Time-lapse Photon Imaging: A New Way to Visualize Deep Chandra X-ray Observations

Joel Kastner
(Credit: Joel Kastner, Rochester Institute of Technology)

We welcome Joel Kastner, a professor at the Center for Imaging Science and School of Physics & Astronomy at the Rochester Institute of Technology, as our guest blogger. In this post, he describes how the final project of an RIT student, Tristan Liberace, for Kastner’s course in astronomical imaging resulted in a novel method to visualize long-duration Chandra observations — and a new view of the many (X-ray) faces of the active galaxy Centaurus A.

This past May, I once again sat down to grade more than 50 final projects for the course ‘Fundamentals of Astronomical Imaging’ at Rochester Institute of Technology (RIT). Astro Imaging Fun, as our imaging science department staff and faculty prefer to call it, is a General Education course of my own design. Each spring I get to guide 50-60 RIT students, with majors ranging from graphic arts to business to computer science to engineering, through the wonderful world of imaging in astronomy across the electromagnetic spectrum, from radio to X-rays.

Though (like many professors) I abhor assigning grades to student work, the final project images and image descriptions for Astro Imaging Fun are always, well, fun to see and read. It’s rewarding to see what the students have learned during the semester and how they’ve made use of that new knowledge, and there are always gems and pleasant surprises among the students’ image submissions.

Tristan Liberace (Credit: Tristan Liberace, RIT) 

This spring, one Astro Imaging Fun final project — from Tristan Liberace, a second-year electrical engineering major — really grabbed my attention. Tristan had generated a time-lapse video rendering of a set of deep Chandra X-ray Observatory exposures of the nearby, famous active galaxy Centaurus A (NGC 5128).

As I watched his video, my jaw hit the floor: in all my years teaching this class, I’d never seen anything remotely like it. Indeed, I’d never seen anything remotely like Tristan’s time-lapse view of a Chandra observation in the entire, 25-plus-year lifetime of that storied NASA Great Observatory.

Timelapse of Centaurus A Chandra Data:

Over the semester, Tristan and the rest of the students taking Astro Imaging Fun had learned how astronomical sources — from stars to nebulae to phenomena in the early Universe — generate radiation across the electromagnetic spectrum, and how our quest to understand the natures of these sources and their emergent radiation drives the designs of astronomical imaging systems ranging from small telescopes and their cameras to state-of-the-art facilities like Hubble, Webb, Chandra, Gemini, the Very Large Telescope, and the Atacama Millimeter-submillimeter Array.

Through lectures and lab exercises, we broke down the detailed anatomies of these imaging systems, and we explored the myriad ways astronomers (professional and amateur alike) render and scientifically interpret the resulting multiwavelength images. The course is capped by a final project intended to tie things together. Each student selects an astronomical object (typically, a nebula or galaxy) that has captured their interest and then combs the public image archives of major space- and ground-based astronomical observing facilities for images of the object. The student uses these archival image data to generate an original view of their chosen object, and they describe how they generated their image and what it reveals about the object.

Tristan’s work went way above and beyond. As he himself described it, in his final project report (here lightly edited):

This video was produced by compiling six different observing sessions (exposures) from the Chandra X-ray Observatory from 2007, all targeting Cen A, and overlaying them on top of each other, with the precise time of each event detected by its Advanced CCD Imaging Spectrometer (ACIS) instrument taken into account. This effectively results in a time-lapse rendering of the X-rays that were detected by ACIS over the nearly 2-day timespan of the six Cen A observations. Each photon hitting the sensor during an individual 3-second CCD exposure is visualized in the video with a ‘splash’ effect, briefly turning the surrounding pixels white, so as to emphasize for the viewer that a photon has struck the CCD. The accumulating image is then rendered as a heat map of pixel photon counts: as more photons land in a given pixel, that pixel becomes ‘hotter’, with the number of accumulated photons represented by a gradient from red to yellow in the image.

“The result of this process is a composite of over 150 hours of imaging data from Chandra compressed into 90 seconds, producing a majestic representation of X-rays from Centaurus A,” Tristan concludes. Indeed, in viewing the short video, one certainly sees that “majestic representation” — and much more.

As the video begins, only the Cen A core and the brightest surrounding X-ray point sources are apparent. Because the galaxy’s growing supermassive black hole (also known as an active galactic nucleus) emits such an intense stream of X-rays, the pixels in the center of the detector accumulate photon counts much faster than the rest of the field. This causes the central region to “heat up” and become visible much sooner as these high-rate regions accumulate photons far more rapidly than the fainter surrounding space. Then, Cen A’s beadlike jet, protruding out to the upper left, starts to emerge from the blackness. The tremendously bright Cen A core — the galaxy’s active nucleus — grows slowly and continuously, as photons that are scattered into the wings of Chandra’s point spread function, or PSF, splash down into ACIS. (The PSF describes how much a point-like source of light, in this case X-rays, is enlarged by the various elements of an imaging system, thus spreading the source out in the resulting image.)

As the video proceeds, increasingly faint point sources appear, and the Cen A jet begins to take on a searchlight appearance. X-ray background photons also steadily arrive, but — because these photons are spread uniformly over the entire CCD sensor area — this background remains subdued relative to the Cen A core/jet system and its surrounding flotilla of point sources.

About halfway through the video, a horizontal streak begins to appear. This is an ACIS “readout streak”, the result of X-rays from Cen A’s intense core striking pixels even as photon-generated charge is sped down the rows of the CCD during its few thousandths of a second readout period. This streak is soon joined by a diagonal readout streak from an exposure obtained at a different orientation angle (rotation) of Chandra’s view of the sky, and then by several more streaks, each at a slightly different angle. By the end of the video, the PSF of the Cen A core has reached its full bloat, the jet is seen to stretch halfway across the image, and X-ray point sources from exotic binary systems in NGC 5128 are sprinkled all across the otherwise X-ray-invisible host galaxy. As Tristan put it:

In the final image, a jet of matter can be seen spewing out from the energy-dense center of the galaxy. This jet is caused by the supermassive black hole in the center of the galaxy, which is estimated to have a mass of 55 million solar masses. The highly energetic environment surrounding the black hole flings particles at relativistic velocities, which then collide with surrounding gases and produce the X-ray emission detected by Chandra.

Each time I watch Tristan’s video, I find myself rewinding and watching it again. And his video makes me want to see other deep Chandra X-ray exposures rendered in the same, time-lapse fashion that Tristan has so creatively and ingeniously engineered for Cen A. By animating individual photon events, Tristan has given us a more intuitive sense of how a telescope like Chandra collects data over time and then passes along this information to us. Ultimately, this visualization approach reminds us that astronomical data isn't static—it’s a living record of time, energy, and light. That’s why seeing a 2-day Chandra X-ray observation materializing in front of your eyes, photon by photon — over the span of just 90 seconds — is so profoundly moving.