Researchers at Lawrence Livermore perform experiments using laboratory facilities and space-based observatories to address fundamental questions in x-ray astrophysics.
Developing and applying high-resolution x-ray spectroscopy
X rays from astrophysical objects provide a view into some of nature’s most violent and extreme environments. Telescopes sensitive to the x-ray waveband (0.1−100 kiloelectronvolts, or keV) provide unique probes of accreting black holes and the warped spacetime around them, clusters of galaxies—the universe’s most massive gravitational potential wells—and the growth and evolution of galaxies.
To unravel the physics governing these sources, researchers not only want to take images in the x-ray band, but they also want to know the precise energy of every photon in the image. Imaging spectroscopy allows the use of spectroscopic plasma diagnostics to learn details about these systems, such as their temperature, velocity, composition, and mass distributions.
At LLNL, we have been working to advance x-ray spectroscopic capabilities and the associated plasma diagnostics for astronomy for several decades. Our team works on instrument development and science teams for x-ray astrophysics missions and leads x-ray laboratory astrophysics investigations. Our work receives funding from the NASA Astrophysics research program to support the general x-ray astrophysics community, and is funded directly to support the X-ray Imaging and Spectroscopy Mission (XRISM), an x-ray observatory that will launch in 2023, and the Athena X-ray Integral Field Unit (X-IFU).
Benchmarking spectral modeling code to enable precision measurements
Our laboratory astrophysics program centers around LLNL’s electron beam ion trap (EBIT) facility, which currently includes the EBIT-I and SuperEBIT. Invented and developed by LLNL researchers, EBIT allows high-accuracy measurements of x-ray emission from isolated, highly charged ions to derive atomic physics parameters needed for the interpretation of complex spectra. It operates in the coronal density limit, with electron densities of about 5 x 1011 per cubic centimeter. These conditions are perfectly suited for astrophysics-related studies.
LLNL’s EBIT facility has a full suite of x-ray and extreme ultraviolet (EUV) spectrometers to aid in diagnosing the trap conditions and measuring the photon emission from the trapped ions. The instruments include grating and crystal spectrometers for very high resolution (about 0.1 eV) measurements, as well as microcalorimeter spectrometers for moderate resolution (about 5 eV) with a broad waveband.
Our laboratory astrophysics measurements help benchmark the leading spectral modeling packages used by the high-energy astrophysics community, such as:
- The Atomic Database for Astrophysicists (AtomDB), maintained at the Harvard–Smithsonian Center for Astrophysics (CfA)
- SPEX, maintained at the Netherlands Institute for Space Science (SRON)
- CHIANTI, maintained at the University of Michigan, George Mason University, the University of Cambridge, and NASA Goddard Space Flight Center
- CLOUDY, maintained at the University of Kentucky
LLNL scientists are actively engaged with the astrophysics community to address high-priority questions and with the teams at CfA, SRON, and other institutions to ensure the measurements are properly incorporated into the databases and spectral modeling codes.
Lawrence Livermore is home to many students and visiting researchers from institutions around the world who perform experiments at the EBIT facility. We also partner with researchers to perform laboratory astrophysics experiments at other facilities, including experiments coupling portable EBITs to synchrotrons and free-electron lasers.
Developing and calibrating microcalorimeter instruments for space-based astronomy
The coming generation of astrophysical x-ray missions, such as XRISM and Athena, will deploy arrays of x-ray microcalorimeters—a type of x-ray detector that operates at low temperature (50 mK) and is designed for high-energy resolution—to provide imaging spectroscopy. These detectors have an unprecedented combination of high spectral resolution and high quantum efficiency over a broad energy range (0.1–20 keV).
The EBIT facility is home to microcalorimeter spectrometers built by our colleagues at NASA’s Goddard Space Flight Center (GSFC) and used at LLNL for laboratory measurements. Conversely, Lawrence Livermore researchers are members of instrument teams developing microcalorimeter instruments for space missions, including Resolve on XRISM and the X-IFU on Athena.
Examples of our research activities include:
- Developing and deploying x-ray generating equipment for ground calibration.
- We refurbished and deployed an EBIT for Resolve calibration and are designing and building a more capable EBIT for X-IFU calibration. X-ray emission from highly charged ions produced in EBITs, such as helium-like oxygen and helium-like neon, provides a significant improvement in accuracy over characteristic lines produced by standard x-ray tubes often used for calibration purposes.
- We are developing optimized crystal and grating x-ray monochromators for X-IFU ground calibration.
- Supporting detector performance and calibration campaigns, from component-level laboratory testing to on-spacecraft system tests to in-orbit commissioning.
- Using DOE facilities such as synchrotron light sources to calibrate thin-film filters and x-ray absorbers.
- Working with software development teams to implement algorithms and incorporate calibration results for mission data processing software.
Observational astrophysics
Our researchers are engaged in observational astrophysics projects using Chandra and XMM-Newton and are preparing for the launch of XRISM. Our scientists lead target teams for observations to be made during the performance verification phase of the XRISM mission in 2023. We contribute to novel observations using payloads flown on sounding rockets, most recently the Micro-X Sounding Rocket.
LLNL expertise in x-ray plasma physics, atomic physics, and instrumentation position researchers to make key contributions to astrophysics using these new windows into the universe.
