On the Tidal Capture of White Dwarfs by Intermediate-mass Black Holes in Dense Stellar Environments

Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass and supermassive black holes, widely believed to reside in at least some dense star clusters, but not yet observed directly. Tidal disruptions of white dwarfs (WDs) are luminous only for black holes less massive than $\sim {10}^5{M}_{\odot }$, therefore providing a unique smoking gun that could finally prove the existence of IMBHs beyond any reasonable doubt. Here, we investigate the tidal captures of WDs by IMBHs in dense star clusters, and estimate a typical rate of $\sim 1{\mathrm{M}\mathrm{y}\mathrm{r}}^{-1}$ for galactic nuclei and $\sim 0.01{\mathrm{M}\mathrm{y}\mathrm{r}}^{-1}$ for globular clusters. Following the capture, the WD inspirals onto the IMBH producing gravitational waves detectable out to $\sim 100$ Mpc by LISA for $\sim {10}^4{M}_{\odot }$ IMBHs. The subsequent tidal stripping/disruption of the WD can also release bright X-ray and gamma-ray emission with luminosities of at least $\gtrsim {10}^{40}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$, detectable by $\mathit{\text{Chandra}}$, $\mathit{\text{Swift}}$, and upcoming telescopes, such as the $\mathit{\text{Einstein}}\mathit{\text{Probe}}$.