X-ray Spectroscopy of Hot Gas in the Milky Way and Beyond

Webster CashAstrophysical and Planetary Sciencesand Aerospace Engineering Sciences

and Center for Astrophysics and Space AstronomyUniversity of Colorado

Much of the work done by:Randy McEntaffer (now Asst. Prof. Univ of Iowa)

Phil Oakley (now at MIT)Ben Zeiger (bears the brunt of the next flight)

Diffuse X-ray Background Spectrum

Rosat All-sky Survey

Theory of ISM

• Field, Goldsmith and Habing had Cosmic Ray heating in 1969

• McKee and Ostriker had SNR heating in 1977

• And there we still are. Somehow this became orthodoxy.

SNR evolution

• Theory (uniform medium)– 4 phases

• Free expansion– Lasts ~100years

• Adiabatic (Sedov)– MISM > Mejecta

• Radiative– Thermal equilibrium

• Merge Into ISM– Runs into other SNR vs ~ 1000 km/s

SNR evolution

• Theory (uniform medium)– 4 phases

• Free expansion• Adiabatic (Sedov)

– MISM > Mejecta

– Lasts thousands of years• Radiative

– Thermal equilibrium• Merge Into ISM

– Runs into other SNRvs ~ 500 km/s

Slows down, heats up. Pressure driven from insideConserves energy because there’s no way to lose energy.

vs < 200 km/s

SNR evolution

• Theory (uniform medium)– 4 phases

• Free expansion• Adiabatic (Sedov)

– MISM > Mejecta

• Radiative– Thermal equilibrium– Loses energy to x-ray radiation– Lasts 100’s of thousands of years

• Merge Into ISM– Runs into other SNR

SNR evolution

• Theory (uniform medium)– 4 phases

• Free expansion• Adiabatic (Sedov)

– MISM > Mejecta

• Radiative– Thermal equilibrium

• Merge into ISM– Runs into other SNR– Becomes part of ISM

Vela SNR (and Puppis)

Cygnus Loop

Diffuse X-ray Background Spectrum

From Rocket Shot ofDan McCammon(U. Wisconsin)

Rosat All-sky Survey

But There Are Problems

• Nicely summarized by Cox in ARAA 2005

• 25% Variable– Charge Exchange with Solar Wind

• O VI Underabundant by a factor of 100• Soft X-ray Background too faint by x10• B fields out of pressure equilibrium

Spectral Rise Not Explained

From Cash, Malina, and Stern,ApJ (letters), 204, L7, 1976

If we don’t understand the ISM, then

We don’t understand anything about the cycles of matter in the Universe.

Now Let’s Get to the Rockets!“Insanity is doing the same thing over and over again

and expecting different results.”Albert Einstein

“An expert is a man who has made all the mistakes which can be made, in a narrow field.”

Neils Bohr

Rockets allow us toDo things differentlyMake mistakes inexpensively

Project Overview• Proposed in April 2003• Awarded funding in fall 2003• Raytrace, design, fabrication

2004-2005• Calibrations and final assembly

2006• Integration at Wallops Flight

Facility Launched– November 20th, 2006 – November 13th, 2009

White Sands Missile Range,New Mexico

Design overview• Known as EXOS

– Extended X-ray Off-plane Spectrograph• FOV and payload length determine design• Payload length = 3 m (longest payload ever flown)• 3.25° × 3.25° FOV • 1 m to form beam, 2 m to disperse it

Payload components• 3.25° FOV + 3 m length =

6.7” opening aperture• Wire grid collimator• Off-plane grating array• Gaseous Electron Multiplier

(GEM) detectors

Wire-grid collimator

Wire-grid collimator

Off-plane grating array

α β

nλ/d

αγ

Off-plane grating array

The world’s “best” detectors

Reflight Step 1: Rebuild

Reflight Step 2

• Make the damn thing work• No glory, no pictures, just Phil Oakley

working his behind off for a year.• Lots of things break, fix them, something else

breaks• Eventually everything that can go wrong has.

(the upside of Murphy’s Law)

Reflight Step 3: Integration

On the RailShow and Tell with the Commanding General

Ready to Go!

Launch Operations - Blockhouse

Gone!

Helicopter Duty -- 2006Randy, Eric and Travis

Spectrum Nov. 13, 2009

Spectrum is continuum with low percentage narrow lines.That’s interesting!

OVII NVI OVII? NewFe??

Radiation Mechanisms• Blackbody

– Optically thick body in thermal equilibrium• Bremsstrahlung (means “braking radiation”)

– Optically Thin Body– Photons result from collisions (and consequent

deceleration) of charged particles (ions and electrons)• Synchrotron

– Relativistic electrons spiraling in a magnetic field• Protons don’t accelerate as fast – not much emission• Non-relativistic emission called Cyclotron Radiation

Is it Bremsstrahlung?

To resolve: Fly again

No

Maybe

What Next?

• Modeling and Theory– Figure out how this works and what it means

• Reflight – December 2011 Vela– Are x-ray remnants

synchrotron driven?

Other Uses• Comets

– Surprising Discovery in 1990’s of x-rays fromcomets

• Charge Exchange at Heliopause– 25% of Soft X-rays vary with solar wind– Elsewhere?

• Earth Atmosphere?– Most sensitive diffuse spectrograph ever, might see

new features.

Summary• Built EXOS

– Central Role Played by Students• Obtained Spectrum of Cygnus Loop

– Wasn’t what was expected• Possible Indication of Greater Role for High Energy

Particles– SNR seen as birthplace of cosmic rays

• Reassess Energy Balance of ISM• Needs More Flights!