Tracking down ‘annihilation photons’ could lead to unique binary systems

Tracking the sources of photons is a hobby of many astrophysicists. Some types of photons are tied so closely to particular phenomena that tracking their sources would help answer some larger questions in astrophysics itself.

Photons on the 511 keV line are one such type of photon, and they have been overrepresented near the galactic core, with no known source being prolific enough to create them. A new paper posted to the arXiv preprint server by Zachary Metzler and Zorawar Wadiasingh of the University of Maryland and NASA’s Goddard Space Flight Center suggests one potential source—millisecond pulsar (MSP) binaries.

So why are 511 keV photons so interesting? First, it’s best to understand where the name comes from. 511 keV is the energy contained in these photons, which can also be translated into a wavelength of 2.427 picometers, putting it squarely in the gamma-ray range of the electromagnetic spectrum. They are unique because they are created as part of the “annihilation line.”

While that might sound like some apocalyptic Star Wars weapon, “annihilation” in this case means the annihilation of a positron and an electron. When these two oppositely charged fundamental particles collide, they change into energy emitted as a 511 keV photon. So, suppose scientists can find the source of 511 keV photons that are overly present in the galactic core. In that case, they should also find an abundant source of electron-positron annihilation.

Plenty of potential sources of 511 keV photons have been put forward, ranging from binary jet X-rays to dark matter annihilation. However, Drs. Metzler and Wadiasingh believe that a specific type of binary pulsar might significantly contribute. An MSP binary is a binary star system with a pulsar that rotates once every few milliseconds. The extreme forces on these stars make them interesting in their own right, but pairing them up with a companion star, which doesn’t have to be a pulsar, can create even more interesting interactions.

The authors modeled several types of MSP binary systems in the paper. They found some unique properties that they believe would be worth looking for with the newest gamma-ray observatories and gravitational wave detectors (since combining both would be the best way to characterize MSP binaries). Three different potential discoveries are worth discussing in more detail.

First, the authors believe details about any accompanying exoplanets could be discovered by analyzing the output of an MSP binary. The 511 keV signal can fluctuate based on the system’s orbital dynamics, creating red/blue shifts as the stars move around each other, and possibly around a possible exoplanet.

Additionally, astronomers can learn about the composition of the companion star in the system by analyzing changes in the “production efficiencies” of 511 keV photons, which vary with the types of material present in the companion star. The orbital dynamics and material composition information could reveal potential exoplanets being harbored in the system.

A second potential avenue for research is the search for “ultra-compact systems,” where the MSP and its companion star are in very close proximity. These are normally skipped over in pulsar surveys, as the algorithms used to search through astronomical data can’t analyze the interactions between the two stars to differentiate them, essentially making this a partial blind spot in the astronomical literature.

However, ultra-compact systems with MSPs would create massive 511 keV lines, since the pulsar’s beam would pass over the outer atmosphere of its companion star frequently, covering a lot of area. That leaves a lot of room for electron/positron annihilation, and therefore a lot of 511 keV photons, which should be noticeably stronger in these binary configurations.

A pulsar’s beam also leads to the third discovery of pulsars themselves, whose beam doesn’t pass over Earth. Typically, pulsars are discovered because their “beam” of energy passes directly over Earth, and our detectors manage to pick up whatever energy that beam was sending, no matter how far away it is. However, astrophysicists hypothesize that there are plenty of pulsars whose beams don’t pass over us at all; therefore, we wouldn’t be able to collect any data on them.

MSP binaries, though, would allow us to see pulsars from a new angle—from the 511 keV photons created when their beam hits their companion star. Those photons are not nearly as directional as the pulsar beam itself, so even if the beam doesn’t directly point toward Earth, at least some of the 511 keV photons from annihilation of electrons in the star’s upper atmosphere will—allowing us to tangentially identify that a pulsar is hitting the star with its high-powered beam.

As the authors discuss in the paper, their work is just theoretical at this point, with some modeling to go along with it. Another generation of detection instruments is coming online in the next few years, including the Compton Spectrometer and Imager (COSI), expected to launch in 2027. With the additional observational power of these platforms, astronomers should be able to gather enough data to test this theory and should be able to track down even more of these interesting photons, no matter their source.