The cosmic-ray and gas content of the Cygnus region as measured in gamma rays by the Fermi Large Area Telescope
ABSTRACT The Cygnus region hosts a giant molecular-cloud complex which actively forms
massive stars. Interactions of cosmic rays with interstellar gas and radiation
fields make it shine at gamma-ray energies. Several gamma-ray pulsars and other
energetic sources are seen in this direction. In this paper we analyse the
gamma-ray emission measured by the Fermi Large Area Telescope in the energy
range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content
over the scale of the whole Cygnus complex. The signal from bright pulsars is
largely reduced by selecting photons in their off-pulse phase intervals. We
compare the diffuse gamma-ray emission with interstellar gas maps derived from
radio/mm-wave lines and visual extinction data, and a global model of the
region, including other pulsars and gamma-ray sources, is sought. The integral
HI emissivity and its spectral energy distribution are both consistent within
the systematics with LAT measurements in the interstellar space near the solar
system. The average X=N(H2)/W(CO) ratio is consistent with other LAT
measurements in the Local Arm. We detect significant gamma-ray emission from
dark neutral gas for a mass corresponding to ~40% of that traced by CO. Despite
the conspicuous star formation activity and large masses of the interstellar
clouds, the cosmic-ray population in the Cygnus complex averaged over a few
hundred parsecs is similar to that of the local interstellar space. (abridged)
arXiv:1110.6123v2 [astro-ph.HE] 30 Nov 2011
Astronomy & Astrophysics manuscript no. ms
December 1, 2011
c ? ESO 2011
The cosmic-ray and gas content of the Cygnus region as measured
in gamma rays by the Fermi Large Area Telescope
M. Ackermann(1), M. Ajello(1), A. Allafort(1), L. Baldini(2), J. Ballet(3), G. Barbiellini(4,5), D. Bastieri(6,7), A. Belfiore(8),
R. Bellazzini(2), B. Berenji(1), R. D. Blandford(1), E. D. Bloom(1), E. Bonamente(9,10), A. W. Borgland(1),
E. Bottacini(1), J. Bregeon(2), M. Brigida(11,12), P. Bruel(13), R. Buehler(1), S. Buson(6,7), G. A. Caliandro(14),
R. A. Cameron(1), P. A. Caraveo(8), J. M. Casandjian(3), C. Cecchi(9,10), A. Chekhtman(15), S. Ciprini(16,10), R. Claus(1),
J. Cohen-Tanugi(17), A. de Angelis(18), F. de Palma(11,12), C. D. Dermer(19), E. do Couto e Silva(1), P. S. Drell(1),
D. Dumora(20), C. Favuzzi(11,12), S. J. Fegan(13), W. B. Focke(1), P. Fortin(13), Y. Fukazawa(21), P. Fusco(11,12),
F. Gargano(12), S. Germani(9,10), N. Giglietto(11,12), F. Giordano(11,12), M. Giroletti(22), T. Glanzman(1), G. Godfrey(1),
I. A. Grenier(3), L. Guillemot(23†), S. Guiriec(24), D. Hadasch(14), Y. Hanabata(21), A. K. Harding(25), M. Hayashida(1),
K. Hayashi(21), E. Hays(25), G. J´ ohannesson(26), A. S. Johnson(1), T. Kamae(1), H. Katagiri(27), J. Kataoka(28),
M. Kerr(1), J. Kn¨ odlseder(29,30), M. Kuss(2), J. Lande(1), L. Latronico(2), S.-H. Lee(31), F. Longo(4,5), F. Loparco(11,12),
B. Lott(20), M. N. Lovellette(19), P. Lubrano(9,10), P. Martin(32), M. N. Mazziotta(12), J. E. McEnery(25,33), J. Mehault(17),
P. F. Michelson(1), W. Mitthumsiri(1), T. Mizuno(21), C. Monte(11,12), M. E. Monzani(1), A. Morselli(34),
I. V. Moskalenko(1), S. Murgia(1), M. Naumann-Godo(3), P. L. Nolan(1), J. P. Norris(35), E. Nuss(17), T. Ohsugi(36),
A. Okumura(1,37), N. Omodei(1), E. Orlando(1,32), J. F. Ormes(38), M. Ozaki(37), D. Paneque(39,1), D. Parent(40),
M. Pesce-Rollins(2), M. Pierbattista(3), F. Piron(17), T. A. Porter(1,1), S. Rain` o(11,12), R. Rando(6,7), M. Razzano(2),
O. Reimer(41,1), T. Reposeur(20), S. Ritz(42), P. M. Saz Parkinson(42), C. Sgr` o(2), E. J. Siskind(43), P. D. Smith(44),
P. Spinelli(11,12), A. W. Strong(32), H. Takahashi(36), T. Tanaka(1), J. G. Thayer(1), J. B. Thayer(1), D. J. Thompson(25),
L. Tibaldo(6,7,3,45), D. F. Torres(14,46), G. Tosti(9,10), A. Tramacere(1,47,48), E. Troja(25,49), Y. Uchiyama(1),
J. Vandenbroucke(1), V. Vasileiou(17), G. Vianello(1,47), V. Vitale(34,50), A. P. Waite(1), P. Wang(1), B. L. Winer(44),
K. S. Wood(19), Z. Yang(51,52), S. Zimmer(51,52), and S. Bontemps(53)
(Affiliations can be found after the references)
Received ...; accepted ...
Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar
gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction.
Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope in the energy range from 100 MeV to 100 GeV
in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive
stellar clusters and from individual sources is addressed elsewhere.
Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray
emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other
pulsars and γ-ray sources, is sought.
Results. The integral Hi emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06±0.11(stat.)+0.15
photons s−1sr−1H-atom−1, where the systematic error is dominated by the uncertainty on the Hi opacity to calculate its column densities. The
integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space
near the solar system. The average XCO = N(H2)/WCOratio is found to be [1.68 ± 0.05(stat.)+0.87
km s−1)−1, consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass
corresponding to ∼ 40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to
Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus
complex averaged over a few hundred parsecs is similar to that of the local interstellar space.
−0.10(Hi opacity)] × 1020molecules cm−2(K
−1× 106M⊙ at a distance of 1.4 kpc.
Key words. ISM: abundances – ISM: clouds – cosmic rays – Gamma rays: ISM
Regions with conspicuousstar formationactivity are of great in-
terest for understanding the life cycle of interstellar matter and
the properties of cosmic rays (CRs) in the Galaxy. Interstellar
γ-ray emission produced by CR interactions with the interstel-
lar gas via nucleon-nucleon inelastic collisions and electron
Bremsstrahlung can be used to probe their CR and gas content.
High-energyγ-ray observationshave entereda new era since
the launch of the Fermi Gamma-ray Space Telescope in 2008.
The Fermi Large Area Telescope (LAT; Atwood et al. 2009) has
already measured strong γ-ray emission toward the 30 Doradus
starburst region in the Large Magellanic Cloud (Abdo et al.
2010d), and it also pointed out a global correlation between the
γ-ray luminosity and star-formation rate in a few normal galax-
ies (Abdo et al. 2010c).
A primary observational target for Fermi in our Galaxy is
the Cygnus X star-forming region, owing to its proximity (∼
1.4 kpc; Hanson 2003; Negueruela et al. 2008) and the avail-
ability of numerous multiwavelength observations. Named af-
ter the strong emission at X-ray wavelengths (Cash et al. 1980),
Cygnus X is located around the Galactic longitude l = 80◦, tan-
gent to the Local Spur. It contains numerousHii regions and OB
associations (Uyanıker et al. 2001; Le Duigou & Kn¨ odlseder
2002). It has long been debated whether it represents a coherent
complex or the alignment of different structures along the line
of sight. Recent high-resolutionobservations by Schneider et al.
(2006) and Roy et al. (2011) have pointed out that most of the
molecular clouds in the Cygnus X region are connected and
partly show evidence of interactions with the massive stellar
cluster Cygnus OB2 and other OB associations in the region.
Foreground molecular clouds from the Great Cygnus Rift, at
0.6–0.8 kpc, contribute little to the high mass seen in interac-
tion with the Cygnus X region itself, at 1.4 kpc. Therefore, the
molecular cloud complex appears as one of the most massive in
the Galaxy.Atomicgas seen in these directionsis probablymore
widespread along the line of sight.
Abdo et al. (2007, 2008) analyzed Milagro measurements at
energies > 10 TeV and reports an excess of diffuse γ-ray emis-
sion with respect to predictions based on CR spectra equivalent
to those near the Earth. They attribute the excess to the possible
presence of freshly accelerated particles.
The escape of CRs from their sources and the early propa-
gation in the surrounding medium have so far been poorly con-
strained by observations. In particular, particles accelerated in
regions of massive star-formation are likely to be significantly
influenced by the turbulent environment. It is therefore interest-
ing to investigate how the CR populations on the scale of the
massive stellar clusters and on the larger scale of the parent in-
terstellar complexcompare with each other and with the average
CR population of the Local Spur.
This paper reports our analysis of the γ-ray emission mea-
sured by the LAT in the energy range between 100 MeV and
scale propertiesof the interstellar emission to probe the CR pop-
ulation and to complement gas and dust observations at other
wavelengths to constrain the amount of gas in different phases
over the whole Cygnus complex. We also build an improved in-
terstellar background framework for the study of individual γ-
raysources that will be treatedin companionpapers. We discuss
interstellar emission in the star-forming region of Cygnus X in a
dedicated paper (Ackermann et al. 2011a).
In this section we describe the data used in the paper. First of all,
criteria (§ 2.1.1) and the procedure to mitigate the signals from
the brightest pulsars (§ 2.1.2). Then, we present an overview of
ter (§ 2.2), including radio and mm-wave lines (§ 2.2.1) to trace
neutral gas, visual extinction (§ 2.2.2) to trace dark neutral gas,
and the free-free emission intensities obtained from microwave
observations (§ 2.2.3) to trace the ionized gas.
2.1. Gamma-ray data
2.1.1. Observations and data selection
The LAT is a pair-tracking telescope detecting photons from
20 MeV to more than 300 GeV. The instrument is described in
Atwood et al. (2009) and its on-orbit calibration in Abdo et al.
(2009a). The LAT operates most of the time in continuous sky-
survey mode. We accumulated data for our region of interest
from August 5, 2008 (MET1239587201) to August 5, 2010
We selected data according to the tightest available back-
ground rejection criteria, correspondingto the Pass 6 Dataclean
event class (Abdo et al. 2010e)2. In order to limit the contami-
nation from the Earth’s atmospheric γ-ray emission, we selected
events with measured arrival directions within 100◦of the local
zenith and within 65◦of the instrument boresight, taken during
periods when the LAT rocking angle was less than 52◦.
The angular resolution of the LAT strongly depends on the
photon energy,improvingas the energy increases (Atwood et al.
2009). Confusion at low energies is a problem since we aim
to spatially separate the different components in the crowded
Cygnus X region.We thereforeacceptedbelow 1 GeV onlypho-
tons that produced electron-positron pairs in the thin converter
planes ofthe front sectionof the tracker,which providesa higher
angular resolution (Atwood et al. 2009). Above 1 GeV, we kept
all events which converted either in the front or back section of
We analysed data at Galactic longitudes 72◦≤ l ≤ 88◦and
latitudes −15◦≤ b ≤ +15◦. The longitude window contains
the interstellar complexes associated with Cygnus X; the lati-
tude window is large enough to allow a reliable separation of
the large-scale emission from atomic gas, isotropic background
and inverse-Compton (IC) scattering of low-energy radiation
fields by CR electrons. We analysed the data in the 100 MeV–
atics are large (Rando et al. 2009) and the angular resolution is
poor, whereas above 100 GeV we are limited by the low photon
2.1.2. Removal of bright pulsars
Three bright pulsars dominate the γ-ray emission from the re-
gionbelowa few GeV: the radiopulsar J2021+3651(Abdo et al.
2009e) and the two LAT-discovered pulsars J2021+4026 and
J2032+4127 (Abdo et al. 2009b). To increase the sensitivity to
faint sources and to the spatial structure of the diffuse emis-
sion, we reduced their contribution by excluding the periodic
time intervals when their pulsed emission is the most intense.
Removing the intense pulsed flux helps to reduce the impact of
any incorrect modeling of such bright sources on the results.
To assign pulse phases for each of the three pulsars, we pro-
duced timing models using Tempo2 (Hobbs et al. 2006) accord-
ingtothemethoddescribedinRay et al.(2011)3.Figure1shows
the three light curves and the phase intervals with bright pulsed
emission. The phase boundariesare reportedin Table 1, together
with the fraction of time in the off-pulse interval suitable for our
study. There is a considerable level of off-pulse emission toward
1Fermi Mission Elapsed Time, i.e. seconds since 2001 January 1 at
2Performance figures for the Dataclean event selection are given in
3Forthethreepulsars, theRMSof thetimingresidualsisbelow 1.1%
of their rotational period.
0 0.10.2 0.30.40.5 0.6 0.70.80.91
00.1 0.2 0.3 0.40.5 0.6 0.70.80.91
00.10.2 0.30.40.50.60.70.8 0.91
Fig.1. Light curves and off-pulse phase intervals for the three
bright pulsars. The light curves are constructed for illustration
purposes with photons recorded in a circular region of radius
0.5◦around the pulsar positions and energies > 200 MeV.
Table 1. Off-pulse phase intervals and time fractions
of the three bright pulsars.
0–0.12, 0.22–0.59, 0.7–1
0–0.14, 0.2–0.59, 0.79–1
PSRtime fraction (%)
PSR J2021+4026 that cannot be removed (Abdo et al. 2010b);
however, given the brightness of the source, the removal of the
on-pulse interval is useful for our aims.
A total countmap in the off-pulsephase intervalsof the three
brightpulsarsis providedforillustrationin Fig. 2. To removethe
pulsar signal without excessivelysacrificing the photonstatistics
510 1520 2530 3540 45
Galactic longitude (deg)
Galactic latitude (deg)
Fig.2. Total count map in the energy range 100 MeV–100 GeV,
binned over a 0.125◦× 0.125◦grid in Galactic coordinates in
Cartesian projection. Data were selected according to the crite-
ria described in the text (§ 2.1.1) and in the off-pulse phase in-
tervals of the three bright pulsars (§ 2.1.2), whose positions are
marked by diamonds. Counts are saturated between 0 and 50,
and smoothed for display with a Gaussian kernel of σ = 0.25◦.
inotherdirections,we restrictedthetimingselectiontoa circular
grid (described later in § 3.2) the centroids of which lie within
rcut(E) = 2 ·
where the symbol ⊕ indicates addition in quadrature. This is an
approximate representation of the LAT 95% containment angle
as a function of energy. We note that the accurate parametriza-
tion of the LAT point spread function (PSF) depends on energy,
pair-conversion point in the tracker and, to a lesser extent, on
the incidence angle. The PSF is best represented by the LAT in-
strument response functions (IRFs), which are used later for the
likelihood analysis. The above acceptance-averagedapproxima-
tion for the containment angle is only useful for calculating the
radius rcut, and we verified that the results are insensitive to rea-
sonable variations in this parameter.
To take the cut on pulsar phases into account, for each di-
rection in the sky and energy the exposure (see again § 3.2) was
multiplied by the remaining livetime fraction. The remainder of
the pulsar emission was included in the model using
– a point source to represent emission in the off-pulse interval;
– a second point source, for which the number of expected
counts is set to zero at r < rcut(E) from the pulsar position,
to representon-pulseγ-raysspilling overat r > rcut(E) in the
tails of the PSF.
The two sources have free independentfluxes in each energybin
of the analysis. This is particularly important to account for the
also to compensate for any mismatch between the tails of the
model PSF and the emission at large angles from the brightest
sources in the region.
Since the three pulsars have exponentialspectral cutoffs near
2 − 3 GeV (Abdo et al. 2010b) the phase selection was not ap-
plied above 10 GeV where the level of pulsed emission is low
and each pulsar was accounted for by a single point source. On
the other hand, given the abundant statistics but poor angular
resolution at low energies (more than half of the region of in-
terest would be subject to on-pulse event removal), we selected
off-pulsephotonsfor the whole regionbelow316 MeV4. In this
case no “spill-over” source was necessary.
2.2. Ancillary data
2.2.1. Radio/mm-wave lines: neutral gas
Neutral atomic hydrogen, Hi, was traced thanks to its 21-
cm line. Where available5we used data from the Canadian
Galactic Plane Survey (CGPS; Taylor et al. 2003) rebinned onto
the 0.125◦×0.125◦grid used for the other maps. Elsewhere, we
useddatafromtheLeiden/Argentine/Bonn(LAB;Kalberla et al.
2005) survey, with a coarser binning of 0.5◦. We checked the
consistency of the two survey calibrations in the overlap region.
Molecular hydrogen cannot be observed directly in its most
abundant cold phase. The velocity-integratedbrightness temper-
ature of the12CO 2.6-mm line, WCO, is often assumed to lin-
early scale with the N(H2) column density. We used CO data
from the composite survey by Dame et al. (2001), filtered with
the moment-masking technique (Dame 2011) in order to reduce
the noise while preserving the faint cloud edges and keeping the
resolution of the original data.
The Doppler shift of radio/mm-wave lines can be used to
kinematically separate the Cygnus complex from two faint seg-
ments of the Perseus and outer spiral arms seen beyond Cygnus
in the same direction. We applied the kinematic separation pro-
cedure illustrated by Abdo et al. (2010f), starting from a prelim-
inary boundary located at a Galactocentric radius6R = 9.4 kpc
and then adapting the separation to the cloud structures and cor-
recting for the spill-over due to the broad velocity dispersion of
Hi lines. The separation into two regions is accurate enough to
4See § 3.2 for the definition of the energy grid used in the analysis.
5The CGPS coverage is almost complete at −3.5◦≤ b ≤ +5.5◦for
this longitude range.
6Based on the assumption of a flat rotation curve with solar radius
R⊙ = 8.5 kpc and Galactic rotation velocity at the solar circle V⊙ =
220 km s−1
modeltheinterstellarγ-rayemissioninCygnussinceAbdo et al.
(2010f) and Ackermann et al. (2011b) did not find any signifi-
cant gradients of the gas γ-ray emissivities in the outer region of
the Milky Way. We applied the kinematic separation procedure
to prepare maps of the column densities of atomic gas, N(Hi),
and of WCO. The maps are shown in Fig. 3 for Hi and Fig. 4 for
CO. All the gas maps mentionedin the paper have> 10◦borders
around the analysis region used to properly convolve the model
with the LAT PSF.
Substantial uncertainties in the determination of N(Hi) arise
from the choice of spin temperature for the optical depth correc-
tion. We adopted a uniform TS = 250 K as baseline case, which
is the average spin temperature that best reproduces the blend-
ing of cold and warm atomic gas according to observations of
emission-absorptionHi pairs in the regioncoveredbythe CGPS
(Dickey et al. 2009). Other values 100 K ≤ TS< ∞ will be con-
sidered later to evaluate the related systematic uncertainties af-
fecting the results of our analysis.
2.2.2. Visual extinction: dark neutral gas
Multiwavelength observations indicate that the combination of
the Hi and CO lines does not properly trace the total col-
umn densities of the neutral interstellar medium (ISM) (e.g.
Magnani et al. 2003; Grenier et al. 2005; Abdo et al. 2010f;
Langer et al. 2010; Ade et al. 2011). Since the work by
Grenier et al. (2005), dust tracers have been used in γ-ray anal-
yses to complement the Hi and CO lines, under the assumption
that dust grains are well mixed with gas in the warm and cold
phases of the ISM and therefore provide an estimate of total gas
column densities. Grenier et al. (2005) and Abdo et al. (2010f)
adoptedthe E(B − V) color excess map by Schlegel et al. (1998)
as a tracer of the total column densities, and used the E(B − V)
residuals –i.e. E(B − V) minus the best-fit linear combination of
N(Hi) and WCOmaps– as a tracer of the dark-gas column densi-
ties in nearby clouds of the Gould Belt.
The use of the E(B − V) map is problematicin the Cygnus X
region for two reasons:
– numerous infrared point sources contaminate the map;
– the temperature correction used by Schlegel et al. (1998) to
derive the dust column-density map from IRAS/ISSA mea-
surements is highly uncertain in regions of massive star-
formation because of the enhanced radiation fields.
We have therefore adopted the visual extinction AV as
derived from the reddening of near-infrared sources in the
2MASS catalog (Skrutskie et al. 2006). The AVmaps produced
by Rowles & Froebrich (2009) and Froebrich & Rowles (2010)
were used for AV < 5 mag. They exhibit saturation effects
at higher extinction values, so we complemented them with
an AV map obtained from 2MASS data using the code and
method developed by Schneider et al. (2011). The latter use
the Besanc ¸on stellar population model (Robin & Creze 1986;
Robin et al. 2003) to filter out the contribution from the fore-
ground bluest stars7. The second AV map was built in a 12◦
region centered on (l,b) = (80◦,0◦), and, compared with the
first set of maps, it presented an offset of ∼ 0.46 mag at low
extinction. We constructed the final AV map from the direct
Rowles & Froebrich (2009) data below 5 mag and from the sec-
ond map, offset by 0.46 mag, at higher extinction.
7To do so, a distance from the observer needs to be assumed for the
clouds under consideration. We verified that variations of a few hundred
parsecs do not significantly change the results presented in the paper.
88 84 8076 72
Galactic longitude (deg)
Galactic latitude (deg)
88 848076 72
Galactic longitude (deg)
Galactic latitude (deg)
102030 40 5060 708090100
Fig.3. Maps of N(Hi) column densities in the Cygnus complex in the Local Spur (left) and in the outer Galaxy (right), under the
assumption of a uniform spin temperature of 250 K. The color scales with N(Hi) in units of 1020atoms cm−2. The maps were
smoothed with a Gaussian kernel of σ = 0.25◦for display.
The AVmap was binned onto the same 0.125◦× 0.125◦grid
in Cartesian projection as the other maps. The AVmap was fit-
ted with a linear combination of the N(Hi) and WCOmaps pre-
viously described. The input AVmap minus the best-fit linear
combination of the N(Hi) and WCOmaps yielded the AVexcess
map, AV,exc, which will be used to trace the dark neutral gas.
Only residuals corresponding to input AV> 0.3 mag were kept
to limit the noise off the plane. The AVexcess map is shown in
2.2.3. Microwave emission: ionized gas
Away from Hii regions around massive stars and stellar clus-
ters, the ionized gas constitutes a layer of characteristic height
? 1 kpc over the Galactic plane with little mass compared to
the neutral phases (Cordes & Lazio 2002). Therefore, it has of-
ten been neglected in previous γ-ray studies. However, we find
in the Cygnus X region many conspicuous Hii regions excited
by the intense radiation fields of the numerous massive stars
(Uyanıker et al. 2001; Paladini et al. 2003).
Ionized gas masses can be traced by free-free emission fol-
lowing the prescription by Sodroski et al. (1989, 1997) to derive
the N(Hii) column densities:
where Iffis the free-freeemission intensity at the frequencyν, Te
density. We adopted a free-free emission map derived from the
seven-year WMAP data in the Q band (40 GHz) by Gold et al.
(2011) using the maximum entropy method from the prior tem-
plate given by the extinction-corrected Hα map by Finkbeiner
(2003). It was rebinned onto the 0.125◦× 0.125◦grid used for
the other maps, as shown in Fig. 6.
N(Hii) = 1.2×1015cm−2?Te
3.1. Analysis model
3.1.1. Diffuse emission
Since the bulk of Galactic CRs in the relevant energy ranges
are expected to be smoothly distributed on scales exceeding