X-MRIs: Extremely large mass ratio inspirals
EMRIs from stellar-mass black holes have a very low event rate in Milky-Way-like galaxies, of about one every million years. However, the capture of a sub-stellar object such as a brown dwarf (BD) is more frequent because these objects are much more abundant and can plunge without being tidally disrupted. A BD EMRI covers some ∼108 cycles before merger, and hence stay on band for millions of years. About 2 x 106 yrs before merger they have a signal-to-noise ratio (SNR) at the GC of 10, 104 yrs several thousands, and 103 yrs before the merger a few 104. Based on these values, this kind of EMRIs are also detectable at neighbour MBHs, albeit with fainter SNRs. We calculate the event rate of BD EMRIs at the GC taking into account the asymmetry of pro- and retrograde orbits on the location of the last stable orbit. We estimate that at any given moment, and using a conservative approach, there are of the order of ≳20 sources in band.
From these, ≳5 are highly eccentric and are located at lower frequencies, and about ≳15 are circular and are at higher frequencies. Due to their proximity, BD EMRIs represent a unique probe of gravity in the strong regime. The mass ratio for a BD EMRI at the GC is q∼108, i.e., three orders of magnitude larger than stellar-mass black hole EMRIs. Since backreaction depends on q, the orbit follows closer a standard geodesic, which means that approximations work better in
the calculation of the orbit. Depending on the frequency distribution, one of the ≳5 sources in circular orbits could be sufficiently loud and at high enough frequencies during LISA’s operation that the systematic growth of its SNR could be tracked. This could then constitute a foreground signal and might bury that of MBH binaries.