Forever Blowing Bubbles … The Soft X-Ray Background of the Milky Way
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Forever Blowing Bubbles …The Soft X-Ray Background of the Milky Way
Michelle Supper,Richard Willingale
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OverviewOverview
Why look at it?Why look at it?
The observationsThe observations
How to extract the spectraHow to extract the spectra
ModellingModelling
ResultsResults
Ideas and InterpretationsIdeas and Interpretations
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Why look at the SXRB?Why look at the SXRB?
Uncertain: Uncertain: Chemical composition, origin, heating Chemical composition, origin, heating mechanism, morphology. mechanism, morphology.
Featureless:Featureless: Except at soft photon energies Except at soft photon energies
0.1–4.0 keV0.1–4.0 keV
Structures: Structures: Visible inVisible in ROSAT All-Sky Survey (RASS)ROSAT All-Sky Survey (RASS)
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The RASS ¾ KeV MapThe RASS ¾ KeV Map
Galactic Plane
North Polar SpurLoop 1
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Observations: Within Loop 1Observations: Within Loop 1
3 fields in the North 3 fields in the North
5 fields in the South 5 fields in the South
2 fields in the North Polar Spur2 fields in the North Polar Spur
Allows variations to be measured Allows variations to be measured as a function of latitudeas a function of latitude
X3X2X1
B1B2B3B4B5
NPS
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Unusual Steps in Data Reduction:Unusual Steps in Data Reduction:
Light curve heavily filteredLight curve heavily filtered
to remove flares and hot pixels.to remove flares and hot pixels.
Mask out point sourcesMask out point sources
Cosmic ray contamination Cosmic ray contamination estimated from unexposed chip estimated from unexposed chip corners.corners.
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Reduced EPIC Spectra:Reduced EPIC Spectra:
0.5 1.0 2.0Channel Energy (keV)
10-3
Nor
mal
ised
Cou
nts/
sec/
keV
0.01
0.1
1
MOS Data
PN Data
Al K and Si K from EPIC
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Cold Column
LHB (Northern fields projection)
LHB (Southern fields projection)
Galactic BulgeLoop1 Superbubble
The Wall
NPS fields
Southern Bulge Fields
Northern Bulge Fields
What’s going on, and how to model it… What’s going on, and how to model it…
Local Hot Bubble
Earth nHnH nH
Loop 1 Cool Halo Cosmic Background
(Apec) (Wabs x Vapec) (Wabs x Apec) (Wabs x Bknpower)
nH
Galactic Plane
(Wabs x ? )
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The ModelThe Model Apec LHB:LHB: Fixed temperature 0.1keV. Dominates 0.5keV. Fixed temperature 0.1keV. Dominates 0.5keV. + (Wabs NPS:NPS: Temperature ~0.3keV. Dominates 0.4 - 0.75keV. Temperature ~0.3keV. Dominates 0.4 - 0.75keV. x Vapec) Fits the O VIII, Fe XVII, Ne IX and Mg XI emission lines. Fits the O VIII, Fe XVII, Ne IX and Mg XI emission lines. Absorption represents the Wall.Absorption represents the Wall. ++(Wabs x (Wabs x Extra ComponentExtra Component MEKAL) MEKAL) Temperature set at 2 KeV.Temperature set at 2 KeV. Absorption frozen to galactic nH.Absorption frozen to galactic nH. +(Wabs x Galactic Halo:Galactic Halo: fixed temperature 0.1KeV. fixed temperature 0.1KeV. Apec) Dominates the 0.4-0.6 keV region. Dominates the 0.4-0.6 keV region. O VII line is a prominent feature.O VII line is a prominent feature. Absorption frozen to galactic nH.Absorption frozen to galactic nH. + (Wabs x Cosmic BackgroundCosmic BackgroundBknpower) Absorption frozen to galactic nH.Absorption frozen to galactic nH.
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OVII OVIII FeXII NeIX NeX
10
-3
Nor
mal
ised
Cou
nts/
sec/
keV
0.0
10
.11
2.00.50.2 1.0Channel Energy (keV)
Fitted SpectrumFitted Spectrum
Field χν2
X3 1.15
X2 1.05
X1 1.07
B1 1.24
B2 1.17
B3 1.04
B4 1.17
B5 1.07
N4 0.86
N5 0.99
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Analysis: Modelling DistancesAnalysis: Modelling Distances
Cold Column
LHB (Northern fields projection)
LHB (Southern fields projection)
Galactic BulgeLoop1 Superbubble
The Wall
NPS fields
Southern Bulge Fields
Northern Bulge Fields
Loop 1Centre: (325°, 25°)Distance to centre: 210 parsecsDiameter: 276 parsecs (Radius 138 parsecs)
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SNR Interaction?SNR Interaction?LHB emission measure ~ constant (6 x 10LHB emission measure ~ constant (6 x 10-4-4 cm cm-6-6 pc) pc)
LHB electron pressure: increases towards 25LHB electron pressure: increases towards 25° latitude.° latitude.
Distance to the Wall: Distance to the Wall: increases sharply from X3increases sharply from X3 B5 B5
(28 (28 110 parsecs) 110 parsecs)
Density of the Wall: Density of the Wall: higher higher
in N4 than N5, very high in in N4 than N5, very high in
the south (3 x higher).the south (3 x higher).
INTERACTION?INTERACTION?
Loop1LHB The Wall
X1
X3
X2
N4
N5
Increasin
g d
ensity
Increasin
g p
ressure
Latitude = 25°
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The Extra Component…The Extra Component…Hard component, modelled as 2 keV MekalHard component, modelled as 2 keV Mekal
Required only in 6Required only in 6° nearest the Plane° nearest the Plane
Increases quickly near planeIncreases quickly near plane– Associated with Plane or Galactic Centre?Associated with Plane or Galactic Centre?
Can’t check for presence in the North Can’t check for presence in the North
0.000
0.002
0.004
0.006
-14 -12 -10 -8 -6 -4 -2 0
Galactic Latitude (degrees)
Mekal E
mis
sio
n M
easu
re
(cm
-6 p
c)
Southern FieldsSouthern Fields
Northern Fields Northern Fields North Polar Spur FieldsNorth Polar Spur Fields
Loop 1 model boundaryLoop 1 model boundary
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Chemical Abundances in Loop 1Chemical Abundances in Loop 1
0
0.5
1
-20 -10 0 10 20 30 40
Galactic Latitude (degrees)
Ab
un
dan
ce r
ela
tive t
o S
ola
r
O
Ne
Mg
Fe
•Depleted Shell
•Rich, high abundances in centre
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Next Steps: Oxygen in the HaloNext Steps: Oxygen in the HaloGalactic halo = MYSTERY.Galactic halo = MYSTERY.Count photons in rangeCount photons in rangeDetermine flux contribution from each componentDetermine flux contribution from each componentHenceHence– Trace 0.1keV plasma, try to determine its origin.Trace 0.1keV plasma, try to determine its origin.– Investigate the distribution of the Galactic Halo Investigate the distribution of the Galactic Halo
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The End! <wake up>The End! <wake up>
Berkhuijsen E., Haslam C., Salter C., 1971, A&A, Berkhuijsen E., Haslam C., Salter C., 1971, A&A, 1414, 252-262, 252-262Egger R. J., Aschenbach B., 1995, A&A, Egger R. J., Aschenbach B., 1995, A&A, 294294, L25-L28, L25-L28Snowden S. L. et al., 1995, ApJ, Snowden S. L. et al., 1995, ApJ, 454454, 643-653, 643-653Snowden S. L. et al., 1997, ApJ, Snowden S. L. et al., 1997, ApJ, 485485, 125-135, 125-135Willingale R., XMM AO-2 ProposalWillingale R., XMM AO-2 ProposalWillingale R., Hands A. D. P., Warwick R. S., Snowden S. L., Willingale R., Hands A. D. P., Warwick R. S., Snowden S. L., Burrows D. N., 2003, MNRAS, Burrows D. N., 2003, MNRAS, 343343, 995-1001., 995-1001.
ReferencesReferences