Science

My PhD is in Astrophysics, earned at the Columbia Astrophysics Lab in a group led by Steve Kahn, my advisor. He designed the spectrometer of XMM, a European Space Agency observatory. I helped calibrate and build it and document its performance.

The observatory observes X-rays from astronomical sources, like active galactic nuclei, black holes, neutron stars, and white dwarves. I studied supernova remnants, in particular those in the Magellanic Clouds.

Since X-rays don't penetrate the Earth's atmosphere, the observatory is a satellite in high Earth orbit. Launched from Guayana in December 2000 for a ten year mission, it continues to outperform all expectations.

I was awarded time on NASA's complementary X-ray observatory, Chandra, but my real work was with XMM.

The Observatory

The observatory, called XMM, for X-ray Multimirror Mission, consists of a telescope, a spectrometer, and detectors. A spectrometer is a device that splits light into a spectrum, like a prism or the face of a compact disc.

X-ray properties define the design of all optical components, meaning grazing incidence optics and sub-micron tolerances. The telescope consists of a set of nested confocal hyperboloid-paraboloid pairs. The spectrometer consists of hundreds of reflective gratings. The detector is a set of back-illuminated CCDs. The optics lie on a Rowland Circle, a configuration common in spectroscopy.

XMM's Reflection Grating Spectrometer and Me

Every single one of the hundreds of reflection gratings had to be measured, as did their performance as a group installed in the array. That's what I did, as part of Columbia's XMM team.

When reflection gratings differ from the ideal, they scatter some light instead of reflecting or refracting it. Scattering properties depend on the gratings' detailed surface, unfortunately directly immeasurable. By shining X-rays of known wavelengths on each grating, we deduced their scattering properties. We could then calculate the properties of the full array from the averages of all the gratings. We also calibrated the full array independently.

Supernova Remnants

Some stars explode as supernovas, leaving sometimes beautiful remnants of the explosion in their wake, see images at right. The remnants emit light across the entire spectrum. XMM is pioneering detailed spectroscopy in X-rays.

A supernova, one of nature's most spectacularly energetic events, ends a star's period of equilibrium between gravity holding it together and nuclear reactions blowing it apart. For a few days one star's supernova can outshine the rest of a galaxy. (How energetic is that? Ever have a productive day at work? Imagine one day you produce as much as everybody else on Earth combined. You're getting close.)

Supernova remnants were among the first celestial X-ray sources discovered in the early days of X-ray astronomy, and have been among the most heavily studied objects ever since. Detailed investigations of their spatial and spectral structure provide key information not only about the later stages of stellar evolution and the supernova process itself, but also about the numerous other related topics, ranging from the nature of the surrounding interstellar medium, to the physics of colisionless shocks, to the microphysics of partially ionized atoms under extreme conditions.

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