Diffuse Gamma-ray Observations of the Orion Molecular Clouds

0.3 MB·English

by  Akira Okumura

2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1
Diffuse Gamma-ray Observations of the Orion Molecular Clouds
AkiraOkumura∗
DepartmentofPhysics,UniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo,113-0033,Japan
TuneKamae†
SLACNationalAcceleratorLaboratory,Stanford,CA94025,USA
onbehalfoftheFermi/LATCollaboration
Wereportonapreliminaryanalysisofthediffusegamma-rayobservationsoflocalgiantmolecularcloudsOrionA
andBwiththeLargeAreaTelescopeonboardtheFermiGamma-raySpaceTelescope. Thegamma-rayemission
0 ofthecloudsiswellexplainedbyhadronicandelectromagneticinteractionsbetweencosmicraysandnucleiinthe
1 clouds. Consequently,weobtainthetotalmassesoftheOrionAandBcloudstobe(80.6±7.5±4.8)×103M⊙
0 and (39.5±5.2±2.6)×103M⊙, respectively, for the distance to the clouds of 400 pc and the Galactic CR
2 spectrum predicted by GALPROP on the local observations of CRs. The structure of molecular clouds has
been extensively studied by radio telescopes, especially using the line intensity of CO molecules (WCO) and
n a constant conversion factor from WCO to N(H2) (≡ XCO). However, this factor is found to be significantly
a differentforOrionAandB:1.76±0.04±0.02and1.27±0.06±0.01,respectively.
J

5
1 1. INTRODUCTION because they located about only ∼400 pc away from

the Sun, their total mass is of the order of 105M ,
] ⊙
E Diffuse emission of > 100 MeV gamma rays in the and their Galactic coordinates (ℓ ≃ 210◦,b ≃ −15◦)
H Galaxyismainlyinducedbyhadronicinteractionsbe- are far from the Galactic plane and the Galactic cen-
. tweenthe Galactic cosmic rays(CRs) andinterstellar ter.
h
medium(ISM),viathe productionofπ0 particlesand
p In the study of MCs, the biggest difficulty is that
their subsequent decay into photons. This emission
- H , the main component of MCs, cannot be ob-
o can be used to study CRs and the structure of ISM, 2
served directly. Hence, CO molecules which are the
r because its emission can be written as the product
t second abundant molecules, have been widely used
s of the CR flux and the density of ISM, and because
a as a tracer of H2 distribution with a conventional
[ thegamma-rayspectralshapepreservestheCRprop- factor X ≡ N(H )/W which converts veloc-
erties. Observations of the emission have mainly two CO 2 CO
ity integrated CO line intensity, W to H col-
2 uniqueadvantages. First,wecanstudyCRsatthelo- umn density, N(H ). For example, CXO = (21.8 ±
v 2 CO
0 cation of the gamma-ray emission, while direct mea- 0.3)×1020 cm−2(K km s−1)−1 was derived by com-
surements of CRs at the Earth cannot extract their
6 paring a CO survey with HI and dust observations
8 positional information which is already lost during over a Galactic scale [9]. This factor has been also
3 propagation in the interstellar magnetic fields of the determined by diffuse gamma-ray observations, e.g.
2. Galaxy. Second, π0 emission is not affected by the (2.3 ± 0.3)×1020 cm−2(K km s−1)−1 by COS-B[10],
1 gas condition, such as the temperature of interstellar and (1.35 ± 0.15)×1020 cm−2(K km s−1)−1 for the
9 dust, and the CO to H2 ratio which are often used to Orion region by EGRET [6]. However, due to the
0 estimate the column density of ISM.
limitedangularresolutionandphotonstatisticsofthe
: Since the early stage of gamma-ray astronomy,
v precedingtelescopes,theycouldnotstudyX forin-
CO
i diffuse emission from HI gas and molecular clouds dividual clouds. Besides, a mystery of “GeV excess”
X (MCs) has been extensively studied to understand
whichshowsadisagreementbetweentheobserveddif-
r GalacticCRs andthe ISM(e.g. [1,2]). Among them,
a fuse gamma-ray spectrum and the local CR spectra,
the Orion A and the Orion B MCs are two of the
was reported by the EGRET [2].
best targets, and have been well studied by pioneer-
ing gamma-ray telescopes [3, 4, 5, 6]. This is because Since the EGRET era, much progress has been
made in gamma-ray observations and studies on the
theyareconsideredtobethearchetypesofgiantMCs
located near the Earth, and have been surveyed by Orion A/B clouds. The most important progress is
the launch of the Large Area Telescope (LAT) on-
radio telescopes especially using the line emission of
CO molecules (e.g. [7, 8]). From the analysis side, boardtheFermiGamma-raySpaceTelescope (Fermi)
[11]. Its large effective area (∼ 1 m2) and wide en-
theirgamma-rayfluxesarestrongenoughtobedistin-
guished from other diffuse emission components (HI ergy band (20 MeV to > 300 GeV) make it possible
gas,inverseCompton, extragalacticdiffuse emission), to study π0 gamma rays with large photon statistics
above1GeV. Inaddition,theenergy-dependentLAT
angular resolution is a few times better than that of
EGRET.As a result,we areable to resolvethe struc-
∗Electronicaddress: [email protected] ture of the Orion clouds on a scale of ∼ 1 ◦ using
†Electronicaddress: [email protected] higher-energy photons of better angular resolutions,
eConf C091122
2 2009 Fermi Symposium, Washington, D.C., Nov. 2-5
mental response function (IRF) using ScienceTools
b = 0° 5500 xelxel v9r15p4.
pipi
4455 nts/nts/
uu
oo
4400 cc
b = -10° Mono. R2 3355 3. ANALYSIS
3300
3.1. EXTRACTION OF GAMMA-RAY
2255
b = -20° Orion B EMISSION FROM ORION A AND B
2200
Orion A
1155 To study the gamma-ray emission associated with
the Orion MCs (molecular gas), other emission com-
1100
b = -30° ponents must be subtracted from Fig. 1. The emis-
°032 = l °022 =l ° l= 210 l =° 200 l = 1°90 0505 sdioomniinnatnhteornegeisonarceondsiiffstusseofπse0vegraamlmcoam-rpaoyneanntds.eTlehce-
tronbremsstrahlungemissioninduced byinteractions
Figure 1: Gamma-ray (200 MeV to 20 GeV) count map between CRs and HI/H2 gas. In addition, inverse
of the Orion region binned in 0.25◦×0.25◦ pixels with Compton(IC)scatteringofCRelectronsoffinterstel-
Hammer-Aitoff projection. The coordinate centerand lar radiation fields exits in the region. Extragalac-
theradius of theregion are (ℓ,b)=(211◦,−17◦) and 20◦, tic diffuse emission, and residual instrumental back-
respectively. In addition to theGalactic plane, three ground are classified as diffuse emission. We refer to
molecular clouds, Orion A, Orion B and Monoceros R2 the latter two components as “isotropic component”.
are visible as bright extended sources.
TheLATdetectionofsomepointsourcesintheregion
hasbeen reported. However,there is no intense point
source overlapping the Orion MCs [19].
andalsoabletocomparethegamma-rayenergyspec-
In order to extract the Orion A and B, we mod-
trum with the predicted one based on the CR spec-
eledotheremissioncomponentsusingacomputerpro-
tra observed at the Earth. π0 gamma-ray emissiv-
gramcalledGALPROPwhichcalculatesCRflux and
ity of hadronic interactions was modeled in several
diffuse gamma-ray emission in the Galaxy [20, 21].
articles using recent results of accelerators and the-
It uses the data of HI and CO surveys to calcu-
oretical studies (e.g. [12, 13, 14]), so that the mass
late the target mass of CR interaction [9, 22]1. Its
of the clouds can be calculated backwards from the
calculation is known to be consistent with the LAT
gamma-ray emission and the distance to the clouds
gamma-rayobservations in a galactic scale except for
which was measured accurately from recent MASER
the Galactic plane, when reasonable input parame-
observations of the Orion nebula in the Orion A
ters basedon many observations andexperiments are
[15, 16, 17, 18].
given [23, 24, 25]. Each set of input parameters is
referred to as a GALDEF file. In this analysis, we
use GALDEF 54 77Xvarh7Sin which CR proton and
2. OBSERVATIONS AND DATA
electron fluxes are scaled by factors of 1.15 and 1.75,
respectively, to reproduce the LAT observations.
The data used in this analysiswere obtained in the BeforewesubtractedtheHIcontributioninthere-
nominal all-sky survey mode of the LAT between 4 gion, its normalization was estimated in nearby two
August2008and15August2009. AmongalltheLAT 10◦ radius regions near the Orion MCs where no
events,weselectedonesclassifiedasP6 V3 DIFFUSE known large MCs exists. The regions are centered
class [11]. The reconstructedenergyrangeand zenith at(ℓ,b)=(230◦,−16◦) and(210◦,−32◦), and the ob-
anglewerelimitedto200MeV–20GeV,and<105◦, tained fluxes are 1.15 and 0.98 times those predicted
respectively. Gamma rays in a circular region of ra- by GALPROP, respectively. Thus we multiply 1.07
dius 20◦ centered at (ℓ,b) = (211◦,−17◦) were then totheGALPROPpredictionforgamma-rayintensity
selected for later analyses. from HI gas, and add 8% to the systematic uncer-
All events in the selected data are binned in 160× taintyofHIsubtractionprocess. ThescaledHIemis-
160 equalarea pixels with the Hammer-Aitoff projec- sionwasconvolvedwiththeLATIRF,andsubtracted
tionasshowninFig.1. TheOrionAandBareeasily from the pixelized data in each energy bin. In addi-
recognizedbytheircountexcessfromthesurrounding tion to this, IC emission was also subtracted. Since
diffuse emission and point sources.
Thereconstructedenergiesoftheeventsarebinned
in a logarithmic series of 16 between 200 MeV and
20 GeV. The LAT exposure of each energy bin was 1Weassumed aconstant spintemperature of125 K forthe
calculated from the LAT pointing history and instru- HImap.
eConf C091122
2009 Fermi Symposium, Washington, D.C., Nov. 2-5 3
×10-6 ×10-6
0° 90 -2-1srs 0° 90 -2-1srs
80 -2m 80 -2m
c c
s s
n n
70 o 70 o
-10° hot -10° hot
60 p 60 p
de 50 de 50
u u
Latit -20° 40 Latit -20° 40
30 30
20 20
-30° -30°
10 10
°032 °022 °210 °200 1°90 0 °032 °022 °210 °200 1°90 0
Longitude Longitude
(a)Extractedgamma-rayintensity (b)ModeledWCO-basedintensity
Figure 2: (a) Same as Fig. 1, but shown after subtracting theother diffuse components. The unit is integrated photon
intensity between 200 MeV and 20 GeV. Solid lines show thedefinitions of the regions of Orion A and B. Rectangles
with dashed lines show theregions which are used to estimate theisotropic component. A straight dot-dashed line at
ℓ=212◦ is theboundaryof two separated regions in Orion A.(b)A modeled gamma-ray intensity map simulated from
a constant XCO =1.5×1020 cm−2(K km s−1)−1, WCO map [9], theLATresponse, theemissivity of π0 gamma and
electron bremsstrahlung, and theCR fluxpredicted byGALPROP.(These figures are still PRELIMINARY)
the ICcontributionissmallerthanotheremission,its sumed,whichare∼8%smallerthantheobservedCR
uncertainty does not affect our results. flux at the Earth [26, 27, 28]. Gamma-ray inclusive
Finally, the isotropic component was estimated cross sections for proton-He, alpha-H, alpha-He, and
from surrounding regions (background regions) heavier CR metals are scaled using the method de-
around the Orion MCs, then subtracted from the scribed in [14, 29]. A factor 1.02 as a contribution
region-of-interest. Fig. 2a shows the gamma-ray in- from ISM metal was finally multiplied to the calcu-
tensity map after the subtraction method described lated π0 emissivity [13]. The bremsstrahlung spec-
above . It was divided by the LAT exposure, and re- trum was calculated using GALPROP.
binned in 1◦×1◦ pixels. We defined the boundary of Theobtainedfitresultsshowgoodagreementswith
Orion A and B with solid lines in Fig. 2a. the π0 spectrum. This means that gamma-ray emis-
sion from MCs is dominated by known physical pro-
cesses, i.e. interactions between CRs and the gas.
3.2. ENERGY SPECTRA AND TOTAL Therefore, we are able to calculate the total mass of
MASSES OF THE CLOUDS theclouds. AssumingthedistancetotheOrionclouds
to be 400 pc [15, 16, 17, 18], the mass of the OrionA
The energy spectra associated with the Orion A andBareestimatedtobe(80.6±7.5(stat)±4.8(HI))×
and B clouds are shown in Fig. 3. All photons in 103M⊙ and (39.5±5.2(stat)±2.6(HI))×103M⊙, re-
the boundary regions in Fig. 2a are collected for each spectively. The second terms indicate statistical er-
cloud. They are fitted with π0 and bremsstrahlung rors of the fit, and the third ones are systematic er-
components with two free normalization parameters. rors introduced by the uncertainty of HI subtraction
Here, we calculated π0 gamma-ray emissivity using a explained in subsection 3.1.
recent parameterized model [12]. Input CR spectra
at the Orion region2 predicted by GALPROP are as-
3.3. CORRELATION BETWEEN
GAMMA-RAY AND CO INTENSITY
2Corresponding to a cylindrical Galactic location (R = Assumption of a constant XCO in small scales (∼
8.5kpc,Z=−0.14kpc). 10 pc to ∼ 100 pc) or Galactic scale has been widely
eConf C091122
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Pionic Orion A Total
(80.6 ± 7.5 ± 4.8) × 103 M
Electron Bremsstrahlung 400
χ2/ndf = 16.7/10
10-4
Orion A Total
1)
-V (39.5 ± 5.2 ± 2.6) × 103 M
400
Me χ2/ndf = 9.3/10
1
-s
2
-m
c
2
V
e
M
( 10-5
E
d
/
N
d
2
E
10-6
102 103 104
Energy (MeV)
Figure 3: Energy spectra of theOrion A (black) and B (red) fitted with modeled spectra of π0 gamma (dashed) and
electron bremsstrahlung(dotted). Statistical errorsareshown withbars,and thesystematic errorsoftheLATresponse
are shown with shaded area. Polygons of solid lines show the systematic uncertainties of HI subtraction process. (This
figureis still PRELIMINARY)
used in studies of MCs (e.g. [6, 9]), while gradient of 0.04(stat)±0.02(HI))×1020 and (1.27±0.06(stat)±
XCO isalsodiscussed[30,31]. However,thereisroom 0.01(HI))×1020, respectively.
toreconsiderthissimpleassumptionthatW canbe
CO
used to trace the structure of MCs [32, 33].
Utilizingthegoodangularresolutionandlargepho- 4. DISCUSSION
ton statistics of the LAT, the correlation between
gamma-ray intensity and a W map can be stud-
CO Weobtainedtheenergyspectraofthe OrionAand
ied in a scale of ∼ 1◦. Fig. 2b shows a mod-
B, and showed that they can be explained by the CR
eled gamma-ray intensity map based on a CO survey
interactions with the nuclei in the gas. Thus, a lin-
and GALPROP calculation with a constant X of
CO ear correlation between gamma-ray intensity and the
1.5 ×1020 cm−2(K km s−1)−1, where the LAT IRF is
column density of the clouds are expected, because
convolved.
gamma-ray emission is not affected by environmen-
Fig. 4 shows the pixel-by-pixel correlation between tal condition of the gas. In fact, the correlation be-
theobservedgamma-rayintensity(Fig.2a;x)andthe tween gamma-rayintensity and the W -based mod-
CO
WCO model (Fig. 2b; y). The best fit results by a lin- eledmapholdsalinearityforroughlyonedecadewith
ear function (y = p0 +p1x) are also shown. If the only a few 10% deviations. This implies that cosmic
gamma-rayintensity canbe presentedby the product rays of energy above ∼ 1 GeV can penetrate dense
ofa constantCR flux anda constantXCO, the slopes cores of molecular clouds. However, the correlation
of the best-fit functions become constant. However, slopes in the Orion region was found to be not con-
wefoundthattheobtainedslopesaresignificantlydif- stant as shown in Fig. 4.
ferentfortheOrionAandBclouds,whiletheseclouds There are some possible interpretations of the dif-
arethoughttobeinacommonenvironmentsincetheir ferent correlationslopes. In the analysis, we assumed
birth. In Fig. 4a, two additional linear functions are aconstantCRfluxinthe Orionregion. However,ifit
shown: one is fitted with the larger longitude region issignificantlydifferentinthe threeseparatedregions
inOrionA(ℓ>212◦),andtheotherisforthesmaller inFig.2a,thegamma-rayintensityalsovariesaccord-
longitude region (212◦ >ℓ) (see Fig. 2a). ing to the CR flux variation which might be caused
Since we modeled the vertical values using a con- by the strong magnetic field in molecular clouds.
stant X of 1.5 ×1020 cm−2(K km s−1)−1, the cor- On the contrary, if the CR flux is almost constant
CO
responding X of Orion A and B are (1.76 ± in the region, we need to consider nonuniformity of
CO
eConf C091122
2009 Fermi Symposium, Washington, D.C., Nov. 2-5 5
(a) (b)
70 70
χ2 / ndf 257 / 74 χχ22 // nnddff 6699..99 // 2288
60 60
p0 1.2 ± 0.313 pp00 --00..779966 ±± 11..0022
-1-1-2)srsm50 p1 0.85 ± 0.0186 -1-1-2)srsm50 pp11 11..1188 ±± 00..00551166
c40 c40
-6 0 -6 0
1 1
e (30 e (30
ns ns
o o
esp20 esp20
R R
× ×
co 10 co 10
W W
0 0
-10 -10
-10 0 10 20 30 40 50 60 70 -10 0 10 20 30 40 50 60 70
Observed (10-6 cm-2s-1sr-1) Observed (10-6 cm-2s-1sr-1)
Figure 4: Correlations between themeasured gamma-ray intensity (x) and a model prediction byWCO (y)assuming
XCO =1.5×1020 cm−2(K km s−1)−1. Data points are fitted with a linear function y=p0+p1x. (a) Orion A. Circles
correspond to thedata in pixels of ℓ>212◦. (b) Orion B. (This figures are still PRELIMINARY)
X in the region. While the CO line (J = 1−0) Acknowledgments
CO
is the de fact standard of mass tracers of molecular
clouds, the CO to H2 ratio can be varied by the con- The Fermi LAT Collaboration acknowledges gen-
dition of each cloud. By comparing WCO maps with erous ongoing support from a number of agencies
dustobservationsorgamma-rayobservations[32,33], and institutes that have supported both the develop-
itisknownthatthereexistsgaswhichisnottracedby ment and the operation of the LAT as well as scien-
CO or HI observations. The nonuniformity of XCO tific data analysis. These include the National Aero-
in the Orion region can also be explained by this. In nautics and Space Administration and the Depart-
fact, IR emissionfrom interstellardust andvisual ex- ment of Energy in the United States, the Commis-
tinction by dust are stronger in the left half of Orion sariat `a l’Energie Atomique and the Centre National
A than that expected from CO observations [34, 35]. de la Recherche Scientifique / Institut National de
Therefore,if there exist H2 molecules not fully traced Physique Nucl´eaire et de Physique des Particules in
byWCO,buttracedbygamma-rayanddust,theXCO France, the Agenzia Spaziale Italiana and the Isti-
variation shown in Fig. 4 can be explained. In addi- tutoNazionalediFisicaNucleareinItaly,theMinistry
tiontoH2,itispossiblethatapartofHIgaswasnot of Education, Culture, Sports, Science and Technol-
subtracted adequately, because its spin temperature ogy (MEXT), High Energy Accelerator Research Or-
is relatively low in cold molecular clouds compared ganization (KEK) and Japan Aerospace Exploration
to surrounding diffuse HI region. While we assumed Agency (JAXA) in Japan, and the K. A. Wallenberg
a constant spin temperature of 125 K in the region, Foundation, the Swedish Research Council and the
optically thick HI gas whose temperature is low may Swedish National Space Board in Sweden.
not have been corrected properly and contribute to
Additional support for science analysis during the
the gamma-rayemission, especially in the left part of
operations phase is gratefully acknowledged from the
Orion A.
Istituto NazionalediAstrofisicainItaly andthe Cen-
tre National d’E´tudes Spatiales in France.
While the gamma-ray emission from HI and H2
cannotbe distinguishedfromeachother,ourobserva-
tionsrevealednon-linearrelationbetweengamma-ray
and W maps, which will enable us to understand
CO
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