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White Dwarfs Neutron Stars Black Holes INPE Lectures in Sao Jose dos Campos, October 2007 Feryal Ozel University of Arizona Some Warnings • I will assume no background in fluid dynamics, general relativity, statistical mechanics, or radiative processes. (If you’ve seen them, some of this will be easy for you). • Because I’m charged with covering a wide range of topics, I made some choices based on personal preferences. (Really, neutron stars ARE very interesting). • Still, I am leaving out a lot. You can find more background material in e.g., “Black Holes, White Dwarfs, and Neutron Stars: The Physics of Compact Objects” by Shapiro & Teukolsky. For current research on individual subjects, I’ll try to give references as we go, and you’re welcome to ask for more after the lectures. • I’m going to focus on their structure, their interiors, and their appearance as it relates to determining the properties of their interiors. • Please ask questions, interrupt, ask for more explanation, etc. Lives of Stars End Stages of Stellar Evolution Main Sequence stars: H burning in the core, synthesizing light elements Heavier elements form in the later stages, after H in the core is exhausted and core contracts, central T rises to ignite “triple-” reaction 3 He4 --> C12 Which stars can ignite He? If they cannot, what happens during the contraction phase? The stellar mass determines if there is sufficient contraction (and thus heating) to ignite further nuclear reactions or if matter becomes degenerate (at very high densities) before nuclear reactions set in. Let’s first look at equation of state of degenerate Fermions. Kinetic Theory Preliminaries Let’s start with the distribution function and define number density: n df 3 d p 3 3 d xd p All averages, such as energy density are given by df E 3 3 d3 p d xd p ; E ( p 2c 2 m 2c 4 )1/ 2 includes particle rest mass For an ideal fermion/boson gas in equilibrium, 1 f (E) exp[( E ) /kT] 1 Fermion (half-integer spin particles) Boson (integer-spin particles) Some limits of f(E): High temperature, low density: f (E) exp( E kT ) For fermions, chemical potential (energy cost of adding one particle) is the Fermi energy f (E) 1 exp[( E E F ) /kT] 1 where Fermi energy EF is defined such that f (E F ) 1 2 Fermions at zero temperature (complete degeneracy): f(E) ~ { 1 (E EF) 0 (E > EF) For comparison, let’s look at bosons: Statistical distributions of photons detected at different times following the startup of the laser oscillation. At short times the source is chaotic and the distribution is of Bose-Einstein type. At longer times the source is a laser and the distribution becomes Poissonian. Unlike Fermions, as T--> 0, an unlimited number of bosons condense to the ground state. • We can write the available number of cells in terms momentum: N( p)dp 2 V 4 p2dp 3 h or in terms of energy by using E=p2/2m N(E)dE 8 V (2m 3 )1/ 2 E1/ 2dE 3 h Thus, at a given E and for fixed V, the phase space available to the system of particles decreases with the particle mass, and electrons can fill the phase space much more easily than protons. • The pressure associated with the degenerate electron gas is given by 1 Pe 3 N ( p) f ( p)v pdp e 0 1 Pe 3 pF N ( p)v pdp e 0 Which can be evaluated for the nonrelativistic case v<<c (v=p/me) and for extreme relativistic case (v~c) to give (3 2 ) 2 / 3 2 5 Pe n e / 3 , v c 5 me (3 2 )1/ 3 Pe cn e4 / 3 , v ~ c 4 Notice there is no dependence on me. Note: P- relations of the type P=K are called polytropic equations of state. We saw that this is exact for degenerate matter (e.g., inside a white dwarf ) and a good approximation for some normal stars. Now back to the fate of the evolving stars: During the contraction of a star, nuclear reactions must start when P ≥ Pe. (otherwise, pressure due to degenerate electrons stops the contraction before necessary T for the reactions is reached) Because stellar temperatures scale with mass, this condition equates to a minimum mass for the onset of reactions: (i) For H-burning at 106 K: M ≥0.031 M Smaller masses do not become MS (ii) For He-burning at 2x108 K: M ≥0.439 M Just the mass if the He core; smaller mass stars become degenerate before triple- kicks in. Polytropes Polytropes are simple stellar models applicable to degenerate stars as well as normal stars. To obtain the structures of stars, we need to solve hydrodynamic equations along with an “equation of state” relating pressure P to density . When we use a P- relation of the type P=K (i.e., a polytropic equation of state), we can obtain a simple polytropic stellar model. Hydrodynamics Preliminaries: ( ui ) 0 t x i Continuity equation u j u j 1 ij ui aj t xi xi Euler (momentum) equation--a fancy F=ma for pliable materials) What is ij ? ij Pij ij off-diagonal terms: Viscosity-stress diagonal terms: Pressure So Euler equation becomes u j u j 1 P 1 ij ui aj t xi x j xi } } pressure gradient viscosity To study the structures of compact objects, we assume hydrostatic equilibrium (as is the case for stars in general) In steady state: 0 t ui 0 Static: So the momentum eqn reads: (with a j 1 P 0 x j x j Which in spherical coordinates is 1 P GM(r) r r2 and M(r) r 4 r (r)dr 2 0 ) x j Now we’re ready to solve our set of equations for the structure of a polytrope: 1 P GM(r) r r2 dM(r) 4 r 2 (r) dr P K (alternatively 1 P K 1 n ) 3 equations for 3 unknowns P, M, and . We can specify boundary conditions and solve this set of equations. Typically the density at the center (r 0) c and the density derivative at the center d/dr=0 are specified as the two boundary conditions. It is common to express radius in terms of a unit length 1 1 n (n 1)K c a 4 G 1/ 2 r a So that the solution for R and M become 1n (n 1)K R 1 c 2n 4 G 1/ 2 where 1 is the radial point at which density becomes zero (the “surface” of the star) and depends only on the polytropic index n. Note that for 0<n<1, R increases with increasing central density, while for N>1, R decreases. Similarly, 3n (n 1)K M f (1 ) 4 c 2n 4 G 3/2 We can also write a relation for M and R: M R 3n 1n Whether M increases or decreases with R depends on the polytropic index n! Note that for n=3 ( = 4/3), M=constant! (independent of R) Possible Outcomes of Stellar Deaths The possible end states of stellar evolution are (i) White dwarfs (ii) Neutron stars (iii) Black holes Which compact object will form depends on whether electron degeneracy is achieved at high or low Temperature (which in turn depends on the stellar mass). M ≤ 1.4 M : Electron degeneracy is reached at a relatively low T. Consequently, advanced nuclear burning is not reached. Support against gravity is provided by Pe. 1.4 M ≤ M ≤ 4 M : At the red giant phase, H burns in a shell, and He in another shell. Prad supports against gravity. Mass loss at this stage. Subsequent evolution to a white dwarf. M > 8 M : C12 ignites prior to the development of a degenerate core. Advanced burning stages can be reached. The core eventually collapses to form a compact object. How many of each form?? A LARGE NUMBER OF UNCERTAINTIES: • late stages of evolution (especially some mass regimes) • mass loss during evolution • differential rotation of the star as the core collapses • explosion energies for supernovae • fallback during supernovae • whether dynamo and/or flux freezing play a role in generating magnetic fields • theoretical uncertainties in maximum NS mass Inverse -decay One other reaction we should briefly talk about in the evolution of stars into compact objects is the inverse -decay p + e n + e In “ordinary” environments, -decay _ n p + e + e also proceeds efficiently and enables an equilibrium between electrons, protons, and neutrons. But at high densities, when electron Fermi energy is high and the electron produced by -decay does not have sufficient energy, the inverse decay proceeds to primarily create more neutrons. Formation of Neutron Stars QuickTime™ and a YUV420 codec decompressor are needed to see this picture. A supernova simulation from Burrows et al. Formation of Neutron Stars QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Dividing Lines From Fryer et al. 99 N.B. This is mostly to show uncertainties and possibilities than hard numbers! How do Compact Objects Appear? The most important factor that determines the observable properties of a compact object is whether or not it is in a (interacting) binary. Isolated White Dwarfs: Accreting White Dwarfs: Cooling white dwarfs Cataclysmic variables, dwarf novae, ….. Isolated Neutron Stars: Accreting Neutron Stars: Radio pulsars, millisecond radio pulsars, magnetars (AXPs and SGRs), CCOs, nearby dim isolated stars low-mass X-ray binaries, high-mass X-ray binaries, bursters Isolated Black Holes: Accreting Black Holes: Not visible!! (except in gravity waves) X-ray binaries AGN, low-luminosity AGN This is a non-trivial statement because accretion seems to alter the properties of the compact object permanently. Cooling white dwarfs in globular cluster M4 The Crab Pulsar -- A “Prototypical” Rotation Powered Pulsar QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Chandra Hubble An Accretion Powered Pulsar QuickTime™ and a Video decompressor are needed to see this picture. - magnetic field of the primary (the neutron star) channels the flow to the polar caps: an X-ray pulsar - the angular momentum transported to the neutron star causes it to spin up P=Pdot diagram of pulsars A Galactic Black Hole QuickTime™ and a Sorenson Video decompressor are needed to see this picture. Our Supermassive Black Hole - this is the longest Chandra exposure image of our Galactic Center Sagittarius A* - supermassive black holes feed off of nearby stars and ISM gas - timescale of flaring events suggests they are occurring near the event horizon INPE Advanced Course on Compact Objects Lecture 2 Structures of White Dwarfs And Neutron Stars Structure of White Dwarfs and the Chandrasekhar Limit: Electron degeneracy pressure: Remember that pressure of a degenerate gas is given by Pe pF 1 3 2 v( p) p dp 3 3 0 Which can be evaluated for the nonrelativistic case v<<c (v=p/me) and for extreme relativistic case (v~c) to give (3 2 ) 2 / 3 2 5 / 3 Pe n e , v c 5 me (3 2 )1/ 3 Pe cn e4 / 3 , v ~ c 4 We solve this equation along with the continuity and force equations: dPe GM(r) (r) dr r2 To obtain R c 1n 2n M c 3n 2n Structure of White Dwarfs and the Chandrasekhar Limit: We can plug in some numbers for low-density white dwarfs 5 3 , n 3 2 and the constants to obtain 1/ 6 R 1.122 10 6 c 3 10 gcm 4 km M 0.4964 6 c 3 M 10 gcm 1/ 2 In the relativistic limit 4 , n3 3 1/ 6 c R 3.347 10 6 3 10 gcm 4 M 1.457 M km Chandrasekhar limit Remember there was no dependence on me, c, or R in the extreme relativistic limit. Chandrasekhar Limit for White Dwarfs: A Quick Treatment The existence of a maximum mass for degenerate stars is very fundamental. Let’s understand it in two ways: I. M ~ 3n 2n c So as matter under extreme density gets more and more relativistic, mass can no longer increase by increasing the central density but asymptotes to a constant. II. Another way to look at it is the Fermi energy at the quantum limit where the volume per fermion is 1/n = R3/N (Pauli principle), momentum per fermion is n1/ 3 so that 1/ 3 EF ~ cN R while the gravitational energy per baryon is EG ~ GMmB R Setting EF + EG = 0 gives N max 2 10 57 M max N max mB 1.5 M Important Note on the Chandrasekhar Limit: White dwarfs and neutron stars have maximum masses for different reasons!! 1. MCh for degenerate neutron gas is ~0.7 M ! 2. Neutron stars have a maximum mass because of general relativity (as we will see) 3. White dwarfs do not reach the Chandrasekhar mass (the absolute maximum) because inverse -decay kicks in at lower densities. 4. Neutron stars can exceed “their Chandrasekhar limit” because there are other sources of pressure (not just pressure of degenerate neutrons) White Dwarf Cooling Interiors of white dwarfs are roughly isothermal because of high thermal conductivity of degenerate matter. No heat generation ==> outer layers are in radiative equilibrium, photons carrying the thermal flux There is also local thermodynamic equilibrium (electrons and photons are thermalized) Finally, hydrostatic equilibrium holds for the star Solve photon diffusion equation (along with hydrostatic equilibrium + EOS) c d L 4 r (aT 4 ) 3 dr 2 where opacity is provided mainly by free-free and bound-free transitions. For values appropriate for a white dwarf, we find M L 2 106 erg s1 T*7 / 2 M T* 10 6 10 7 K How long does it take the White Dwarf to Cool? 3 M U k T* 2 A mu L dU dt Combining this with our expression for L and solving for the cooling time gives L 5 / 7 M Or about ~109 yrs for typical white dwarf luminosities. Two (most important) effects that we neglected: 1. When T falls below the melting temperature Tm, the liquid crystallizes and releases q ~ kTm per ion. 2. Crystallization also changes the heat capacity, adding additional 1/2 kT per mode from the lattice potential energy. The overall effect is to increase the thermal lifetime of the white dwarf. Observations of White Dwarf Cooling • Very detailed studies of white dwarfs in globular clusters are carried out • Detailed cooling models are applied to, e.g., HST data • One such study of NGC 6397 (Hansen et al. 2007) finds a cluster age of Tc=11.47 ± 0.47 Gyrs. A typical luminosity function N magnitude Observations of White Dwarf Cooling • Sloan Digital Sky Survey discovers “ultracool” WDs • At some arbitrarily low T, we start calling them “black dwarfs” • Spectral fits (and in some cases binary companions) allow us to determine WD masses as well from Kepler et al. 07 Neutron Stars Density Regimes in Neutron Stars 1. Atmosphere ( 104 g /cm3): Matter in gaseous form, filamentary if B 1010 G) 2. 104 ≤ ≤ 107 g /cm3 : Matter as in white dwarfs. A lattice of nuclei embedded in a degenerate relativistic electron gas. 3. 107 ≤ ≤ 1011 g /cm3 : Inverse -decay transforms protons into n in nuclei. As nuclei get n-rich, the most stable configuration is no longer A=56 but shifts to higher values. 4. 1011 ≤ ≤ 5x1012 g /cm3 : Nuclei become so heavy (A~122) and so neutron-rich (n/Z=83/39) that they “drip” neutrons, forming a free neutron gas. 5. 5x1012 g /cm3: Mixture of degenerate n gas, ultrarelativstic electrons and heavy nuclei. Pn ~ Pe at this density. 6. 5x1012 g /cm3: Nuclei disappear, p, e, and n exist in -equilibrium. These density regimes are found in the “crust” of the neutron star, which is ~few hundred km thick and makes up a few percent of the star’s mass. 7. 1013 ≤ ≤ 5x1015 g /cm3 : Free neutrons dominate. 8. 1015 g /cm3: ??? Neutron Star Structure and Equation of State Structure of a (non-rotating) star in Newtonian gravity: dM(r) 4 r 2 (r) dr M(r) r 2 4 r (r) dr 0 dP(r) GM(r) (r) 2 dr r Need a third equation relating P(r) and (r ) (called the equation of state --EOS) P P( ) Solve for the three unknowns M, P, (enclosed mass) Equations in General Relativity: dM(r) 4 r 2 (r) dr 3 G M(r) 4 r P(r) dP(r) P (r) 2 2GM(r) dr c 2 r 1 rc 2 } OppenheimerVolkoff Equations Two important differences between Newtonian and GR equations: 1. Because of the term [1-2GM(r)/c2] in the denominator, any part of the star with r < 2GM/c2 will collapse into a black hole 2. Gravity ≠mass density Gravity = mass density + pressure (because pressure always involves some form of energy) Unlike Newtonian gravity, you cannot increase pressure indefinitely to support an arbitrarily large mass Neutron stars have a maximum allowed mass Equation of State of Neutron Star Matter We saw for degenerate, ideal, cold Fermi gas: P~ { 5/3 (non-relativistic neutrons) 4/3 (relativistic neutrons) Solving Oppenheimer-Volkoff equations with this EOS, we get: R~M-1/3 As M increases, R decreases --- Maximum Neutron Star mass obtained in this way is 0.7 M (there would be no neutron stars in nature) --- There are lots of reasons why NS matter is non-ideal (so that pressure is not provided only by degenerate neutrons) Some additional effects we need to take into account : (some of them reduce pressure and thus soften the equation of state, others increase pressure and harden the equation of state) I. -stability Neutron matter is formed by inverse -decay p + e n + e escape And is also unstable to -decay _ n p + e + e escape In every neutron star, -equilibrium implies the presence of ~10% fraction of protons, and therefore electrons to ensure charge neutrality. The presence of protons softens the EOS and reduces the maximum mass II. The Strong Force The force between neutrons and protons (as well as within themselves) has a strong repulsive core II. The Strong Force At very high densities, this interaction provides an additional source of pressure. The shape of The potential when many particles are present is very difficult to calculate from first principles, and two approaches have been followed: a) The potential energy for the interaction between 2-, 3-, 4-, .. particles is parametrized and and the parameter values are obtained by fitting nucleon-nucleon scattering data. b) A mean-field Lagrangian is written for the interaction between many nucleons and its parameters are obtained empirically from comparison to the binding energies of normal nucleons. III. Isospin Symmetry The Pauli exclusion principle makes it energetically favorable for a system of nucleons to have approximately equal number of protons and neutrons. In neutron stars, there is a significant difference between the neutron and proton fraction and this costs energy. This interaction energy is usually added to the theory using empirical formulae that reproduce the (A,Z) relation of stable nuclei. IV. Presence of Bosons, Hyperons, Condensates As we saw, neutrons can decay via the -decay _ n p + e + e yielding a relation between the chemical potentials of n, p, and e: n p e And they can also decay through a different channel np+ _ when the Fermi energy of neutrons exceeds the pion rest mass EF,n m c 2 140 MeV The presence of pions changes the thermodynamic properties of the neutron star interior significantly. WHY? Because pions are bosons and thus follow Bose-Einstein statistics ==> can condense to the ground state. This releases some of the pressure that would result from adding additional baryons and softens the equation of state. The overall effect of a condensate is to produce a “kink” in the M-R relation: It is very difficult for — to be present in the centers of neutron stars. How about other particles? Nucleon reactions of the form N N N n n n are possible and lead to the creation of other particles with different decay properties. For example, for the K mesons, K 0 2 e e 2 which means that K0 and K+ will spontaneously decay, but so K- can be present. e Hyperons in neutron-star matter Hyperons and the masses of neutron stars V. Quark Matter or Strange Matter Exceeding a certain density, matter may preferentially be in the form of free (unconfined) quarks. In addition, because the strange quark mass is close to u and d quarks, the “soup” may contain u, d, and s. Quark/hybrid stars: typically refer to a NS whose cores contain a mixed phase of confined and deconfined matter. These stars are bound by gravity. Strange stars: refer to stars that have only unconfined matter, in the form of u, d, and s quarks. These stars are not bound by gravity but are rather one giant nucleus. Mass-Radius Relation for Neutron Stars Normal Neutron Stars Stars with condensates Strange Stars •We will discuss how accurate M-R measurements are needed to determine the correct EOS. However, even the detection of a massive (~2M) neutron star alone can rule out the possibility of boson condensates, the presence of hyperons, etc, all of which have softer EOS and lower maximum masses. Effects of Stellar Rotation on Neutron Star Structure Using Cook et al. 1994 Spin frequency (in kHz) Effects of Magnetic Field on Neutron Star Structure Magnetic fields start affecting NS equation of state and structure when B ≥ 1017 G. by contributing to the pressure. For most neutron stars, the effect is negligible. INPE Advanced Course on Compact Objects Lecture 3 Masses and Radii of Neutron Stars Mass-Radius Relation for Neutron Stars Baryonic vs. Gravitational Mass Important point about what we mean by NS mass: We measure “gravitational” mass from astrophysical observations: the quantity that determines the curvature of its spacetime. This is different than “baryonic” mass: the sum of the masses of the constituents of the NS. Remember the equation of structure for the NS: dM grav (r) 4 r 2 (r) dr M grav 4 R r (r) dr 2 0 Here, “r” is not the proper radius (the one a local observer would measure) but the Schwarzschild radius (which is smaller) The baryonic mass can be calculated from 2GM(r) 2 M b 4 1 r (r) dr 2 rc 0 R And is larger than Mgrav. Why is Mgrav< Mb ? Classically, the total energy in the volume of the NS is tot M b c pot 2 Epot < 0 The mass seen by a test particle outside the neutron star is related to the total energy, M grav | pot | tot M Mb b c2 c2 This potential energy is released during the formation of the neutron star and is converted into heat. The heat escapes (mostly) in the form of neutrinos and (a small fraction) as photons. Methods of Determining NS Mass and/or Radius • Dynamical mass measurements (very important but mass only) • Neutron star cooling (provides --fairly uncertain-- limits) • Quasi Periodic Oscillations • Glitches (provides limits) • Maximum spin measurements Dynamical Mass Measurements Use the general relativistic decay of a binary orbit containing a NS (PÝb )GR f (m1,m2 ,sin( i)) The observed binary period derivative can be expressed in terms of the binary mass function. Need a short binary period, preferably a fast pulsar, a long baseline to get accurate timing parameters. Also use Shapiro delay, t f (m2,sin( i)) (For black holes, measurements are more approximate and rely on the binary mass function) Limits on PSR J0751+1807 from Nice et al. 05 M = 2.1 M Methods of Determining NS Mass and/or Radius • Dynamical mass measurements (very important but mass only) • Neutron star cooling (provides --fairly uncertain-- limits) • Quasi Periodic Oscillations • Glitches (provides limits) • Maximum spin measurements Neutron Star Cooling Why is cooling sensitive to the neutron star interior? The interior of a proto-neutron star loses energy at a rapid rate by neutrino emission. Within ~10 to 100 years, the thermal evolution time of the crust, heat transported by electron conduction into the interior, where it is radiated away by neutrinos, creates an isothermal core. The star continuously emits photons, dominantly in X-rays, with an effective temperature Teff that tracks the interior temperature. The energy loss from photons is swamped by neutrino emission from the interior until the star becomes about 3 × 105 years old. The overall time that a neutron star will remain visible to terrestrial observers is not yet known, but there are two possibilities: the standard and enhanced cooling scenarios. The dominant neutrino cooling reactions are of a general type, known as Urca processes, in which thermally excited particles alternately undergo - and inverse- decays. Each reaction produces a neutrino or antineutrino, and thermal energy is thus continuously lost. Neutron Star Cooling The most efficient Urca process is the direct Urca process. This process is only permitted if energy and momentum can be simultaneously conserved. This requires that the proton to neutron ratio exceeds 1/8, or the proton fraction x ≥ 1/9. If the direct process is not possible, neutrino cooling must occur by the modified Urca process n + (n, p) → p + (n, p) + e− + νe p + (n, p) → n + (n, p) + e + + νe Which of these processes take place, and where in the interior, depend sensitively on the composition of the interior. Neutron Star Cooling Caveats: Very difficult to determine ages and distances Magnetic fields change cooling rates significantly Methods of Determining NS Mass and/or Radius • Dynamical mass measurements (very important but mass only) • Neutron star cooling (provides --fairly uncertain-- limits) • Quasi Periodic Oscillations • Glitches (provides limits) • Maximum spin measurements Quasi-periodic Oscillations Accretion flows are very variable, with timescales ranging from 1ms to 100 days! QUASI PERIODIC OSCILLATIONS VAN DER KLIS ET AL. 1997 Power Spectra of Variability: HIGH FREQUENCIES BROAD-BAND VARIABILITY Quasi-periodic Oscillations from Miller, Lamb, & Psaltis 1998 Methods of Determining NS Mass and/or Radius • Dynamical mass measurements (very important but mass only) • Neutron star cooling (provides --fairly uncertain-- limits) • Quasi Periodic Oscillations • Glitches (provides limits) • Maximum spin measurements Limits from Maximum Neutron Star Spin The mass-shedding limit for a rigid Newtonian sphere is the Keplerian rate: N Pmin R 3 1/ 2 M 1/ 2 R 3 / 2 2 ms 0.545 M 10km GM Fully relativistic calculations yield a similar result: Pmin M 1/ 2 R 3 / 2 0.83 ms M 10km for the maximum mass, minimum radius configuration. Depending on the actual values of M and R in each equation of state, the obtainable maximum spin frequency changes. Methods of Determining NS Mass and/or Radius • Dynamical mass measurements (very important but mass only) • Neutron star cooling (provides --fairly uncertain-- limits) • Quasi Periodic Oscillations • Glitches (provides limits) • Maximum spin measurements More promising methods (entirely in my opinion): • Thermal Emission from Neutron Star Surface • Eddington-limited Phenomena • Spectral Features Methods to Determine M and/or R Radius for a thermally emitting object from continuum spectra: 2 F D R2 = T4 RX J1856 Methods to Determine M and/or R Mass from the Eddington limit: 4GcM LEdd = (1+X) At the Eddington Limit, radiation pressure provides support against gravity Methods to Determine M and/or R Globular Cluster Burster Kuulkers et al. 2003 Methods to Determine M and/or R M/R from spectral lines: E = E0 (1 2M R ) Cottam et al. 2003 In reality, Mass and Radius are always coupled because neutron stars lens their own surface radiation due to their strong gravity NS GR n Gravitational Lensing 4GM 2 cb deflection angle b impact parameter Gravitational Self-Lensing NS max= 900+deflection angle A perfect ring of radiation: R/M = 3.52 Self-Lensing The Schwarzschild metric: 1 2M 2M 2 ds2 dt 2 1 dr 1 f ( , ) R R Photons with impact parameters b<bmax can reach the observer: bmax M 1/ 2 R(1 2 ) R General Relativistic Effects QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Lensing of a hot spot on the neutron star surface * Normalized to DC Pulse Amplitudes Two antipodal hot spots at a 45 degree angle from the rotation axis Apparent Radius of a Neutron Star bmax M 1/ 2 R(1 2 ) R Because of lensing, the apparent radius of neutron stars changes GR Modifications The correct expressions (lowest order) R2 = LEdd = D2 F T4 (1 4GcM (1+X) (1 2M R -1 ) 2M R ) 1/2 Effects of GR Modifications to the Eddington limit What if the NS is rotating rapidly? E =E0 (1R/c) Doppler Boosts R t = / ~ R/c /2 ~ 600 Hz v = 0.1 c Time delays Other effects: Frame dragging Oblateness Equation of State (Stergioulas, Morsink,Cook) Effect of Rotation on Line Widths Özel & Psaltis 03 May affect the inferred redshift and detectability BUT E/Eo M/R FWHM R A fourth method! Combining the Methods 1. The methods have different M-R dependences: they are complementary! 2. Surface emission gives a maximum NS mass!! 3. Eddington limit gives a minimum radius!! gravity effects can be undone Özel 2006 INPE Advanced Course on Compact Objects Lecture 4 Surfaces of Neutron Stars & Observations of their Masses, Radii and Magnetic Fields Determining Mass and Radius 1. The methods have different M-R dependences: they are complementary! 2. Surface emission gives a maximum NS mass!! 3. Eddington limit gives a minimum radius!! gravity effects can be undone Özel 2006 A Unique Solution for Neutron Star M and R M and R not affected by source inclination because they involve flux ratios Effect of Systematic Errors A 15% systematic error in the assumed distance will prohibit a unique solution Applying the Methods to Sources: For isolated sources: Can use surface emission from cooling to get area contours (and possibly a redshift) For accreting sources: Can possibly apply all these methods, especially if there is Eddington limited phenomena Pros and Cons of Surface Emission from Isolated vs. Accreting: Isolated: Pros: No heavy elements --atmospheres simple No accretion luminosity Accreting: Eddington-limited phenomena (Redshifted) spectral features more likely Surface emission likely to be uniform Bright Cons: Strong magnetic fields --atmospheres complicated Non-thermal emission often dominates Heavy elements may not be present -- redshifted lines unlikely Surface emission non-uniform Heavy elements --atmospheres complicated Accretion luminosity can be high Good Isolated Candidates • Nearby neutron stars with no (or very low) pulsations • No observed non-thermal emission (as in a radio pulsar) • (Unidentified) spectral absorption features have been observed in some Sources have been dubbed “the magnificient seven” initially, even though they are more numerous now. Good Accreting Candidates: LMXBs Showing Thermonuclear Bursts Sample lightcurves, with different durations and shapes. Spectra look pretty featureless and are traditionally fit with blackbodies of kT~few keV. Thermonuclear Bursts and Eddington-limited Phenomena QuickTime™ and a BMP decompressor are needed to see this picture. from Spitkovsky et al. Burst proceeding by deflagration Bursts propagate and engulf the neutron star at t << 1 s. Thermonuclear Bursts and Eddington-limited Phenomena QuickTime™ and a Video decompressor are needed to see this picture. from Zingale et al. Thermonuclear Bursts and Eddington-limited Phenomena Theoretical reasons to think that the emission is uniform and reproducible Magnetic fields of bursters (in particular 0748-676) are dynamically unimportant (for EXO 0748: Loeb 2003) --> fuel spreads over the entire star Emission from neutron stars during thermonuclear bursts are likely to be uniform and reproducible Thermonuclear Bursts and Eddington-limited Phenomena An Eddington-limited (i.e., a radius-expansion) Burst A flat-topped flux, a temperature dip, a rise in the inferred radius Thermonuclear Bursts and Eddington-limited Phenomena The Constant Peak Luminosity The peak luminosity is constant to 2.8% for 70 bursts of 4U 1728-34 Galloway et al. 2003 Measuring the Eddington Limit: The Touchdown Flux Luminosity (arbitrary) An “H-R” diagram for a burst 4R 2R R 3 2 Temperature (keV) Reliability of the Inferred “Radius” Fcool Constant inferred radius from 4 Tc Savov et al. 2001 Need a model for the surface emission! N.B. This is both to determine NS mass and radius but also to understand a wide range of phenomena happening on neutron stars! Emission from the Surfaces of Neutron Stars: Isolated NS I. Composition of the Surface: 1. How much material is necessary to cover the surface and dominate the emission properties? Assume zero magnetic field, need material to optical depth =1. m V 2 4 RNS h 2 N p m p 4 RNS h Ne= Np and d = Ne Tdz ==> = Ne T z (assuming electron density is independent of depth) 2 m m p 4 RNS T For typical values, m=10-17 M for an unmagnetized neutron star. 2. How long does it take the cover the NS surface with a 10-17 M hydrogen or helium skin by accreting from the ISM? Using Bondi-Hoyle formalism: 2 4 (GM) ISM Ý M v3 If we take v 10 7 cm /s m p /cm 3 1.71024 g /cm 3 M 1.5 2 10 33 g ÝISM 7 10 8 g /s 1017 M / yr M taccr= 1 yr. Assuming magnetic fields do not prevent accretion, very quickly, NS surfaces can be covered by H/He. 3. Settling of Heavy Elements (Bildsten, Salpeter, & Wasserman) Heavy elements settle by ion diffusion, as they are pulled down by gravity and electron current. How long does it take for them to settle below optical depth ~1 (where they no longer affect the spectrum?) 3 / 2 g kT tsettle 13s 14 10 1keV 1 (T enters because it affects the speed of ions and the inter-particle distances) II. Ionization State of the Atmosphere and Magnetic Fields: 1. The ionization state of a gas is given by the Saha equation: nH VZ H n p ne Ze Z p Partition function Z defined for each species: 2 2 1/ 2 Ze , e ( ) 2 3e me kT V / kT Zp e 2 3p V When we consider H atoms at kT ≈ 1keV, <<kT so the atmosphere is completely ionized. For lower temperatures (kTeff ~ 50 eV), need to consider the presence of neutral atoms. 2. Magnetic Fields At B ≥ 1010 G, magnetic force is the dominant force, >> thermal, Fermi, Coulomb energies. Photon-Electron Interaction in Confining Fields B e-- parallel mode perp mode Magnetic Opacities 2 s / NeT 1 ( E / Eb ) T 1 s 1 2 T 2 s 1 Energy, angle & polarization dependence expect non-radial beaming and deviations from a blackbody spectrum Vacuum Polarization Resonance Vacuum-dominated Plasma-dominated -- at B ~ Bcr virtual e+ e- pairs affect photon transport -- resonance appears at an energy-dependent density -- proton cyclotron absorption features appear at ~keV, and are weak Emission from the Surfaces of Neutron Stars: Accreting Case I. Composition of the Surface: A steady supply of heavy elements from accretion as well as thermonuclear bursts Atmosphere models need to take the contribution of Fe, Si, etc. II. Ionization State: Temperatures reach ~few keV. Magnetic field strengths are very low (108--109 G) Light elements are fully ionized. Bound species of heavy elements. III. Emission Processes: Compton Scattering Most important process is non-coherent scattering of photons off of hot electrons Bound-bound and bound-free opacities also important for heavy elements Compton Scattering “Compton” scattering is a scattering event between a photon and an electron where there is some energy exchange (unlike Thomson scattering which changes direction but not the energies) By writing 4-momentum conservation for a photon scattering through angle , we find Ef 1 i cos i E i 1 cos E i (1 cos ) i f mc 2 Energy gain from the electron Recoil term Typical to expand this expression in orders of , and average over angles. To first order, photons don’t gain or lose energy due to the motion of the electrons (angles average out to zero) Compton Scattering To second order, we find on average E 1 2 E i i Ei 3 mc 2 Energy gain from K.E. of electron If electrons are thermal, i2 3kT mc 2 E kT E i Ei mc 2 If Ei < kT, photons gain energy If Ei > kT, photons lose energy Energy loss from recoil Model Atmospheres: Hydrostatic balance: Gravity sustains pressure gradients dP g 2 d yG N e T ( h N e T dz) 0 yG is the correction to the proper distance in GR 2GM 1/ 2 yG 1 Rc 2 Equation of State: Assume ideal gas P = 2NkT Equation of Transfer: dIEi i i i BE yG a I a si I i d es 2 ij j ( , ) I d s j 1,2 for i = 1, 2 Radiative Equilibrium : H( ) Teff 4 I( , , E) d dE Techniques for solving the Transfer equation (with scattering): Feautrier Method, Variable Eddington factors, Accelerated Lambda Iteration… Techniques for achieving Radiative Equilibrium: Lucy-Unsold Scheme, Complete Linearization… Typical Temperature Profiles: magnetic field strengths Typical Spectra (Isolated, Non-Magnetic): From Zavlin et al. 1996 Typical Spectra (Isolated, Magnetic): T=0.5 keV B=4•1014 G B=6•1014 G B=8•1014 G B=10•1014 G B=12•1014 G From Madej et al. 2004, Majczyna et al 2005 Typical Spectra (Accreting, Burster): • Comptonization produces high-energy “tails” beyond a blackbody • Heavy elements produce absorption features Color Correction Factors From Madej et al. 2004, Majczyna et al 2005 EXO 0748-676 Redshifted lines with XMM: z=0.35 Cottam, Paerels, & Mendez 2003 Four Eddington-limited bursts with EXOSAT and RXTE: Fcool / Tc4 1.14 0.10 FEdd 2.25 0.23 108 erg cm2 s1 Gottwald et al. 1986, Wolff et al. 2005 Slow rotation 0= 44.7Hz Villareal & Strohmayer 2004 Özel 2006 Mass and Radius of EXO 0748-676 M-R limits: M = 2.10 ± 0.28 M R = 13.8 ± 1.8 km Neutron star equations of state need to allow heavy and large neutron stars Future Prospects - Monitoring and long exposure observations of bursters and isolated stars necessary - Distance determination to sources (or their companions) would eliminate the need for the redshift - Atmosphere models are getting more and more sophisticated -In the meantime, we can try to understand emission mechanisms and magnetic field strengths of isolated NS, bursters, AXPs, SGRs, and even surface properties of some radio pulsars. Spectral Analysis Fits to seven epochs of XMM data on XTE J1810-197 Guver, Ozel, Gogus, Kouveliotou 07 Temperature Evolution and Magnetic Field of XTE J1810-197 Magnetic field remains nearly constant; is equal to spindown field! Temperature declines steadily and dramatically No changes in magnetospheric parameters during these observations Guver, Ozel, Gogus, Kouveliotou 07 INPE Advanced Course on Compact Objects Lecture 5 Black Holes General Relativity Basics General relativity is a relativistic theory of gravity. In special relativity, we specify events by 4 spacetime coordinates (ct, x) The invariant distance between two (nearby) events is given by ds2 c 2 dt 2 dr 2 r 2 d 2 r 2 sin 2 d 2 or ds2 dx dx Minkowski space In GR, events are still specified by 4 coordinates, with the invariant distance given by ds2 g (x ) dx dx g is the metric that specifies the properties of the (curved) spacetime in the presence of matter (energy). GR has two ingredients Newtonian Gravity Analog Newton’s Second Law The Equivalence Principle Ý xÝ mG Ý xÝ g 0 mI xÝ xÝ 0 Einstein’s Equation G 8 T Poisson’s Equation 2 4 Predictions of General Relativity G 8 T Solutions of this equation yield metrics that describe the properties of the spacetime. T contains all forms of energy Let’s first look at validations / tests of GR. Will 2001 The Equivalence Principle Has Been Tested to a Very High Degree The field equations have only been tested in the weak field limit. What measures the strength of the gravitational field? Matter Gravity No scale in the theory! No field is either weak or strong! Weak Field Strong Field Potential GM ~ Rc 2 1 1 Velocity, u /c 1 1 Slide credit: D. Psaltis GENERAL RELATIVISTIC PHENOMENA LIGO Neutron Stars Galactic Black Holes LISA GP-B Eclipse Hulse-Taylor AGN Moon Mercury Redshift: 1 E E0 The Schwarzschild Metric Solution of the Einstein field equation G 8 T in spherical symmetry, in the absence of any matter (in the region of solution) is the Schwarzschild metric: 2GM 2 2GM 1 2 ds 1 dt 1 dr r 2 d 2 r 2 sin 2 d 2 2 2 rc rc 2 customary to set G=c=1 2M 2 2M 1 2 2 2 2 2 2 ds 1 dt 1 dr r d r sin d r r 2 • The Schwarzschild metric describes the exterior spacetime of any spherical mass distribution (not just black holes) • However, if the enclosed mass is so concentrated that it is within M<r/2, and you extend the vacuum solution to r=2M, this equation predicts an “event horizon” • This region of spacetime cannot communicate with the external universe. • At r=0 (all the way inside a black hole), field equations predict a singularity of infinite density. Event Horizons Proper time in this metric: 2M 1/ 2 d 1 dt r gravitational time dilation At r=2M, proper time is infinite; gravitational redshift is infinite ==> it takes infinite amount of time for a signal emitted at the event horizon to reach a distant observer Event Horizons What happens to an observer getting close to the event horizon? Collapse to a Black Hole QuickTime™ and a Animation decompressor are needed to see this picture. Spacetime during Collapse QuickTime™ and a Animation decompressor are needed to see this picture. Astrophysical Evidence for Black Holes & Event Horizons Evidence for Event Horizons Menou et al. 98 Garcia et al. 01 . Orbital period is a measure of the mass accretion rate M Signatures of a Black Hole Charles 1999 A Very Heavy Compact Object CAUSALITY LIMIT (?) Signatures of a Black Hole Kinematics of gas around black hole Signatures of a Black Hole Water MASERs Signatures of a Black Hole Reverberation mapping All kinematic measurements indicate ~billion solar mass black holes The Case for Supermassive Black Holes from Stellar Dynamics QuickTime™ and a YUV420 codec decompressor are needed to see this picture. Using Kepler’s law, Ghez et al. and Gebhart et al. find 4x106 M inside this volume The mass of the black hole in the center of the Milky Way The Case for the Existence of Galactic Black Holes •Variability timescale: t ~ 1-10 ms ==> emission zone size R ≤ c t ≤ 108 cm. (coherent variable phenomena must occur over the size of the compact object/emission zone) • Orbital parameters obtained from the Doppler curve give a mass function and a minimum primary mass Seeing Black Holes IMAGING OF SELF LENSING C. REYNOLDS EVEN FOR ACTIVE GALACTIC NUCLEI REQUIRES arcsec X-RAY INTERFEROMETRY: THE BLACK HOLE IMAGER Broderick & Loeb 2006 What do Black Holes look like? Other Strange Phenomena The existence of an Innermost Circular Stable Orbit For orbits in central potentials, we define Veff G M 1 L2 r 2 r2 In GR, GR eff V G M 1 L2 2GM (1 ) r 2 r2 r Other Strange Phenomena For any L, there are no stable orbits inside r=6M. Correlations between black hole and galaxy properties From Kormendy et al.