Implications from Late-time X-Ray Detections of Optically Selected Tidal Disruption Events: State Changes, Unification, and Detection Rates
Year: 2020
Type: article
Abstract: We present Chandra X-ray observations of four optically selected tidal disruption events (TDEs) obtained 4–9 yr after discovery. Three sources were detected with luminosities between 9 × 1040 and 3 × 1042 erg s−1. The spectrum of PTF09axc is consistent with a power law of index 2.5 ± 0.1, whereas the spectrum of PTF09ge is very soft. The power-law spectrum of PTF09axc and prior literature findings provide evidence that TDEs transition from an early-time soft state to a late-time hard state many years after disruption. We propose that the time to peak luminosity for optical and X-ray emission may differ substantially in TDEs, with X-rays being produced or becoming observable later. This delay helps explain the differences in observed properties such as L opt/L X of optically and X-ray-selected TDEs. We update TDE rate predictions for the eROSITA instrument: it ranges from 3 to 990 yr−1, depending sensitively on the distribution of black hole spins and the 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. The details of the observational biases can contribute to observed differences between optically and X-ray-selected TDEs (with optically selected TDEs being fainter in X-rays for retrograde TDE disks).
Source: The Astrophysical Journal
Institutions: Radboud University Nijmegen, Space Research Organisation Netherlands, Hebrew University of Jerusalem, University of Maryland, College Park, Columbia University, National Institute of Standards and Technology, Joint Institute for Laboratory Astrophysics, University of Colorado Boulder, Joint Space Science Institute, New York University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: bronze
Awards: Do intermediate-mass black holes exist?, 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.
OpenAlex ID: W2955219680