The author at La Verna


I am a Principal Research Scientist with the Henry A. Rowland Department of Physics and Astronomy of The Johns Hopkins University and I also work with the X-ray Laboratory of the Astrophysics Science Division of the Goddard Space Flight Center. I am a member of the XMM Guest Observer Facility, where I have inherited the responsibility for the Extended Source Analysis Software (ESAS) from my former advisor and long-time colleague, Steve L. Snowden. I am deeply involved with the proposed Solar-Terrestrial Observer for the Response of the Magnetosphere as well as it's cousins, Solar wind Magnetosphere Ionosphere Link Explorer, CuPID, and LEXI. I have developed a simulators for both the slumped micropore optics used for wide-field X-ray imaging, and for wide-field imagers using such optics to study the Earth's magnetosheath.

The Bastille Day Storm as imaged by a idealized wide-field X-ray telescope.
The log of the integrated emissivity is shown as the color scale. The solar
wind flux is shown in the upper left, while the direction fo the IMF is
shown in the lower left. The square is the FOV; the black circle is the
Earth. No background or foreground components have been shown.

Research Area:

Although my research interests began as curiosity about the hot Galatic halo, observational exigencies led me to study both the hot ISM in nearby galaxies and the X-ray emission that arises from the near-Earth environment. As a result I am deeply mired in understanding the instrumental background for XMM and other X-ray missions in astrophysics. However, in 2004, disaster struck, and we discovered that X-ray emission from the near-Earth environment can be a significant "contaminating" signal for observations of extended diffuse X-ray emission. As a result, I have gotten sidetracked into studying solar wind charge exchange emission, both in the magnetosphere and in the heliosphere (see my review article).

My interests lie primarily in the study of the hot Galactic ISM using X-ray observations, and I am particularly interested in isolating and studying the hot Galactic halo. Understanding the extent and nature of the hot Galactic halo places constraints on models of galaxy formation, will be crucial for studies of the Warm-Hot Intergalactic Medium, and may resolve the missing baryon problem. Isolating the hot Galactic halo requires characterization of the Galactic foreground components such as the Local Hot Bubble and the Galactic bulge by a combination of absorption and broad-band photometric techniques. Those foreground components are also of interest and X-ray observations directly address such active questions as the filling factor of hot gas, but can give only indirect answer to questions of the total energy or mass in the hot component. Further progress can be made only if the distance to the gas can be determined, and that can only be determined by shadowing studies.

The current generation of X-ray instruments makes possible isolation of the various emission components by spectroscopic techniques, given sufficiently long exposures and sufficiently accurate instrumental backgrounds and calibration. Thus, I have been working on the characterization of the XMM instrumental background.

However, the temporally variable solar wind charge exchange (SWCX) emission make observations quite difficult as the SWCX emission lines tend to be the strongest emission lines, which are also the lines to be used to diagnose the plasma conditions of the more distant emission. Thus, I had developed an unhealthy interest in the solar wind, the magnetosheath, and the heliosphere. This interest has consequently led to my involvement with a number of heliospheric missions to study the SWCX X-ray emission from the magnetosheath, as well as a new interest in wide-field X-ray imaging.

In parallel, I have been pursuing a study of the diffuse X-ray emission in galaxies similar to the Milky Way (M101, M83, M51, M33, M31) in order to constrain the amounts/types of emission from various structural componentsr: arms, bulge, extended spheroid, etc., as well as different emission components: star-forming regions, diffuse hot gas, unresolved stars, and other unresolved sources. I hope to get back to these studies soon!


Positions Held

Current Missions

These are proposed, approved, or operating missions with which I am involved in some capacity or another.

STORM logo Co-I Solar-Terrestrial Observer for the Response of the Magnetosphere is a proposed heliospheric MIDEX mission which will, among other things, use a large array of 4 cm by 4 cm slumped micropore optics to image the magnetosheath in the X-ray. Launch would be in the 2026-2027 timeframe.
SMILE logo Consultant Solar wind Magnetosphere Ionosphere Link Explorer is a joint ESA/Chinese Academy of Science mission intended to image the Earth's magnetosheath in the X-ray using slumped micropore optics. SMILE is expected to launch in November 2023.
CuPID logo Co-I CuPID is a 6U cubesat scheduled for a launch into low Earth orbit in 2020. It will measure the width and location of the cusp in X-rays using a single 4 cm by 4 cm slumped micropore optic. The prototype of this instrument previously flew on DXL-II in 2015.
LEXI logo Co-I Lunar Environment Heliospheric X-ray Imager (LEXI) will have a mosaic of 9, 4 cm by 4 cm slumped micropore optics providing a 9 by 9 degree FOV which will be imaged by a microchannel plate detector. The instrument will operate for 8 continuous days on the lunar surface. Deployment is expected in December 2022. This instrument was previously flown on DXL in 2012.
HaloSat logo Co-I HaloSat is a 6U Cubesat currently in orbit, measuring the soft X-ray spectrum (0.4-8 keV) over the entire sky. Its primary goal is to measure the OVII and OVIII line emiission with a unique strategy to minimize the contributions by SWCX. Some observations are also scheduled to measure SWCX emission from the Earth's magnetosheath and from the heliospheric focussing cone. It uses single element Si drift detectors and a colimator to restrict the FOV to roughly 10 degrees. HaloSat was launched in 2018.

Past Missions

DXL logo DXL was launched from White Sands Missle Range on 12/12/12. It carried the same intrument which was used to create the Wisconsin All-Sky Survey. That instrument is a pair of proportional counters, each with an effective area of 500 square centimeters at 1 keV and an energy resolution similar to that of ROSAT. The flight scanned across the heliopsheric helium focussing cone to measure the strength of the SWCX emission from the cone. This observation produced a measure of the X-ray production factor of the solar wind interaction with the neutral ISM within the solar system. Using this production factor, the contribution of the heliopsheric SWCX to the soft X-ray background could be estimated. It also carried the STORM/LEXI prototype which observed perpendicular to the primary payload.
DXL logo DXL-II launched in December 2015. The payload included the large area proportional counters, one with the standard C band filter, the other with a B band filter. The payload also included two smaller proportional counters with Be band filters. Data analysis is on-going. Finally, this payload also included the CuPID prototype, which observed in the same direction as the principle instruments.
DXL logo DXL-III was launched from Poker Flat Research Range in January 2018.

Current Projects

Extended Source Analysis Software (ESAS)

The Extended Source Analysis Software (ESAS) began as a little project with Steve Snowden to create background spectra for XMM observations. In the course of this "little" project, we discovered the first XMM example of solar wind charge exchange due to the nose of the Earth's magnetosheath, and built a tool used by many X-ray astronomers. However, such a tool is never really finished, and we continue to modify and improve ESAS. Due to our recent work in heliophyscis as well as extended interaction with the Milano group, we have characterized the quiescent particle background (QPB) as a function of the XMM spacecraft location, and found that most of the temporal variation in the background can be understood as a function of the spacecraft location and the solar wind conditions.

ESAS has become even more tightly integrated into the XMM SAS, and that work continues. Some of the extensive scripting that had been part of ESAS is being shed in favor or smaller, more flexible, applications. However, the original scripts, or their counterparts are not disappearing; they will be available here once they are tested.

A rough draft of the next publication describing the XMM background is available, but is subject to severe revision.

PBP: a Pore-By-Pore Simulator for Slumped Micropore (Lobster-Eye) Optics

Slumped micropore optics are a light-weight alternative for applications requiring a large field of view and a PSF on the order of 30 arcseconds. Many simulators of X-ray optics generate photons using Monte Carlo techniques, and then trace their progress through the optics. For slumped micropore optics, however, one can calculate the point spread function efficiently by calculating the distribution of photons due to a fully filled aperture on a pore-by-pore basis. The mathematics behind the simulator is fully described in the "SMO Vade Mecum", which may become available here next year. Questions can always be directed to the author.
A gratuitous movie showing the PSF as a function of off-axis viewing angle
for four different optical configurations. (The bulk of the difference is in the stray-light baffles.)

Simulating the Earth's Magnetosheath in X-rays

I currently have a very robust simulator in C and IDL that is based (alas!) on no-longer-supported C code that will only run on old Linux machines. The code ingests BATS-R-US models and produces simulations of X-ray images. Although this code has been used by others, I will no longer support it. I hope to start building a new version soon. The movie at the top of the page was built with this software. See also David Sibeck's review for other examples.

Other Interests

I live in and am restoring a Victorian house designed by Charles Casselli. As a result, I am interested in how the original residents would have lived their lives there. I am particularly interested in period tableware and its uses, but not so interested in Victorian cuisine. My culinary interests are strongly Franco-Italian and I and my partner, are known for producing seven or eight course meals at the high holidays.

I maintain a woodland garden in the narrow space between two rowhouses. It is filled primarily with species native to the mid-Atlantic, though it also includes some more exotic Arisaema. I also collect members of the genus amorphophallus after having seen some of their cousins (the dracunculus vulgaris) at the Cloisters.