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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. Overview. Why look at it? The observations How to extract the spectra Modelling Results Ideas and Interpretations. Why look at the SXRB?. - PowerPoint PPT Presentation

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

  • Forever Blowing Bubbles The Soft X-Ray Background of the Milky WayMichelle Supper,Richard Willingale

  • OverviewWhy look at it?The observationsHow to extract the spectraModellingResultsIdeas and Interpretations

  • Why look at the SXRB?Uncertain: Chemical composition, origin, heating mechanism, morphology.Featureless: Except at soft photon energies 0.14.0 keVStructures: Visible in ROSAT All-Sky Survey (RASS)

  • The RASS KeV MapGalactic PlaneNorth Polar SpurLoop 1

  • Observations: Within Loop 13 fields in the North 5 fields in the South 2 fields in the North Polar SpurAllows variations to be measured as a function of latitude



  • 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.

  • Reduced EPIC Spectra:

  • Whats going on, and how to model it

  • The Model Apec LHB: Fixed temperature 0.1keV. Dominates 0.5keV. + (Wabs NPS: Temperature ~0.3keV. Dominates 0.4 - 0.75keV. 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 0.4-0.6 keV region. O VII line is a prominent feature. Absorption frozen to galactic nH. + (Wabs x Cosmic BackgroundBknpower) Absorption frozen to galactic nH.

  • Fitted Spectrum


  • Analysis: Modelling DistancesLoop 1Centre: (325, 25)Distance to centre: 210 parsecsDiameter: 276 parsecs (Radius 138 parsecs)

  • 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).


  • The Extra ComponentHard component, modelled as 2 keV MekalRequired only in 6 nearest the PlaneIncreases quickly near planeAssociated with Plane or Galactic Centre?Cant check for presence in the North

    Southern FieldsNorthern Fields North Polar Spur FieldsLoop 1 model boundary

  • Chemical Abundances in Loop 1Depleted ShellRich, high abundances in centre

  • Next Steps: Oxygen in the HaloGalactic halo = MYSTERY.Count photons in rangeDetermine flux contribution from each componentHenceTrace 0.1keV plasma, try to determine its origin.Investigate the distribution of the Galactic Halo

  • The End! Berkhuijsen E., Haslam C., Salter C., 1971, A&A, 14, 252-262Egger R. J., Aschenbach B., 1995, A&A, 294, L25-L28Snowden S. L. et al., 1995, ApJ, 454, 643-653Snowden S. L. et al., 1997, ApJ, 485, 125-135Willingale R., XMM AO-2 ProposalWillingale R., Hands A. D. P., Warwick R. S., Snowden S. L., Burrows D. N., 2003, MNRAS, 343, 995-1001.

    [email protected]

    Three data sets, downloaded from the XMM archive 4.0 keV (background subtraction unreliable here)

    Astrophysical Plasma Emission CodeThe 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 ChandraA 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 SuperbubbleThe 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).

    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].

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