A large population of X-ray binaries (XRBs) was recently discovered within the central parsec of the Galaxy by Hailey et al. While the presence of compact objects on this scale due to radial mass segregation is, in itself, unsurprising, the fraction of binaries would naively be expected to be small because of how easily primordial binaries are dissociated in the dynamically hot environment of the nuclear star cluster (NSC). We propose that the formation of XRBs in the central parsec is dominated by the tidal capture of stars by black holes (BHs) and neutron stars (NSs). We model the time-dependent radial density profiles of stars and compact objects in the NSC with a Fokker-Planck approach, using the present-day stellar population and rate of in situ massive star (and thus compact object) formation as observational constraints. Of the ∼ 1 − 4 × 10 4 BHs that accumulate in the central parsec over the age of the Galaxy, we predict that ∼ 60 − 200 currently exist as BH-XRBs formed from tidal capture, consistent with the population seen by Hailey et al. A somewhat lower number of tidal capture NS-XRBs is also predicted. We also use our observationally calibrated models for the NSC to predict rates of other exotic dynamical processes, such as the tidal disruption of stars by the central supermassive black hole (∼ 10 −4 per year at z=0).

We present Chandra X-ray observations of four optically-selected tidal disruption events (TDEs) obtained 4-9 years after their discovery. Three sources were detected with luminosities between 9×10 40 and 3 × 10 42 erg s −1 . The spectrum of PTF09axc is consistent with a power law with index of 2.5±0.1, whereas the spectrum of PTF09ge is consistent with the Wien tail of a soft black body best described over the 0.3-7 keV range with a power law of index 3.9±0.5 (the best-fit black body temperature is 0.18 ± 0.02 keV). The power law spectrum of PTF09axc may signal that TDEs transition from an early-time soft state to a late-time low-hard state many years after disruption. The mismatch in Eddington fractions of these sources (≈ 5% for PTF09axc; ≈ 0.2% for PTF09ge) could indicate that, as is the case for X-ray binaries, mass accretion rate is not the sole parameter responsible for TDE state changes. These detections can be used to shed light on the difference between optically selected vs. X-ray selected TDEs. We propose that the time to peak luminosity for optical and X-ray emission may differ substantially in an individual TDE, with X-rays being produced or becoming observable later. This delay can serve to explain the differences in observed properties such as L opt /L X of optically and X-ray selected TDEs. Using our observations to calibrate simple models for TDE X-ray light curves, we update predictions for the soon-to-be-launched eROSITA instrument, finding an eROSITA TDE detection rate of 3 yr −1 to 990 yr −1 , a range that depends sensitively on (i) the distribution of black hole spins and (ii) the typical time delay between disruption and peak X-ray brightness. We further predict an asymmetry in the number of retrograde and prograde disks in samples of optically and X-ray selected TDEs, even if the intrinsic number of stars on pro-and retrograde TDE orbits is equal. X-ray selected TDE samples will have a strong bias towards prograde disks (up to 1-2 orders of magnitude if most supermassive black holes spin rapidly, and less so if most spin slowly). On the other hand, in flux-limited samples of optically-selected TDEs, there seems to exist a more modest (typically factor of a few) bias for either retrograde or prograde disks, depending on the underlying optical emission mechanism and regime of loss cone repopulation. These observational biases can contribute to observed differences between optically and X-ray selected TDEs (with optically selected TDEs being fainter in X-rays if the TDE disk is retrograde).

Recent observations suggest that stellar tidal disruption events (TDE) are strongly overrepresented in rare, post-starburst galaxies. Several dynamical mechanisms have been proposed to elevate their TDE rates, ranging from central stellar overdensities to the presence of supermassive black hole (SMBH) binaries. Another such mechanism, introduced here, is a radial velocity anisotropy in the nuclear star cluster produced during the starburst, which temporarily enhances the stellar flux into the loss cone of a solitary SMBH. These, and other, dynamical hypotheses can be disentangled by comparing observations to theoretical predictions for the TDE delay time distribution (DTD). We show that SMBH binaries are a less plausible solution for the post-starburst preference, as they predict an unrealistically top-heavy distribution of primary SMBH masses, and can only reproduce the observed DTD with extensive fine-tuning. The overdensity hypothesis produces a reasonable match to the observed DTD (based on the limited data currently available), provided that the initial stellar density profile created during the starburst, ρ(r), is exceptional in both steepness and normalization. In particular, explaining the post-starburst preference requires ρ ∝ r −γ with γ ∼ > 2.5, i.e. much steeper than the classic Bahcall-Wolf equilibrium profile of γ = 7/4. For "ultrasteep" density cusps (γ ≥ 9/4), we show that the TDE rate decays with time measured since the starburst aṡ N ∝ t −(4γ−9)/(2γ−3) / ln t. Radial anisotropies also represent a promising explanation, provided that initial anisotropy parameters of β 0 ≈ 0.5 are sustainable against the radial orbit instability. TDE rates in initially anisotropic cusps will decay roughly asṄ ∝ t −β 0 . As the sample of TDEs with well-studied host galaxies grows, the DTD will become a powerful tool for constraining the exceptional dynamical properties of post-starburst galactic nuclei.

Dozens of stellar tidal disruption events (TDEs) have been identified at optical, UV and X-ray wavelengths. A small fraction of these, most notably Swift J1644+57, produce radio synchrotron emission, consistent with a powerful, relativistic jet shocking the surrounding circumnuclear gas. The dearth of similar non-thermal radio emission in the majority of TDEs may imply that powerful jet formation is intrinsically rare, or that the conditions in galactic nuclei are typically unfavorable for producing a detectable signal. Here we explore the latter possibility by constraining the radial profile of the gas density encountered by a TDE jet using a one-dimensional model for the circumnuclear medium which includes mass and energy input from a stellar population. Near the jet Sedov radius of 10 18 cm, we find gas densities in the range of n 18 ∼ 0.1−1000 cm −3 across a wide range of plausible star formation histories. Using one-and two-dimensional relativistic hydrodynamical simulations, we calculate the synchrotron radio light curves of TDE jets (as viewed both on and off-axis) across the allowed range of density profiles. We find that bright radio emission would be produced across the plausible range of nuclear gas densities by jets as powerful as Swift J1644+57, and we quantify the relationship between the radio luminosity and jet energy. We use existing radio detections and upper limits to constrain the energy distribution of TDE jets. Radio follow up observations several months to several years after the TDE candidate will strongly constrain the energetics of any relativistic flow.