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Neutron Star Fundamental Physics with Constellation-X

Neutron Star Fundamental Physics with Constellation-X. Tod Strohmayer, NASA/GSFC. r ~ 1 x 10 15 g cm -3. Neutron Stars: Nature’s Extreme Physics Lab. Neutron stars, ~1.5 Solar masses compressed inside a sphere ~20 km in diameter. Highest density matter observable in universe.

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Neutron Star Fundamental Physics with Constellation-X

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  1. Neutron Star Fundamental Physics with Constellation-X Tod Strohmayer, NASA/GSFC r ~ 1 x 1015 g cm-3

  2. Neutron Stars: Nature’s Extreme Physics Lab • Neutron stars, ~1.5 Solar masses compressed inside a sphere ~20 km in diameter. • Highest density matter observable in universe. • Highest magnetic field strengths observable in the universe. • Among the strongest gravitational fields accessible to study. • General Relativity (GR) required to describe structure. Complex Physics!!

  3. Neutron Stars: A (very) Brief Introduction and History • Neutron stars, existence predicted in the 1930’s, Zwicky & Baade (1933), super-nova, neutron first discovered in 1932 (Chadwick). • Theoretical properties and structure, Oppenheimer & Volkoff (1939), TOV eqns. • Cosmic X-ray sources discovered, accreting compact objects, X-ray binaries (Giacconi et al. 1962). Nobel Prize, 2002. • First firm observational detection, discovery of radio pulsars, 1967 (Bell & Hewish). Hewish wins Nobel Prize in 1974, Bell does not. • Binary Pulsar discovered, 1974, Hulse-Taylor win Nobel Prize, 1993, gravitational radiation • X-ray bursting neutron stars discovered (1976), Grindlay et al. Belian, Conner & Evans, predicted by Hansen & van Horn (1975).

  4. Inside a Neutron Star The physical constituents of neutron star interiors remain a mystery. Superfluid neutrons ??? Pions, kaons, hyperons, quark-gluon plasma? r ~ 1 x 1015 g cm-3

  5. The Neutron Star “Zoo” • Rotation Powered: Radio Pulsars, some also observed at other wavelengths (eg. Crab pulsar). • Accretion Powered: X-ray binaries • High Mass X-ray Binaries (HMXB): X-ray pulsars (young, high B-field) • Low Mass X-ray Binaries (LMXB): Old (~109 yr), low B-field (109 G ) some are pulsars. • Nuclear Powered: X-ray burst sources • Magnetically Powered: Magnetars: Soft Gamma Repeaters (SGR), and Anomalous X-ray Pulsars (AXP). Young, ultra-magnetic 1014-15 G • Thermally Powered: Isolated (cooling) neutron stars.

  6. QCD phase diagram: New states of matter • Theory of QCD still largely unconstrained. • Recent theoretical work has explored QCD phase diagram (Alford, Wilczek, Reddy, Rajagopal, et al.) • Exotic states of Quark matter postulated, CFL, color superconducting states. • Neutron star interiors could contain such states. Can we infer its presence?? Rho (2000), thanks to Thomas Schaefer

  7. The Neutron Star Equation of State dP/dr = -r G M(r) / r2 • Mass measurements, limits softening of EOS from hyperons, quarks, other exotic stuff. • Radius provides direct information on nuclear interactions (nuclear symmetry energy). • Other observables, such as global oscillations might also be crucial. Lattimer & Prakash 2004

  8. Observational properties <=> Fundamental physics constraints • Mass - radius relation, maximum mass • Equation of state • Cooling behavior (Temperature vs Time) • QCD phase structure, degrees of freedom (condensates) • Maximum rotation rates • Equation of state, viscosity • Spin-down, glitches • Superfluidity

  9. Current Tests of GR using Neutron Stars (double pulsar, PSR J0737-3039A/B) • Exquisite radio timing measurements give accurate NS masses, but no radius information. • Still at 1PN order, but future measurements (2-5 yrs) will probably be sensitive to 2PN corrections. But do not directly probe near rg • Additional data could yield direct measure of NS moment of inertia (constrains EOS). Kramer et al. (2006) r ~ 1 x 1015 g cm-3

  10. Sources of Thermonuclear X-ray Bursts • Accreting neutron stars in low mass X-ray binaries (LMXBs). • Approximately 80 burst sources are known. • Concentrated in the Galactic bulge, old stars, some in GCs (distances). • Bursts triggered by thermally unstable He burning at column of few x 108 gm cm-2 • Liberates ~ 1039 – 1043 ergs. • Recurrence times of hours to a few days (or years). accreting neutron star binary Credit: Rob Hynes (binsim) Fun fact: a typical burst is equivalent to 100, 15 M-ton ‘bombs’ over each cm2 !! Accretion should spin-up the neutron star!

  11. Why Study Bursting Neutron Stars • Surface emission! • Eemit / Eobs = (1+z) = 1/ (1 – 2GM/c2R)1/2 => m/R • Continuum spectroscopy; Lobs = 4pR2s Teff4 = 4p d2 fobs • Eddington limited bursts; LEdd = 4pR2s TEddeff4 = g(M, R) • For most likely rotation rates, line widths are rotationally dominated, measure line widths and can constrain R (if W known). • If detect several absorption lines in a series (Ha, and Hb, for example), can constrain m/R2 . • Timing (burst oscillations) can also give M – R constraints. • In principle, there are several independent methods which can be used to obtain M and R (Con-X can do several).

  12. Thermonuclear X-ray Bursts • 10 - 200 s flares. • Thermal spectra which soften with time. • 3 - 12 hr recurrence times, sometimes quasi-periodic. • ~ 1039 ergs • H and He primary fuels 4U 1636-53 Intensity Time (sec) He ignition at a column depth of 2 x 109 g cm-2

  13. X-ray Spectroscopy of Neutron Stars: Recent Results XMM/Newton RGS observations of X-ray bursts from an accreting neutron star (EXO 0748-676); Cottam, Paerels, & Mendez (2002). Features consistent with z=0.35

  14. Discovery of Neutron Star Spin Rates in Bursting LMXBs • Discovered in Feb. 1996, shortly after RXTE’s launch (review in Strohmayer & Bildsten 2006). • First indication of ms spins in accreting LMXBs. • Power spectra of burst time series show significant peak at frequencies 45 – 620 Hz (unique for a given source).

  15. Burst Oscillations reveal surface anisotropies on neutron stars Cumming (2005) Strohmayer, Zhang & Swank (1997) Surface Area Spreading hot spot. Intensity • Oscillations caused by hot spot on rotating neutron star. • Modulation amplitude drops as spot grows. • Spectra track increasing size of X-ray emitting area on star.

  16. EXO 0748-676: Burst Oscillations, 45 Hz spin rate • 38 RXTE X-ray bursts. • Calculated Power spectra for rise and decay intervals Villarreal & Strohmayer (2004) • Averaged (stacked) all 38 burst power spectra. • 45 Hz signal detected in decay intervals.

  17. Mass (M) Rotational Broadening of Surface Lines • Rotation broadens lines, if Spin frequency known, can constrain R (with caveats). • For Fe XXVI Ha, and 45 Hz, fine structure splitting of line is comparable to rotational effect. Need good intrinsic profile (Chang et al 2006). Chang et al. (2006)

  18. Constellation-X Capabilities • Con-X will provide many high S/N measurements of X-ray burst absorption spectra: measure gravitational red-shift at the surface of the star for multiple sources, constrains M/R. • Relative strength of higher-order transitions provides a measure of density  unique M, R. • Absorption line widths can constrain R to 5 – 10%. z = 0.35

  19. No frame dragging Frame dragging Line Spectroscopy: Neutron Stars • Line features from NS surface will be broadened by rotational velocity. • Asymmetric and double-peaked shapes are possible, depending on the geometry of the emitting surface. • Shape of the profile is sensitive to General Relativistic frame dragging (Bhattacharyya et al. 2006).

  20. Neutron star cooling: Isolated neutron stars • Cooling rates are sensitive to interior physics (EOS and composition). • Compare surface temps and ages with theoretical cooling curves (isolated neutron stars, SN remnant sources). • Difficulties: high B field, atmosphere complicated (how to infer T), ages are difficult to measure accurately. • Con-X will advance these efforts: • Confirm new INS candidates • Deep spectra may clarify atmosphere models, emission processes, for example in enigmatic CCOs (as in Cas A). Cumming (2005)

  21. Cooling Neutron Star Transients Markwardt et al. KS 1731-260 Cackett et al. 2006 • Accretion heats the crust (Haensel & Zdunik, Brown et al). When it ceases the cooling of the crust can be tracked. • kT “floor” related to core temperature, neutrino emissivity, EOS Cackett et al. (2006)

  22. Cooling transients: Surface spectra and radius constraints Cackett, Miller (2006) Simulations for MXB 1659-29 • Con-X can obtain high S/N spectra with modest exposures(20 ksec). • Yield statistical uncertainties in radii of a few tenths of a km. • Deep spectra can help to refine atmosphere models.

  23. Pulse Profiles Probe the Structure of Neutron Stars • Pulse strength and shape depends on M/R or ‘compactness’ because of light bending (a General Relativistic effect). • More compact stars have weaker modulations. • Pulse shapes (harmonic content) also depend on relativistic effects (Doppler shifts due to rotation, which depends on R (ie. spin frequency known). GM/c2R = 0.284

  24. Rotational Modulation of Neutron Star Emission: millisecond rotation-powered pulsars • Emission from small, thermal hot spots (pulsar polar cap heating) • Spectra consistent with non-magnetic, hydrogen atmospheres. • Modelling allows constraints on M/R (recent work by Bogdanov et al.) • Soft X-ray spectra excellent match to Con-X band-pass

  25. Rotational Modulation of Neutron Star Emission: PSR J0437-4715 • 5.76 ms pulsar, with both parallax and kinematic distance, 157 pc • Radio timing data suggest M = 1.76 +- 0.2 Msun (Verbiest et al. 2008) • X-ray pulse profile consistent with two small, thermal spots (Bogdanov et al. 2007). • Possibility of tighter mass constraints and deep Con-X data could tightly constrain M and R.

  26. PSR J0437-4715: Con-X simulations • 1 Msec Con-X observations could achieve few percent radius measurement (1) • Several other promising targets with possible mass measurements.

  27. Science Objectives Flow Into Key Performance Requirements

  28. 1.3 m Flight Mirror Assembly Representative XGS Gratings XGS CCD Camera X-ray Microcalorimeter Spectrometer (XMS) Mission Implementation 4 Spectroscopy X-ray Telescopes • To meet the requirements, our technical implementation consists of: • 4 SXTs each consisting of a Flight Mirror Assembly (FMA) and a X-ray Microcalorimeter Spectrometer (XMS) • Covers the bandpass from 0.6 to 10 keV • Two additional systems extend the bandpass: • X-ray Grating Spectrometer (XGS) – dispersive from 0.3 to 1 keV (included in one or two SXT’s) • Hard X-ray Telescope (HXT) – non-dispersive from 6 to 40 keV • Instruments operate simultaneously: • Power, telemetry, and other resources sized accordingly

  29. Spectroscopy X-ray Telescope (SXT) • Trade-off between collecting area and angular resolution • The 0.5 arcsec angular resolution state of the art is Chandra • Small number of thick, highly polished substrates leads to a very expensive and heavy mirror with modest area • Constellation-X collecting area (~10 times larger than Chandra) combined with high efficiency microcalorimeters increases throughput for high resolution spectroscopy by a factor of 100 • 15 arcsec angular resolution required to meet science objectives (5 arcsec is goal) • Thin, replicated segments pioneered by ASCA and Suzaku provide high aperture filling factor and low 1 kg/m2 areal density

  30. X-ray Microcalorimeter Spectrometer (XMS) • X-ray Microcalorimeter: thermal detection of individual X-ray photons • High spectral resolution • E very nearly constant with E • High intrinsic quantum efficiency • Non-dispersive — spectral resolution not affected by source angular size • Transition Edge Sensor (TES), NTD/Ge and magnetic microcalorimeter technologies under development High filling factor 8 x8 development Transition Edge Sensor array: 250 m pixels 2.5 eV ± 0.2 eV FWHM Exposed TES Suzaku X-ray calorimeter array achieved 7 eV resolution on orbit

  31. The Constellation-X Mission Tod Strohmayer (NASA/GSFC) • Quantum to Cosmos 3, Airlie Center, VA July 2008

  32. Fundamental Physics: The Neutron Star Equation of State (EOS) • R weakly dependent on M for many EOSs. • Precise radii measurements alone would strongly constrain the EOS. • Radius is prop. to P1/4 at nuclear saturation density. Directly related to symmetry energy of nuclear interaction (isospin dependence). Lattimer & Prakash 2001

  33. Why Study Bursting Neutron Stars I • X-ray bursts: we see emission directly from the neutron star surface. • “Low” magnetic fields, perhaps dynamically unimportant < 109 G (from presence of bursts, accreting ms pulsars). • Accretion supplies metals to atmosphere, spectral lines may be more abundant than in non-accreting objects. • Models suggest several tenths Msun accreted over lifetime, may allow probe of different neutron star mass range, mass – radius relation, neutron star mass limit. • However, presence of accretion may also complicate interpretation of certain phenomena.

  34. X-ray Spectroscopy of Neutron Stars • One of the most direct methods of determining the structure of a neutron star is to measure the gravitational redshift at the surface. • Extensive searches have been conducted for gravitationally redshifted absorption features in isolated neutron stars. • Most neutron stars (so far) show no discrete spectral structure. • Several isolated neutron stars (including;1E1207.4-5209, RX J0720.4-3125, RX J1605.3+3249, RX J1308.6+2127) show broad absorption features, but these have not yet been uniquely identified. • X-ray bursting neutron stars are excellent targets for these searches: • During the bursts, the neutron star surface outshines the accretion-generated light by an order of magnitude, or more. • Continuing accretion provides a source of heavy elementsat the neutron star surface, that would otherwise gravitationally settle out quickly. • Low magnetic fields in accreting neutron star systems vastly simplify the spectral analysis.

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