Title: Revealing terrestrial-mass primordial black holes with the Nancy Grace Roman Space Telescope
Authors: William DeRocco, Evan Frangipane, Nick Hamer, Stefano Profumo, Nolan Smyth
First author’s institution: Department of Physics, University of California Santa Cruz and Santa Cruz Institute for Particle Physics, Santa Cruz, CA, USA
Status: Published in Physical Review D [closed access]
The universe is (mostly) dark. Ordinary matter (made of protons, neutrons, and electrons) is thought to contribute 5% of the mass-energy density of the universe. It was Fritz Zwicky who first suspected that the gravitational pull of dark matter (“dunkle Materie”) could keep the Coma Cluster of galaxies from flinging apart. Vera Rubin later confirmed that dark matter is required to explain the rotational properties of galaxies. The dark matter problem has existed for decades, having been the subject of at least one rap, and numerous candidates including sterile neutrinos, WIMPs, axions, and massive astrophysical compact halo objects (MACHOs) remain.
Proposed MACHO candidates include non- or faintly luminous objects, including primordial black holes (PBHs), which are hypothesized to have been formed in the early universe. Since these objects do not emit photons, we need another method to find them. One such method is gravitational microlensing, in which a background star appears temporarily brighter due to alignment with a foreground massive object (in this case, a PBH). Just as a pair of glasses bends light to focus it on your retina, a gravitational lens bends light towards the observer, as shown in Figure 1.
Microlensing is one of the most powerful tools for finding non-luminous objects, but such close alignment of two astrophysical bodies is intrinsically rare. To maximize our chances of observing microlensing events, we want to look towards a field dense with stars over a long time period. Fortunately, NASA is planning to do just that with the Galactic Bulge Time Domain Survey to be conducted by the Roman Space Telescope once it launches by 2027.
However, PBHs are not the only objects that will create microlensing signals. Free-floating planets (FFPs) are planetary bodies wandering the galaxy, unbound to any star. Studies have suggested FFPs are exceedingly common, and, while it is possible to estimate the mass of a lens, it is not possible to distinguish whether that lens is an FFP or PBH. Since the possible masses (dwarf planet – gas giant range) of FFPs and PBHs overlap, the authors of today’s paper investigate whether Roman will be able to identify a distinct population of PBHs from the FFP background.
The distribution of masses, or the mass function, for each class of object will affect how well we can distinguish them. Duration and magnification are two observables of microlensing events, and they each depend on many parameters, including prominently the mass of the lens. Today’s authors use a log-normal PBH mass function, as predicted by the theory of gravitational collapse of overdensities in the early universe. FFPs, on the other hand, are thought to be formed in an ejection process, and the authors use the ubiquitous power law to model their mass function.
Next, the authors estimate the event rates of FFP and PBH microlensing events and how they vary with mass function parameters. Using estimates of Roman’s capabilities, they are then able to estimate the number and properties of FFP and PBH events detected. In order to distinguish the distributions, the authors of today’s paper investigate whether the distribution of event durations has signs of two distinct subpopulations using the Anderson-Darling statistical test. As shown in Figure 2, our ability to disentangle the distributions is highly dependent on mass function parameters.
So, how well Roman can identify PBHs depends on the underlying properties of the PBH population. The authors make a sensitivity plot, shown in Figure 3, demonstrating that Roman will be better than any previous mission at detecting PBHs over many possible masses, though the best constraints will be set for PBHs approximately the mass of Earth.
Even with conservative assumptions about the FFP background and Roman’s capabilities, Roman should be able to set stronger limits on PBH occurrence than any previous survey. The Galactic Bulge Time Domain Survey will not only detect microlensing planets but also illuminate PBHs, bringing us closer to understanding these elusive objects and setting stronger limits on their occurrence.
Astrobite edited by Victoria Bonidie
Featured image credit: NASA
