$\gamma$-Ray Flashes from Dark Photons in Neutron Star Mergers

In this Letter we begin the study of visible dark sector signals coming from binary neutron star mergers. We focus on dark photons emitted in the 10 ms–1 s after the merger, and show how they can lead to bright transient $\gamma$-ray signals. The signal will be approximately isotropic, and for much of the interesting parameter space will be close to thermal, with an apparent temperature of $\sim 100\mathrm{keV}$. These features can distinguish the dark photon signal from the expected short $\gamma$-ray bursts produced in neutron star mergers, which are beamed in a small angle and nonthermal. We calculate the expected signal strength and show that for dark photon masses in the 1–100 MeV range it can easily lead to total luminosities larger than ${10}^{46}\mathrm{ergs}$ for much of the unconstrained parameter space. This signal can be used to probe a large fraction of the unconstrained parameter space motivated by freeze-in dark matter scenarios with interactions mediated by a dark photon in that mass range. We also comment on future improvements when proposed telescopes and midband gravitational detectors become operational.

Figure 1

Luminosity of dark photons with $\epsilon ={10}^{-10}$, produced in BNS mergers using the simplified profile. We use $T=30\mathrm{MeV}$ (blue curve) for calculations in this Letter. The dashed and dotted lines show luminosity for other values of temperature. The enhancement near 10 MeV comes from resonant mixing between the dark photon and the photon.

Figure 2

The left (right) plot shows the conditions produced when the merger emits dark photons for 10 ms (1 s). The red region marks where the dark photons form a fireball which produces a thermal spectrum; below it dark photons generate a dimmer nonthermal signal. The dark gray region shows where the dark photon decay length is less than 1000 km (close enough to the merger remnant that baryons could affect the signal). The purple line shows the minimum $\epsilon$ needed to produce a signal visible to Fermi GBM for a merger 100 Mpc away ($\sim 2\times {10}^{46}\mathrm{ergs}$), and the blue line shows parameters for a ${10}^{44}\mathrm{erg}$ signal. The light gray regions marked SN Cooling and SN Emission show the parameter space already ruled out by astrophysical observations [37, 39]. The light blue regions labeled $N_{\mathrm{eff}}$ and BBN mark space excluded by cosmological observations [35, 57, 58, 59]. For potential additional complementary constraints, see also Refs. [39, 60, 61].