroid
ofHESSJ1809−193isindicatedwithagreencross.ThewhitedashedcircledenotesthemeasuredextensionoftheVHEsourceintermsofthe
sigma(rms)ofasymmetric2DGaussianfittotheTeVdata(Renaudetal.2008).
3. Resultsanddiscussion ris et al. 2002), we obtain a pulsar magnetic field strength
B =3.2×1019(PP˙)1/2=1.5×1012Gingoodagreementwiththat
p
3.1. TeVemissionfromaPWN derived for most young pulsars in SNRs (Gaensler & Slane
2006).On the otherhand,to quantifythe propertiesofthe am-
Figure 1 shows the region covered by the bient medium where the pulsar is propagating we inspected
PSR J1809−1917/HESS J1809−193 system. From the new the structure of the HI emission in a large region surrounding
image no evidence is found, down to the noise level of our PSR J1809−1917in the velocityinterval28-31km s−1 (all the
data, for the radio counterpart to either the X-ray PWN or the velocitiesmentioned in this work are relative to the local stan-
surroundingradioshellassociatedwithitshostSNR. dard of rest) corresponding to the distance range estimate to
TherearemanypulsarsforwhichthesearchofradioPWNe PSRJ1809−1917of3.5-3.7kpc(accordingtotherotationcurve
yieldednegativeresults(e.g.,Gaensleretal.2000;Giacanietal. model of the Milky Way of Fich et al. 1989, with the galac-
2009,2014).Inthesecases,thefailuretodetectthenebularemis- tic distance R0=8.5 kpc and the rotation velocity at the Sun
sion has been explained as a consequence of two main phys- Θ=220 km s−1). Figure 2 shows a HI large-scale feature sur-
ical conditions based on either a high magnetic field that in- roundingthepulsar,whichatfirstglancelookslikeablownwind
hibits the production of synchrotron radiation at longer wave- bubblegeneratedby the stellar wind of the progenitorstar. We
lengthsorsevereadiabaticlossesthatoccurinyoungandener- crudelyestimate the volume density of the ambientatomic gas
geticpulsarslyinginaverylowambientdensity(∼0.003cm−3). in the cavity by integrating the column density distribution NH
In the case of PSR J1809−1917, using the parameters previ- withinthementionedvelocityrangeandassumingforthecavity
ously derived for this object (P=82.7 ms, P˙=2.55×1014, Mor- asphericalgeometrywitharadiusof∼9′atadistanceof3.6kpc
Articlenumber,page3of7
A&Aproofs:manuscriptno.astroph-castelletti
calsimulationsof thisprocessestimate a crushingtimescaleof
≤ 103E−1/2(M /M )5/6η−1/3 yr,whereE isthetotalmechan-
51 ej ⊙ 51
icalenergyof the SN explosionin unitsof 1051 erg, M is the
ej
ejectedmass,andηistheambienthydrogendensityincm−3(van
65 70 74 79 83 88 93 97 102 106 111
derSwaluwetal.2004).InthecaseofPSRJ1809−1917,ifwe
assume that E is equal to unity, M =10 M and we use the
51 ej ⊙
0
0 hydrogendensity derivedfromour study of the HI distribution
0:
9:0 in the region of the pulsar (η=1.5 cm−3), we obtain a crushing
1
- timescale of ∼6 kyr. This time is considerably lower than the
characteristic age of the pulsar even if we consider an initial
0)
J2000.20:00 reenqeurgiryesotfhteheexeixstpelnocseioonfoafS1N05R0,ebrgu.tTtahkiisngreilnictoPcWonNsisdceernaatiroion
on ( thecharacteristicageofPSRJ1809−1917,theradioemissionof
nati theallegedremnantmighthavedissipatedthroughtheactionof
Decli0:00 radiativeshocks, makingthe SNR forwardshock undetectable.
4 Thus,a relic TeV PWN originfor HESSJ1809−193cannotbe
dismissed. Itshouldbe pointed,however,outthatthis scenario
00 is not completely consistent with the location of the pulsar in
00: the cometary shaped nebula observed in X-rays in the vicinity
0:
-2 ofPSRJ1809−1917.Indeed,asnoticedbyAnadaetal.(2010)
13:00 12:00 11:00 18:10:00 09:00 08:00 07:00
bothChandraandSuzakuobservationssuggestthatthepulsaris
Right Ascension (J2000.0)
movingtowardsthecenteroftheHESSJ1809−193source.
Fig.2.NeutralhydrogendistributiontowardsPSRJ1809−1917andits
nebula. The overlaid contours represent the HI emission at 100, 103,
3.2.TeVemissionfromahadronicmechanism
107, 110, and 120 K. The blue dashed annulus approximately delim-
itsthehigher densitygasof theregion,whiletheplussignmarksthe
Inoursearchforanalternativeexplanationoftheobservedγ-ray
positionofPSRJ1809−1917.
emission,the SNR G11.0−0.0thatlies in projectionwithin the
brightest region of the VHE source appears to be a promising
(an average between the 3.5 and 3.7 kpc). After subtracting an candidatefortheaccelerationofparticlesviaπ0-decayemission
appropriatebackgroundleveltoaccountfordiffuseemissionthat arisingfromproton-protoncollisions.Inthiscontext,wesearch
maycomefromfargaswhoseemissionisdetectedatthesame for spatial correspondences between matter concentration and
radialvelocitiesinthisdirectionoftheGalaxy,theobtainedam- theγ-rayphotonsobservedinHESSJ1809−193.
bientmediumdensityis∼1.5cm−3. Regardlessofwhetherthis Afteracarefulinspectionofthewhole12CO(J=3−2)data
HIstructurerepresentstheimprintleftintheISMbyapastevent cube in the environment of HESS J1809−193, we identified
that ended in a SN explosionor simply inhomogeneitiesin the molecularmaterialatdifferentvelocitiesthatshowstightspatial
medium,theobtainedvaluerulesoutthepossibilityofanexpan- correlation with both the SNR G11.0−0.0 and the γ-ray peak.
sioninanareaofverylowdensity.Weconsequentlynotethatthe Theobservedmolecularcloudsarelimitedtothevelocityinter-
conditionsinvokedtoaccountforthelackofanassociatedradio vals(+16kms−1, +27kms−1) and(+71kms−1, +78kms−1).
nebulafailtoexplainthecaseofPSRJ1809−1917. Figure 3(a) shows the distribution of the 12CO emission inte-
Nevertheless, we can still speculate that if the radio pulsar gratedinthelowervelocityrangeoverlaidwithcontoursdelin-
wind nebula existed and if it were at least 4′ across (that is, a eatingthelocationoftheSNRG11.0−0.0.Atleastthreeclouds,
sizesimilartothatofthefaintestX-rayemissionobservedwith whichwe refertoin thisworkasA,B, andCforconvenience,
Chandra),thentheresultingupperlimitforthenebularfluxden- canbedistinguishedinthelarge-scaledistributionofthemolec-
sity at 1.4GHz – consistentwith ournon-detection(ata sensi- ular gas. These components seen in projection on the plane of
tivity of 0.2 mJy beam−1) – would be ∼250 mJy. Assuming a theskyoverlapthewesternouterborderofG11.0−0.0inallits
typicalPWNradiospectralindexα=−0.3(S∝ να,Gaensler& extension. Feature A overlaps the flattened and intense region
Slane 2006),we find thatthe mentionedfluxat1.4GHz corre- of the radio shell towards the south and delineates the western
spondstoabroadbandradioluminosityintegratedbetween107 borderoftheSNR.CloudBcoincideswiththebrightestregion
and1011HzofL ∼1.4×1032ergs−1(atadistanceof3.6kpc) of HESS J1809−193 (depicted as a 5σ significance contour in
R
and to an efficiencyǫ ≡ L /E˙∼ 7.7×10−5; both estimates are Fig.1ofKominetal.2008)andoneofitsextremesisincontact
R
significantlylowerthanthetypicalvaluesof L ∼ 1034 ergs−1 with the northwestern rim of G11.0−0.0. Finally, the molecu-
R
andǫ ≥ 10−4 observedinotherradioPWNe(Gaensler&Slane lar componentC, while located in the northern extreme of the
2006). However, this analysis should be taken with caution as remnant,stilllieswithintheextensionoftheγ-rayemission.We
exceptions exist; for example, the pulsars PSR B1706−44 and adopt∼21kms−1asthesystemicvelocityofthemolecularmate-
PSRB0906−49bothpowerobservableradionebulaewithvery rial;becauseoftheso-calledkinematicdistanceambiguityinthe
lowefficienciesofabout10−6(Giacanietal.2001). innerGalaxy,thisresultsinanearandfarkinematicdistancesof
As mentioned in Sect. 1, PSR J1809−1917 appears to be ∼2.9and∼13.6kpc,respectively.Figure3(b)showstheintensity
the most viable counterpart to the HESS J1809−193 source in ofthe12COintegratedinthevelocityrangeof71-78kms−1,su-
a leptonic scenario where the reverse shock from a surround- perimposedwithsomecontoursoftheradiocontinuumemission
ing SNR has collided with the entire shock surface bound- from G11.0−0.0.From this image, a good correspondencecan
ing the nebula powered by PSR J1809−1917. After this phase be seen between the flat western boundary of the remnant and
an offset nebula is formed and HESS J1809−193 may corre- the moleculargas.Interestingly,partof the cloudprojectsonto
spond to such nebula emitting in the VHE domain. Numeri- thebrightestγ-rayemission.Forthiscloudweadoptasystemic
Articlenumber,page4of7
G.Castellettietal.: UnveilingtheoriginofHESSJ1809−193
2.4 0.93
(a) (b)
5:00 Cloud C 2.3 0.85
1
0.76
0 Cloud B 2.1
0
0)20: 0.68
0.9: 1.9
01
0-
2 0.60
nation (J25:00 SNR G11.0-0.0 11..67 0.52
ecli 0.43
D
00 1.4
0: 0.35
3 Cloud A
1.2
0.27
0
0
35: 1.1 0.18
0.89 0.10
40.0 20.0 18:10:00.0 40.0 20.0 09:00
Right Ascension (J2000.0)
0.72
Fig.3.Distributionofthe12COJ=3−2gasinthefieldcontainingtheSNRG11.0−0.0andtheTeVsourceHESSJ1809−193.a)Averagedintensity
distributionofthe12COinthevelocityrangefrom16to27kms−1.b)Averagedintensitymapofthemoleculargasforvelocitiesbetween71and
78kms−1.Inbothpanelsthewhitecontoursdelineatetheradiocontinuumemissionat1.4GHz.Thewedgetotherightofeachpanelshowsthe
12COintensitiesinKkms−1.
velocityof∼75kms−1,whichcorrespondstonear/fardistances
)
of∼6/11kpc. nits
FollowingRoman-Duvaletal.(2009)weresolvedthekine- u
y 20
maticdistanceambiguityforeachcloudusingthe12COinforma- ar
r
tioninconjunctionwiththecoldatomichydrogengasembedded bit
within the molecular cloud. Because HI is present throughout ar
(
the Galaxy, if the cloud is located at a near distance, a corre- ure 0
lation between the 12CO emission line from the cloud and an at
r
HIself-absorptionfeatureshouldbenoticedinthespectrumbe- pe
m
causetheatomicgasintheinteriorofthecloudiscolderthanthe e
T
rest of the Galactic atomic hydrogen located behind the cloud
s-20
s
alongthelineofsight.Onthecontrary,noabsorptionfeaturein e
the HI spectrumshouldbe presentfora cloudplacedat the far ghtn 12HCIO
kinematicdistancebecausethereisnowarmGalacticHIatthe ri
B
velocity of the molecular cloud to be absorbed. From the rota- -40
tion curve it is evident that the warm HI that could be located 0 20 40 60 80 100
beyondthefarmolecularcloudcanonlybeassociatedwithve- Velocity (km s-1)
locities completely different from the velocity of the cloud of
interest. In Fig. 4 we show the HI 21 cm and 12CO spectra in Fig.4.12COandHI21cmspectratowardsthemolecularmaterialiden-
the direction of the molecular clouds identified at ∼21 km s−1 tifiedinthefieldof SNRG11.0−0.0.Thepeakbrightnesscentered at
∼21kms−1 correspondstothenearbycloudslocatedat∼3kpc,while
and ∼75km s−1. At the lowervelocity,the HI 21cm spectrum
thepeakat∼75kms−1iscausedbybackground12COgaslocatedatthe
exhibits a deep self-absorption throughoutthe molecular emit-
farkinematicdistance∼11kpcinthelineofsight.
ting regionandhencewe assign to this cloudthe near distance
(d∼3 kpc). Conversely, the emission feature in the HI 21 cm
spectrum centered at 75 km s−1, which is in coincidence with
a peakinthe12CO emission,revealedthatthiscloudislocated sition(Bolattoetal.2013).Themassforeachcloudisthende-
atthefarkinematicdistance(d∼11kpc). terminedthroughthe relation M = µmHΣ[d2ΩN(H2)],with µ
WeestimatethemassofthestructuresA,B,andCobserved beingthemeanmolecularmassequalto2.8assumingarelative
at ∼21 km s−1 and the cloud identified at ∼75 km s−1. To do heliumabundanceof25%, mH the massof the hydrogenatom,
Ωtheangularareasubtendedbythesource,anddthedistanceto
this,wefirstcalculatetheH columndensityusingtheempirical
2
relation N(H ) = X ×W(12CO), where W(12CO) is the inte- thecloud.Inordertoestimatethetotalambientprotondistribu-
2 CO
tion,weextendouranalysisoftheISMpropertiesbyexploring
gratedlineCO fluxandX is theCO toH conversionfactor
CO 2
forthe12COJ=3−2transition.Byassumingthatthegasisinlo- thecontributionoftheatomicgastothemassanddensityofthe
calthermodynamicequilibrium,itmeansthatallmolecularlev- cloudsdefinedbythevelocityrangesusedforthe12COdata.Un-
elsareexpectedtobeequallypopulated,andhencewecanuse dertheassumptionthatthe neutralatomicgasis opticallythin,
XCO =(2±0.6)×1020cm−2Kkms−1,whichisthecanonicalcon- theHIcolumndensityisgivenasN(HI)=1.823× 1018 R Tbdv
versionfactorbetweentheH andthe12COintheJ=1−0tran- (Dickey & Lockman 1990), where T [K] and v[km s−1] repre-
2 b
Articlenumber,page5of7
A&Aproofs:manuscriptno.astroph-castelletti
sent the intensity and velocity of the neutral gas, respectively. Thequestiontobeaddressed,therefore,iswhetherthecloud
Thevolumedensitieswereestimatedassumingellipticrepresen- componentstracedbythe12CO emissionat∼21kms−1 canbe
tationsofeachmolecularcloudwithadepthcomparabletothe considereda plausibleastronomicalenvironmentforproducing
sizeoftheminoraxis.Theindividualestimatesforthemassand thedetectedphotonsinHESSJ1809−193.Topursuethispoint
density of all the clouds are summarized in Table 2. The error we first calculate the γ-ray flux from HESS J1809−193taking
in the cloud masses are estimated to be on the order of 45%, intoaccountthatthespectrumofthissourceisfitwellbyapure
which mainly arises from the uncertainty in distance measure- power law (dF (E)/dE=N (E/1 TeV)−Γ) with a photon index
γ 0
ments(∼30%). Γ=2.23andN =6.4×10−12TeV−1cm−2s−1(Renaudetal.2008),
0
Before discussing the possible role of the identified molec- whichimpliesFγ(>1TeV)=5.2×10−12phcm−2s−1.Then,we
ular material as target ISM protons to produce the observed quantifytheambientdensityrequiredtopowertheobservedγ-
γ rays, we estimate the distance to SNR G11.0−0.0 using the rayfluxthroughtheEq.(16)ofTorresetal.(2003),givenoures-
HI absorption technique. While G11.0−0.0 itself is relatively timateofthedistancetothemolecularmaterial(d∼3kpc)along
weak and has a thin shell, we construct the emission spectrum with other assumptions such as an efficiency θ=3% to convert
againstthesouthernpartoftheSNRshellwherethereisadom- theacceleratedhadronsintocosmicraysandasupernovaenergy
inantbrightfeature.Theabsorptionprofilewasobtainedbysub- outputof1051erg.Wefindthatatadistanceof3kpcanambient
tractingthebrightradioshellemissionfromnearbyregionsde- densityofη3kpc∼40 cm−3 itwouldbe necessaryto producethe
void of continuumemission. Both spectra are shown in Fig. 5; γ-rayemissiondetectedabove1TeV.Bycomparingthisdensity
the most positive-velocityreliable absorptionfeature is seen at withthatmeasuredinthemolecularmaterialcomprisingClouds
∼20kms−1.Regardingthelowerlevelabsorptiondipintheve- A,B,andC,weconcludethatitisdenseenoughtogeneratethe
locityrange∼50-65kms−1,webelievethatinviewofitsextent VHE emission hadronically.We also infer an expectedvolume
and its association with relative low-amplitude emission, this densityη11kpc∼470cm−3forthemolecularmaterialidentifiedat
featuremayrepresentfluctuationsintheHIemissionandhence ∼75kms−1,whichfavorsourinterpretationthatthisbackground
wearenotconfidentthattheabsorptionisreal.Additionally,no gasatd∼11kpcisnotrelatedtotheproductionofγ-raysphotons
absorptionisobservedatnegativevelocities.Therefore,wecon- inHESSJ1809-193.
siderthevalue20kms−1asalowerlimittothesystemicvelocity
ofG11.0−0.0,correspondingtonearandfarkinematicdistances
of ∼3 and ∼13.8 kpc, respectively. However, if we regard the 4. Summary
featureat50-65kms−1asreliable,theabsenceofabsorptionbe-
tweenthesevelocitiesandthetangentpointat∼167kms−1indi- Atpresent,themostplausibleoriginfortheγ-rayemissionfrom
catesthatanupperlimitonthedistancetotheSNRof∼5.2kpc HESSJ1809−193hasbeenexplainedbyaPWNpoweredbythe
can be adopted.The distanceof ∼17kpc asderivedbyBrogan pulsarPSRJ1809−1917;however,thereisnoconclusiveobser-
et al. (2004) using the Σ-D correlation between the linear di- vationalevidencetosupporttheproposedleptonicscenario.This
ameter of the G11.0−0.0 shell and its radio surface brightness motivatedusto performthe first high-sensitivityradiostudyof
isnotconsistentwiththerangeofpossiblevalues(3.0-5.2kpc) a large region containing the TeV source at 1.4 GHz using the
thatwehavedeterminedforSNRG11.0−0.0throughHIabsorp- JVLA.Inaddition,weanalyzedtheISMofthisarea.Although
tion.Suchadisagreementisnotsurprising,sinceithaslongbeen ourradioobservationssurpassthe sensitivity of available radio
knownthattheΣ-Drelationshiphasalargeintrinsicdispersion. imagesby morethanan orderofmagnitude(noise levelof our
image ∼0.17 mJy beam−1), there is no trace of radio emission
associatedwiththeX-rayPWNdrivenbyPSRJ1809−1917nor
anindicationoftheexistenceofa hostSNR. We alsoanalyzed
the 12CO in the vicinity of HESSJ1809−193to investigatethe
)
K
possibilityofanyothercandidateinthefieldofviewthatcould
re (100 explaintheproductionmechanismofthehigh-energyγ-rays.As
u
at a result of this analysis, we discovered a system of molecular
per clouds(centeredatv∼21kms−1)thatincludesthreemaincom-
m
e 50 ponents(calledinthisworkcloudsA, B,andC).Thismaterial
T
iswellcorrelatedwiththeTeVsourceandmatchesthemorphol-
s
es ogy of the western edge of the SNR G11.0−0.0. The distance
n
ht to these clouds was constrained to be ∼3 kpc based on the as-
g 0
ri sociation between 12CO emission and HI-self absorption fea-
B
tures, which also coincides within uncertainties with the kine-
maticaldistancederivedforG11.0−0.0.Wecalculatedthetotal
-100 -50 0 50 100 150 200 massandtotalprotondensityoftheA,B,andCclouds(taking
Velocity (km s-1) intoaccountcontributionsfrombothHIand12CO,thevaluesare
M ≃ 3×103 M andη ≃ 7.4×103cm−3,seeTable2),whichwe
⊙
Fig. 5. HI emission (thin line) and absorption (thick line) spectra to- assumedareinteractingwiththeremnantG11.0−0.0,andfound
wardsSNRG11.0−0.0.Thedashedlineindicatesabsorptionatthe5σ
thattheyindeedsatisfytherequiredamountoftargetmaterialto
level,whereσisthenoiselevelintheHIdatacube.
explaintheobservedTeVγ-rayfluxfromHESSJ1809−193as
producedbyhadrons.
Takingintoaccountthegoodmorphologicalcorrespondence We finally note that our analysis does not rule out a hy-
betweentheremnantG11.0−0.0andthemolecularmateriallo- brid scenario in which contributions from relativistic leptons
catedatthesamesystematicvelocity,wethereforeargueaphys- via inverse Compton process in the PSR J1809−1971 operate
icalconnectionbetweenthem.Thisallowsustoresolvetheam- together with the hadronic process that we have proposed in
biguityofthedistancetoG11.0−0.0:itislocatedatabout3kpc. thiswork.Inaddition,star formationactivitycanplayarolein
Articlenumber,page6of7
G.Castellettietal.: UnveilingtheoriginofHESSJ1809−193
Table2.Derivedparametersforthemolecularclouds
Feature CentralPosition N N Mass Totalprotondensity
H2 H
R.A.,decl.(J2000.0) ×1021[cm−2] ×1021[cm−2] ×103[M ] ×103[cm−3]
⊙
CloudAat21kms−1 18h09m46.3s,−19◦26′53′′ 5.3 2.2 0.8 1.9
CloudBat21kms−1 18h09m41.8s,−19◦21′38′′ 9.6 2.1 1.3 3
CloudCat21kms−1 18h09m51.3s,−19◦17′45′′ 6.4 2.5 0.7 2.5
Cloudat75kms−1 18h09m41.5s,−19◦24′33′′ 1.5 0.3 4.2 0.1
HESS J1809−193 as several bright HII regions lie within the
boundaryoftheVHEsource.
Acknowledgements. This research was partially funded by Argentina Grants
awarded byANPCYT: PICT 0571/11 and 0902/13; CONICET:0736/11; and
UniversityofBuenosAires(UBACYT).G.C.,E.G.,andA.P.areMembersof
theCarreradelInvestigadorCientíficoofCONICET,Argentina.
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