A Bright Electromagnetic Counterpart to Extreme Mass Ratio Inspirals

The extreme mass ratio inspiral (EMRI), defined as a stellar-mass compact object inspiraling into a supermassive black hole (SMBH), has been widely argued to be a low-frequency gravitational-wave (GW) source. Providing accurate measurements of the black hole mass and spin of EMRIs is one of the primary interests for the Laser Interferometer Space Antenna (LISA). However, it is currently understood that there are no electromagnetic (EM) counterparts to EMRIs. Here we show a new formation channel of EMRIs with tidal disruption flares as EM counterparts. In this scenario, flares can be produced from the tidal stripping of the helium (He) envelope of a massive star by an SMBH. The remaining compact core of the massive star then evolves into an EMRI. We find that, under a certain initial eccentricity and semimajor axis, the GW frequency of the inspiral can enter the LISA band within 10 ∼ 20 yr, which makes the tidal disruption flare an EM precursor to an EMRI. Although the event rate is just $2\times {10}^{-4}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$, this association not only improves the localization accuracy of LISA and helps us to find the host galaxy of EMRI, but it also serves as a new GW standard siren for cosmology.