Table Of ContentTobe resubmitted toApJ:v18.0,January8, 2016
PreprinttypesetusingLATEXstyleemulateapjv.04/20/08
H. Katagiri1,2, K. Yoshida1,2, J. Ballet3, M.-H. Grondin4, Y. Hanabata5, J.W. Hewitt6,7, H. Kubo8,
M. Lemoine-Goumard4
Tobe resubmitted toApJ: v18.0, January 8, 2016
ABSTRACT
We report the discovery of extended gamma-ray emission measured by the Large Area Tele-
scope (LAT) onboard the Fermi Gamma-ray Space Telescope in the region of the supernova rem-
6
nant (SNR) HB 3 (G132.7+1.3) and the W3 HII complex adjacent to the southeast of the remnant.
1
W3 is spatially associated with bright 12CO (J=1-0) emission. The gamma-ray emission is spatially
0
correlated with this gas and the SNR. We discuss the possibility that gamma rays originate in inter-
2
actionsbetweenparticles acceleratedin the SNR andinterstellargasorradiationfields. The decayof
n neutral pions produced in nucleon-nucleon interactions between accelerated hadrons and interstellar
a gasprovidesareasonableexplanationforthegamma-rayemission. TheemissionfromW3isconsistent
J
with irradiation of the CO clouds by the cosmic rays accelerated in HB 3.
7
Subject headings: cosmic rays — acceleration of particles — ISM: individual objects (HB 3, W3) —
ISM: supernova remnants — gamma rays: ISM
]
E
H
1. INTRODUCTION that of W3, estimated to be 2.0–2.4 kpc (see e.g. Rout-
h. Diffusive acceleration by supernova shock waves can ledge et al. 1991; Xu et al. 2006). In this paper, we
p accelerate particles to very high energies (e.g., Bland- adopt2.2kpc. Theagewasestimatedtobe∼3×104 yr
- ford & Eichler 1987). However, the processes of accel- based on X-ray data and applying different evolution-
o eration, release from the shock region, and diffusion in ary models to the SNR (Lazendic & Slane 2006). The
r
X-ray spectrum was well described by a thermal
t the interstellar medium of such particles has not been
s well understood so far. Gamma-ray observations in the plasma model (Lazendic & Slane 2006).
a
[ GeV regime are a powerful probe of these mechanisms. In the COS B era, W3 was associated with
The Large Area Telescope (LAT) on board the Fermi the gamma-ray source CG135+1 (Strong 1977) and
1 Gamma-ray Space Telescope has recently detected GeV during the EGRET era it was associated with
v gammaraysfromSNRs(e.g., Thompsonetal.2012,and 3EG J0229+6151 (Hartman et al. 1999). However, de-
7 tailed spatial association and extension measurements
references therein).
0 HB 3 (G132.7+1.3)is a well-knownmiddle-agedSNR. could not be made due to the limited angular resolution
4 The 1◦.3 diameter (Green 2014) makes it a good can- of COS B and EGRET. Five LAT sources positionally
1 didate for detailed morphological studies in high-energy associated with HB 3 and W3 are listed in the 2FGL
0
gamma rays since it is larger than the LAT angular res- catalog (Nolan et al. 2012). In this paper, we report
.
1 olution above 1 GeV (smaller than 1◦). The W3 a detailed analysis of Fermi LAT observations around
0 HIIcomplexisadjacentandtothe southeastoftherem- HB 3. First, we briefly describe the observations and
6 nant. Bright12CO(J=1-0)lineemissionnear−43kms−1 data selection in Section 2. The analysis procedure and
1 aroundW3ispartlysurroundedbyaregionofenhanced the results are presented in Section 3, with the study
: of the morphology and spectrum of the emission associ-
v radio continuum emission from HB 3, suggesting that
i HB3isinteractingwiththisgas(Routledgeetal.1991). ated with HB 3 and W3. Results are then discussed in
X The distance to the SNR is therefore considered to be Section4andourconclusionsarepresentedinSection5.
r
a 1College of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 2. OBSERVATIONS AND DATA SELECTION
310-8512,Japan
2Corresponding author: H. Katagiri, The LAT is the main instrument on Fermi, detecting
[email protected]; K. Yoshida, gamma rays from ∼ 20 MeV to > 300 GeV. It is an
[email protected] electron-positron pair production telescope, built with
3Laboratoire AIM, CEA-IRFU/CNRS/Universit´e Paris tungsten foils and silicon microstrip detectors to mea-
Diderot, Service d’Astrophysique, CEA Saclay, 91191 Gif sur
sure the arrival directions of incoming gamma rays, and
Yvette, France
4Centre d’E´tudes Nucl´eaires de Bordeaux Gradignan, a hodoscopiccesium iodide calorimeterto determine the
IN2P3/CNRS,Universit´eBordeaux1,BP120,F-33175Gradignan gamma-ray energies. They are surrounded by 89 seg-
Cedex,France mented plastic scintillators that serve as an anticoinci-
5Institute forCosmic-RayResearch, UniversityofTokyo, 5-1-5
dence detector to reject charged particle events. The
Kashiwanoha,Kashiwa,Chiba,277-8582,Japan
6Department of Physics and Center for Space Sciences and on-orbit calibration, event classification and instrument
Technology,UniversityofMarylandBaltimoreCounty,Baltimore, performance are described in Abdo et al. (2009a) and
MD21250,USA Ackermann et al. (2012). Compared to earlier high-
7Center for Research and Exploration in Space Science and
energygamma-raytelescopes, the LAT has a largerfield
Technology (CRESST) and NASA Goddard Space Flight Center,
Greenbelt,MD20771, USA of view (∼ 2.4 sr), a larger effective area (∼ 8000 cm2
8Department of Physics, Graduate School of Science, Kyoto for > 1 GeV on-axis) and improved point-spread func-
University,Kyoto,Japan tion (PSF; the 68% containment angle above 1 GeV is
2 Fermi LAT collaboration
smaller than 1◦). W3 (2FGL J0224.0+6204 , 2FGL J0218.7+6208c and
RoutinescienceoperationswiththeLATbeganonAu- 2FGL J0225.9+6154c) free. Additionally, the normal-
gust 4, 2008. We have analyzed events in the region of izations of the Galactic diffuse and isotropic component
the HB 3 SNR collected from August 4, 2008, to Jan- were left free. All other background sources had their
uary 30, 2014. The LAT was operated in sky-survey indices fixed to the 2FGL values and normalizations set
mode for almost the entire period. In this observing free. Weusedonlyeventsabove1GeV(comparedtothe
mode the LAT scans the whole sky, obtaining complete 0.2 GeV used in the spectral analysis) for the morpho-
sky coverage every 2 orbits (∼ 3 hr) and approximately logical study to take advantage of the narrower PSF at
uniform exposure. The analysis was performed over a higher energies.
square region of 14◦×14◦ with a pixel size of 0◦.1. We Figure 1 shows the counts map in a 4◦ × 4◦ region
set the centroid of the region to the SNR center: (R.A., centered on HB 3, after subtracting the Galactic emis-
Dec.) = (34◦.42,62◦.75) (J2000). sion, the isotropic component and all the sources from
We used the standard LAT analysis software, the the 2FGL catalog but the five ones spatially coincident
ScienceTools version 09-32-05, publicly available from with HB 3 and W3. The image in the 12CO (J=1-
the Fermi Science Support Center (FSSC)9. We used 0) line (Dame et al. 2001) is overlaid, where the
the post-launch instrument response functions (IRFs) COline intensities wereintegratedfor velocitieswith re-
P7REP SOURCE V15 (Ackermann et al. 2012) and applied spect to the local standard of rest −44.2 km s−1 <V <
the following event selection criteria: 1) events should −33.8 km s−1, corresponding to the clouds associated
be classified as reprocessedPass 7 Source class (Atwood with W3 (Routledge et al. 1991). The correlation be-
et al. 2009), 2) events have a reconstructed zenith an- tweengammaraysandthe CO line emissionaroundW3
gle less than 100◦, to minimize the contamination from is evident.
Earth-limbgamma-rayemission, and 3) only time inter- ToquantitativelyevaluatethecorrelationwiththeCO
vals when the center of the LAT field of view is within line emission, we fitted the LAT counts with the spatial
52◦ ofthe localzenithareacceptedtofurtherreducethe template of the CO line emission instead of the three
contaminationbyEarth’satmosphericemission. Wealso 2FGL sources within W3. The shape of CO emission
eliminated a short period of time (06:25:45 to 06:42:25 was extracted only around W3. Since the edge of the
UTC on 2012 September 11) during which the LAT de- CO emission was unclear due to due to the noise of the
tected the bright GeV-emitting GRB 120911B (Racusin spectral measurements, we varied the threshold value of
et al. 2012) within 15◦ of HB 3. The resulting total the CO intensity to extract the shape to maximize the
exposure was ∼ 2 × 1011 cm2 s (at 1 GeV). We likelihood value with a penalty for one additional pa-
restricted the analysis to the energy range > 200 MeV rameter. The spectral shape of W3 was assumed to be
to avoid possible large systematics due to the rapidly a log-parabola function, which is used for the sources
varying effective area and much broader PSF at lower corresponding to W3 (Nolan et al. 2012). The resulting
energies. maximumlikelihoodvalueswithrespecttothemaximum
likelihoodforthenullhypothesis(noemissionassociated
with the SNR and W3) are summarized in Table 1. A
3. ANALYSIS AND RESULTS
test statistic (TS) value is defined as 2ln(L/L ), where
0
3.1. Morphological analysis L and L are the maximum likelihoods for the model
0
with/without the source component, respectively (e.g.
In order to study the morphology of gamma-ray emis-
Mattox et al. 1996). Although the spatial models are
sion associated with the HB 3 SNR we performed
not nested so there is no rigorous way to directly com-
a binned likelihood analysis based on Poisson statis-
tics10 (see e.g. Mattox et al. 1996). The likelihood is parethem,theTSvaluefortheCOimageissignificantly
larger than for the three 2FGL sources and is physically
theproductoftheprobabilitiesofobservingthe gamma-
motivated.
ray counts within each spatial and energy bin for a
We further characterized the morphology of gamma-
specified emission model. The probability density func-
ray emission associated with the SNR. Figure 2 shows
tion for the likelihood analysis includes (1) individual
sources detected in the 2FGL catalog within 20◦ of the thecountsmapina4◦ ×4◦ regioncenteredonHB3,af-
ter subtractingthe Galactic emission, the isotropic com-
SNR, (2) the Galactic diffuse emission resulting from
ponent, the 2FGL point sources except for the 2 sources
CR interactions with the interstellar medium and ra-
associated with the SNR, and W3 modeled by the CO
diation based on the LAT standard diffuse background
template. This highlights the diffuse emission around
model, gll iem v05 rev1.fitavailable from the FSSC,
the SNR. To quantitatively evaluate the detection sig-
and(3)anisotropiccomponenttorepresentgammarays
nificance of the diffuse emission from HB 3, we adopted
and residual CR background using a tabulated spec-
a uniform disk as a spatial template assuming a simple
trumwritteniniso source v05 rev1.txtalsoavailable
power-law spectrum. We varied the radius and location
from the FSSC. We set the spectral parameters of the
of the disk and evaluated the maximum likelihood val-
2FGL catalog sources located within the SNR (2FGL
ues. The best-fit disk has the radius of 0◦.80+0.18 cen-
J0214.5+6251c and 2FGL J0221.4+6257c) and within −0.17
tered on (R.A., Dec.) = (35◦.36, 62◦.69) (J2000). The
9 Software and documentation of the Fermi Science- errorof the centroidis 0◦.06 at 68%confidence level. We
Tools are distributed by Fermi Science Support Center at note that the maximum likelihood value for a 408 MHz
http://fermi.gsfc.nasa.gov/ssc radio template with suppression of emission from W3
10 As implemented in the publicly available Fermi Sci-
was significantly worse than the best-fit disk model, in-
ence Tools. The documentation concerning the analysis
tools and the likelihood fitting procedure is available from dicating that the gamma-ray emission around HB 3 is
http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/.
Fermi LAT Discovery of Extended Gamma-Ray Emissions in the Vicinity of the HB 3 SNR 3
not strongly spatially associated with the shock region gamma-ray energy fluxes between 0.2–300 GeV inferred
of HB 3 as traced by radio. The best-fit disk is off- from our analysis are 3.7×10−11erg cm−2 s−1 for the
set from the SNR center (R.A., Dec.) = (34◦.51, SNR and 1.0×10−10erg cm−2 s−1 for W3, respectively.
62◦.79) (J2000) (Kothes et al. 2006) by 0◦.40, and The gamma-ray luminosities between 0.2–300 GeV as-
the offset is significant at the 4.2σ level. The TS suming the distance is 2.2 kpc are 2.1×1034erg s−1 for
valueforthebest-fitdiskislargerthanforthetwo2FGL the SNR and 5.9×1034erg s−1 for W3, respectively.
sources. In addition, the detection significance for the
disk with energies of > 1 GeV is at the ∼ 12 σ level. 4. DISCUSSION
Therefore we finally adopted the uniform disk template
There is a correspondence between gamma-ray emis-
withmaximumlikelihoodparametersforthe wholeSNR
sionandtheSNR.The gamma-ray emission around
in the following spectral analysis.
HB 3 is extended, located off the Galactic plane
3.2. Spectral analysis and far from the Galactic center, so the prob-
ability of chance superposition of other Galac-
Tomeasurethe spectraforthe HB 3SNRandW3,we
tic sources is small. Also, the spectral shape
performed maximum likelihood fits in logarithmically-
of gamma rays around HB 3 is similar to other
spacedenergybandsfrom0.2GeVto300GeV,usingthe
gamma-raysourcesassociatedwitholdSNRs, i.e.
spatial model described above with the spectral indices
a soft spectrum with a spectral break at ener-
in each bin fit to 2. Figure 3 and 4 show the resulting
gies of around 1–10 GeV (e.g., Abdo et al. 2009b,
spectral energy distributions (SEDs). Upper limits at
2010a,b,c). These facts suggest the association
the 2 sigma confidence level are calculated assuming a
between the gamma-ray emission and HB 3. As-
photon index of 2 if the detection is not significant in
suming this, enhancement of gamma-ray emission from
an energy bin, i.e., the TS value with respect to the null
W3, which contains the molecular clouds traced by CO
hypothesis is less than 4 (corresponding to 2 σ for one
lineemission,canbereasonablyexplainedbyinteraction
additional degree of freedom).
between high-energy particles and the dense material in
Weidentifyatleastfourdifferentsourcesofsystematic
the clouds. Thus we argue that the bulk of gamma-ray
uncertainties affecting the estimate of the fluxes. Un-
emissioncomesfrominteractionsofhigh-energyparticles
certainties in the LAT effective area are evaluated by
acceleratedattheshocksofHB3withinterstellarmatter
comparing the efficiencies of analysis cuts for data and
or fields in the regions. First, we discuss the gamma-ray
simulation of observations of Vela and the limb of the
spectrum of the SNR which was obtained by using the
Earth, among other consistency checks (Ackermann et
disk spatial template. To model the broadband emis-
al. 2012). For P7REP SOURCE V15, these studies suggest
sionfrom the entire SNR we adoptthe simplest possible
a10%systematicuncertaintybelow100MeV,decreasing
assumption that gamma rays are emitted by a popula-
linearly with the logarithmof energy to 5% in the range
tion of accelerated protons and electrons distributed in
between 316 MeV and 10 GeV and increasing linearly
the same region and characterized by constant matter
with the logarithm of energy up to 15% at 1 TeV (Ack-
density and magnetic field strength. We assume the in-
ermann et al. 2012). We estimated the systematic er-
jectedelectronstohavethesamemomentumdistribution
rors by changing the effective area of the IRFs energy-
as protons. This assumptionrequiresa breakin the mo-
dependently considering the above uncertainties.
mentum spectrum because the spectral index in the ra-
We adopted the strategy described in Ackermann et
dio domain, correspondingto lowerparticle momenta,is
al.(2013)anddePalmaetal.(2013)toevaluatethesys-
much harder than for gamma rays, which correspond to
tematic uncertainties due to the modeling of interstellar
higherparticlemomenta. Therefore,weusethefollowing
emission. We compared the results obtained using the
functional form to model the momentum distribution of
standard model in Section 3.1 with the results based on
injected particles:
eightalternative interstellaremissionmodels. We varied
tuhmenundiefnosrimtiesspoinf tinetmerpsetrealltaurreatuosmedictohyedsrtoimgeant,etthheevceorl-- p −sL p 2 −(sH−sL)/2
Q (p)=a 1+ ,
tical height of the CR propagation halo, and the CR e,p e,p 1 GeV c−1 p
source distribution in the Galaxy. We similarly gauged (cid:16) (cid:17) ( (cid:18) br(cid:19) )
(1)
the uncertainties due to the morphological template by
where p is the break momentum, s is the spectral
comparing the results with those obtained by changing br L
indexbelowthebreakands abovethebreak. Notethat
the radius of the disk template by ± 1σ. Lastly, we here we consider a minimumH momenta of 100 MeV c−1
obtained the uncertainties due to the assumption of the
since the details of the proton/electroninjection process
photonindices. Wechangedtheindicesfrom0to4. The
are poorly known.
total systematic errors shown in Figure 3 and 4 are set
Electrons suffer energy losses due to ionization,
by adding the above uncertainties in quadrature.
Coulomb scattering, bremsstrahlung, synchrotron emis-
WeprobedforaspectralbreakintheLATenergyband
sionandinverseCompton(IC)scattering. Wecalculated
by comparing the likelihood values of a spectral fit over
the evolution of the electron momenta spectrum by the
the whole energy range considered based on a simple
following equation:
power law and a log parabola function. The TS values
and best-fit parametersare summarizedin Table 2. The ∂N ∂
e,p
= (b N )+Q , (2)
TS values for the log-parabola function correspond to e,p e,p e,p
∂t ∂p
improvements at the >4 σ confidence level for the SNR
and >16 σ for W3, respectively. Assuming the spectral where b = −dp/dt is the momentum loss rate, and
e,p
shape can be described by a log parabola function, the Q is the particle injection rate. We assume Q to be
e,p e,p
4 Fermi LAT collaboration
constant, i.e., that the shock produces a constant num- ground (CMB). We used in our calculations the first
berofparticlesuntiltheSNRentersthe radiativephase, two components as they are modeled in Porter et al.
at which time the source turns off. This prescription (2008) at the location of HB 3. Since their spectra
approximatesthe weakeningofthe shock and the reduc- are very complex, they are approximated by two in-
tion in the particle acceleration efficiency, which would frared blackbody components (T =33.3,1.00×103 K,
IR
be properly treated by using a time-dependent shock U =0.17,0.04eV cm−3, respectively), andtwo optical
IR
compression ratio (Moraal & Axford 1983). To derive blackbody components (T =4.02×103,1.34×104 K,
opt
theremnantemissionspectrumwecalculatedNe,p(p,T0) U =2.71×10−1,8.80×10−2 eV cm−3, respectively).
opt
numerically, where T is the SNR age assumed to be
0 The flux ratio betweenthe IC andthe synchrotroncom-
3×104 yr. Notethatweneglectedthe momentumlosses
ponentsconstrainsthemagneticfieldtobelessthan1µG
for protons since the timescale of neutral pion produc- andrequiresalowgasdensityofn¯ ∼2.0×10−3cm−3to
tionis∼107/n¯ yrwheren¯ isthegasdensityaveraged H
H H suppress the electron bremsstrahlung, which is unlikely.
over the entire SNR shell and is much longer than the
Increasing the intensity of the interstellar radiation field
SNR age. Also we neglect the gamma-ray emission by
otherthanCMBwouldloosentheconstraintonthemag-
secondarypositronsandelectrons fromchargedpionde-
netic field and the gas density. A radiation field about
cay, because the emission from secondaries is generally
100 times more intense is required to satisfy the above
faintcomparedtothatfromprimaryelectronsunlessthe
assumption on the magnetic field and the gas density.
gas density is as high as that in dense molecular clouds Assuming that π0-decay produced by the interactions
andtheSNRevolutionreachesthelaterstages,orthein-
of protons with ambient hydrogen is responsible for the
jectedelectron-to-protonratioismuchlowerthanlocally
bulk of the gamma-ray emission from the SNR, the
observed. The gamma-ray spectrum from π0 decay pro-
gamma-rayspectrumofW3canbereasonablyexplained
ducedbytheinteractionsofprotonswithambienthydro- by a π0-decay dominated model. If the protons are ac-
genis calculatedbasedonDermer(1986)usingascaling
celerated in the whole SNR in the same manner and
factor of 1.68 to account for helium and heavier nuclei
are not largely affected by spectral deformation due to
in target material and cosmic rays (Mori 2009). Contri-
cosmic-ray diffusion process, the gamma-ray spectrum
butions frombremsstrahlungandinverseComptonscat-
of W3 is expected to be modeled with a similar pro-
tering by accelerated electrons are computed based on
ton spectrum. The curved lines in Figure 4 show the
Blumenthal & Gould (1970), whereas synchrotron radi- gamma-ray spectrum of W3 with the π0-decay dom-
ation is based on Crusius & Schlickeiser (1986).
inated model assuming the density in the molecular
First, we consider a π0-decay dominated model. The clouds is 100 cm−3. The spectrum can be reproduced
number index of protons in the high-energy regime is without any change from the proton momentum spec-
constrained to be sH ≈ 4.0 from the gamma-ray spec- trum of the SNR. The resulting total proton energy,
tral slope. sL ≈ 1.5 by modeling the radio spectrum Wp ∼ 0.29 × 1049 · (100 cm−3/n¯H)·(d/2.2kpc)2 erg,
as synchrotron radiation by relativistic electrons (under is about 5.5 % of that for the π0-decay model of the
theassumptionthatprotonsandelectronshaveidentical
SNR, which is likely considering the solid angle of W3
injectionspectra). Thespectralindexαoftheradiocon-
from the center of the SNR. We also estimated the
tinuum emission is ∼ 0.34 (Tian & Leahy 2005), where
energy density of cosmic rays in W3 in order to
α is defined as S ∝ ν−α where S and ν are the flux
ν ν confirm the enhancement of the energy density
density and the frequency, respectively. The gamma-
due to the SNR compared to that from the lo-
ray spectrum provides an upper bound for the momen-
cal cosmic rays. The mass of the whole clouds of
ttuhme bbesrte-afikt avtalu∼e, 1103GGeeVVc−c−11..ThWeegaasdodpetnsaitybriseafikxeadt W3 is 4.4×105 M⊙ (Polychroni et al. 2012). We
assume the cloud mass related to the gamma-ray
stokii2.2000c5m).−3Tbhaesedresounltitnhge tHotIalobpsreortvoantioennser(gGy,osWachin∼- emissionto be about 104 M⊙ since the gamma-ray
p emission only comes from a small part of the W3
5.2 × 1049 · (2.0 cm−3/n¯H)·(d/2.2kpc)2 erg, is less giant molecular cloud. The resulting density is
than 10 % of the typical kinetic energy of a supernova 15·(104M⊙/M)eVcm−3,whereMisthecloudmass
explosion. For an electron-to-protonratio Kep =0.01 at related to the gamma-ray emission. This is much
1GeVc−1,whichistheratiomeasuredattheEarth,the higher than the energy density of the local cos-
magnetic field strength is constrained to be B ∼ 25 µG mic rays, indicating a strong association between
by radio data. Using the parameters summarized in Ta- thegamma-ray emissionand the SNR. Tosumma-
ble 3, we obtained the SEDs shown in Figure 5 (a). rize, it is most natural to assume that gamma-ray emis-
The gamma-ray spectrum can be reproduced by sion from HB 3 is dominated by decay of π0 produced
an electron bremsstrahlung dominated model in Fig- in nucleon-nucleon interactions of hadronic cosmic rays
ure5(b),althoughitisnoteasytofitthedatainthera- with interstellar matter. It should be emphasized that
diobandsimultaneously. The densityusedinthis model ourobservationsaroundHB3providearareandvaluable
is the same as that for the π0-decay dominated model, example where there are detections of gamma rays from
while Kep is set to 1 so that the leptonic emission domi- both the adjacent interacting molecular cloud and the
nates. SNRitself. This differsfrom W44(Uchiyama etal.
An inverse Compton dominated model can also re- 2012) and W28 (Hanabata et al. 2014) where the
produce the gamma-ray spectrum as shown in Fig- adjacent gamma-ray emitting clouds are thought
ure 5 (c). Gamma-ray emission of IC origin is due to to be outside the SNRs.
interactionsofhigh-energyelectronswithopticalandin-
frared radiation fields and the cosmic microwave back- 5. CONCLUSIONS
Fermi LAT Discovery of Extended Gamma-Ray Emissions in the Vicinity of the HB 3 SNR 5
TABLE 1
Test Statistics forDifferentSpatialModels Comparedwith the Nullhypothesis (nogamma-rayemission associated
neither with HB3 nor W3) in the 1–300GeVenergyrange.
Model TestStatistic AdditionalDegreesofFreedom
3pointsources+2pointsourcesa 2025.5 13
COimageb +2pointsourcesc 2115.8 9
COimage+Uniformdiskd 2142.2 9
COimage+Radiotemplatee 2111.8 6
aThefivesources listedinthe2FGLsourcelistassociated withtheHB3SNRandtheW3(Nolanetal.2012). Thespectral shapes are
apower-lawfunctionfor2sources,alogparabolafunctionfor3sources.
bTheadditionaldegreesoffreedomfortheCOimageare2forthespectralshape,1forthethresholdvalueoftheCOintensitytoextract
theshape. Thethresholdvaluetomaximizethelikelihoodvalueis3K·kms−1. Thedetailsareshowninthetext.
cThetwosourceslistedinthe2FGLsourcelistassociatedwithHB3.
dTheadditional degreesoffreedomfortheuniformdiskare5,2forthespectralshape(apowerlaw),3forthediskradiusandposition.
eTheradiotemplatewasobtainedfrom408MHzradiodata(Tayloretal.2003)byexcludingtheregionaroundW3,wheretheemission
ispredominantlythermal. Theadditionaldegreesoffreedomfortheuniformdiskare2forthespectralshape(apowerlaw).
We detected extended gamma-ray emissions by using ticules in France, the Agenzia Spaziale Italiana and the
theLATdataintheregionoftheSNRHB3andtheW3 Istituto Nazionale di Fisica Nucleare in Italy, the Min-
HII complex. The decay of π0 produced by interactions istryofEducation,Culture,Sports,ScienceandTechnol-
of hadrons accelerated by the remnant with interstellar ogy(MEXT),HighEnergyAcceleratorResearchOrgani-
gas naturally explains the gamma-ray emission around zation(KEK)andJapanAerospaceExplorationAgency
HB 3. Assuming this, the gamma-rayemissionfromW3 (JAXA)inJapan,andtheK.A.WallenbergFoundation,
is also reasonably explained by the interactionof cosmic the Swedish ResearchCouncil and the Swedish National
rays accelerated by the SNR with the dense gas in the Space Board in Sweden.
clouds associated with W3. Additional support for science analysis during the op-
erations phase is gratefully acknowledged from the Is-
tituto Nazionale di Astrofisica in Italy and the Centre
The Fermi LAT Collaborationacknowledges generous
National d’E´tudes Spatiales in France.
ongoing support from a number of agencies and insti-
The research presented in this paper has used data
tutesthathavesupportedboththedevelopmentandthe
from the Canadian Galactic Plane Survey, a Canadian
operation of the LAT as well as scientific data analysis.
project with international partners, supported by the
These include the National Aeronautics and Space Ad-
Natural Sciences and Engineering Research Council.
ministrationandtheDepartmentofEnergyintheUnited
We wish to thank Dave Green (University of
States, the Commissariat `a l’Energie Atomique and the
Cambridge) for advising us about the reference
Centre National de la Recherche Scientifique / Institut
for the position of the SNR.
National de Physique Nucl´eaire et de Physique des Par-
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6 Fermi LAT collaboration
[counts/pixel]
0 20
0
)
0 5 18
0 .
4
0
6 16
2
J
( 0 14
n 0
o 0 12
i .
t 3
a 6 10
n
i
l 0 8
c
0
e
5
D 6
.
1
6 4
38.000 34.000 30.000 2
Right Ascension (J2000) 0
Fig.1.—Background-subtractedLATcountsmapin the1–10 GeVenergyrangearound theHB3SNR.Thefive2FGL point
sourcesassociatedwithHB3(cyansquares)andW3(cyancrosses)arenotincludedinthebackgroundmodel. Theredpluses
are the other 2FGL point sources. The countsmap is binned using a grid of 0◦.1 and smoothed with a Gaussian kernel of
σ =0◦.25. The inset of thefigureshows the simulated LATPSFwith a photon index of 2.5 in thesame energy range, adopting
the same smoothing. Note that throughout the paper the analysis is conducted on unsmoothed data taking into account the
instrumentPSFinthelikelihoodanalysis. Thewhite contours are 408MHzradiodata (Tayloretal.2003) excluding
the region around W3, where the emission is predominantly thermal. The contour interval is 2 K from 2 K.
Contoursofintegratedintensityofthe12CO(J=1-0)line(Dameetal.2001) areingreen. Thecontourintervalis2.7K·kms−1
from 2.7 K·km s−1. TheCO image was smoothed using a Gaussian kernelwith σ = 0◦.125.
TABLE 2
Test StatisticsandParameters forSpectralModels (0.2–300 GeV)
Spectral Model TestStatistica Degrees SpectralParameters
ofFreedom
HB3
PowerLaw 0 2 E−p;p=2.39±0.05
LogParabola 18 3 E −p1−p2log(1GEeV)
(cid:16)1GeV(cid:17)
p1=2.29±0.09
p2=0.30±0.11
W3
PowerLaw 0 2 E−p;p=2.42±0.02
LogParabola 267 3 E −p1−p2log(1GEeV)
(cid:16)1GeV(cid:17)
p1=2.40±0.03
p2=0.33±0.03
a2ln(L/L0),whereLandL0arethemaximumlikelihoodvaluesforthemodelunderconsiderationandthepower-lawmodel,respectively.
Fermi LAT Discovery of Extended Gamma-Ray Emissions in the Vicinity of the HB 3 SNR 7
[counts/pixel]
0
0
) 5 2.5
0
.
0 4
6
0
2
2
J 0
( 0
1.5
0
n
.
o 3
6
i 1
t
a
0
n
0
0.5
i
l 5
c
.
e 1
6 0
D
38.000 34.000 30.000
-0.5
Right Ascension (J2000)
Fig.2.—Background-subtracted LATcountsmapinthe1–10GeVenergyrange. ThetwoLATpointsourcesassociatedwithHB3are
notincludedinthebackground model,buttheCOtemplate forW3is. Thecounts mapisbinnedusingagridof0◦.1andsmoothedwith
a Gaussian kernel of sigma = 0◦.40. The green circle indicates the best-fit diskfor HB 3 used forthe analysis as a spatial template. The
detailsoftheotheroverlaysaredescribedinthecaptionofFigure1.
TABLE 3
Modelparametersforthe HB 3SNRandW3.
HB3Model Kepa sLb pbrc sHd B n¯He Wpf Wef
(GeV c−1) (µG) (cm−3) (1049 erg) (1049 erg)
(a)Pion 0.01 1.5 10 4.0 25 2.0 5.2 0.074
(b)Bremsstrahlung 1 1.5 4.0 4.0 5.0 2.0 0.46 0.84
(c)InverseCompton 1 1.5 60 40 0.64 2.0×10−3 12 18
(d)InverseCompton 1 1.5 60 40 15 2.0 0.11 0.16
(photon field×100)
W3Model
Pion 0.01 1.5 10 4.0 <200g 100 0.29 3.4×10−3
aTheratioelectrons-to-protons at1GeVc−1.
bThemomentumdistributionofparticlesisassumedtobeasmoothlybrokenpowerlaw,wheretheindicesandthebreakmomentumare
identicalforbothacceleratedprotons andelectrons. sL isthespectralindexinmomentumbelowthebreak.
cpbr isthebreakmomentum.
dSpectral indexinmomentum abovethebreak.
eAveragehydrogennumberdensityofambientmedium.
f Thetotal energyiscalculatedforparticles>100MeVc−1.
gThis upper limit was obtained by assuming the flux of the non-thermal radio emissionaround W3 is the same as that from the whole
SNRsincetheradioemissionfromW3ispredominantlythermal.
8 Fermi LAT collaboration
)
1
-
s
2
-
m
c 10
V
e
(
E
d
/
F
d 1
2
E
10-1
10-1 1 10 102
Energy (GeV)
Fig. 3.— Spectral energy distribution of the gamma-ray emission measured by the LAT for the HB 3 SNR. Red squares are LAT flux
points. Horizontal bars indicate the energy range the flux refers to. Vertical bars show statistical errors in red and systematic errors in
black. Inenergybinswherethedetectionisnotsignificant(TS<4)weshowupperlimitsatthe2sigmaconfidencelevel. Theblueregion
isthe68%confidence range(nosystematicerror)oftheLATspectrumassumingthatthespectralshapeisalogparabola.
Fermi LAT Discovery of Extended Gamma-Ray Emissions in the Vicinity of the HB 3 SNR 9
)
1
- Total
s
2
- π0
m
c
10 Bremsstrahlung
V
e
(
E
d
/
F
d
1
2
E
10-1
10-1 1 10 102
Energy (eV)
Fig.4.— Spectral energy distribution of the gamma-ray emission measured by the LAT for W3. The details of the plots and bars are
described in the caption of Figure 3. The blue region is the 68 % confidence range (no systematic error)of the LAT spectrum assuming
thatthespectralshapeisalogparabola. Thelinesshowsaπ0-decaydominatedmodel. Thedetailsofthemodellinesaredescribedinthe
captionofFigure5.
10 Fermi LAT collaboration
)
1 (a) Pion-decay dominated
-s 10
2
-m
c 1
V
e
(
E (b) Brems dominated
10
d
/
F
d
1
2
E
(c) IC dominated
10
1
10-5 10-2 10 104 107 10101011
Energy (eV)
Fig.5.— Multi-band spectrum of the HB 3 SNR. In the GeV band LAT measurements are reported as in Figure 3. The radio
continuum emission(Tian & Leahy 2005) is shown by black dots. Radio emissionis modeled as synchrotron radiation, whilegamma-ray
emissionismodeledbydifferentcombinationsofπ0-decay(long-dashedcurve),bremsstrahlung(dashedcurve),andinverseCompton(IC)
scattering(dottedcurve). DetailsofthemodelsaredescribedinthetextandnumericalvaluesaregiveninTable3: a)π0-decaydominated
model,b)bremsstrahlungdominatedmodel,c)ICdominatedmodel.
