Constraints on Primordial Black Holes from Galactic Diffuse Synchrotron Emissions

TL;DR

Primordial black holes with masses above the evaporation threshold can emit electrons/positrons via Hawking radiation, which then propagate through the Galaxy as CRs. By adopting diffusive re-acceleration CR propagation models with sizeable Alfvén velocities, evaporated particles are boosted to energies where their synchrotron emission in the Galactic magnetic field is detectable at radio frequencies; the authors constrain the PBH fraction $f_\text{PBH}$ by comparing model predictions to low-frequency radio sky maps. They fit CR propagation parameters to AMS-02 and Voyager-1 B/C data, find $V_a \,\approx\,20\,\mathrm{km s^{-1}}$ for several models, and show that the resulting PBH-induced synchrotron signals can yield stringent limits on $f_\text{PBH}$, often stronger than Voyager-1 all-electron and AMS-02 positron constraints for $M_\text{PBH} \gtrsim 10^{16}\,\mathrm{g}$. The results depend on DM density profiles and GMF models, but the radio bounds generally exceed competing constraints across a wide PBH mass range, highlighting the power of low-frequency synchrotron observations for PBH DM studies.

Abstract

We investigate the possibility of constraining primordial black holes (PBHs) with masses $M_\mathrm{PBH}\gtrsim 10^{15}\,\mathrm{g}$ through Galactic diffuse synchrotron emissions. Due to Hawking radiation, these types of PBHs are expected to be stable sources of cosmic-ray (CR) electrons and positrons with energies below $\mathcal{O}(10\,\mathrm{MeV})$. In many CR propagation models with diffusive re-acceleration characterized by a significant Alfvén velocity $V_a\sim \mathcal{O}(10)\,\mathrm{km/s}$, the energies of the evaporated electrons/positrons can be further enhanced to $\mathcal{O}(100)\,\mathrm{MeV}$ through their scattering with the Galactic random magnetic fields. Consequently, the observation of Galactic synchrotron emissions at frequencies above $\sim 20\,\mathrm{MHz}$ can provide useful constraints on the abundance of PBHs. Using the AMS-02 and Voyager-1 data on the boron-to-carbon nuclei flux ratio, we confirm that a significant Alfvén velocity $V_a \sim 20\,\mathrm{km/s}$ is favored in several benchmark diffusive re-acceleration models. We show that, in this scenario, the observed low-frequency synchrotron emissions (from 22 MHz to 1.4 GHz) can provide stringent constraints on PBH abundance. The obtained conservative constraints are stronger than those derived from the Voyager-1 all-electron (electron plus positron) data by more than one order of magnitude for $M_\mathrm{PBH}\gtrsim 1\times 10^{16}\,\mathrm{g}$, and also stronger than our previous constraints derived from the AMS-02 positron data for $M_\mathrm{PBH}\gtrsim 2\times 10^{16}\,\mathrm{g}$.

Figures (17)

• Figure 1: Model predictions and residuals of B/C flux ratio for the benchmark models. Left: for Voyager-1. Right: for AMS-02
• Figure 2: The LIS energy spectra of evaporated all-electrons at the Solar position for CR propagation models adopted in this work, assuming a monochromatic mass function with $M_c=9\times 10^{15}\,\mathrm{g}$, $f_\mathrm{PBH}=1.0$, and NFW DM profile.
• Figure 3: Sky maps of synchrotron signals of PBHs with NFW and Burkert DM profiles (left and right) at 22 MHz, 150 MHz, and 408 MHz (top to bottom), assuming a monochromatic mass function with $M_c=5\times 10^{14}\,\mathrm{g}$, GH CR propagation model, and SUNE GMF model. All maps are computed with $f_\mathrm{PBH}=10^{-8}$, and are plotted with $N_\mathrm{side}=64$ and linear color mapping.
• Figure 4: Same as Fig. \ref{['fig: sky_DM_SUNE']} but for JF12 GMF model.
• Figure 5: Spectra of synchrotron signals of PBHs for GH, DRz4, DRz6 and DBz4 CR propagation models, assuming SUNE GMF model, NFW DM profile, and monochromatic mass functions of selected $M_c=5\times 10^{14}\,\mathrm{g}$, $1\times10^{15}\,\mathrm{g}$, and $5.6\times 10^{16}\,\mathrm{g}$. The spectra are averaged within the intermediate latitudes $10^\circ<|b|<40^\circ$. Observed intensities averaged within the same region are also shown. For a tight plot, the spectra of GH model are scaled by $f_\mathrm{PBH}=2\times 10^{-8}$, $2\times 10^{-8}$ and $2\times 10^{-4}$ for each $M_c$, and the spectra of DBz4 model are scaled by $f_\mathrm{PBH}=2\times 10^{-4}$ and $1.0$ for the smaller two $M_c$ (as labeled). Only the spectra with $M_c=5.6\times 10^{16}\,\mathrm{g}$ for DRz4 and DRz6 models are shown, which are also scaled by $f_\mathrm{PBH}=2\times 10^{-4}$ as the spectrum of GH model with the same $M_c$ (the $f_\mathrm{PBH}$ is not labeled).