Fast Optical Transients from Stellar-mass Black Hole Tidal Disruption Events in Young Star Clusters

Abstract

Observational evidence suggests that the majority of stars may have been born in stellar clusters or associations. Within these dense environments, dynamical interactions lead to high rates of close stellar encounters. A variety of recent observational and theoretical indications suggest stellar-mass black holes may be present and play an active dynamical role in stellar clusters of all masses. In this study, we explore the tidal disruption of main-sequence stars by stellar-mass black holes in young star clusters. We compute a suite of over 3000 independent N-body simulations that cover a range of cluster mass, metallicity, and half-mass radii. We find stellar-mass black hole tidal disruption events (TDEs) occur at an overall rate of up to roughly 200 Gpc−3 yr−1 in young stellar clusters in the local universe. These TDEs are expected to have several characteristic features, namely, fast rise times of order a day, peak X-ray luminosities of at least 1044 erg s−1, and bright optical luminosities (roughly 1041–1044 erg s−1) associated with reprocessing by a disk wind. In particular, we show these events share many features in common with the emerging class of Fast Blue Optical Transients.

Periodic Fast Radio Bursts from Luminous X-ray Binaries

“…However, if similar transients are generated from BH/NS merger events (as suggested by numerical simulations; e.g., Law-Smith et al 2020), then some FRB sources-particularly the most luminous ones-could be accompanied by an LRN in the months to years after "turning off" as FRB sources. More speculatively, the very high BH accretion rates in such events -which are broadly similar to those achieved in tidal disruption events by stellar-mass BHs-could power luminous optical/X-ray transients (e.g., Perets et al 2016;Kremer et al 2021), perhaps similar to the "fast blue optical transients" (FBOTs; e.g., Drout et al 2014;Margutti et al 2019). FRBs could then serve as unique sign posts to this special type of stellar merger event.…”

Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

“…We assume primarily a black hole mass of 10 M and additionally run a small subset of models that adopt a black hole mass of 30 M (with the exception of Figure 1, all figures in the paper show results exclusively for the M BH = 10 M case). These choices are motivated by recent N-body studies of black hole TDEs in star clusters (e.g., Kremer et al 2019aKremer et al , 2021. We assume a wide range in stellar masses from 0.5 − 20 M , which reasonably trace the range of the initial mass function expected for clusters at birth.…”

“…Assuming for simplicity a single component mass function with slope α = 2.35 (e.g., Salpeter 1955) 2 , a main sequence star mass-radius relation of R ∝ M 0.6 (appropriate for M 0.7 M and a reasonable approximation for 0.1 M 0.7 M ), and a fixed black hole mass of 10 M , we estimate the total rate of events with main sequence masses in the range m 1 to m 2 and pericenter distances in the range r p,1 = β 1 r T to r p,2 = β 2 r T as Γ ≈ 4.5 (β 2 − β 1 ) m2 m1 m −2.08 dm Gpc −3 yr −1 (22) where we have additionally assumed the total rate integrated over all masses and pericenter distances is roughly 100 Gpc −3 yr −1 as estimated in Kremer et al (2021). Here β 1 and β 2 are the dimensionless penetration factors of the lower and upper bounds in pericenter distance.…”

“…If during the encounter material is stripped from the star and becomes bound to the black hole, a fraction of the stellar material may be accreted by the black hole, producing a luminous flare. Previous studies (e.g., Perets et al 2016;Kremer et al 2019aKremer et al , 2021 have demonstrated that in the tidal disruption regime, bright X-ray transients with optical/ultraviolet counterparts likely result. The peak optical luminosities and rates of these tidal disruption events (TDEs) make them notable sources for optical transient observatories such as the Vera Rubin Observatory (VRO) and the Zwicky Transient Facility (ZTF).…”

“…The peak optical luminosities and rates of these tidal disruption events (TDEs) make them notable sources for optical transient observatories such as the Vera Rubin Observatory (VRO) and the Zwicky Transient Facility (ZTF). Indeed, transients already detected such as fast blue optical transients like AT2018cow (Margutti et al 2019) may be explained by such TDEs (Kremer et al 2021). Furthermore, a number of recent observations have linked TDEs of white dwarfs by massive black holes with massive star clusters (e.g., Krolik & Piran 2011;Jonker et al 2013;Lin et al 2018) Aside from their intrinsic interest as electromagnetic transient sources, close encounters between black holes and stars may have an important effect on the overall demographics of black holes in clusters.…”