Transcript Tod E. Strohmayer - UCLA Physics & Astronomy

Beyond Einstein: From the Big Bang to Black Holes
Neutron Star Fundamental Physics with Constellation-X
Tod Strohmayer, NASA/GSFC
r ~ 1 x 1015 g cm-3
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!!
http://constellation.nasa.gov
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).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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
http://constellation.nasa.gov
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.
http://constellation.nasa.gov
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??
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
The Neutron Star Equation of State
dP/dr = -r G M(r) / r2
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.
http://constellation.nasa.gov
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
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Current Tests of GR using Neutron Stars
(double pulsar, PSR J0737-3039A/B)
• Exquisite radio timing
Kramer et al. (2006)
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
r ~ 1 x 1015 g cm-3
• Additional data could yield
direct measure of NS
moment of inertia
(constrains EOS).
http://constellation.nasa.gov
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).
Credit: Rob Hynes (binsim)
• Bursts triggered by
thermally unstable He
burning at column of few x
108 gm cm-2
• Liberates ~ 1039 – 1043
ergs.
Fun fact: a typical burst is equivalent to 100,
15 M-ton ‘bombs’ over each cm2 !!
Accretion should spin-up the neutron star!
• Recurrence times of hours
to a few days (or years).
http://constellation.nasa.gov
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 = 4pR2 s Teff4 = 4p d2 fobs
• Eddington limited bursts; LEdd = 4pR2 s 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).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Thermonuclear X-ray Bursts
4U 1636-53
• 10 - 200 s flares.
• Thermal spectra
Intensity
which soften with
time.
• 3 - 12 hr
recurrence times,
sometimes quasiperiodic.
Time (sec)
He ignition at a column depth of 2 x 109 g cm-2
• ~ 1039 ergs
• H and He primary
fuels
http://constellation.nasa.gov
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
http://constellation.nasa.gov
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).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Burst Oscillations reveal surface anisotropies on neutron stars
Cumming (2005)
Surface Area
Strohmayer, Zhang & Swank (1997)
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.
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
EXO 0748-676: Burst Oscillations, 45 Hz spin rate
• 38 RXTE X-ray bursts.
Villarreal & Strohmayer (2004)
• Calculated Power spectra
for rise and decay intervals
• Averaged (stacked) all 38
burst power spectra.
• 45 Hz signal detected in
decay intervals.
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Rotational Broadening of Surface Lines
Mass (M)
• 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)
http://constellation.nasa.gov
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
http://constellation.nasa.gov
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 doublepeaked shapes are
possible, depending on the
geometry of the emitting
surface.
No frame dragging
Frame dragging
• Shape of the profile is
sensitive to General
Relativistic frame dragging
(Bhattacharyya et al.
2006).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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
Cumming (2005)
processes, for example in enigmatic CCOs (as in Cas A).
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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)
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
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.
http://constellation.nasa.gov
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
http://constellation.nasa.gov
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
http://constellation.nasa.gov
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 (1s)
• Several other promising targets with possible mass measurements.
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
Science Objectives Flow Into Key Performance Requirements
Bandpass:
0.3 – 40 keV
15,000 cm2 @ 1.25 keV
Effective Area:
6,000 cm2 @ 6 keV
150 cm2 @ 40 keV
Spectral
Resolution:
1250 @ 0.3 – 1 keV
2400 @ 6 keV
15 arcsec 0.3 – 7 keV
Angular Resolution
(5 arcsec goal)
30 arcsec 7.0 – 40 keV
Field of View
5 x 5 arcmin
http://constellation.nasa.gov
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)
4 Spectroscopy X-ray Telescopes
1.3 m
Flight Mirror Assembly
• 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)
Representative XGS Gratings
• Hard X-ray Telescope (HXT) – nondispersive from 6 to 40 keV
 Instruments operate simultaneously:
– Power, telemetry, and other resources sized
accordingly
XGS CCD Camera
X-ray Microcalorimeter Spectrometer (XMS)
http://constellation.nasa.gov
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
http://constellation.nasa.gov
Beyond Einstein: From the Big Bang to Black Holes
X-ray Microcalorimeter Spectrometer (XMS)
 X-ray Microcalorimeter: thermal detection
of individual X-ray photons
High filling
factor
– 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
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
http://constellation.nasa.gov
The Constellation-X Mission
Tod Strohmayer
(NASA/GSFC)
Quantum to Cosmos 3, Airlie Center, VA
July 2008
Beyond Einstein: From the Big Bang to Black Holes
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
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.
http://constellation.nasa.gov
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.
http://constellation.nasa.gov