X-ray Observations of Transient Neutron Stars in Quiescence

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1 X-ray Observations of Transient Neutron Stars in Quiescence Bob Rutledge McGill University Graduate Students: Sebastien Guillot (MSc 09; PhD) Christian Haakonsen (MSc 09) Collaborators: Lars Bildsten (ITP/UCSB) Ed Brown (MSU) George Pavlov (PSU) Slava Zavlin (MSFC) Greg Ushomirsky (Lincoln Lab.)

Signal to noise required is out of reach

2 Summary Neutron Star Masses and Radii can be measured - independently, simultaneously - from Hydrogen atmosphere neutron stars, to < few% accuracy, each. This can be used to determine the cold dense matter equation of state, at and above nuclear density. Signal to noise required is out of reach of existing X-ray observatories, but can be done with a proposed mission (International X-ray Observatory).

3 Solving Quantum Chromodynamics At Finite Density - Neutron Stars are Unique Astrophysical Laboratories 1932: Since discovery of neutron (Chadwick 1932), how the strong force works to mediate attraction between protons and neutrons has been a major question. 1930s-1960s: Strong force is a non-superpositional force, multi-body in nature -- parameterization with Skyrme potential with limited success explaining the properties of small nuclei. 1960s: Discovery of quarks, and development of QCD explained the formation of neutrons, protons, mesons, hyperons... It does an outstanding job of explaining the existence of these particles, and their one-to-one interactions, but we have no exact theoretical apparatus to perform the many-body interaction calculations. Thus we rely on approximate methods of effective-field-theories (Brueckner-Betha-Goldstone Theory, Green s function theory, and relativistic mean-field theory). Today: QCD is not an exact theory at finite density (QCDd). Where this becomes relevant is above nuclear density (>2.35x10 14 g cm -3 ), and for asymmetric matter (many more neutrons than protons). It is technically impossible to create cold matter above nuclear density in terrestrial laboratories. In the cores of neutron stars, gravitational force compresses cold matter to supernuclear densities. Thus: Neutron Stars are unique sites of cold QCDd in the universe, precisely as black holes are the unique sites for strong field gravity. If we want to solve QCDd, neutron stars will be the astrophysical tool for how we do it.

4 Neutron Star Structure: The Cartoon Picture Atmosphere Envelope Crust Outer Core Inner Core Credit: Dany Page

5 From Neutron Star Mass-Radius Relation to the Equation of State P = f(ρ) Lindblom (1992) showed that each Dense Matter Equation of State maps to a unique Mass-Radius relationship for neutron stars. Postnikov, Prakash and Lattimer (in progress) and Ozel and Psaltis (2009) demonstrate how to perform the inverse problem: take the mass-radius relationship, and produce an equation of state. Only ~5-7 such objects are needed, but with different masses, to derive a new dense matter equation of state. Thus, measurement of the neutron star mass-radius relationship would implicate a unique deos. Short Course: Gravity pulls inward Pressure Pushes Outward Result: R=f(M) F g M(< R)

$function of fractional neutron excess).$

6 Estimated Equations of State Different calculational (approximation) methods Different input physics Different nuclear parameters (example: nuclear compressibility as a function of fractional neutron excess). Measurements of neutron star radii alone can place useful constraints on the deos (Lattimer and Prakash 2000) Lattimer and Prakash (2000)

7 Mass-Radius Relation from the Equation of State Lattimer & Prakash (2000)

8 measurements Some systems with masses measured to 1 part in 10 6 Pulsars However, not so useful for radius measurements.

9 Quiescent Low Mass X-ray Binaries When accreting (pictured) we mostly observe X-rays from the disk. In some sources ( transients ) accretion can stop -- and then we see only the neutron star. System Names: LMXBs, Soft X-ray Transients, Neutron-Star Binaries.

10 Soft X-ray Transients Outbursts are due to disk instability; peak luminosities are ergs s -1. Outbursts last ~30 days (or as long as years). Exhibit type-i X-ray bursts (thermonuclear flashes). After outburst, X-ray sources return to quiescence ( ergs s -1 )

11 Why are qlmxbs promising for measuring NS radii? Brown, Bildsten & RR (1998) First detection: transient neutron star was discovered in quiescence (Cen X-4; L x ~10 33 erg s -1. Van Paradijs et al 1984), resulted in two problems : Glen & Sutherland (1980) 10 6 yr 1. The neutron stars should be cold. Luminosity provided by accretion? (van Paradijs et al 1984) Alternative: Deep Crustal Heating

12 ρ (g cm -3 ) Non-Equilibrium Processes in the Outer Crust Beginning with 56 Fe (Haensel &Zdunik 1990, 2003) Reaction Δρ ρ Q (Mev/np) Fe 56 Cr - 2e- + 2ν e Cr 56 Ti - 2e- + 2ν e Ti 56 Ca - 2e- + 2ν e Ca 56 Ar - 2e- + 2ν e Ar 52 S +4n - 2e- + 2ν e Non-Equilibrium Processes in the Inner Crust ρ Reaction X Q (g cm -3) n (Mev/np) S 46 Si +6n - 2e- + 2ν e Si 40 Mg + 6n - 2e- + 2ν e Earlier work by K. Sato (1979) Deep Crustal Heating Begins Here Ends Here Mg 34 Ne + 6n - 2e- + 2ν e 34 Ne+ 34 Ne 68 Ca Ca 62 Ar +6n - 2e- + 2ν e Ar 56 S + 6n - 2e- + 2ν e S 50 Si + 6n - 2e- + 2ν e Si 44 Mg + 6n - 2e- + 2ν e Mg 36 Ne + 6n - 2e- + 2ν e 36 Ne+ 36 Ne 72 Ca 68 Ca 62 Ar + 6n - 2e- + 2ν e Ar 60 S + 6n - 2e- + 2ν e S 54 Si + 6n - 2e- + 2ν e Si 48 Mg + 6n - 2e- + 2ν e Mg+ 48 Mg 96 Cr Cr 88 Ti + 8n - 2e- + 2ν e Mev per np Brown, Bildsten & RR (1998)

13 ρ (g cm -3 ) Non-Equilibrium Processes in the Outer Crust Beginning with 56 Fe (Haensel &Zdunik 1990, 2003) Reaction Δρ ρ Q (Mev/np) Fe 56 Cr - 2e- + 2ν e Cr 56 Ti - 2e- + 2ν e Ti 56 Ca - 2e- + 2ν e Ca 56 Ar - 2e- + 2ν e Ar 52 S +4n - 2e- + 2ν e Non-Equilibrium Processes in the Inner Crust ρ Reaction X Q (g cm -3) n (Mev/np) S 46 Si +6n - 2e- + 2ν e Si 40 Mg + 6n - 2e- + 2ν e Mg 34 Ne + 6n - 2e- + 2ν e 34 Ne+ 34 Ne 68 Ca Ca 62 Ar +6n - 2e- + 2ν e Ar 56 S + 6n - 2e- + 2ν e S 50 Si + 6n - 2e- + 2ν e Si 44 Mg + 6n - 2e- + 2ν e Earlier work by K. Sato (1979) Deep Crustal Heating Schatz et al (2002) Gupta et al 2007 Yakovlev 2006 Haensel and Zdunik 2003, Mg 36 Ne + 6n - 2e- + 2ν e 36 Ne+ 36 Ne 72 Ca 68 Ca 62 Ar + 6n - 2e- + 2ν e Ar 60 S + 6n - 2e- + 2ν e S 54 Si + 6n - 2e- + 2ν e Si 48 Mg + 6n - 2e- + 2ν e Mg+ 48 Mg 96 Cr Cr 88 Ti + 8n - 2e- + 2ν e Mev per np Brown, Bildsten & RR (1998)

14 Deep Crustal Heating Reactions in the crust provide ~1 MeV/np. Because the crust is in close thermal contact with the NS core, this will heat a cold core until a steady-state is reached (10 4 yr; cf. Colpi 1999) in which the energy emitted between outbursts (the quiescent luminosity) is equal to the energy deposited in the crust during outbursts. L q M M sun yr -1 Q 1 MeV erg s-1 F q F Q 200 1MeV Heinke et al 2007 Brown, Bildsten & RR (1998)

15 Why are qnss promising for measuring NS radii? 2. Spectral fits using blackbody spectra produced too small of radii for a neutron star (<1 km vs. ~10-20 km, with kt eff ~100 ev; Asai et al 1996). Solution: qnss are not blackbodies. When the accretion rate onto the NS drops below a certain rate (~10 34 erg s -1 ) metals settle out of the photosphere on a timescale of sec (Bildsten et al 1992). This leaves a photosphere of pure Hydrogen. The dominant opacity of a ~100 ev H photosphere is free-free processes, which is strongly energy dependent. κ E ff 114 kt 50 ev 3/2 E 1 kev 3 cm 2 g -1 Brown, Bildsten & RR (1998)

16 Emergent Spectra from Neutron Star Hydrogen Atmosphere For H atmospheres, see also: Rajagopal and Romani (1996) Pons et al (2002) Heyl (Thesis), work by Heinke et al Gaensicke, Braje & Romani (2001) Zavlin et al (1996) R F = σteff, 4 D R = R 1 2GM Rc 2 2 R 2 F = σteff, 4 D R = R 1 2GM Rc 2 RR et al (1999,2000)

17 Improved Calculations for the Hydrogen Atmosphere Spectrum McGill Planar Hydrogen Atmosphere Code (McPHAC) -- Haakonsen et al 2009 (submitted) T eff = K M=160 T eff = K M=40 T eff = K M=40 T eff = K M=40 T eff = K M=40 Two major improvements: 1) Inclusion of true dipole scattering instead of isotropic scattering -- a 1-3% effect. 2) Code will be (after acceptance of the paper) distributed under public license. F Anisotropic / F Isotropic F ν, dipole scattering F ν, isotropic scattering h*nu [kev] Goal: Produce spectra accurate to 0.01% in Flux

18 Chandra X-ray Observatory Launched 1999 (NASA) 1 resolution XMM/Newton Launched 1999 (ESA) 6 resolution ~4x area of Chandra. Every photon is time tagged (~1 sec), with its energy measured (E/deltaE = 10) with full resolution imaging.

19 Aql X-1 with Chandra -- Isolated Field Source (α p =1) F pl =15% ( kev) RR et al (2001b) R (d/5 kpc) kt eff, (1e20 cm -2 ) N H km ev 35 7

20 H atmosphere Spectral Analyses/DCH Interpretation of qlmxbs Cen X-4 (Campana et al. 2000; Rutledge et al. 2001b); Aql X-1 (Rutledge et al. 2001a) 4U (Rutledge et al. 1999); 4U (Rutledge et al. 2000); SAX J , in NGC 6440 (in t Zand et al. 2001); X in Terzan 1 (Wijnands et al. 2002); XTE J (Tomsick et al. 2004); EXO (Tomsick et al. 2005); MXB (Cackett et al. 2006); 1M (Jonker et al. 2007a); 2S (Cornelisse et al. 2007); 4U (Tomsick et al. 2007); 1H (Jonker et al. 2007b). This interpretation is now the dominant theory to explain the properties of quiescent Low Mass X-ray Binaries. But uncertainties in distance remain the dominant uncertainty in qlmxbs in field sources. F = σt 4 eff R D 2

21 The LMXB Factories: Globular Clusters GCs : overproduce LMXBs by 1000x vs. field stars -- contain 10% of the known LMXBs vs. 0.01% of the stars in the galaxy. Accurate distances are important for a number of studies (Stellar evolution, WD cooling). qlmxbs can be identified by their soft X-ray spectra, and confirmed with optical counterparts. NGC D (kpc) +/-(%) Carretta et al (2000)

22 NGC 5139 (Omega Cen) The optical counterpart has been identified! (second one) R c = R c An X-ray source well outside the cluster core DSS

23 NGC 5139 (Omega Cen) R (d/5 kpc) kt eff, (1e20 cm -2 ) N H 14.3± 2.1 km ev (9) RR et al (2002)

24 The Best Measured Neutron Star Radii Name omega Cen (Chandra) omega Cen** (XMM) M13** (XMM) 47 Tuc X7 (Chandra) M28** (Chandra) R (km/d) 13.5 ± ± ± 0.4 D (kpc) 5.36 ±6% 5.36 ±6% 7.80 ±2% ±4% ±10% kt eff, (ev) N H (10 20 cm -2 ) (9) 67 ±2 9 ± ±3 (1.1) ± 4 Ref. Rutledge et al (2002) Gendre et al (2002) Gendre et al (2002) Heinke et al (2006) Becker et al (2003) R < 5% Caveats: All IDd by X-ray spectrum (47 Tuc, Omega Cen now have optical counterparts) calibration uncertainties M30 (Chandra) Lugger et al (2006) NGC 2808 (XMM)?? 9.6 (?) Webb et al (2007) Distances: Carretta et al (2000), Thompson et al (2001)

25 Best Mass-Radius Constraints on the Equation of State R = R NS 1 2GM NS c 2 R ns Lattimer & Prakash (2000) 47 Tuc X7 47 Tuc X7 Ω Cen M13 47 Tuc X7 - Heinke et al (2006) M13 - Gendre et al (2002a) Omega Cen - Gendre et al (2002b)

26 Observationally important qlmxbs in Globular Clusters R < 5% 2010 Name kt_eff(infty) (ev) NH Fx (10-13 cgsflux) Band (kev) Ref. R (infty) 47 Tuc X7 105(5) 0.04(2) Heinke et al (2006) <: 5 47 Tuc X5 100(20) 0.09(7) Heinke et al (2003) < 5 M28 90(+30-10) 0.26(4) Becker et al (2003) <5 NGC 6304 X9 115(20) [0.266] Guillot et al (2009) +++ NGC 6304 X4 120(20) [0.266] Guillot et al (2009) <15 NGC 6397 (U24) 74(18) Grindlay et al (2001 < 15 M13 76(3) [0.011] Gendre et al (2003) DONE NGC 3201 X (50) [0.14] Webb et al (2006) Data taken NGC 6553 X9 100 (20) [0.35] Guillot et al (in prep) +++ Omega Cen 67(2) 0.09(3) Rutledge et al (2002), Gendre DONE et al (2003) NGC 6637 X3 100 (40) [0.11] Guillot et al (in prep) +++ M30 A-1 94(15) 0.03(1) Lugger (2007) <10 NGC 6553 X3 127(+7-45) [0.35] Guillot et al (in prep) <: 5 >500 ksec with XMM NGC 6304 X5 70(25) [0.266] Guillot et al (2008) +++ NGC 6553 x35 88 (60) [0.35] Guillot et al (in prep) +++ NGC 2808 C Servillat et al (2008) <15 M80 CX2 82(2) 0.09(2) Heinke et al (2003) <5 M80 CX6 76(6) 0.22(7) Heinke et al (2003) <15

27 Observationally important qlmxbs in Globular Clusters R < 5% 2011 Name kt_eff(infty) (ev) NH Fx (10-13 cgsflux) Band (kev) Ref. R (infty) 47 Tuc X7 105(5) 0.04(2) Heinke et al (2006) <: 5 47 Tuc X5 100(20) 0.09(7) Heinke et al (2003) < 5 M28 90(+30-10) 0.26(4) Becker et al (2003) <5 NGC 6304 X9 115(20) [0.266] Guillot et al (2009) +++ NGC 6304 X4 120(20) [0.266] Guillot et al (2009) <15 NGC 6397 (U24) 74(18) Grindlay et al (2001 < 15 M13 76(3) [0.011] Gendre et al (2003) DONE NGC 3201 X (50) [0.14] Webb et al (2006) Data taken NGC 6553 X9 100 (20) [0.35] Guillot et al (in prep) +++ Omega Cen 67(2) 0.09(3) Rutledge et al (2002), Gendre DONE et al (2003) NGC 6637 X3 100 (40) [0.11] Guillot et al (in prep) +++ M30 A-1 94(15) 0.03(1) Lugger (2007) <10 NGC 6553 X3 127(+7-45) [0.35] Guillot et al (in prep) <: 5 NGC 6304 X5 70(25) [0.266] Guillot et al (2008) +++ NGC 6553 x35 88 (60) [0.35] Guillot et al (in prep) +++ NGC 2808 C Servillat et al (2008) <15 M80 CX2 82(2) 0.09(2) Heinke et al (2003) <5 M80 CX6 76(6) 0.22(7) Heinke et al (2003) <15

28 Mass Measurements with Continuum R Spectra T Normalization Peak of the spectrum You cannot measure a redshift from blackbody emission due to photon energy (E) temperature (kt) degeneracy. z = GM NS c 2 Second Derivative at the Peak of the Spectrum R NS I(E γ ) E γ kt 3 1 E γ kt e 1 T 2 T 1 T 2 =T 1 /(1+z) But, (Rthe = free-free opacity,z = breaks this degeneracy. This spectrum, redshifted, 1 2GM NS permits c 2 ) (R NS,M NS ) (in R NS c principle) determination 2 R ns of the redshift. T T 2!=T 1 /(1+z) I(E γ ) E γ kt R NS 1 E γ kt e 1 κ ff,o E o E γ GM NS T 2

29 The Proposed International X-ray Observatory Source: NASA Science Objectives NASA, ESA and JAXA proposal Matter Under Extreme Conditions Neutron Star Equation of State Basic specifications: Black Area: Hole 3 m kev Cosmic 1 m Feedback 6-7 kev Galaxy Cluster cm 2 40 kev Cosmology Energy resolution: (Dark Energy) < Cosmic kev Web of Baryons Source: Spatial The Resolution: International <5 arcsec X-ray halfpower diameter White, Parmar, Kunieda, Observatory, Nandra, Launch: Ohashi 2021 and Bookbinder.

Calorimeter response curves Simultaneous Mass and Radius Measurement M-R plot of EOSs from Lattimer & Prakash 47 Tuc Requirement: 250k-300k counts with calorimeter (2.

30 Calorimeter response curves Simultaneous Mass and Radius Measurement M-R plot of EOSs from Lattimer & Prakash 47 Tuc Requirement: 250k-300k counts with calorimeter (2.5 ev) energy resolution. M Constellation X Ω Error Ellipses (R=10 km, M=1.4) Field Source (5 ksec) Omega Cen (230 ksec) M13 (150 ksec) M28 (155 ksec) NGC 2808 (310 ksec) (At XMM/pn resolution: 1M -2M counts)

V # 3% σµ arcsec Distance (millions) (kpc) 10 0.34 7 4.2 15 26 22 1.

31 Measuring Distances: GAIA Mission Capabilities An European Space Agency Cornerstone Mission, with a launch on schedule for V # 3% σµ arcsec Distance (millions) (kpc) GAIA Are there enough qlmxbs within this distance? SIM Lite can do better!

32 IXO Simultaneous Mass and Radius Measurements of Neutron Stars R 1 Neutron Star Masses and Radii can be measured from Hydrogen atmosphere neutron stars with ~few% accuracy with IXO. Field sources are the most promising targets, due to their brightness, but they must be identified first. Constellation X A clear path forward: a handful of simultaneous mass-radius measurements will determine the cold dense matter equation of state. Questions: Is there any reason to be pessimistic? What other observational support for Deep Crustal Heating is there? Why not perform these analyses on isolated neutron stars (with no companion)?

33 Known Challenges to this Approach Accurate Distances (<2-3%) are required. Astronomical Challenges Background Spectral Contributions (the powerlaw?) Coronal activity in the low-mass companion -- ~1-10% of the luminosity? Non-DCH related Intensity Variability (observed in active field qlmxb Aql X-1, Cen X-4; but not yet detected in GC qlmxbs). Will atmospheric isotropy hold? Effects of rotation in the NS (simple doppler boosting) Systematic Challenges (instrumental Calibration) H Atmosphere Spectral models --- Heinke (Rybicki) et al (2006); Zavlin et al (2006) are calculated to ~1.5% accuracy in Lbol; disagree with each other at the 1-3% level at each photon energy, not likely to be resolved. Next Generation H atmosphere modeling is needed, with <0.1% uncertainty (Haakonsen et al 2009, submitted).

Measuring the Neutron Star Mass-Radius Relationship with X-ray Spectroscopy

Measuring the Neutron Star Mass-Radius Relationship with X-ray Spectroscopy Bob Rutledge McGill University Collaborators: Lars Bildsten (ITP/UCSB) Ed Brown (MSU) George Pavlov (PSU) Slava Zavlin (MSFC)