Illuminating gravitational wave sources with Sgr A* flares

TL;DR

This work tests whether grazing tidal stripping of brown dwarfs by Sgr A* can fuel daily non-thermal flares and how such events connect to low-frequency gravitational waves from XMRI systems detectable by LISA. The authors combine high-resolution SPH simulations of BD–Sgr A* encounters with a post-processing, parametric non-thermal radiation model to produce synthetic light curves that reproduce observed flare peaks and durations, finding best-fit parameters $E_{\text{strip}} \approx 10^{-9}$, $\eta_{\text{NT}} \approx 10^{-8}$, and $t_{\nu} \approx 90$ minutes. The inferred physical accreted mass is $M_{\text{phys}} \approx 1.5\times10^{-7}\,M_{\odot}$, requiring extreme radiative inefficiency and implying a two-temperature RIAF where most energy is advected. The results imply a hidden, dynamically important BD population in the Galactic Center, compatible with tight XMRI orbits, and suggest that EM flares could serve as advance electromagnetic alerts for high-SNR GW signals to LISA, enabling precise tests of general relativity in the strong-field regime near a supermassive black hole.

Abstract

Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, exhibits daily energetic flares characterized by non-thermal emission in the infrared and X-ray bands. While the underlying accretion flow is a Radiatively Inefficient Accretion Flow (RIAF) peaking at radio frequencies, the mechanism powering these non-thermal transients remains debated. Stellar dynamics predict a population of faint brown dwarfs orbiting Sgr A*. These objects are progenitors of Extremely Large Mass Ratio Inspirals (XMRIs), crucial sources of low-frequency gravitational waves for the future Laser Interferometer Space Antenna (LISA) mission. We investigate whether the tidal stripping of brown dwarfs provides a viable fueling mechanism for the observed flares. Here we present high-resolution hydrodynamic simulations of grazing tidal interactions coupled with a parameterized non-thermal radiation model. We demonstrate that the dynamics of the tidal fallback and subsequent viscous evolution naturally reproduce the fundamental temporal characteristics of observed flares: the peak luminosity and the characteristic 1-hour duration. We show that this fueling mechanism is dynamically viable and energetically consistent, placing strong constraints on the required efficiency of the non-thermal emission process, suggesting extreme radiative inefficiency. These findings provide compelling evidence for a hidden population of brown dwarfs in the Galactic Center. Crucially, the observed high flare frequency implies tight orbits characteristic of advanced inspirals. This establishes a direct link between electromagnetic transients and active gravitational wave sources, alerting the LISA consortium years in advance to the presence of specific XMRI systems promising exceptionally high signal-to-noise ratios for precision tests of general relativity.

Figures (2)

• Figure 1: Visualization of the Smoothed Particle Hydrodynamics (SPH) simulation near periapsis passage ($R_p=120\,R_{\odot}$). The colour scheme represents the logarithmic column density of the gas in code units. The tidal forces exerted by Sgr A* (located off-frame) significantly stretch and deform the $0.01\,M_{\odot}$ brown dwarf structure. Material is stripped from the outer layers, forming the tidal streams that will subsequently fuel the accretion event.
• Figure 2: Temporal profile of the optimized simulation light curve. We plot the non-thermal luminosity (erg/s) against time (hours) on a semi-logarithmic scale. The solid line represents the simulated light curve, generated by imposing stochastic variability on the hydrodynamic envelope using optimized parameters: non-thermal radiative efficiency $\eta_{\text{NT}}=10^{-8}$, rescaling factor $E_{\text{strip}}=10^{-9}$, and viscous timescale $t_{\nu}=90$ minutes. The shaded area indicates the target observational luminosity range ($10^{34}$ to $10^{36}\,\text{erg/s}$). A horizontal marker illustrates the Full Width at Half Maximum (FWHM). The inset summarizes the key temporal metrics: FWHM duration (1.03 h), rise time (15.5 min), and fall time (23.3 min).