New constraints on primordial black holes abundance from femtolensing of gamma-ray bursts

TL;DR

This work targets primordial black hole dark matter in the mass range 5e17–1e20 g by searching for femtolensing-induced fringes in GRB spectra observed by Fermi/GBM. By analyzing 20 GRBs with known redshifts and modeling detectability through simulations, the authors convert a null femtolensing result into an upper bound on the PBH density parameter: Omega_CO < 0.04 at 95% C.L. for two cosmologies. The study demonstrates that femtolensing of GRBs is a powerful, independent probe of tiny PBHs and improves previous constraints by about a factor of four in the targeted mass window. With more GBM data over time, the method promises to reach subpercent sensitivity, significantly narrowing the PBH dark matter parameter space.

Abstract

The abundance of primordial black holes is currently significantly constrained in a wide range of masses. The weakest limits are established for the small mass objects, where the small intensity of the associated physical phenomenon provides a challenge for current experiments. We used gamma- ray bursts with known redshifts detected by the Fermi Gamma-ray Burst Monitor (GBM) to search for the femtolensing effects caused by compact objects. The lack of femtolensing detection in the GBM data provides new evidence that primordial black holes in the mass range 5 \times 10^{17} - 10^{20} g do not constitute a major fraction of dark matter.

Figures (6)

• Figure 1: Simulated spectrum obtained with GRB 090424592. The spectrum was fitted with femtolensing$+$BAND model. The fit has $\chi^2$ = 92 for 73 d.o.f. The fit parameters are: $A=0.34\pm0.02\,$ph$\,$s$^{-1}\,$cm$^{-2}\,$keV$^{-1}$, $E_{peak}=173\pm12\,$keV, $\alpha=-0.84\pm0.03$ and $\beta=-3.9\pm7.5$. The simulated femtolensing effect is caused by a lens at redshifts $z_L=0.256$ and a source at $z_S=0.544$. The simulated mass is $M = 5 \times 10^{18} \,$ g and the mass reconstructed from the fit is $5.8 \times 10^{18}\,$g. The source is simulated at position $r_S=2$. The position reconstructed from the fit is $r_S =1.9$.
• Figure 2: Simulated femtolensed spectrum fitted with the BAND model. The fit has $\chi^2 = 810$ for $75$ d.o.f. The fit parameters are: $A= 0.5\pm0.03\,$ph$\,$s$^{-1}\,$cm$^{-2}\,$keV$^{-1}$, $E_{peak}=147\pm5\,$keV, $\alpha=-0.58\pm0.03$ and $\beta=-2.4\pm0.1$. The SBKN model fit is almost indistinguishable from the BKN model fit.
• Figure 3: The spectrum of GRB 090424592 using NaI detector n7, with the BAND and femtolensing fits superimposed. The parameters are $r_{S} = 1$, $2$, and lens mass $5\times10^{18} \,$g. The models are convolved with the response matrix.
• Figure 4: The spectrum of GRB 090424592 using NaI detector n7. The BAND and femtolensing fits are superimposed. The parameters are $r_{S} = 3$, $4$, and lens mass $5\times 10^{18} \,$g. The excess at $33\,$keV (K-edge) is an instrumental effect seen on many bright bursts.
• Figure 5: Minimum and maximum detectable $r_S/r_E$ as a function of lens mass for GRB 090424592.