Diffuse X-ray Emission from the Carina Nebula Observed with Suzaku
Diffuse X-ray Emission from the Carina Nebula Observed with
Suzaku
Kenji Hamaguchi1,2, the Suzaku η Carinae team and the Carinae D-1 team
1CRESST and X-ray Astrophysics Laboratory NASA/GSFC, Greenbelt, MD 20771
2Universities Space Research Association, 10211 Wincopin Circle, Suite 500,
Columbia, MD 21044
A number of giant HII regions are associated with soft diffuse X-ray emission. Among
these, the Carina nebula possesses the brightest soft diffuse emission. The required plasma
temperature and thermal energy can be produced by collisions or termination of fast winds
from main-sequence or embedded young O stars, but the extended emission is often observed
from regions apart from massive stellar clusters. The origin of the X-ray emission is unknown.
The XIS CCD camera onboard Suzaku has the best spectral resolution for extended
soft sources so far, and is therefore capable of measuring key emission lines in the soft band.
Suzaku observed the core and the eastern side of the Carina nebula (Car-D1) in 2005 Aug and
2006 June, respectively. Spectra of the south part of the core and Car-D1 similarly showed
strong L-shell lines of iron ions and K-shell lines of silicon ions, while in the north of the core
these lines were much weaker. Fitting the spectra with an absorbed thin-thermal plasma
model showed kT∼0.2, 0.6 keV and NH∼1−2×10
21 cm−2 with a factor of 2-3 abundance
variation in oxygen, magnesium, silicon and iron. The plasma might originate from an old
supernova, or a super shell of multiple supernovae.
§1. Extended X-ray Emission from the Star Forming Region
Soft X-ray emission nebulae with kT∼0.1–0.8 keV, log LX∼33-35 ergs s
−1, and
size of ∼1–103 pc accompany a number of giant HII regions (see Table 4 of Ref. 6).
Chandra observations of extended emission in a few star forming clusters indicate
that the emission may arise from the fast O star stellar winds thermalized either by
wind-wind collisions or by a termination shock. However, the emission is often found
outside of the massive stellar clusters, so that another origin, such as an otherwise
unrecognized supernova remnant, cannot be ruled out.
In principle, the origin of the diffuse emission can be determined by measuring
its composition. For example, the plasma should be overabundant in nitrogen and
neon if it originates from winds from nitrogen-rich Wolf-Rayet stars (WN), while it
would be overabundant in oxygen if it arises from a Type II SNR. The temperature
of the plasma, typically a few million degrees, makes soft X-ray band studies highly
desirable, because of the presence in this band of strong lines from these elements,
plus carbon, silicon and iron.
The Carina Nebula, which contains several evolved and main-sequence massive
stars such as η Car, WR 25 and massive stellar clusters such as Trumpler 14 (Tr
14), emits soft diffuse X-rays 10–100 times stronger than any other Galactic giant
HII region (LX ∼10
35 ergs s−1).4) The high surface brightness made possible the
discovery of the diffuse emission by the Einstein Observatory in the late 1970’s.
The Einstein observations revealed that the diffuse emission tends to be associated
typeset using PTPTEX.cls 〈Ver.0.9〉
http://arxiv.org/abs/0704.0346v1
2 K. Hamaguchi et al.
with optically bright regions containing massive stars. Recent Chandra observations
provided a point source free measurement of the diffuse flux,1) and suggested the
presence of a north-south Fe and Ne abundance gradient.5)
The X-ray CCD cameras (XISs: X-ray Imaging Spectrometer) onboard the
Suzaku observatory have the best spectral resolution for extended soft X-ray emission
and thus they provide good diagnostics of emission lines especially below ∼1 keV.
§2. Suzaku and XMM-Newton Observations of the Carina Nebula
Figure 1 shows a mosaic image of the Carina nebula between 0.4−7 keV created
from 32 XMM-Newton observations. The image depicts several bright X-ray point
sources: η Car (an LBV), WR25, WR22 (Wolf-Rayet stars), HD 93250, HD 93043
(O3 stars), and Tr 14, Tr 16 (massive stellar clusters). The image also clearly shows
apparently extended emission toward the east-west direction. In a color image (e.g.
Figure 1 of Ref. 2), XMM-Newton Image Gallery∗)) the emission is softer between
Tr 14, WR 25 and η Car.
We analyzed the Suzaku data of the core and the eastern side (named Car-D1)
of the Carina nebula taken on 2005 Aug. 29 and 2006 June 5. The XIS FOVs
of these observations are shown in Figure 1 with dotted lines. To investigate the
color variation in detail, we divided the core region into two and thus extracted
three spectra from two Suzaku observations (core-north, core-south and Car-D1).
The background was reproduced with the night earth data. The spectra showed
strong emission between 0.3 and 2 keV, which is probably dominated by soft diffuse
emission associated with the Carina nebula, while the spectra above 2 keV may be
explained with CXB, Galactic Ridge X-ray Emission, X-ray point sources resolved
with Chandra and unresolved pre-main-sequence stars.
Figure 2 shows an overlay of the BI spectra between 0.3–2 keV. The left panel
compares spectra of the core-north region with the core-south region. A strong differ-
ence is seen between 0.7 keV and 1.2 keV, which apparently is the source of the two
colors of diffuse emission. The band in which the difference is found is dominated by
emission lines from the iron L-shell complex. Additionally, the core-south spectrum
shows a stronger Si line. The Car-D1 spectrum shows similar intensity in the Si and
Fe lines to the core-south spectrum (right panel of Figure 2) while it shows relatively
strong magnesium and oxygen lines. All these spectra look similar except for these
emission lines. This suggests that the differences represent an elemental abundance
variation, and not a temperature difference.
This is supported by spectral fits of the individual spectra. All three spectra
between 0.3−2 keV were reproduced by an absorbed 2T thin-thermal plasma models
although the best-fit models are not formally acceptable. The plasma tempera-
tures of all three regions are ∼0.2 and ∼0.6 keV, and their column densities are
∼3×1021 cm−2, which is consistent with extinction toward the Carina nebula.3) The
abundances of some elements show a factor of 2−4 variations: the core-north region
has a factor of 2 lower silicon abundance and a factor of 4 lower iron abundance
∗) http://xmm.esac.esa.int/external/xmm science/gallery/public
Diffuse X-rays from the Carina nebula 3
Fig. 1. Mosaic image (∼90′×60′) of the Carina nebula between 0.4−7 keV created from 32 XMM-
Newton observations. The image is created with the ESAS package, divided by the exposure
map and smoothed with the adaptive smoothing technique. The dotted lines show the XIS
FOVs of the Suzaku observations of η Car (right) and the Car-D1 field (left). The solid lines
show source extraction regions for the spectral analysis.
than the core-south region, while the Car-D1 region has a factor of 2 higher oxygen
and magnesium abundances. On the other hand, spectral fits of the core region
with higher sensitivity around 0.5 keV gave small upper-limits (.0.02 solar) of the
nitrogen abundance.
§3. Origin of the Diffuse Plasma
The N/O abundance ratio inferred from the spectral fits is .0.4, over 20 times
less than around η Car. The abundance distribution is totally contrary to that
expected from stellar winds from evolved massive stars, unless the winds somehow
heat the interstellar matter without enriching it, thus leaving the X-ray plasma with
abundances typical of interstellar matter. At the same time, the X-ray luminosity
of the Carina Nebula is about two orders of magnitude higher than that of other
Galactic star forming regions, but the number of early O stars is only an order of
magnitude higher (see Table 4 in Ref. 6). These results suggest an additional energy
source is needed to power the X-ray emission in the Carina Nebula.
An obvious possibility is one or more core-collapse supernovae (i.e. Type Ib,c
or II), mentioned as a possibility by Ref. 6). The regions vary strongly in oxygen,
magnesium, silicon, and iron abundances. These elements are products of core-
collapse supernovae, and young SNRs such as Cas A and Vela show strong abundance
4 K. Hamaguchi et al.
Fe L complex Fe L complex
Fig. 2. Comparison of the XIS1 spectra between the fields – left: the core-north region (black) and
the core-south region (grey), right: the Car-D1 field (black) and the core-south region (grey).
The above labels demonstrate energies of emission lines detected (black) or concerned (grey)
with this result. Emission lines with the solid lines showed variation in their line intensity. Low
count rates of the Car-D1 spectrum below 1 keV is caused by degradation of soft response by
progressive contamination on the XIS.
variation from location to location. The total energy content in the hot gas of
∼2×1050 ergs is a modest fraction of the ∼1051 ergs of kinetic energy produced by a
canonical supernova, while assuming an iron abundance of 0.30 solar, the total iron
mass in the diffuse gas requires at least 3-5 supernovae.
Acknowledgements
K. H. is financially supported by a US Chandra grant No. GO3-4008A and US
Suzaku grant.
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http://arxiv.org/abs/astro--ph/0608173
	Extended X-ray Emission from the Star Forming Region
	Suzaku and XMM-Newton Observations of the Carina Nebula
	Origin of the Diffuse Plasma
