Title: Dimming Starlight with Dark Compact Objects
Authors: Joseph Bramante, Melissa D. Diamond, J. Leo Kim
First Author’s Institution: Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, Ontario K7L 3N6, Canada
Status: Published in Physical Review Letters [open access]
The brightening and dimming of distant stars
Light travels in a straight line through spacetime. However, matter bends spacetime, causing light to travel along a curve. Gravitational microlensing occurs when a massive object passes between Earth and a distant star, acting like a lens that focuses the starlight. Figure 1 illustrates how gravitational microlensing bends and converges the light rays as they travel to Earth. As a result, the starlight appears brighter for us on Earth.
For some diffuse or extended objects, other effects can dominate over microlensing. Photons from the star can interact with the particles of the passing object, for example, by bouncing off them or converting into other kinds of particles. These interactions may cause the starlight to dim, as illustrated in Figure 1.
Dark matter throws shade
Dark matter is an invisible type of matter that is only known to interact with ordinary matter through gravity. Any other interactions, if present, are thought to be extremely weak. However, if a dense ball of dark matter particles travels between a star and Earth, even weak interactions between light and dark matter could dim the starlight. Therefore, the amount of dimming we observe (or do not observe) can reveal properties of the interactions between dark matter and ordinary matter.
Microlensing surveys are designed to measure changes in the brightness of stars. These surveys, such as EROS-2 and OGLE-III and IV, traditionally look for increases in brightness due to microlensing. Nevertheless, they are also sensitive to dimming, making them suitable to detect these dark matter clumps that might dim the star instead.
Modeling dimming from dark matter clumps
The amount of starlight that is allowed to pass through a clump depends on the size of the clump and its optical depth. Optical depth is a measure of how much the clump impedes the passage of light. It depends on the density of the clump and how strongly its particles interact with light. For a given optical depth, we can calculate the size of the clump by measuring the dip in brightness of the source.
To determine how often we should detect dimming events, the authors assume all dark matter clumps have the same mass and float around space like particles in a gas. They use a realistic model for the dark matter distribution in the Milky Way to obtain an expression for the dimming event rate. Their expression depends on the number of stars we observe and the duration and sensitivity of the microlensing survey.
The authors consider surveys with different durations and sources. These include EROS-2 observations of the Large (LMC) and Small Magellanic Cloud (SMC), OGLE-IV observations of the Milky Way (MW) bulge, and combined OGLE-III and OGLE-IV observations of the LMC.
In Figure 2, the outlined regions show the radii and masses of the dark matter clumps that would cause dimming and microlensing detectable by each survey. We see that dimming can be used to find clumps that have much larger radii and smaller masses than objects that cause microlensing, opening a new region of the parameter space that microlensing cannot detect.
What dimming (or the lack of it) tells us about dark matter
Dimming can also constrain properties of the particles that make up the dark matter clumps. Dark matter models predict how strongly light interacts with dark matter particles, which determines the amount of dimming we observe. To demonstrate how the dimming effect can constrain different models, the authors consider two scenarios: elastic scattering and millicharged dark matter.
In elastic scattering, a photon simply bounces off a dark matter particle. A quantity called the scattering cross section measures how much area is “blocked out” by the particle when the light hits it. A larger cross section means that light is more often scattered, while a smaller cross section means that light passes right through.
If we never observe any dimming due to scattering, that must mean that the cross section is small. For dark matter clumps of fixed masses and radii, the authors compute the constraints we could place on the cross section if no dimming is observed. To illustrate, for the smallest clumps and lightest dark matter particles considered, the cross section would be, at most, 10-36 cm2. This is over ten orders of magnitude smaller than the cross section for photon-electron elastic scattering!
On the other hand, millicharged dark matter is a model that assumes dark matter has a small but nonzero electric charge. Since light is an electromagnetic wave, photons would interact with this electric charge. Analogous to the cross section for elastic scattering, the charge controls how much dimming occurs. Therefore, the absence of dimming also places constraints on the charge of this type of dark matter.
Takeaway
This study shows how we can readapt existing tools to help in the ongoing effort to understand dark matter. Microlensing surveys are well-suited to detect light dimming caused by dark matter clumps between us and distant stars. Even the absence of dimming events can constrain the masses, radii, and abundance of these clumps, as well as the properties of the dark matter that composes them. By revealing the potential of microlensing surveys to study dark matter, this work shows us that sometimes the tools we need are already in front of us. We just need to learn to recognize them.
Astrobite edited by Sandy Chiu.
Featured image credit: ESO/L. Calçada
