Forever Blowing Bubbles …The Soft X-Ray Background of the Milky Way

Michelle Supper,Richard Willingale

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

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)

The RASS ¾ KeV MapThe RASS ¾ KeV Map

Galactic Plane

North Polar SpurLoop 1

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

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.

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

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 ? )

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.

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

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)

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°

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

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

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

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

mar23@star.le.ac.uk