Primordial black holes as a dark matter candidate are severely constrained by the Galactic Center 511 keV gamma-ray line

Abstract: We derive the strongest constraint on the fraction of dark matter that can be composed of low mass primordial black holes by using the observation of the Galactic Center 511 keV gamma-ray line. Primordial black holes of masses $\lesssim$ 10$^{15}$ kg will evaporate to produce $e^\pm$ pairs. The positrons will lose energy in the Galactic Center, become non-relativistic, and then annihilate with the ambient electrons. We derive robust and conservative bounds by assuming that the rate of positron injection via primordial black hole evaporation is less than what is required to explain the SPI/ INTEGRAL observation of the Galactic Center 511 keV gamma-ray line. Depending on the primordial black hole mass function and other astrophysical uncertainties, these constraints are the most stringent in the literature and show that primordial black holes contribute to less than 1\% of the dark matter density. Our technique also probes part of the mass range which was completely unconstrained by previous studies.

• The paper derives stringent constraints on primordial black holes as a dark matter candidate by analyzing the 511 keV gamma-ray line from positron annihilation in the Galactic Center.
• It finds that primordial black holes with masses less than approximately 10^14 kg contribute less than 1% to the total dark matter density.
• This analysis introduces a new electromagnetic method using gamma-ray emissions to probe primordial black holes and constrains mass ranges previously unconstrained by other methods.

Primordial Black Holes and Galactic Center 511 keV Gamma-Ray Line Constraints

In this paper, Laha addresses the constraints on the fraction of dark matter that can be composed of low mass primordial black holes (PBHs) through the analysis of the Galactic Center 511 keV gamma-ray line. This gamma-ray line arises from positron-electron annihilation in the Galactic Center, indicating a significant presence of non-relativistic positrons. The results suggest that PBHs, with masses less than approximately $10^{15}$ kg, produce positrons via evaporation. These positrons lose energy, become non-relativistic, and subsequently annihilate with ambient electrons to contribute to the observed gamma-ray line.

The paper derives robust, conservative bounds based on the assumption that the rate of positron injection from PBH evaporation does not exceed the rate required to account for the SPI/INTEGRAL observations of the gamma-ray line. The constraints presented are particularly stringent within the literature, indicating that PBHs contribute to less than 1% of the dark matter density. Notably, the analysis covers mass ranges previously unconstrained, and the approach introduces new electromagnetic probing methods for PBHs through gamma-ray emissions.

Key Numerical Results and Claims

The paper presents several pivotal findings:

• Positron Injection Rate: It is derived that PBHs cannot account for the entire positron injection rate observed in the Galactic Center due to the constraints posed by the gamma-ray line intensity.
• PBH Mass Range: PBHs contributing to dark matter are constrained to be less than 1% of the total dark matter density, specifically in the mass range less than $10^{14}$ kg.
• Comparison to Previous Constraints: The technique yields constraints stronger than those from Voyager 1 observations, cosmic microwave background measurements, and gamma-ray data, especially in the mentioned mass ranges.

Implications and Future Developments

The implications of this research are substantial, both in the understanding of dark matter composition and in probing PBH physics:

• Dark Matter Composition: The constraints imply a negligible contribution of evaporating PBHs to the total dark matter density, effectively ruling them out as dominant dark matter candidates in specific mass ranges. This finding redirects focus towards alternative dark matter models, necessitating exploration into other possibilities within the broad dark matter candidate spectrum.
• PBH Physics: The technique highlights the utility of electromagnetic observations in probing black hole evaporation, typically viewed through gravitational effects alone. This could lead to innovative methodologies in black hole research, emphasizing the complementarity of gamma-ray data to gravitational observations, especially concerning PBH mass ranges beyond traditional detection capabilities.

Future work could explore refined dark matter distribution models, positron propagation studies in different astrophysical environments, and the potential identification and exclusion of contributing astrophysical sources to the gamma-ray line, improving sensitivity on PBH constraints significantly. Additionally, integration with gravitational wave data could further test PBH models across varying mass spectrums, particularly as gravitational wave observatories advance.