Constraints on Earth-mass primordial black holes from OGLE 5-year microlensing events (1901.07120v2)

Abstract: We constrain the abundance of primordial black holes (PBH) using 2622 microlensing events obtained from 5-years observations of stars in the Galactic bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of microlensing events display a single or at least continuous population that has a peak around the light curve timescale $t_{\rm E}\simeq 20~{\rm days}$ and a wide distribution over the range $t_{\rm E}\simeq [1, 300]~{\rm days}$, while the data also indicates a second population of 6 ultrashort-timescale events in $t_{\rm E}\simeq [0.1,0.3]~{\rm days}$, which are advocated to be due to free-floating planets. We confirm that the main population of OGLE events can be well modeled by microlensing due to brown dwarfs, main sequence stars and stellar remnants (white dwarfs and neutron stars) in the standard Galactic bulge and disk models for their spatial and velocity distributions. Using the dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic bulge/disk models, we obtain the tightest upper bound on the PBH abundance in the mass range $M_{\rm PBH}\simeq[10^{-6},10^{-3}]M_\odot$ (Earth-Jupiter mass range), if we employ null hypothesis that the OGLE data does not contain any PBH microlensing event. More interestingly, we also show that Earth-mass PBHs can well reproduce the 6 ultrashort-timescale events, without the need of free-floating planets, if the mass fraction of PBH to DM is at a per cent level, which is consistent with other constraints such as the microlensing search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our result gives a hint of PBH existence, and can be confirmed or falsified by microlensing search for stars in M31, because M31 is towards the MW halo direction and should therefore contain a much less number of free-floating planets, even if exist, than the direction to the MW center.

• The paper analyzes OGLE microlensing events to place tight constraints on the abundance of Earth-mass primordial black holes.
• It employs Galactic bulge, disk, and halo models to distinguish between microlensing events from known stellar objects and those possibly caused by PBHs.
• The study finds that ultrashort-timescale events could be attributed to PBHs if they account for roughly 1% of dark matter, offering a viable alternative to free-floating planets.

Constraints on Earth-Mass Primordial Black Holes from OGLE 5-Year Microlensing Events

The paper investigates the potential contribution of primordial black holes (PBHs) to dark matter (DM) by analyzing microlensing data obtained over a five-year period from the Optical Gravitational Lensing Experiment (OGLE). Specifically, it examines whether Earth-mass PBHs could be responsible for a subset of the microlensing events observed within the Galactic bulge.

Utilizing 2,622 microlensing events, the authors explore the distribution of event timescales, categorized into two main populations. Most events have timescales around 20 days, while a distinct group of six events exhibits ultrashort timescales of 0.1 to 0.3 days. Conventionally, these latter events are attributed to free-floating planets. The authors, however, examine an alternative explanation involving Earth-mass PBHs.

Under the assumption that PBHs constitute dark matter, the paper applies Galactic bulge and disk models along with Milky Way halo models to calculate microlensing event rates due to PBHs. The authors analyze the spatial and velocity distributions of brown dwarfs, main sequence stars, and stellar remnants to model expected contributions from known stellar populations.

The most significant findings occur through two hypotheses:

1. Null Hypothesis: Assuming no PBH microlensing contribution, the paper establishes strict upper limits on the abundance of PBHs in the Earth-Jupiter mass range, with particular stringency in the region of $M_{\rm PBH}\simeq[10^{-6},10^{-3}]M_\odot$. These limits enhance previously existing constraints from the MACHO/EROS experiments due to the OGLE’s expansive dataset.
2. PBH Explanation Hypothesis: By conjecturing that the ultrashort-timescale events might be due to PBHs, the authors found that these events could be explained by PBHs with masses around Earth’s if they constitute about 1% of the DM. This interpretation remains consistent with the OGLE observations for longer timescale events and with bounds from the analysis of Andromeda galaxy via Subaru Hyper Suprime-Cam.

While previous studies suggested the short events might originate from unbound planets, the Earth's PBHs scenario provides an intriguing alternative, suggesting PBHs as DM could explain these events without necessitating free-floating planets.

The implications of this research lie in potentially confirming PBHs as DM candidates, thus offering a viable avenue for understanding the unresolved nature of DM. Future endeavors should focus on microlensing searches in the direction of Andromeda (M31), as this would help discriminate between free-floating planets and PBHs due to the different expected star densities.

Overall, the paper contributes significantly to our understanding of PBHs, providing compelling arguments for their role in explaining certain microlensing events while setting strong constraints on their abundance across specific mass ranges.