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



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!!



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).



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.



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.



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??



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



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



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).



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!



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).



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



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



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).



• 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)



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)



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).



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



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



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).



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)



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



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



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



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.



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.



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



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



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



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



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).



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.



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.

Http://constellation.nasa.gov Beyond Einstein: From the Big Bang to Black Holes Neutron Star...

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