A future very-high-energy view of our Galaxy

0.4 MB·English

by  S. Funk

A future very-high-energy view of our Galaxy
S. Funk , J.A. Hinton†, G. Hermann and S. Digel‡
∗ ∗∗
KavliInstituteforParticleAstrophysicsandCosmology,Stanford,CA94025,USA
∗
†SchoolofPhysics&Astronomy,UniversityofLeeds,Leeds,LS29JT,UK
9
Max-PlanckInstitutfürKernphysik,P.O.Box103980,D-69029,Heidelberg,Germany
0 ∗∗
‡KavliInstituteforParticleAstrophysicsandCosmology,Stanford,CA-94025,USA
0
2
Abstract. ThesurveyoftheinnerGalaxywithH.E.S.S.[1,2]wasremarkablysuccessfulindetectingawiderangeofnew
n
very-high-energygamma-raysources.NewTeVgamma-rayemittingsourceclasseswereestablished,althoughseveralofthe
a
J sourcesremainunidentified,andprogresshasbeenmadeinunderstandingparticleaccelerationinastrophysicalsources.In
this work, we constructed a model of a population of such very-high-energy gamma-ray emitters and normalised the flux
3
and size distribution of this population model to theH.E.S.S.-discoveredsources. Extrapolating that population of objects
1
to lower flux levels we investigate what a future array of imaging atmospheric telescopes (IACTs) such as AGIS or CTA

might detect inasurveyof theInner Galaxywithanorder ofmagnitude improvement insensitivity.Thesheer number of
]
A sourcesdetectedtogetherwiththeimprovedresolvingpowerwilllikelyresultinahugeimprovementinourunderstanding
ofthepopulationsofgalacticgamma-raysources.AdeepsurveyoftheinnerMilkyWaywouldalsosupportstudiesofthe
G
interstellardiffusegamma-rayemissioninregionsofhighcosmic-raydensity.Inthefinalsectionofthispaperweinvestigate
. thesciencepotentialfortheGalacticCentreregionforstudyingenergy-dependentdiffusionwithsuchafuturearray.
h
p Keywords: Gamma-rayobservations,AGIS,CTA,IACT,H.E.S.S.,Ground-basedGamma-rayastronomy,GalacticCentre,SgrA*
- PACS: 95.55.Ka,95.85.Pw
o
r
t BUILDINGTHE POPULATION
s
a Radial distribution
[ The first step in the simulation of a future survey of 1.6

1 theGalactic planeistoestablish amodelforthesource 1.4
v population. For this purpose, the sources detected in
1.2
5 the H.E.S.S. survey of the inner Galaxy ( 30 ) were
◦
8 assumed to be attributable to a single pop±ulation and 1
8
thefollowingobservableswerecomparedbetweenthese
1 0.8
sourcesandasyntheticpopulation:
.
1 0.6
0 • Numberofdetectedsources
0.4
9 • Galacticlongitudeandlatitudedistributions
0 0.2
• Gamma-rayfluxdistribution
:
v • Angularsizedistribution 00 2 4 6 8 10 12 14 16 18 20
i
X
Two different source population models have so far
r been tested separately: an SNR-type and a PWN-type FIGURE1. RadialdistributionofsourcesintheSNRmodel
a
model.In thefollowingwe will focuson theSNR pop- ingalactocentriccoordinates.Theunitsonthey-axisarearbi-
trary.
ulationmodel.Tomatchthenumberofsources,thefre-
quencyofSNeinourGalaxywasfixedto10percentury.
For the distribution of Galactic longitudesand latitudes
inourGalaxyasgivenby[4]withtheefficiencyofcon-
of the underlying population we used a 3-dimensional
vertingthattotalexplosionenergyintogammaraysasa
distributionofsourcesinourGalaxy.Theradialdistribu-
singleadjustableparameter.Thelinearsizeofthemodel
tionweretakenfromamodelofofSNRsinourGalaxy
sourcesasafunctionoftimewastakenfromtheSedov-
(see Fig. 1) as given in [3] and the height distribution
solutionforSNRs[5,6].Usingadistributionofdensities
above the galactic disc was assumed to be exponential
ofinterstellargasproportionalton 1 aroundtheSNRs,
withascaleheightof30kpc,approximatelymatchedto −
the gamma-ray flux from p 0-emission was then calcu-
thetypicalscaleheightofmoleculargas.
lated based on the prescription of Drury, Aharonian &
For the distribution of gamma-ray luminosities we
Völk [7]. As mentioned before, in this model, the ad-
usedthemodeloftotalexplosionenergiesofSupernovae
justableparameters(tomatchtheH.E.S.S.distributions)
are:
es 7
c
• SNRTeVgamma-raylifetime Sour 6
• Efficiencyoftransferringexplosionenergyintoki- #
neticenergyofprotons 5
4
Withtheseparametersadistributionofsourcesbased
on the SNR-modelcan be constructedandcomparedto 3
theabovementionedH.E.S.S.distributions.
2
1
MATCHINGTHEH.E.S.S. POPULATION
0
-5 -4 -3 -2 -1 0 1 2 3 4 5
Galactic Latitude (deg)
Thedistributionsofgamma-rayfluxes,galacticlatitudes
and angular sizes of the H.E.S.S. sources in the inner
Galaxywerefittedtotherespectivedistributionsfromthe es
population. It seems clear from the fit (as already sug- urc 18
o
gested in the initial publication about the H.E.S.S. sur- # s 16
vey [2]), that the scale height of the sources above the 14
Galaxymustberathersmalltomatchthenarrowdistri- 12
butionofH.E.S.S.sourcesinGalacticlatitude.Figures2 10
showthedistributioninGalacticlatitudeandinLog(N)- 8
Log(S)forthemodelandtheH.E.S.S.data.TheH.E.S.S. 6
Log(N)-Log(S)distributionfollowsapower-lawwithin- 4
dex -1.Areasonableagreementbetweenthemodeland 2
∼
theH.E.S.S.datacanbeachievedwiththefollowingpa- -013 -12.5 -12 -11.5 -11 -10.5 -10 -9.5 -9
rameters: Log10 (Flux)
• NumberofSNexplosions/century:10 FIGURE2. Top:DistributionofGalacticlatitudesformodel
• SNRTeVgamma-raylifetime:104years (black)andH.E.S.S.data(red).Bottom:Log(N)-Log(S)distri-
butionsformodel(black)andH.E.S.S.data(red).
• ScaleheightoftheSNRdistribution:30pc
• Efficiencyofconvertingexplosionenergyintocos-
micrays:9%
backgroundweightedbyanadjustableparameterwhich
• Averagedensityofsurroundingmedium:0.5cm−3 denotes how much better the background-rejectionof a
futureinstrumentwillberelativetotheH.E.S.S.system.
Withthispopulationofsourceswecanreproducethe
Inadditiontothepopulationofsourceswealsoadddif-
distributionofsourcesandwiththisandtheH.E.S.S.ex-
fusegamma-rayemissionfromp 0-interactionsusingCO
posuremap of the InnerGalaxyand the H.E.S.S. back-
maps [8] and assuming solar system cosmic ray (CR)
groundwe can simulate the survey of the inner Galaxy
densitiesthroughouttheGalaxy.
withbothaH.E.S.S.-likeandafutureinstrument.
Figure 3 shows simulatedmaps forthe populationof
sourcesderivedintheprevioussection,oneforaninstru-
A FUTURE VIEWOFTHE INNER mentwithH.E.S.S.effectiveareas,backgroundrejection
and angular resolution (top) - it should be noted that in
GALAXY
this case instead of using the sources from the popula-
tion, we used the measured parameters of the H.E.S.S.
Usingthepopulationmodelderivedintheprevioussec-
sourcesintheinnerGalaxy(intermsoffluxnormalisa-
tionandextrapolatingthispopulationtolowerfluxes,we
tion,spectralpower-lawindex,positionandextension)to
can now make an educated guess at what a future TeV
enhancethesimilaritytothewell-knownH.E.S.S.survey
gamma-rayinstrumentmight be able to detect in a sur-
picture[2].Thisiswhy,e.g.,weseeashell-typeSNRat
veyof the inner Galaxy.Starting fromthe modelpopu-
the position of RXJ1713.7-3946.Nevertheless, the dis-
lation,wehavetoincludetheinstrumentresponsefunc-
tribution of these parameters for the population model
tion,i.e.,theeffectiveareaandthepoint-spreadfunction
matchesthedistributionofH.E.S.S.sourcesasderivedin
of the instrument (as adjustable parameters) to predict
theprevioussection.ThebottomplotofFigure3shows
thenumberofdetectedgammaraysforthegivenpopu-
a simulated significance map for a future survey of the
lationmodel.WeusetheH.E.S.S.residual(photon-like)
3 Simulation Significance H.E.S.S. (Real Exposure)
2
1
0
-1
-2
Galactic Longitude (deg.)
3 Simulation Significance AGIS/CTA (Flat Exposure)
2
1
0
-1
-2
-3
Galactic Longitude (deg.)
FIGURE3. SignificanceMapsoftheinnerl= 30 andb= 3 forthepopulationmodeldescribedinthispaperusingthe
◦ ◦
± ±
H.E.S.S.background. Top: for an array withthe H.E.S.S.survey exposure and the H.E.S.S.angular resolution and background
rejection.Bottom:foranarraywithafactorof10largerarea,afactorof2betterbackgroundrejectionandafactorof2improved
angularresolution(resultinginafactorof 9improvementinsensitivity).
innerGalaxy.Again,forthebrightestsourcestheparam-
eters of the H.E.S.S. sources have been used, whereas
to extrapolateto lowerfluxeswe used the sourcesfrom
thepopulationmodel.Forthisbottomplot,theexposure
is a maximumof 5 hoursat anygivenpointonthe sur-
vey map was required (i.e., a more even exposure than
intheoriginalH.E.S.S.survey).Forthisparticularmap,
theeffectiveareaofthefuturearraywas10timesthatof
H.E.S.S. and both the angular resolution and the back-
groundrejection were conservativelyimprovedonly by
a factor of 2, resulting in an overallsensitivity increase
byafactorof2 √10 √2 9.
× × ∼
PROBINGENERGY-DEPENDENT
DIFFUSION
In the plots shown in the previous paragraph we have
integrated over the whole energy range of a future
Cherenkov array. A natural extension to that scheme
wouldbe to splitthe energyrangeintoseveralbandsto FIGURE4. Simulatedgamma-raymapintwoenergybands
intheGalacticCentreregion,assumingenergy-dependentdif-
seehowwellspectralparameterscanbemeasured.Asa
fusionofcosmicraysandaburst-likesourceinthecenter.
firstexampleofsuchanapproach,wesplitthesimulated
diffuseemission frominteractionof cosmic rayswithin
interstellargasintheGalacticCenterregionintoseveral
target material distribution (note that the unknown z-
energybandsandshowinFigure4onlythehighestand
distributionwasassumedtobeflat).TheCRdistribution
the lowest range (0.1-0.3 TeV, and E > 3 TeV). With
waschosenasa3-dimensionalGaussianwithenergyde-
this approachwe can test how well a future Cherenkov
pendent width, corresponding to burst-like injection in
instrumentmightstudyenergy-dependentdiffusion-as-
the past(e.g.a SN explosion).Thediffusioncoefficient
sumingacentralsourceilluminatesthecloudsintheGC
normalisationat1TeVwaschosentomatchtheH.E.S.S.
region.
data in the GC regionand an energydependenceof the
For Figure 4, CS data have been used to trace the
formD(E) E0.6 wasassumed.Usinga power-lawin- 7. L.O.Drury,F.A.Aharonian,andH.J.Voelk,A&A287,
∼
jectionenergyspectrumoftheCRswederivedthespatial 959–971(1994),astro-ph/9305037.
distributionofCRsintheGC-region.Usingthisdistribu- 8. T.M.Dame,D.Hartmann, andP.Thaddeus, ApJ 547,
tion of CRs we calculated the distribution of p 0-decay 792–813(2001),arXiv:astro-ph/0009217.
9. S.R.Kelner,F.A.Aharonian,andV.V.Bugayov,Phys.
gamma-rays following the parametrisation of Kelner
Rev.D74,16(2006).
etal.[9].Thegamma-raymapsshowninFigure4have
beensmearedwiththeestimated(energy-dependent)an-
gularresolutionofaCTA/AGIS-likeinstrumentandthe
probabilitydensitydistributionforgamma-raydetection
hasbeenplottedinthetwoenergybandsspecifiedabove.
Amissingstepinthisstudy(tobeaddedinafuturestep)
is the additionof backgroundand the sampling of pho-
tonswithPoissonstatistics.
SUMMARY
This study provides a first glimpse of what a future ar-
rayofground-basedCherenkovtelescopessuchasAGIS
or CTA might be able to study in the inner part of the
Galaxy. We have constructed a populationmodelbased
on a distribution of SNR-like sources that is able to
explain the global properties of the H.E.S.S.-detected
sources.Extrapolatingthispopulationtogamma-rayflux
values below the H.E.S.S.-sensitivity gives a prediction
oftherichnessofobjectsthatwemightbeabletostudy
withafutureinstrument.Dependingontheexactparam-
eters of the simulation we expect 300 sources above
∼
thefluxsensitivitylimitofCTA/AGISfortheinner30
◦
of the Galaxy. Next steps will include trade-off studies
betweenangularresolution,energythresholdand effec-
tiveareatoguidethechoiceofarrayparameters.Detailed
studiesareneededforawiderangeofastrophysicstopics
toeducatesuchaparameterchoice.Wehavebegunwork
onthespecificareaoftheenergy-dependentdiffusionin
theGalacticCentreregion,asasteptowardsthis.
ACKNOWLEDGMENTS
The authors would like to thank the members of the
H.E.S.S. CTA and AGIS collaborations for help and
interestingdiscussions.
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