Which Near Future Telescope Can Discover Primordial Black Holes? (Astronomy)

According to Regina Caputo and colleagues near future MeV telescopes like AMEGO can discover asteroid-mass primordial black hole dark matter.

Unequivocal evidence of a non-baryonic form of matter, known as dark matter (DM), as a dominant component of the Universe has been confirmed by numerous astrophysical and cosmological observations. Experimental searches for the elusive DM have thus far shown no firmly preferred model. Primordial black holes (PBHs), possibly formed via gravitational collapse of large overdensities in the early universe or via other exotic mechanisms, are one of the earliest proposed and well-motivated DM candidates. PBHs have a wide range of masses and can constitute a large fraction or even the entirety of the DM density. The idea of PBH DM has recently received renewed attention with the first detection of a BH merger by the LIGO-Virgo collaboration, argued to have a PBH rather than astrophysical origin. Several techniques have been implemented to probe the DM fraction of PBHs over a wide mass range. These have resulted in a multitude of observational constraints, along with several future projections along a broad range of PBH masses.

Due to their Hawking emission, extremely light PBHs would have evaporated by today, setting a lower limit on the mass PBH DM of ∼ 5 × 10¹⁴ g for non-rotating PBHs (or ∼ 7 × 10¹⁴ g if maximally rotating). Heavier PBHs would still evaporate, albeit slowly, acting as decaying DM, Ultra-light PBHs with masses in between 5 × 10¹⁴ g – 2 × 1017 g, are typically probed via searches of their Hawking radiation. Non-observations of such Hawking-produced photons, neutrinos, and electrons/ positrons provide the leading constraints on ultra-light PBHs. Additional constraints in this mass range are also obtained via precise observations of the cosmic microwave background and Big Bang Nucleosynthesis. PBHs in the mass range of ∼ 2×1017 g – 10²³ g, often known as the asteroid-mass range, are currently allowed to compose the entirety of the DM. Unlike solar-mass BHs, these ultra-light BHs cannot be produced by any known astrophysical processes (even with the continued accumulation of asymmetric DM particles in compact objects, and thus would be a smoking gun of new physics, be it during the early Universe or in a complex dark sector.

The Galactic and extra-Galactic photon contributions from Hawking evaporation off a non-rotating PBHs of mass 1017 g. It is assumed that PBHs make up the entirety of DM and follow an NFW density profile. The blue and red lines correspond to the Galactic and extra-Galactic contributions in the region of interest (|l| ≤ 5 deg and |b| ≤ 5 deg) respectively. © Regina Caputo et al.

Now, Regina Caputo and colleagues, propose a technique to decisively probe a part of the parameter space for PBH DM in the asteroid-mass range. They showed that observation of the Galactic Center by future MeV telescopes, such as an instrument with the sensitivity of AMEGO, can probe the DM fraction of asteroid-mass PBHs.

At the lower end of this mass range, PBHs with masses ∼ 1017 g – 1018 g have Hawking temperatures in the range of 0.01 MeV to 0.1 MeV, implying that substantial evaporated photons are produced by them around these energy scales. Near-future soft gamma-ray telescopes like AMEGO, with its large effective area and improved background rejection capabilities, can search for these photons and investigate this hard-to-probe parameter space.

The most efficient search strategy involves observations of the region around the Galactic Center.

Galactic and extra-Galactic astrophysical backgrounds are shown as a function of the emitted photon energy. Dashed black line corresponds to the Galactic background. Dashed red line corresponds to the extra-Galactic background which is a single power law fit to the Cosmic X-ray background measurements. Total background, sum of the Galactic and extra-Galactic backgrounds, is shown by the solid blue line. Evaporation signals from non-rotating PBHs of mass 1017 g with dark matter fraction of 10-⁴ and a non-rotating PBH of mass 7 × 1017 g with dark matter fraction of unity are shown for comparison. © Regina Caputo et al.

We include the Galactic astrophysical background produced by cosmic-rays and the measured extra-galactic gamma-ray background in our projected search strategy.

— Said Regina Caputo

Their projections showed that AMEGO can exclude non-rotating PBHs as the sole component of DM upto ∼ 7 × 1017 g. They demonstrated that maximal rotation as well as extended mass distribution of the PBHs allow them to explore larger ranges of PBH masses.

They also predicted that the projected exclusions on PBH DM in the mass range ∼ 1016 – 1017 g will be much stronger than the existing limits. The projections presented in their work are robust to the different choices of DM density profiles. At higher PBH masses in this range, the Hawking radiation flux gets smaller and thus much larger instruments need to be built in order to detect the evaporation signature.

They concluded that in the absence of much larger telescopes, other techniques need to be developed in order to probe the complete parameter space of asteroid-mass PBHs.

Reference: Anupam Ray, Ranjan Laha, Julian B. Muñoz, Regina Caputo, “Closing the gap: Near future MeV telescopes can discover asteroid-mass primordial black hole dark matter”, Astronomical Journal, pp. 1-10, 2021. https://arxiv.org/abs/2102.06714

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