Published on November 15, 2007
Black Hole Event Horizon: Black Hole Event Horizon Ramesh Narayan Can We Be Sure That We Have Discovered Black Holes?: Can We Be Sure That We Have Discovered Black Holes? Not really… Astronomers have shown that black hole candidates (BHCs) have M 3M, so they are not NSs The mass gives us a good reason to suspect that the objects may be BHs But we need to find some independent evidence that our BHCs actually possess Event Horizons, before we can be sure that they are really BHs How to do this? Compare BH and NS systems and look for some dramatic difference that implies lack of surface Signatures of the Event Horizon: Signatures of the Event Horizon Differences in quiescent luminosity (Narayan, Garcia & McClintock 1997, Garcia et al. 2001; Narayan et al. 2002;…) Differences in variability power spectra (Sunyaev & Revnivtsev 2000) Differences in Type I X-ray bursts (Narayan & Heyl 2002) Differences in X-ray colors (Done & Gierlinsky 2003) Differences in thermal surface emission (McClintock, Narayan & Rybicki 2003) Quiescent Transients: Quiescent Transients BHCs are at least 100 times fainter than NSs (Narayan, Garcia & McClintock 1997; Garcia et al. 2001;…) Such a large difference can be explained if BHs have event horizons and NSs do not. Hard to explain otherwise Spectra of Quiescent NS SXTs: Spectra of Quiescent NS SXTs Quiescent NS SXT spectra have two components: soft thermal + power-law Soft component typically has L ~ 1033 erg/s Interpreted as heat from deep crustal nuclear reactions (Brown, Bildsten & Rutledge 1998), which is generated during accretion outbursts, and escapes during quiescence Predicted flux agrees with observations If BHCs have surfaces, then they should show a similar thermal component --- unavoidable Spectra of Quiescent BH SXTs: Spectra of Quiescent BH SXTs Spectra are all consistent with pure power-law Severe limit L < 1031 erg/s on thermal component in XTE J1118+480 (McClintock, Narayan & Rybicki 2003) Predicted luminosity: 3x1032 to 1034 erg/s. Huge discrepancy Obvious explanation: no surface in BHCs, i.e., event horizon Type I X-ray Bursts: Type I X-ray Bursts Discovered by Grindlay et al. (1976) Sudden brightening, once every several hrs; lasts about 10-100 s Physics understood: unstable nuclear burning of accreted gas Very common in NS XRBs No Type I Bursts in BHCs!!: No Type I Bursts in BHCs!! No BHC has ever shown a Type I burst Obvious explanation: They have event horizons, so material cannot pile up, and there can be no bursts But, is the lack of a surface the only reason why the sources do not burst? Not obvious… Some NS XRBs don’t have bursts Modeling Type I Bursts(Narayan & Heyl 2002, 2003): Modeling Type I Bursts (Narayan & Heyl 2002, 2003) We follow the accretion layer as it builds up. For each choice of the accretion column: We solve for the equilibrium profiles of density , temperature T, radiative flux F, and H/He fractions X, Y, as a function of depth (column density g/cm2) in the accretion layer We then carry out a formal linear stability analysis of the layer (this is new):-- if stable, the object will have no bursts if unstable, the object will have Type I bursts The Governing Equations: The Governing Equations There are 5 coupled partial differential equations: hydrostatic balance, radiative transfer, energy conservation, hydrogen burning, helium burning (just like stellar astrophysics!) Plus equation of state, expressions for radiative and conductive opacity, and hydrogen and helium burning rates Parameters: gravity g, mass accretion rate d/dt, initial H and He fractions X0 and Y0, and core temperature Tcore (inner bc) What is New in this Work?: What is New in this Work? We carry out a full stability analysis, using the (complex) growth rate as an eigenvalue We are more quantitative than previous work (e.g., Hansen & van Horn 1975; Fujimoto et al. 1981, 1987; Paczynski 1983; Fushiki & Lamb 1988; Cumming & Bildsten 2000;…) But, we cannot study the burst itself --- that requires full time-dependent simulations Results for a 1.4M Neutron Star: Results for a 1.4M Neutron Star We have tried a variety of accretion rates (0.001 -- 1 LEdd) and NS radii (1.6 -- 4 Schw. radii, i.e., 6.5 -- 16 km), for three choices of the core temperature We find that accreting NSs will produce Type I bursts over a range of accretion rates, but no bursts when close to Eddington Pretty good agreement with data M=1.4M, Tcore=108K Narayan & Heyl (2003) Structure of Black Hole Candidates: Structure of Black Hole Candidates If BHCs are not black holes, then what kind of objects are they? BHCs may be just like NSs (though more massive), with a hard surface on which gas accumulates BHCs may be made of porous (dark) matter through which gas falls to collect at the center 10M Object With a Hard Surface: 10M Object With a Hard Surface We tried a variety of accretion rates (0.001 -- 1 Eddington) and BH radii (9/8 -- 3 Schw. radii, i.e., 33 -- 85 km), with different choices of Tcore We find that BH candidates should exhibit Type I bursts over a wide range of conditions (wider than NSs!) M=10M, Tcore=107.5K Narayan & Heyl (2002) So Why Don’t BH XRBs Burst?: So Why Don’t BH XRBs Burst? Wrong fuel (no H, He) X Wrong core temperature X Wrong burst recurrence time X Wrong accretion rate X Rotation X Magnetic fields X Burning front propagation ? Exotic matter ? Rotation?: Rotation? Could the objects be rotating rapidly, thereby avoiding bursts? Even a maximally rotating object has only a factor ~2 variation in g from pole to equator, and only modest changes in local Mdot Cannot eliminate bursts Magnetic Fields?: Magnetic Fields? Could there be a strong magnetic field, which quenches bursts, just as in NS X-ray pulsars? Unlikely, since no pulsations have been seen during accretion outbursts of BH SXTs Magnetic field must be aligned with rotation --- conspiracy! Burning Front Propagation?: Burning Front Propagation? We have only proved that BH candidates with surfaces are thermonuclearly unstable. Perhaps the instability does not lead to a Type I burst? The physics of the detonation/deflagration front in a burst is uncertain (Timmes & Neimeyer 1999, Spitkovsky et al. 2001) But, NSs and White Dwarfs burst with no difficult Is it possible for the burning front to work fine for NSs and WDs but not for 10M BH candidates (with surface)? Implausible, since gravity in BH candidates is in between those in NSs and WDs Exotic Matter?: Exotic Matter? What if our BH candidate has some exotic interior? --- pion or kaon condensate or hyperons or quark star matter (or something even more exotic) The burst phenomenon is limited to quite a low density (typically ~106 g/cm3) on the surface. Weird interior at 1014 -- 1015 g/cm3 has no effect on surface bursts Non-Interacting Exotic Matter?: Non-Interacting Exotic Matter? What if BH candidates are made of non-interacting particles? Accreting gas may simply sink to the center and form a separate compact sphere Would these objects burst? Our calculations indicate Yes (Yuan, Narayan & Rees 2003, in preparation) Fermion-Fermion and Boson-Fermion Stars: Fermion-Fermion and Boson-Fermion Stars Assume that BHC consists of two independent fluids interacting via gravity Fermionic dark matter (mf ~ 200 MeV) plus gas (this is a fermion-fermion star) Bosonic dark matter (mb ~ 10-18 MeV) plus gas (this is a boson-fermion star) Calculate surface gravity, redshift, etc. at the surface of the gas sphere Evaluate burst properties of the models (Yuan, Narayan & Rees 2003) Results: Results Bursts are unavoidable on fermion-fermion and boson-fermion stars (Yuan et al. 2003) So Why Don’t BH XRBs Burst?: So Why Don’t BH XRBs Burst? We have eliminated virtually every other explanation The only remaining explanation is that BHCs have event horizons: no surface, no burst Summary: Summary Are BHCs really black holes with event horizons? Several independent arguments suggest that the answer is Yes. Absence of Type I X-ray bursts is an especially strong argument. Each argument has loopholes (hardly any for bursts) If BHCs are not black holes, they must negotiate so many different constraints, it becomes highly implausible We can be pretty confident that they are black holes!