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Transcript of Http://constellation.nasa.gov Beyond Einstein: From the Big Bang to Black Holes Neutron Star...
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Neutron Star Fundamental Physics with Constellation-X
~ 1 x 1015 g cm-3
Tod Strohmayer, NASA/GSFC
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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!!
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Beyond Einstein: From the Big Bang to Black Holes
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).
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Beyond Einstein: From the Big Bang to Black Holes
Inside a Neutron Star
???
~ 1 x 1015 g cm-3
Superfluid neutrons
Pions, kaons, hyperons,
quark-gluon plasma?
The physical constituents of neutron star interiors remain a mystery.
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Beyond Einstein: From the Big Bang to Black Holes
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.
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Beyond Einstein: From the Big Bang to Black Holes
QCD phase diagram: New states of matter
Rho (2000), thanks to Thomas Schaefer
• 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??
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Beyond Einstein: From the Big Bang to Black Holes
The Neutron Star Equation of State
Lattimer & Prakash 2004
• 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.
dP/dr = - G M(r) / r2
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
Current Tests of GR using Neutron Stars (double pulsar, PSR J0737-3039A/B)
~ 1 x 1015 g cm-3
Kramer et al. (2006)• 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).
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Beyond Einstein: From the Big Bang to Black Holes
Sources of Thermonuclear X-ray Bursts
accreting neutron star binary• 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).
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!
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Beyond Einstein: From the Big Bang to Black Holes
Why Study Bursting Neutron Stars
• Surface emission!
• Eemit / Eobs = (1+z) = 1/ (1 – 2GM/c2R)1/2 => m/R
• Continuum spectroscopy; Lobs = 4R2 Teff4 = 4 d2 fobs
• Eddington limited bursts; LEdd = 4R2 TEddeff
4 = g(M, R)
• For most likely rotation rates, line widths are rotationally dominated, measure line widths and can constrain R (if known).
• If detect several absorption lines in a series (H, and H, 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).
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Beyond Einstein: From the Big Bang to Black Holes
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
Time (sec)
4U 1636-53
Inte
nsity
He ignition at a column depth of 2 x 109 g cm-2
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
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).
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Beyond Einstein: From the Big Bang to Black Holes
• 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.
Sur
face
Are
aIntensitySpreading
hot spot.
Strohmayer, Zhang & Swank (1997)
Burst Oscillations reveal surface anisotropies on neutron starsCumming (2005)
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Beyond Einstein: From the Big Bang to Black Holes
EXO 0748-676: Burst Oscillations, 45 Hz spin rate
• 38 RXTE X-ray bursts.
• Calculated Power spectra for rise and decay intervals
• Averaged (stacked) all 38 burst power spectra.
• 45 Hz signal detected in decay intervals.
Villarreal & Strohmayer (2004)
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Beyond Einstein: From the Big Bang to Black Holes
Rotational Broadening of Surface Lines
Mass
(M
)M
ass
(M
)
Chang et al. (2006)
• Rotation broadens lines, if Spin frequency known, can constrain R (with caveats).
• For Fe XXVI H, and 45 Hz, fine structure splitting of line is comparable to rotational effect. Need good intrinsic profile (Chang et al 2006).
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
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).
No frame dragging
Frame dragging
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Beyond Einstein: From the Big Bang to Black Holes
Cumming (2005)
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).
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Beyond Einstein: From the Big Bang to Black Holes
Cooling Neutron Star Transients
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
Markwardt et al.
Cackett et al. (2006)
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Beyond Einstein: From the Big Bang to Black Holes
Cooling transients: Surface spectra and radius constraints
Cackett, Miller (2006)
•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.
Simulations for MXB 1659-29
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
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.
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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.
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Beyond Einstein: From the Big Bang to Black Holes
Science Objectives Flow Into Key Performance Requirements
Bandpass: 0.3 – 40 keV
Effective Area:
15,000 cm2 @ 1.25 keV
6,000 cm2 @ 6 keV
150 cm2 @ 40 keV
Spectral Resolution:
1250 @ 0.3 – 1 keV
2400 @ 6 keV
Angular Resolution
15 arcsec 0.3 – 7 keV
(5 arcsec goal)
30 arcsec 7.0 – 40 keV
Field of View 5 x 5 arcmin
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Beyond Einstein: From the Big Bang to Black Holes
Mission Implementation 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
4 Spectroscopy X-ray Telescopes
1.3 m
X-ray Microcalorimeter Spectrometer (XMS)
Representative XGS Gratings
XGS CCD Camera
Flight Mirror Assembly
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Beyond Einstein: From the Big Bang to Black Holes
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
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Beyond Einstein: From the Big Bang to Black Holes
Exposed TES
X-ray Microcalorimeter Spectrometer (XMS)
Suzaku X-ray calorimeter array achieved 7 eV resolution on orbit
8 x8 development Transition Edge Sensor array: 250 m pixels
2.5 eV ± 0.2 eV FWHM
High filling factor
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
The Constellation-X MissionThe Constellation-X Mission
Tod Strohmayer(NASA/GSFC)
Quantum to Cosmos 3, Airlie Center, VA July 2008
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Beyond Einstein: From the Big Bang to Black Holes
Fundamental Physics: The Neutron Star Equation of State (EOS)
Lattimer & Prakash 2001
• 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).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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.
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Beyond Einstein: From the Big Bang to Black Holes
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 elements at the neutron star surface, that would otherwise gravitationally settle out quickly.
– Low magnetic fields in accreting neutron star systems vastly simplify the spectral analysis.