Year: 2018
Type: article
Abstract: 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 × 104 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 BH (∼10−4 per year at z = 0).
Authors: A Generozov, N C Stone, B D Metzger, J P Ostriker
Institution: Columbia University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Awards: Signatures of Shocks and Particle Acceleration from Novae in our Galaxy, STARS IN GALACTIC NUCLEI WHICH ARE DEFLECTED TOO CLOSE TO THE CENTRAL SUPERMASSIVE BLACK HOLE (SMBH) CAN BE TORN APART BY TIDAL FORCES. FOLLOWING SUCH A TIDAL DISRUPTION EVENT (TDE) STELLAR DEBRIS ACCRETES ONTO THE SMBH POWERING A LUMINOUS FLARE.ALTHOUGH THE ORBIT OF A TIDALLY DISRUPTED STAR OFTEN BRINGS IT WITHIN TEN GRAVITATIONAL RADII OF THE SMBH STUDIES OF THE DISRUPTION DYNAMICS OFTEN NEGLECT THE EFFECTS OF STRONG-FIELD GENERAL RELATIVITY (GR) SUCH AS BH SPIN. VERY FEW THEORETICAL WORKS HAVE INVESTIGATED THE MOST GENERAL AND COMMON CASE OF INCLINED ORBITS IN THE KERR SPACETIME. MISALIGNED ORBITS AROUND SPINNING SMBHS BREAK IMPORTANT SYMMETRIES OF THE RELATIVISTIC TIDAL TENSOR OPENING THE DOOR TO QUALITATIVELY NEW DISRUPTION PHENOMENA.WE WILL INVESTIGATE HOW TDE DYNAMICS CHANGES IN RELATIVISTIC SPACETIME USING HYDRODYNAMICAL SIMULATIONS COUPLED WITH LOCAL ACCELERATIONS DERIVED FROM THE FULLY RELATIVISTIC TIDAL TENSOR IN THE SCHWARZSCHILD AND KERR METRICS. FOR A RANGE OF DIFFERENT STELLAR TYPES (MAIN SEQUENCE WHITE DWARF) AND ORBITAL PARAMETERS WE WILL QUANTIFY HOW PHYSICAL QUANTITIES SUCH AS THE VERTICAL COLLAPSE VELOCITY AZIMUTHAL ANGLE OF TOTAL VERTICAL COLLAPSE AND NUMBER OF HYDRODYNAMICAL BOUNCES CHANGE IN GR. OUR INITIAL CALCULATIONS WILL EMPLOY A SERIES OF ONE-DIMENSIONAL SIMULATIONS FOR EACH COLLAPSING COLUMN OF THE STAR. BY COUPLING THE HYDRODYNAMICS TO A NUCLEAR REACTION NETWORK WE WILL EXPLORE THE IMPACT OF COMPRESSION-INDUCED NUCLEAR BURNING ON THE COMPOSITION OF THE UNBOUND DEBRIS. WE WILL ALSO QUANTIFY HIGH FREQUENCY GRAVITATIONAL WAVES PRODUCED BY THE ABRUPT HYDRODYNAMIC BOUNCE AND ASSESS THEDETECTABILITY OF WHITE DWARF TDES BY ADVANCED LIGO IN ORDER TO PLACE CONSTRAINTS ON THE EXISTENCE OF INTERMEDIATE MASS BLACK HOLES. FINALLY WE WILL EXPLORE THE IMPACT OF SPIN-INDUCED NODAL PRECESSION ON THE ENERGY SPREAD OF THE DEBRIS AN ISSUE WITH POTENTIALLY IMPORTANT RAMIFICATIONS FOR THE MAXIMUM ACCRETION LUMINOSITY OF A TDE. GUIDED BY THE RESULTS AND INTUITION OBTAINED FROM OUR 1D HYDRODYNAMICAL CALCULATIONS WE WILL PERFORM FULLY THREE-DIMENSIONAL GENERAL RELATIVISTIC HYDRODYNAMICAL SIMULATIONS OF THE DISRUPTION PROCESS USING GRHYDRO WITH ADAPTIVE MESH REFINEMENT.THE MOST ROBUSTLY UNDERSTOOD DYNAMICAL PROCESS FOR FEEDING STARTS TO THE SMBH IS TWO-BODY RELAXATION WHICH SLOWLY DIFFUSES STARS THROUGH ORBITAL PHASE SPACE AND INTO THE LOSS CONE WHERE STARS ARE TIDALLY DISRUPTED. HOWEVER TWO-BODY RELAXATION CALCULATIONS BASED ON THE MEASURED INNER STELLAR DENSITY PROFILES OF NEARBY GALAXIES FIND TDE RATES WHICH ARE TYPICALLY MORE THAN AN ORDER OFMAGNITUDE HIGHER THAN OBSERVATIONS. WE WILL SYSTEMATICALLY EXPLORE THE CAUSE OF THIS `RATE DISCREPANCY' BY INVESTIGATING THE EFFECTS OF BIASES IN THE GALAXY SAMPLE AND THE MASS-TO-LIGHT RATIO. BY CONSTRUCTING NON-PARAMETERIC SURFACE BRIGHTNESS PROFILES TO ARCHIVAL HSTIMAGING WE WILL EXPLORE THE IMPACT OF UNRESOLVED NUCLEAR STAR CLUSTERS ON THE INFERRED TDE RATES.SEPARATELY WE WILL EXPLORE A MORE EXOTIC MECHANISM FOR SUPPRESSING THE TDE RATE DUE TO THE PRESENCE OF A SUB-CLUSTER OF STELLAR MASS BHS IN THE GALACTIC NUCLEUS. THROUGH STRONG SCATTERINGS SUCH A BH CUSP COULD EJECT STARS ON ORBITS WHICH MUST REPEATEDLY PASSTHROUGH THE SUBCLUSTER BEFORE BEING TIDALLY DISRUPTED. WE WILL QUANTIFY THE IMPACT OF THE BH SUBCLUSTER ON THE LOSS CONE FLUX BY SOLVING THE TIME-DEPENDENT FOKKER-PLANCK EQUATION USING A SINK TERM FOR STRONG BH SCATTERING. A SIMILAR TECHNIQUE WILL ASSESS WHETHER BH SHIELDING EFFECT IS REDUCED AFTER A SMBH BINARY SCOURS THE NUCLEUS FOLLOWING A MAJOR MERGER PROVIDING A POSSIBLE EXPLANATION FOR THETDE RATE ENHANCEMENT IN E+A (POST-STARBURST) GALAXIES.OUR INVESTIGATION IS RELEVANT TO OUTCOMES HIGHLIGHTED IN THE 2011 NASA STRATEGIC PLAN E.G. ``TO UNDERSTAND... THE EDGES OF SPACE AND TIME NEAR BHS". NASA'S HUBBLE SWIFT AND CHANDRA MISSIONS HAVE PLAYED A KEY ROLE IN OBSERVING TDE FLARES AND GALACTIC NUCLEI., NUCLEAR REACTIONS PLAY A KEY ROLE IN THE ACCRETION DISKS AND OUTFLOWS ASSOCIATED WITH THE MERGER OF BINARY COMPACT OBJECTS AND THE CENTRAL ENGINES OF, Testing the Millisecond Magnetar Model for Gamma-Ray Bursts and Superluminous Supernovae, G7715071082, G8142478605, G8143966986
OpenAlex ID: W2796292935