Highlights of the X-Ray Universe
Chandra's 25 Years of Observations and More to Come
Scott Wolk
Senior Astrophysicist
Harvard-Smithsonian Chandra X-Ray Center
Sponsored by PSW Science Member AC Charania
About the Lecture
This lecture will discuss the legacy and destiny of The Chandra X-ray Observatory. As part of NASA’s “Great Observatories” program, Chandra was designed and built to observe X-rays, alongside observations by the Hubble Space Telescope in the ultraviolet, visible and infrared, the Spitzer Space Telescope in the infrared, and the Compton Gamma-ray Observatory in the gamma ray regime. And it is now joined by the James Webb Space Telescope observations in the infrared.
X-ray astronomy is a product of the space age, and Chandra began as an almost unimaginably bold proposal to build a space-borne observatory based on X-ray optics whose resolution and sheer size represented leaps by orders of magnitude over any such mirrors ever built. This vision was fully realized in July of 1999 with the launch and deployment of NASA’s Chandra X-ray Observatory by the Space Shuttle Columbia.
In the 25 years since then, Chandra has revealed many discoveries, from resolving the hazy X-ray background into a speckled array dominated primarily by black holes, to the quiet black hole at the center of our galaxy, to peering through a gravitational lens to peek at supermassive black hole formation at the edge of time. Chandra has made movies of the motions of supernova remnants, studied planets near and far, and seen star clusters that flare and sparkle like Christmas lights.
A full quarter century after launch, Chandra remains the world’s premier X-ray astrophysics facility. It continues to do things that no other X-ray observatory can do. Many of the phenomena Chandra now investigates were not even known when the telescope was being developed and built. For example, astronomers now use Chandra to study the effects of dark energy, test the impact of stellar radiation on exoplanets, and observe the outcomes of gravitational wave events. Today, Chandra has a rapid target of opportunity response and its imaging capabilities and observing efficiency still exceeds pre-launch requirements. The observatory is capable of many more years of operation and scientific discovery.
This lecture will discuss the many current themes in astrophysics, along with new NASA facilities to address these questions, and their reliance on the unique information that Chandra provides.
Selected Reading & Media References
Chandra’s Cosmos: Dark Matter, Black Holes, and Other Wonders Revealed by NASA’s Premier X-Ray Observatory. By Wallace Tucker
Secrets of the Hoary Deep: A Personal History of Modern Astronomy. By Riccardo Giaconni
About the Speaker
Scott Wolk is a Senior Astrophysicist at the Harvard / Smithsonian Astrophysical Observatory. He currently serves as one of the flight directors of Chandra. He has held several roles in the mission, planning ground calibration tests, as a monitoring & trends scientist, and running the uplink support program. Previously he served as a Principal Investigator, co-investigator, and team member on several missions in the NASA Explorer program. And he is currently part of the AXIS probe team, recently selected for a phase A study.
He began in graduate school studying to be a planetary scientist, but a lack of appropriate missions at the time drove him towards the role of stellar X-ray scientist. Immediately after graduation, he was hired into the role of Operations Scientist for the upcoming (at the time) NASA’s Chandra X-ray Observatory. Over the last 25 years discoveries of exoplanets and the unique capabilities of Chandra have opened a new door for exploration and he is back to studying planets – this time exoplanets.
Scott is an author on more than 160 technical papers on a wide range of subjects. He authored a series of papers on star formation in clusters that outline top-level disk evolution. Along with Carey Lisse, he discovered charge exchange in comets, elsewhere in the solar system, and beyond. He and colleagues pioneered the study of exoplanets in X-rays, measuring the first transit of an exoplanet at high energy. Most recently he has been involved in a series of observations of protostars with the James Webb Space Telescope.
Among other honors and awards, Scott has served on several NASA panels including a NASA Study Analysis Group: Great Observatories, the NASA New Great Observatories SAG, and the NASA (New) Athena Science Team.
Scott earned an AB at Cornell University and a PhD in Astrophysics at the State University of New York at Stony Brook.
Social Media
Webpage(s): https://hea-www.harvard.edu/~swolk/
LinkedIn Profile: https://www.linkedin.com/in/scott-wolk-922139b/
Bluesky account(s) handle(s): @unknown-astronomer.bsky.social
Facebook page(s): https://www.facebook.com/scott.wolk.9
Minutes
On January 24, 2025, Members of the Society and guests joined the speaker for a reception and dinner at 5:45 PM in the Members’ Dining Room at the Cosmos Club. Thereafter they joined other attendees in the Powell Auditorium for the lecture proceedings. In the Powell Auditorium of the Cosmos Club in Washington, D.C., President Larry Millstein called the lecture portion of the 2,508th meeting of the Society to order at 8:02 p.m. ET. He began by welcoming attendees, thanking sponsors for their support and announcing new members. Scott Mathews then read the minutes of the previous meeting which included the lecture by Javier Garay, titled “Fusion Energy: Meeting the Engineering Challenges”. The minutes were approved, pending a minor correction.
President Millstein then introduced the speaker for the evening, Scott Wolk, of the Harvard-Smithsonian Chandra X-Ray Center. His lecture was titled “Highlights of the X-Ray Universe: Chandra’s 25 Years of Observations and More to Come”.
The speaker began by describing the size of the Chandra spacecraft: approximately the size of the Powell Auditorium. He indicated that most X-rays do not penetrate the Earth’s atmosphere, and must be observed from space. Wolk described the operating principle of an X-ray telescope, which uses specialized mirrors to reflect the X-rays at a glancing angle, and thereby focus the X-rays on a detector. He showed images of the precise polishing and alignment procedures used to fabricate and assemble Chandra’s X-ray optics. Wolk described the two X-ray detectors: a micro-channel plate, which provides high spatial resolution but no spectral information, and the ACIS camera, a CCD device that provides energy sensitivity, as well as images. He claimed the X-ray background of these detectors is so low that they average 2 counts per pixel per one million seconds of integration time. He showed a video of Chandra being deployed from the cargo bay of Shuttle Columbia in 1999. He described the use of Chandra’s onboard rockets to move the observatory from low Earth orbit to its final orbit, where although it cannot be reached for servicing, it can “stare” at celestial objects for several days, uninterrupted.
Wolk showed spectrally separated images from a super nova remnant, identifying specific elements from their characteristic X-rays. He described the four main sources which produced celestial X-rays: shock waves in plasma, synchrotron radiation, accretion energy, and super nova remnants. He indicated that X-rays are emitted by “nearly everything in Astronomy”.
The speaker presented what he called the three pillars of the “Pathways of Discovery”, the 2020 Decadal survey for astronomy and cosmology. These were:
• Worlds and Suns in Context
• The Nature of Extreme Cosmic Events, and
• Cosmic Ecosystems
Wolk began with “Cosmic Ecosystems”, which he described as understanding the evolution and interactions of galaxies. The speaker discussed the role of dark matter in cosmic evolution. He compared images from Chandra and Hubble, showing how the collision of two galaxies created dramatic differences in the distribution of normal matter and dark matter. He discussed how observations from Chandra allowed a more accurate determination of the dark energy constant of the universe.
The speaker then discussed “Extreme Cosmic Events”. He showed images of the collision between two neutron stars which created gravity waves detected by LIGO in 2017. He said that two days after the event was observed by LIGO, Chandra observed no X-rays. However, nine days later the X-ray source was clearly visible. He discussed possible mechanisms to explain the delay in the observations. He showed X-ray images of super nova remnants, including: type Ia, and type II (or core collapse) supernovae. He showed how data from Chandra can be used to map out the distributions of various elements in the supernova remnants.
And finally, Wolk discussed “Worlds and Suns in Context”. He explained that Chandra images have provided key insights into star formation and stellar evolution. As examples, he showed images of the Carina nebula, the Lobster-eye nebula, and the Orion nebula: all star forming regions. He described the use of Chandra’s high-resolution spectral sensors to measure both composition and Doppler broadening in 36 hot, young stars in the Orion nebula. The speaker then described the use of Chandra for finding and studying exoplanets.
Wolk ended his talk by presenting a video, produced in collaboration with the Perkins School for the Blind, wherein Chandra images were used to synthesize the accompanying audio track, saying that science should not be limited to sighted people.
The lecture was followed by a Question and Answer session:
A member asked if there were plans for using Coded Aperture imaging or diffractive optics for X-ray telescopes. Wolk responded by saying that each of these approaches provides some advantages and disadvantages, and are therefore complementary. He said that while these particular techniques are not employed on Chandra, they are used in a number of other instruments.
A member asked “What are you finding about exoplanets?” Wolk responded “Exoplanets are difficult”, saying that many exoplanets turn out to be barren rocks. He indicated that the JWST has been crucial in these observations. He discussed the details of how energetic X-rays from the host star can strip away the atmosphere of a nearby planet, but that exoplanets orbiting M-class stars were more likely to retain a thick atmosphere.
After the question and answer period, President Millstein thanked the speaker and presented him with a PSW rosette, a signed copy of the announcement of his talk, and a signed copy of Volume 1 of the PSW Bulletin. He then announced speakers of up-coming lectures, made a number of housekeeping announcements, and invited guests to join the Society. He adjourned the 2,508th meeting of the society at 10:05 pm ET.
Temperature in Washington, DC: -0.5° Celsius
Weather: Cloudy
Audience in the Powell auditorium: 60
Viewers on the live stream: 40
For a total of 100 viewers
Views of the video in the first two weeks: 772
Respectfully submitted, Scott Mathews: Recording Secretary