The solar gravitational lens could map white dwarfs and black holes

It feels like every few months we get to report on another academic paper singing the praises of the Solar Gravitational Lens (SGL). Partly, this is due to Dr. Slava Turyshev’s astounding productivity in pumping out academic articles, but partly because such a groundbreaking mission has lots of positive aspects—as well as challenges that need to be addressed. A new paper, posted to the arXiv preprint server from Dr. Turyshev, stresses an often overlooked feature of the SGL: how useful it can be for imaging things other than faraway exoplanets.

As a bit of background, the SGL itself uses a feature of general relativity: how the mass of our sun bends and magnifies light. A spacecraft positioned around 550 AU from our sun could use that lensing effect as a magnifying glass, allowing us to reconstruct megapixel-scale images of Earth-like exoplanets at a distance of a few dozen light-years.

Exoplanets have been the focus of SGL development up to this point, but Turyshev points out that there are plenty of other targets for the SGL to capture high-resolution images of. Exoplanets in particular face one large challenge: “photon starvation.” Even a telescope as powerful as the SGL would have to stare at an exoplanet for a long time to gather enough signal to beat the background noise caused by the sun’s corona.

There are plenty of astronomical objects that wouldn’t have that problem, though, particularly those that create their own light. For these targets, the math switches from photon counts to focal-line navigation, the detector’s dynamic range and subtracting out the corona’s glare. To prove the point, Turyshev dove into the math on three particularly interesting use cases.

First, consider mapping the surface of a magnetic white dwarf. These dead stars are incredibly bright but physically small, roughly the size of Earth. Currently, we can measure details on a white dwarf’s surface only down to the microarcsecond scale. According to Turyshev, the SGL would be capable of mapping the surface of a white dwarf 10 parsecs away down to the nanoarcsecond. This would allow features like temperature differentials and rocky debris within the accretion belt to become visible for the first time.

Another use case features the famous M87* supermassive black hole, first captured by the Event Horizon Telescope (EHT). While a historic achievement, the original EHT image has a resolution on the order of tens of microarcseconds. Turyshev shows that the SGL could improve this resolution to 0.66 microarcseconds per pixel—an improvement of several orders of magnitude over the original EHT picture.

There is a lot of activity going on in a protoplanetary disk, and the SGL could help us focus on a particular subfield of it to better map out what is happening. Attempting to scan an entire 100 AU protoplanetary disk would be infeasible for the SGL, as it requires the telescope capturing the images to physically move along a focal line to map out the images. But according to Turyshev, the SGL would be perfect for focusing on specific parts of the disk that might be of particular interest, such as where planets are actively forming.

But that points out one of the main difficulties in selecting the target for the SGL: It must travel up and down a focal line (not a plane) to get an accurate image of an object. For example, shifting its view by a single degree when out at 650 AU would require moving the spacecraft a greater distance than the distance from Earth to Saturn. Any such trip would take any existing propulsion system years, if not decades, and that’s just to change a target in the sky by 1 degree.

So until we get a better propulsion system—and work through a myriad of other technical hurdles—the SGL will remain a dream. But with every new paper, and every year’s worth of technological advances, we’re moving closer to where that dream could eventually become a reality. This paper slots nicely into that hopefully linear path and provides even more reasons why such a powerful telescope is worth pursuing.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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