Local Hot Bubble ScienceDoes it Exist?

Steve SnowdenNASA/GSFC

RASS ¼ keV Band

RASS ¾ keV Band

From Near to Far:1) Geocoronal emission from scattered

solar X-rays and solar wind charge exchange, and auroral X-rays

2) Heliospheric solar wind charge exchange (SWCX) emission

3) Local Hot Bubble (LHB)4) Nearby supernova remnants (SNRs) and

superbubbles (SBs)5) More distant SNRs and SBs6) Stars (unresolved so “diffuse”)7) Galactic Bulge8) Galactic Ridge9) Galactic Halo, part 1 (cooler)10) Galactic Halo, part 2 (hotter)11) Local Group12) Extragalactic power law (unresolved

AGN and so also “diffuse”)13) Clusters and structure of the Universe

Nearly all diffuse emission is thermal in origin, and therefore dominated by line emission.

What makes up the soft X-ray background?

The ¼ keV BackgroundThe ¼ keV diffuse background was first observed in the late 1960’s using sounding rocket data.

During the 1970’s and early 1980’s the sky was mapped using both sounding rockets and satellite observatories.

All maps agreed quite well and showed a distinct negative correlation between the ¼ keV background and the column density of Galactic material.

Mapping culminated in the 1990’s with the ROSAT All-Sky Survey and the IRAS 100 m data.

The negative correlation was first interpreted as an extragalactic background absorbed by Galactic material.

Next it was realized that there was a real background in the Galactic plane, and that due to the short MFP at ¼ keV it had to originate locally

IRAS 100 m

RASS ¼ keV

The Local Hot BubbleThis led to the concept of the Local Hot Bubble

Local - meaning surrounding the Sun with an extent of ~50 to ~150 pcHot - meaning T~0.1 keVBubble - meaning there isn’t much else within the bubble except for the hot plasma and a few partially ionized clouds (e.g.,the Local Fluff) 21 cm observations ISM absorption line studiesNo significant communication with Loop I (different temperatures)Emission appears to be boundedSupernova origin (deep sea floor sediment, OB association movement through the ISM)

In the late 1980’s it was suggested that nearly all of the ¼ keV background could be produced by the LHB. This model was “Not inconsistent with any of the available data” (Dan McCammon). This was actually a step forward.

But the RASS changed all that, and the Galaxy got a whole lot more complicated.

Nearby Supernova Remnants and Superbubbles

North Polar Spur/Loop I

Cygnus Superbubble

Cygnus Loop

Vela/Puppis SNRsMonogem SNR

Eridanus Superbubble

Galactic Bulge LMCRASS ¼ keV Band

Cartoon of the ISM

The solar neighborhood. The locations and extents of the various objects have come from a variety of sources.

Most striping from SWCX

XMM-Newton SWCX emission spectrum

SWCX emission occurs when highly ionized solar wind particles charge exchange with either exospheric or ISM material. Carbon, oxygen, neon, and magnesium emission is common.

The SWCX is both distributed throughout the solar system and associated with Earth’s magnetosheath. There is, perhaps, additional emission at the heliopause.

Strongly time variable

O VII and O VIII emission is particularly problematic for astrophysical observations.

Solar System Emission

XMM-Newton SWCX emission and associated light curves

SWCX emission can also be problematic to detect with relatively constant fluxes over extended periods (tens of ks).

Multiple observations of the same direction can give some idea of the level of contamination.

Abnormally strong O VII, O VIII, and Mg XI can also suggest the presence of SWCX emission

But, space astronomy folks actually care about SWCX emission as it can possibly provide remote sensing on phenomena which currently rely upon in-situ measurements.

Solar System Emission

SWCX emission is relatively commonExpected from the RASS LTEsHowever, we can only easily identify the brighter episodes

=> What is the zero level of SWCX emissionCurrently we are only able to observe at E>0.5 keV

=> Not in general the same spectrum as the LHB

Suzaku

Pros and Cons

LHBPro

Explains many observational issues wellNegative correlation between the ¼ keV intensity and HI column

densityLack of spectral hardness variation with intensity

ConRequires a high thermal pressure inconsistent with local ISM

SWCXPro

We know it exists, it has to contribute some to the observed fluxModels seem to predict the observed amount of O VII and O VIII emission

ConGood agreement between surveys with radically different geometriesSWCX cross sections uncertain at lower energiesOdd geometry at higher latitudes, two separate emission regions

ROSAT Spectra

The ROSAT All-Sky Survey gave us the best available view of the¼ keV diffuse background. It used a proportional counter and so Had very poor spectral resolution.

LHB EmissionModel vs Observed

The observed spectrum (DXS) is dominated by line emission.

The spectrum, though, is completely different from what has been used for modeling.

The spectrum is not from a thermal equilibrium spectrum, strange abundances and non-equilibrium ionization states.

DXS Spectrum

Model Spectrum

SWCX EmissionModel vs Observed

DXS Data vs. Model SWCX Emission

Not many similarities here either

DXS Spectrum Model SWCX