1. Forever Blowing Bubbles …
The Soft X-Ray Background of the Milky Way
Michelle Supper,
Richard Willingale

2. Overview Why look at it? The observations How to extract the spectra
Modelling
Results
Ideas and Interpretations

3. Why look at the SXRB?
Uncertain: Chemical composition, origin, heating mechanism, morphology.
Featureless: Except at soft photon energies
0.1–4.0 keV
Structures: Visible in ROSAT All-Sky Survey (RASS)

4. The RASS ¾ KeV Map
Loop 1
North Polar Spur
Galactic Plane

5. Observations: Within Loop 1
3 fields in the North
5 fields in the South
2 fields in the North Polar Spur
Allows variations to be measured as a function of latitude
NPS X3 X2 X1 B1 B2 B3 B4 B5
Three data sets, downloaded from the XMM archive

6. Unusual Steps in Data Reduction:
Light curve heavily filtered
to remove flares and hot pixels.
Mask out point sources
Cosmic ray contamination estimated from unexposed chip corners.

7. Normalised Counts/sec/keV
Reduced EPIC Spectra:
0.5
1.0
2.0
Channel Energy (keV)
10-3
Normalised Counts/sec/keV
0.01
0.1 1
MOS Data
PN Data
Al K and Si K from EPIC
<0.3 keV (Detector noise too high)
>4.0 keV (background subtraction unreliable here)

8. What’s going on, and how to model it…
Local Hot Bubble
Earth nH
Loop 1
Cool Halo
Cosmic Background
(Apec)
(Wabs x Vapec)
(Wabs x Apec)
(Wabs x Bknpower)
Galactic Plane
(Wabs x ? )
Cold Column
LHB (Northern fields projection)
LHB (Southern fields projection)
Galactic Bulge
Loop1 Superbubble
The Wall
NPS fields
Southern Bulge Fields
Northern Bulge Fields

9. The Model + (Wabs NPS: Temperature ~0.3keV. Dominates 0.4 - 0.75keV.
Apec LHB: Fixed temperature 0.1keV. Dominates 0.5keV. +
(Wabs NPS: Temperature ~0.3keV. Dominates keV.
x Vapec) Fits the O VIII, Fe XVII, Ne IX and Mg XI emission lines.
Absorption represents the Wall.
(Wabs x Extra Component
MEKAL) Temperature set at 2 KeV.
Absorption frozen to galactic nH.
(Wabs x Galactic Halo: fixed temperature 0.1KeV.
Apec) Dominates the keV region.
O VII line is a prominent feature.
(Wabs x Cosmic Background
Bknpower) Absorption frozen to galactic nH.
Astrophysical Plasma Emission Code
The power law is given two fixed photon indices (), 2.0 before the break at 0.7keV, and 1.4 thereafter. The higher value before the break represents the contribution of the background quasar population, which has now been partly resolved at very faint fluxes in observations by ROSAT and Chandra

10. Fitted Spectrum χν2 Field X3 1.15 X2 1.05 X1 1.07 B1 1.24 B2 1.17 B3
OVII OVIII FeXII NeIX NeX
10-3
Normalised Counts/sec/keV
0.01
0.1 1
2.0
0.5
0.2
1.0
Channel Energy (keV)
Field
χν2 X3
1.15 X2
1.05 X1
1.07 B1
1.24 B2
1.17 B3
1.04 B4 B5 N4
0.86 N5
0.99

11. Analysis: Modelling Distances
Cold Column
LHB (Northern fields projection)
LHB (Southern fields projection)
Galactic Bulge
Loop1 Superbubble
The Wall
NPS fields
Southern Bulge Fields
Northern Bulge Fields
A model of the local ISM depicts a local bubble filled with a cool, tenuous gas (nH ~0.2)
surrounded by an absorbing Wall
lying at some distance dw from the Earth
The gas density within the Wall is assumed to be ~25 times greater than that within the bubble. This model, utilised by the wall_info.qin script, was used to calculate dw for each of the three fields.
9.2 Dimensions of the NPS Superbubble
The NPS is located on the edge of the Loop 1 Superbubble, an enormous feature within the central region of the Milky Way that encompasses the bright x-ray structures visible in figure 2. The Loop can be modelled by a circle of radius 42o, centred at lll=352o, bll=15o [6]. The bubble.qin script constructs a spherical volume based on this circle. The sphere encloses the bulk of the x-ray emission in this area as required, but also contains some emission from the Galactic Bulge and absorbing regions of the Galactic Plane. However, since the observations are closely placed, it is assumed that the conditions along the lines of sight will be uniform, rendering negligible any variations due to the Bulge and Plane. A distance of 210 parsecs was set as the distance to the centre of the bubble, consistent with polarisation observations of the NPS. The radius of the bubble was set to 140 parsecs. The predicted entrance and exit distances, dlo and dhi, were then calculated for each field (table 5, figure 5).
Loop 1
Centre: (325°, 25°)
Distance to centre: 210 parsecs
Diameter: 276 parsecs (Radius 138 parsecs)

12. SNR Interaction? LHB emission measure ~ constant (6 x 10-4 cm-6 pc)
LHB electron pressure: increases towards 25° latitude.
Distance to the Wall: increases sharply from X3 B5
(28  110 parsecs)
Density of the Wall: higher
in N4 than N5, very high in
the south (3 x higher).
INTERACTION?
Loop1
LHB
The Wall X1 X3 X2 N4 N5
Increasing density
Increasing pressure
Latitude = 25°
The LHB emission measure is almost constant, and quite small, of the order 10-4 cm-6 pc over the three fields (table 6), as would be expected. Since the Solar System lies embedded within the LHB, it is unlikely that we would see significant variation when observing it over so small a region. The electron density is similarly consistent. The pressure within the LHB varies considerably, increasing abruptly in SXRB3. Both figure 4 and the calculated dw (28 parsecs) reveal that the Wall is significantly closer in SXRB3 than the other two fields. The increase in pressure may indicate an interaction between the LHB and the absorbing Wall at this location.
The distance to the Wall decreases sharply as the observations move northwards, but the Wall density shows the opposite trend, being more than four times higher in the SXRB1 than in the other two fields. Since the distance (dlo) to the near side of the NPS is constant at ~73 parsecs, it appears that the increase in density is due to compression of the cold material lying between the two expanding shells of the LHB and the NPS. In the SXRB1 field, the LHB and NPS are much closer than in the other two fields, leading to higher compression of the intervening cold material, producing a higher density. The fourfold increase in density is consistent with a compression factor in the range of 5 – 11, suggested in [6].

13. North Polar Spur Fields
The Extra Component…
Hard component, modelled as 2 keV Mekal
Required only in 6° nearest the Plane
Increases quickly near plane
Associated with Plane or Galactic Centre?
Can’t check for presence in the North 
North Polar Spur Fields
Northern Fields
Southern Fields
Loop 1 model boundary

14. Chemical Abundances in Loop 1
Depleted Shell
Rich, high abundances in centre

15. Next Steps: Oxygen in the Halo
Galactic halo = MYSTERY.
Count photons in range
Determine flux contribution from each component
Hence
Trace 0.1keV plasma, try to determine its origin.
Investigate the distribution of the Galactic Halo

16.  The End! <wake up>
References
Berkhuijsen E., Haslam C., Salter C., 1971, A&A, 14,
Egger R. J., Aschenbach B., 1995, A&A, 294, L25-L28
Snowden S. L. et al., 1995, ApJ, 454,
Snowden S. L. et al., 1997, ApJ, 485,
Willingale R., XMM AO-2 Proposal
Willingale R., Hands A. D. P., Warwick R. S., Snowden S. L., Burrows D. N., 2003, MNRAS, 343,
Any Questions?