High Energy Gamma Rays from Nebulae Associated with Extragalactic Microquasars and Ultra-Luminous X-ray Sources

0.18 MB·

by  Yoshiyuki Inoue

High Energy Gamma Rays from Nebulae Associated with Extragalactic Microquasars and
Ultra-Luminous X-ray Sources
YoshiyukiInouea,Shiu-HangLeea,b,YasuyukiT.Tanakac,ShogoB.Kobayashid
aInstituteofSpaceandAstronauticalScienceJAXA,3-1-1Yoshinodai,Chuo-ku,Sagamihara,Kanagawa252-5210,Japan
b2DepartmentofAstronomy,KyotoUniversity,Kitashirakawa,Sakyo-ku,Kyoto606-8502,Japan
c3HiroshimaAstrophysicalScienceCenter,HiroshimaUniversity,1-3-1Kagamiyama,Higashi-Hiroshima,Hiroshima739-8526,Japan
dDepartmentofPhysics,TheUniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo113-0033,Japan
7
1
0Abstract
2
Intheextragalacticsky,microquasarsandultra-luminousX-raysources(ULXs)areknownasenergeticcompactobjectslocating
n
at off-nucleus positions in galaxies. Some of these objects are associated with expanding bubbles with a velocity of 80–250
a
Jkms 1. Weinvestigatetheshockaccelerationofparticlesinthoseexpandingnebulae. Thenebulaehavingfastexpansionvelocity
−
1 &120kms−1areabletoacceleratecosmicraysupto 100TeV.If10%oftheshockkineticenergygoesintoparticleacceleration,
∼
3powerfulnebulaesuchasthemicroquasarS26inNGC7793wouldemitgammaraysuptoseveraltensTeVwithaphotonindex
of 2. These nebulae will be good targets for future CherenkovTelescope Array observationsgiven its sensitivity and angular
] ∼
Eresolution. They would also contribute to 7% of the unresolved cosmic gamma-ray background radiation at 0.1 GeV. In
∼ ≥
Hcontrast,particleaccelerationinslowlyexpandingnebulae. 120kms−1 wouldbelessefficientduetoion-neutralcollisionsand
resultinsofterspectraat&10GeV.
.
h
pKeywords: astroparticlephysics,ISM:bubbles,gammarays:ISM,stars: blackholes
-
o
r
t1. Introduction off-nucleuspositions[27].
s
a
[ In the veryhighenergy(VHE;& 50GeV) gamma-raysky, Microquasars are X-ray binary systems having relativistic

a few hundreds of objects have been detected by the Large bipolar outflows or jets whose kinetic power is greater than
1
vAreaTelescope(LAT)onboardtheFermigamma-raytelescope 1039 erg s−1 [63]. ULXs are also compact X-ray binary sys-
2(Fermi) [7] and by the imaging atmospheric Cherenkov tele- temshavingX-rayluminositiesgreaterthan1039 ergs−1 [27].
8scopes[seee.g.84]. Furtherprogressisanticipatedinthenear AlthoughX-rayemission mechanismsfrom microquasarsand
8future by the Cherenkov Telescope Array (CTA) [8]. Its im- ULXsarenotfullyunderstoodyet2,someofthemareknown
8provedfluxsensitivityandangularresolutionwillenableusto to be associated with expanding nebulae with a velocity of
0
1.unvIneilthneewexptarartgiacllaecatcicceslkeyra,tovrasriionutshesoUunrciveercslea.sses have been 8k0ine−ti2c5p0owkmerso−f1thaonsdeanesbizuelaoefis∼kn2o0w0nptco[b2e0c,o6m9]p.arSabinlecetothoer
0
considered for future CTA observations such as active galac- evengreaterthantheradiativeorjetkineticpower[20],theneb-
7
tic nuclei [76], gamma-ray bursts [38], star forming galaxies ulae are good candidatesas new cosmic-ray acceleration sites
1
:[3],andclusterofgalaxies[3]. Here,theangularresolutionof and possibly they would emit gamma rays through hadronu-
v
CTA will achieve . 3 arcmin at & 1 TeV1. As angular sizes clearinteractionswithambientgases.
i
Xofnearbygalaxiesupto 10Mpcis 10arcmin,itwouldbe
∼ ∼ Inthispaper,weinvestigatethediffusiveshockacceleration
rpossibletospatiallyresolveparticleacceleratorsinthosegalax-
a of particles in the expanding nebulae associated with extra-
ies.
galactic microquasarsand ULXs. And, we estimate expected
It would be difficultto detect supernovaremnantsor pulsar
gamma-ray and neutrino signals from those nebulae. We fur-
wind nebulae in extragalacticgalaxies considering their lumi-
ther consider their contribution to the cosmic gamma-ray and
nosities (L . 1036 ergs 1). Here, some galaxies are known
γ −
neutrinobackgroundradiationandfuturedetectabilitybyCTA.
to host more powerful compact objects such as microquasars
Throughoutthispaper,wedefineQ =Q/10x.
and ultra-luminousX-ray sources (ULX) whose kinetic or ra- x
diativepoweris&1039ergs 1. Theseobjectsarediscoveredat
−
Emailaddresses:[email protected](YoshiyukiInoue), 2There are three distinct ideas widely considered to interpret high X-ray
[email protected](Shiu-HangLee), luminositiesofULXs,althoughthereisnogeneralagreementontheirnature;
[email protected](YasuyukiT.Tanaka), sub-ortrans-Eddington accretion ontointermediate massblackholes witha
[email protected](ShogoB.Kobayashi) massofMBH 10M [e.g.59],supercriticalmassaccretionontostellarmass
1https://portal.cta-observatory.org/Pages/CTA-Performance.aspxblackholes[e.≫g.30,5⊙1],oraccretingpulsars[e.g.11].
PreprintsubmittedtoAstroparticlePhysics February1,2017
2. X-raySourceEmbeddedBubblesinthelocalUniverse Here, the magnetic field in the shock upstream can be de-
scribedas
The size of the microquasar and ULX bubbles Rb is of an Bu =1.7b√ngas,0.5µG, (8)
order of 200 pc and the expansion velocity of the bubbles is
knowntobev =80 250kms 1[20,69]. Followingtheself- wherethedimensionlessparameterbis 1fromZeemanmea-
s − ∼
− surements of self-gravitating molecular clouds in the Galaxy
similarexpansionlaw[47,86],thebubblesizecanbedescribed
[19]. Themagneticfieldinthedownstreamregionwillbeam-
as
plifiedbycompressionas
R 0.76(P /µm n )1/5t3/5 200P0.2 n 0.2 t0.6 pc,
b ≈ kin p gas ∼ kin,40.5 −gas,0.5 13.5 (1) B = 1 + 2r2B 5.7n1/2 µG. (9)
where P isthetime-averagedkineticpower,µ = 0.61isthe d r3 3 u ∼ gas,0.5
kin
mean molecularweight, m is the protonmass, n is the gas
p gas Here, if Alfve´n-wave turbulence is fully generated, B
d
density,andtistheageofthesystem. Thus,thecharacteristic
can be amplified locally around the shocks [see e.g. 12–
ageofthebubbleis
14, 75]. In such cases, B would become as high as B
d d
(4πξ f µn m v2)1/2 74ξ1/2f1/2 n1/2 v µG, where ξ≈
τ=3Rb/5vs ∼4.9×105Rb,20.8v−s,17.1yr. (2) istheBmioangnegatiscfipelsdam∼plificaBti,o0niofna,c0tograsa,0n.5dsf,7.1 n /(n +nB)
ion n n i
≡
This age is comparable to the expected ULX lifetime [e.g. istheionizationfraction. nn andni istheneutralparticleden-
42, 62]. Once we measure the bubble size and the expanding sityandtheionizedparticledensity,respectively. However,we
velocity,thetime-averagedkineticpowerofthebubblecanbe note that ion-neutralcollisions suppress Alfve´n wave [56, 74]
describedas at vs . 120 kms−1. In this paper, we take Equation9 as the
fiducialvalueforthedownstreammagneticfield.
P 18µm n R2v3 3.6 1040R2 v3 n ergs 1. Thediffusiveshockaccelerationtimescalecanbewrittenas
kin ≈ p gas b s ∼ × b,20.8 s,7.1 gas,0.5 −
(3)
10ηcr
TheMachnumberoftheshockduetotheexpandingnebulae tacc ≈ 3 v2g ∼1.3×106Z−1n−ga1s/,02.5v−s,27.1Ecr,14yr, (10)
isestimatedas s
wherer = E /ZeB isthegyroradiusandη 1. Forsimplic-
M≈vs/(γkBTu/µmp)1/2 ∼8.0vs,7.1Tu−,14/2, (4) ity, wegtake ηcr = 1din this paper, i.e. the B≥ohm limit, since
ULXs are known to be associated with star forming regions
where γ = 5/3 is the specific heat ratio, k is the Boltzmann
B [e.g.33,72,81]whichwouldamplifytheupstreamturbulence.
constant,andT isthetemperatureoftheupstreamoftheshock.
u SuchanefficientaccelerationispossiblyseeninaGalacticsu-
is larger than unity for the upstream cold gas component
M pernova remnant by assuming a simple diffusive shock accel-
withT < 104 K.Thedynamicaltimescaleinwhichtheshock
u eration model [83], althoughη can be different from unity by
dissipatesis
considering escape-limited acceleration [34, 68] or stochastic
R acceleration[26].
t b 8.1 105R v 1 yr. (5)
dyn ≈ vs ∼ × b,20.8 −s,7.1 Theattainablemaximumcosmic-rayenergyisgivenbytacc =
t (SeeEquation5andEquation10):
dyn
Thedownstreamtemperatureisestimatedas
3ZeB R v
E = d b s (11)
T 2(γ 1)µm v2/k /(γ+1)2 2.0 105v2 K. (6) cr,max 10ηc
d ≈ − p s B ∼ × s,7.1
1.3 1014η n1/2 v R eV, (12)
Thus, radiative cooling plays an important role. For a solar ∼ × 0 gas,0.5 s,7.1 b,20.8
metallicityinterstellarmedium(ISM)environment,thecooling TeVcosmicrayscanbeaffordablefromtheextragalacticX-ray
timescaleisgivenby[Eq. 34.4of24] source nebulae. If the Alfve´n-wave turbulence is fully gener-
ated,itwillreachto 1PeV.
t 1.4 103n 1 v17/5yr, (7) ∼
rad ∼ × −gas,0.5 s,7.1 The ionization state in the shock downstream strongly de-
pendsonitsvelocity[36,56,74]. Ifneutralparticleexists,i.e.
for 105 . T . 107.3 K. If the metallicity is 0.1Z , the time
v . 120 kms 1, ion-neutral collisions will lead damping of
⊙ s −
scale will become a factor of 3 longer. From Equation5 and
themagneticturbulenceintheshockprecursorandthosecolli-
Equation7, the downstream plasma cools within dynamical
sionswillhamperaccelerationofparticlesatthehighestener-
timescale. In other words, these nebula shocks are radiative.
gies. The expectedbreakenergycan be estimated as [60, and
UV photons from the radiative zone would significantly ion-
referencestherein]
ize the upstream ISM. At v & 120 kms 1, shock induced
s −
ionizing radiation is strong enough to completely ionize the Ecr,br ∼1.9×109Tu−,04.4B2u, 6(1− fion)−1fi−on1/2n−ga3s/,20.5eV. (13)
upstream gas [36, 56, 74]. At the downstream, the tempera- −
ture decreases and the density increases as the radiationcools Thus, at v . 120 kms 1, the particle spectrum will have a
s −
the gas in the downstream. As the shock compression ratio break at 10 GeV and become steeper by one power above
∼
r =(γ+1) 2/[(γ 1) 2+2] 4inourcase,weassumethe the break. The ion fraction f is estimated according to the
ion
M − M ∼
spectralindexof2forsimplicityinthispaper. steady-statemodelbyHollenbachandMcKee[36].
2
10-10
Table1: ObservedPropertiesofNebulaeAssociatedwithExtragalacticCom- S26 (ngas = 1 cm-3)
pactX-rayObjects IC 342 X-1 (ngas = 1 cm-3)
Object (Mdpc) (ergLXs−1) (Rpcb) (kmvss−1) 10-11 IC 1F0e Xrm-1i/ L(nAngTgaa s(s 1= =0 1 3y c ecmamr-s3-3))
UMLicXroIqCua3s4a2rXS2-16 33..99 16..62×11004306 220000 120500 2cm/s) 10-12 CTA-South (50 hr)
HMXBIC10X-1 0.79 1.5××1038 170 80 N/dE (erg/ 10-13
d
Table2: ThetabledataaretakenfromPakulletal.[70,forS26],Allenetal. 2E
[10],Csehetal.[20,forIC342X-1],andLozinskayaandMoiseev[58,for
IC10X-1]. Thedataforthetime-averaged X-rayluminosityarefromCseh 10-14
etal.[20].
10-15
10-2 10-1 100 101 102 103 104 105 106 107
Energy (GeV)
Let us consider energyloss processesfor cosmic rays. The
energy loss timescale due to pp interactions can be estimated
as Figure1: Expected gamma-rayspectra fromnebulae associated withmicro-
1
t 6.0 106n 1 yr, (14) quasarS26,ULXIC342X-I,andHMXBIC10X-Ishownbysolid,dashed,
pp ≈ κ 4n σ c ∼ × −gas,0.5 and dot-dashed curve, respectively. Thick curves represent the case with
pp gas pp
ngas = 1 cm−3, while thin curves show ngas = 3 cm−3. The sensitivity of
where the cross section σpp 3 10−26 cm2 for 1010−12 eV Fermi/LATandCTAinthesouthernsiteisshownfor10yrand50hrintegra-
∼ ×
[46,48]andtheinelasticityκ 0.5.Thefactorof4infrontof tion,respectively.
pp
≈
n isduetocompressioninthedownstream. Wenotethatthe
gas
ppcrosssectiondependslogarithmicallyontheprotonenergy.
Figure1showstheexpectedgamma-raysignalsfromnebu-
By consideringthe steady-state for simplicity, the hadronu-
lae hosting compact X-ray sources in the nearby Universe to-
clearinteractionefficiencyisestimatedas
gether with the Fermi/LAT3 and CTA-South4 sensitivities for
f = tdyn 0.28R v 1 n . (15) 10-yrintegrationanda50hrobservation,respectively. Wese-
pp t ∼ b,20.8 −s,7.1 gas,0.5 lectrepresentativesourcesfromCsehetal.[20]whichinclude
pp
microquasar S26 [70], ULX IC 342-X-1 [10, 20], and high-
Inhadronuclearinteractions,chargedandneutralpionsaregen-
mass X-ray binary (HMXB) IC 10-X-1 [58]. The parameters
eratedattheratioofπ+ :π0 2:1. Gammaraysareproduced
≈ are summarized in Table1. For the calculation of the pp in-
bythedecayofneutralpionsasπ0 2γ.
→ teraction,wefollowtheprescriptionprovidedbyKamaeetal.
Ifweneglection-neutralcollisions,gamma-rayfluxfroman
[48] setting the fraction of the energy received in the pion as
expandingnebulacanbeestimatedas
0.17 [52]. For the gas density, we consider two cases with
E2dNγ 1ǫMξcrPkinmin[1, fpp], (16) ngas = 1 cm−3 and 3 cm−3. Here, ULXs are known to be as-
γdE ≈ 3 4πd2 sociatedwithstarformingregions[e.g.33,72,81]andthegas
γ
C densityintheHIIregioncanbeashighas 10cm 3 forthe
∼ 1.1×10−13ergcm−2s−1 1C2!−1 2Mdpc!−2 sizTehoef∼n1eb0u0lpaca[s3s7o]c.iated with the nearby∼microqu−asar S26
ξ R3 v2 n2 (17) shows a flat spectrum in E2dN/dE extending to 100 TeV
× cr,−1 b,20.8 s,7.1 gas,0.5 andisdetectableevenforn = 1cm 3. Thus, S2∼6wouldbe
where ξ is the cosmic-ray acceleration efficiency and = gas −
cr C agoodtargetforfutureCTAobservation. Onthecontrary,the
ln(E /E ).WesetE =1014eVandE =109eV.
p,max p,min p,max p,min nebulaeassociatedwithIC342X-1andIC10X-1wouldhave
We set the nuclear enhancement factor ǫ = 1.85 as in our
M soft gamma-rayspectra at & 10 GeV due to ion-neutralcolli-
Galaxy [e.g., 65, 78]. ULXs are known to be hosted by low-
sions,astheexpansionvelocityisslowerthan120kms 1.Such
metallicitygalaxiesatthemetallicityofZ .1/2Z [61]. Thus, −
slowlyexpandingnebulaewouldbedifficulttobeobservedby
⊙
ǫ wouldbelowerthanthatinourgalaxy.However,itisknown
M CTA.
that a galaxy is not chemically homogeneous and can have
Figure2showstheexpectedneutrinosignalperflavourfrom
metallicitydispersionbyafactorof10inagalaxy[e.g.67,73].
thenebulasurroundingS26togetherwiththeIceCubesensitiv-
WenotethatanX-raymeasurementoftheULXNGC1313X-1
ity[2]. Ifthegasdensityisashighasn = 3cm 3,anorder
revealedthatthelocaloxygenabundanceis 50%ofthesolar gas −
∼ ofmagnitudesensitiveneutrinodetectorswillbeabletoseethe
valueusingthelowenergyX-rayabsorptionfeature[64].
signal.
Wecanalsoevaluatetheexpectedneutrinofluxperflavoras
Bordasetal.[15]reportedthemarginaldetectionofgamma
E dN /dE 2 E dN /dE (E =0.5E ),or
γ γ γ ≈ × ν ν ν ν γ rays from the direction of the Galactic microquasar SS 433
Eν2ddNEν ∼ 5.6×10−14ergcm−2s−1 1C2!−1 2Mdpc!−2
ν 3https://www.slac.stanford.edu/exp/glast/groups/canda/lat_Performanc
× ξcr,−1R3b,20.8v2s,7.1n2gas,0.5. (18) 4https://portal.cta-observatory.org/Pages/CTA-Performance.aspx
3
10-10 regime[22]. TheICfluxisapproximately
10-11 Eγ2ddNEγ ∼ 1.4×10−13ergcm−2s−1 1C2!−1 2Mdpc!−2
γ
2g/cm/s) 10-12 × Kep,−2ξcr,−1R3b,20.8v2s,7.1ng7a/4s,0Eγ1/,122, (20)
er
E ( Therefore, the IC contribution from primary electrons would
N/d 10-13 become important at E 1 TeV if K & 0.01. Since
d γ ep
2E secondary electrons carry∼only 0.8% of the primary pro-
∼
10-14 S26 (ngas = 1 cm-3) ton energy, their contribution would be less important. If the
ngas = 3 cm-3 local optical or infrared (IR) photon energy density is com-
IceCube (Northern Sky)
IceCube (Southern Sky) parable to the CMB energy density, the total IC contribution
10-15
102 103 104 105 106 107 108 would be enhanced. The local nebular luminosity of S26 is
Energy (GeV) 5 1039ergs 1[23]correspondingtothephotonenergyden-
−
∼ ×
sityof 0.020eVcm 3.Thus,theICfluxforS26wouldnotbe
−
∼
significantlyenhancedevenifwetakeintoaccountlocalphoton
Figure 2: Expected neutrino spectrum per flavorfromthe nebula associated
withmicroquasar S26. Thethickanddashedcurverepresents thecase with fieldsotherthanCMBphotons.
ngas = 1cm−3 and3cm−3,respectively. Thesensitivities ofIceCubeforthe
northernskyandthesouthernsky[2]arealsoshown,respectively.
3. Discussions
and the related supernova remnant W50 using Fermi. These
gamma-raysignalsmay arise fromthe particle accelerationin 3.1. SpectralIndexofAcceleratedCosmicRays
theassociatednebula. However,theshellexpansionvelocityis
knowntobe16 40kms 1 [57]whichwouldbeinefficientto Inthispaper,weassumethespectralindexof2foracceler-
−
−
accelerateparticlesintheexpandingshell. ated particles. This assumption would be violated by various
Electrons are also expected to be accelerated together with possible mechanisms. As the source S26 expands at the ve-
protons. To evaluate the leptonic contribution, we need an locity of 250 kms−1 correspondingto the sonic and Alfve´nic
electron-to-protonratio K , where K is defined as the ratio mach number of 17 and 35, respectively, we can expect the
ep ep
of electron spectrum dN /dp and proton spectrum dN /dp at compression ratio r 4 for the sources S26 in NGC 7793.
e p ∼
p = 1 GeVc 1. In this paper, we take K = 0.01 as the However,forsourceshavinglowerexpansionvelocity,thecom-
− ep
fiducialvalue,sincethisvalueisfavoredforGalacticSNRsin pressionratiowouldbecomesmallerresultinginsofterspectra
thehadronuclearscenario[5]. Theexpectedradiosynchrotron [e.g.17]. Inadditiontothiseffect,theion-neutralcollisionef-
flux by electrons is given by f cσ U γ3 N (γ )/6πd2 fect[36,56, 74]andtheeffectoffinitevelocityofthecounter
e,syn ≈ T B e,s e e,s
[22].Thus,theradiofluxbyprimaryelectronsisapproximately streamingmagneticturbulenceinfrontoftheshock[e.g.16,55]
givenas wouldsteepenthecosmic-rayspectralindex. Onthecontrary,
in the radiative region, we expect a larger compression ratio
Eγ2ddNEγγ ∼ 1.2×10−16ergcm−2s−1 1C2!−1 2Mdpc!−2 4th.3an×r1=0−42.n−gTa1sh,0e.5svi2sz,27e/.51opfct,hwehraildeiatthievesirzeegioofnthiselaracdc≈elevrsattriaod/n4re∼-
× Kep,−2ξcr,−1R3b,20.8v2s,7.1n7ga/4s,0ν1s,/92.7, (19) gthioenspiseclatrccal≈invdsetaxccw∼ou4l.d8b×ec1o0m−2eZh−a1rnd−gea1sr/,02t.h5va−sn,172.1Eabcro,1v0e.5pc3.0TGheuVs,
∼
andmorephotonswouldbedetectableattheTeVband.
where ν is a frequency of the synchrotron component. By
s
adoptingtheparametersforS26andassumingK = 0.01,we
ep
get 2.6mJyand 2.0mJyat5.5and9.0GHz,respectively, 3.2. CosmicGamma-rayandNeutrinoBackgroundRadiation
∼ ∼
withn = 3cm 3 whichareconsistentwiththemeasuredra-
gas −
diofluxofS26of2.1mJyat5.5GHzand1.6mJyat9.0GHz ThecosmicGeVgamma-raybackgroundradiationisknown
[77]. Thus,determinationofgamma-rayspectrumwillbealso to be composed of blazars [e.g. 9, 43], radio galaxies [39],
usefultounderstandtheoriginoftheradioemissionandtocon- and star forming galaxies [6]. However, there is still an un-
strain the environmentof the nebula. If the downstreammag- resolvedcomponentatthefluxlevelof7.2 10 6cm 2s 1sr 1
− − − −
×
neticfieldisamplifiedbyAlfve´n-waveturbulence,therequired above 0.1 GeV [4]. If nebulae associated with microquasars
K becomes7 10 5 tobeconsistentwiththeradioobserva- andULXsemitgammarays,theywouldalsocontributetothe
ep −
×
tions. unresolvedcosmicgamma-raybackgroundradiation.
Here, the inverseCompton(IC) flux by electronsscattering ThelocalX-rayluminosityfunctionsofULXsarewellstud-
thecosmicmicrowavebackground(CMB)photonsisgivenas iedinliterature[e.g.32,62,80,85]. Thelocalnumberdensity
f cσ γ3 N (γ )u /6πd2,whereamonochromaticradi- of ULXs is n 1.5 10 2 Mpc 3 [80]. Then, the cos-
IC ≈ T e,s e e,s CMB ULX ∼ × − −
ationfieldassumedfortheCMBandweassumetheThomson micgamma-raybackgroundfluxfromULX/microquasarnebu-
4
laecanbeestimatedas may accelerate particles near the polar cap region of the pul-
sars. However,& 10GeV gammaraysfromthe polarcapre-
E2 dIγ 1ctHξzǫMξcrPkinnULXmin[1, fpp] (21) gion emission will be attenuated due to one-photon pair cre-
γdE ≈ 3 4π
γ C ation. The threshold magnetic field strength is B & 0.1BQ
5.2 10−6MeVcm−2s−1sr−1 ξz −1 C −1 [21,35],whereBQ =m2ec3/e=4.4×1013G.Therefore,strong
∼ × (cid:18)3(cid:19) 12! gamma-rayemissionwouldnotbeexpectedforhighmagnetic
ξ R3 v2 n2 n , (22) fieldULXpulsars.
× cr,−1 b,20.8 s,7.1 gas,0.5 ULX,−1.8
where t 13.8 Gyr is the Hubble timescale assuming H =
H ∼ 0 4. Conclusions
67.8 kms 1 Mpc 1 and Ω = 0.308 [71]. ξ is a factor ac-
− − M z
counting cosmological evolution of the source. We assume Inthe comingdecade, CTA willdeepenourunderstandings
vs 120kms−1 forallsources. Therefore,ULX/microquasar ofhighenergyastrophysicalphenomena. Inthispaper,wein-
≥
nebulaewouldcontribute5.2 10−7cm−2s−1sr−1above0.1GeV vestigatehighenergysignalsfromnebulaeassociatedwithmi-
×
( 7%oftheunresolvedbackgroundflux). croquasarsand ULXs in the nearby Universe whose available
∼
Thecosmicneutrinobackgroundfluxwouldbecome poweris&1039ergs 1.
−
Eν2ddEIνν ∼ 2.6×10−9GeVcm−2s−1sr−1 (cid:18)ξ3z(cid:19)−1 1C2!−1 speonwAseilttrihfvouiutlygnhaenbgudalaamnemgausalsaorracryieassteofdrlouwmtioitnhhoamslltiocgwraolquausxaytosaidtrsseetalefncdteaxUnisLdt,XreCss.ToFAlvo’esr
× ξcr,−1R3b,20.8v2s,7.1n2gas,0.5nULX,−1.8, (23) example, the nebula associated with the microquasar S26 in
NGC7793[70]willbeagoodtargetwhoseexpectedgamma-
where we note that the neutrino spectra from ULX nebulae
rayfluxis 1 10 13–1 10 12ergcm 2s 1extendingtosev-
wouldhaveacutoffatseveraltensTeV(seeEquation12). Due ∼ × − × − − −
eraltensTeVwithaphotonindexof 2. At& 1TeV,theIC
tothiscutoff,thenebulaewouldnotcontributetotheIceCube ∼
contributionwouldbecomeimportantif K & 0.01. We note
detected neutrino fluxes [1]. However, if Alfve´n-wave turbu- ep
that the spectral index of cosmic rays would be harder than 2
lenceattheshockisfullygenerated,sub-PeVneutrinoscanbe
above 30GeVsinceitistheradiativeshock. Wealsofound
affordable. ∼
thattheexpectedsynchrotronradiationfluxfromprimaryelec-
trons is consistent with the measured radio flux. If all micro-
3.3. Gamma-rayEmissionandAttenuationinHostGalaxies
quasarsandULXsareassociatedwithpowerfulexpandingneb-
The total IR luminosity of NGC 7793 which hosts the mi-
ulae,theywouldmake 7%oftheunresolvedcosmicgamma-
croquasar S26 is 2.07 109L [29]. Adopting the scaling re- ∼
ray background flux. However, they would not significantly
× ⊙
lation between gamma-ray and IR luminosities [6], we have
contribute to the TeV-PeV neutrino background flux reported
L 3.8 1038 ergs 1 for galactic diffuse emission leading
γ − byIceCubeduetocutoffsatseveraltensTeVinneutrinospec-
∼ ×
EγdNγ/dEγ 2.2 10−13 ergcm−2s−1 which is comparable tra. If the expansionvelocity is v . 120 kms 1, ion-neutral
to expectedfl∼uxfro×mS26 assumingn = 1cm 3. Here, the s −
gas − collisionswillsuppressefficientparticleaccelerationandresult
angularsizeofNGC7793is93 63andS26locatesattheout-
′ ′ infainterTeVgamma-rayfluxabove 10GeV.
×
skirtregionofthegalaxy( 3arcminawayfromthenucleus). ∼
∼
Given the CTA angular resolutionof . 3 arcminat & 1 TeV,
Acknowledgements:
gamma-raysignalsfromS26canbedistinguishedfromgamma
The authors thank Chris Done for useful comments and
rays due to galactic cosmic rayswhich would be brightin the
discussions. YI is supported by the JAXA international top
starformingnucleusregion.
young fellowship and JSPS KAKENHI Grant Number 420
Itiswell-knownthatgammarayspropagatingtheintergalac-
JP16K13813.
ticspaceareattenuatedbythecosmicoptical/IRbackgroundra-
diationfieldviathepairproductionprocess[31,45,79]. Since
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