Neutron star physics with X-ray
observations of accreting systems
Craig Heinke
University of Alberta
NewCompStar, Warsaw, 2017
NS manifestations
Young cooling NS
(central compact
object)
Accretion
(X-ray binary)
Magnetic field
(radio pulsar,
magnetar)
See Kaspi 2010, PNAS
Physics of Dense Matter
Neutron Star Structure
• Behavior of matter at high
densities unclear; how “stiff” is matter?
Do exotic particles or
quarks condense?
What is superfluid?
Properties of crust?
by Bennett Link
X-ray potential
• Often see direct emission
from NS surface
• If see spectral lines—directly
measure redshift
• If know atmosphere &
distance, measure radius
• If know age or how heated,
measure cooling rate
•
If see rotating hot spot,
constrain compactness
Puppis A SNR & NS,
XMM (Becker+03)
Spectral lines
• Identification of an atomic
spectral line from surface
could give redshift, M/R
• Spectral lines from bursts
claimed to give z=1.35
(Cottam+02).
• However, fast NS spin
would smear surface lines;
absorption lines not
replicated
EXO 0748-676 high-res
spectra, Cottam+02
Spectral lines
• 2-4 absorption lines
identified from young NS
1E1207 (Sanwal+01,
Bignami+03)
• Identified as cyclotron
lines (Suleimanov+10), so
constrain B field, not
redshift
• Seen in several nearby
NSs (e.g. Borghese+17)
1E1207, Bignami+03
Hot spot lightcurves
Bogdanov+09
Gravitational light-bending smooths profile
•
• Pulsation profile constrains compactness of NS (e.g.
Bogdanov+13; but cf. Guillot+16)
• NICER to launch in May (see Guillot talk)
Bogd.+13
Thermal radiation
• Blackbody-like radiation
• F =σT (R /4D )
obs
4
2
∞
2 From Fobs, T, get angle;
if know d, get ‘radius’.
• Issues: Atmosphere will modify spectrum.
Anisotropic emission will change result.
(Pavlov+00)
Radius constraints
Grav. redshift gives
radius/mass
degeneracy; R∞=RNS(1+z) [orange lines]
Surface gravity changes atmosphere
Lattimer & Prakash 2004
X-ray observations: accretion
Outburst
Get lots of photons, study bursts, pulsations
RXTE
Quiescence
Get few photons,
study NS surface
Chandra
Suleimanov+11
X-ray bursts
Touchdown
• Burning of He and/or H
to heavy elements
• Photosphere can expand;
inferred blackbody radius
~constant after
Galloway+08
Color corrections
• At high T, Compton
scattering alters
spectrum
• Depends on composition
• Compare kT to BB
prediction, get color
correction fC
• Should vary with L
Suleimanov+11
Mass/radius constraints
•
• Radiation at Eddington
Area: F=σT4*R∞2/4D2
limit lifts mass;
•
•T
2
FEdd=GMc/(σD [1+z])
Edd; corresponding
kT.
Constraints from 3 methods; Suleimanov+11
X-ray bursts
• X-ray flux in tail
• Reproducible areas & F
4
∝T
Edd,
permit well-constrained M/R
• Uncertainties in atmosphere
modeling, fraction of surface,
radius of emission, etc. (Steiner
+10, Suleimanov+11, Zamfir+12)
Guver+2012
Ozel/Guver burst fits
• Ozel & Guver used
RXTE bursts (overlap
of area and FEdd
constraints),
measured M, R for 3
NSs with known
distances
• Initial results gave low
radii, below 10 km
Ozel+10 (1σ, 2σ constraints)
Criticisms
• Suleimanov+11 argue Ozel
bursts don’t evolve as
theory predicts
• Ozel bursts in different
accretion state—problem?
• Guver & Ozel argue
Suleimanov’s burst poorly
fit by blackbody spectrum
Kajava+14
Suleimanov+11
Which bursts?
• Poutanen+14 study all
bursts from 4U 1608-52
• Use longer bursts that
Hard state
Poutanen+14
follow predictions
• Find larger radius, >14 km
Soft state
Ozel+ arguments
• Ozel: bursts evolve
too quickly to see
spectral change
Ozel+15
Ozel+ arguments
• Ozel: 5% scatter in
area could hide
predicted spectral
evolution
Ozel+10 (1σ, 2σ constraints)
Cooling tail fits:
EOS constraints
• Suleimanov+11 fit 1 burst;
imply large (>14 km) radius
• Kajava+16 fit several bursts,
find smaller (10-13 km)
radii
•
Systematic uncertainties:
data selection, atmosphere
composition (H, He, solar
metallicity)
Kajava+16
Suleimanov+11
Inferences about EOS
• Kajava+16 infer most
likely nuclear EOS
(w/Steiner, Bayesian)
• Parametrize EOS as
polytrope (A), or
with phase
transitions (B)
Kajava+16
Ozel; add systematics
• Guver+12a,+12b,+16
measured systematics, e.g.
radii variation.
• Ozel+16 included these,
updated Edd. limit,
recomputed burst M,R
constraints
Ozel; final burst
constraints
• Ozel+16 combine their
burst constraints from
6 NSs
• Including systematics,
looser constraint—
radius of 9-12 km
Ashes
• A few bursts show signs of
spectral features
• Kajava+14 find edge in
RXTE burst; convection lifted ashes to
surface?
Flux
Metallicity
• Could substantially alter
inferred radii
Fit
Kajava+17
X-ray observations: accretion
Outburst
Get lots of photons, study bursts, pulsations
Advantage of quiescence:
simple atmosphere
Disadvantage:
fewer photons
Quiescence
Get few photons,
study NS surface
Low-B Quiescent NS Atm
At 106 K, H, He ionized; -3
free-free opacity ∝ ν . Fe not fully ionized, opacity
line-dominated.
H, He shift flux to higher E vs.
blackbodies.
Infer larger radius for given
2
4
spectrum (L~4πR σT ).
Zavlin+96; H, He, Fe atmospheres
Quiescent LMXB Spectra
• Thermal component; NS surface, H atmosphere
(elements stratify quickly)
Thermal
Nonthermal
• Deep crust heated by
accretion, reradiates heat in
quiescence
• Nonthermal (“power-law”)
component; low-level
X-ray spectrum of qLMXB,
(unfolded) Rutledge+02b
Quiescent LMXBs
• Use globular clusters
(known distances)
• Identified by thermal
spectra; best targets lack
nonthermal component,
have little gas/dust
Chandra image of 47 Tuc,
Heinke+05
High-quality
spectra
• Excellent spectra for half
dozen qLMXBs
ω Cen, XMM, Webb+07
• Well-fitted with H
atmospheres, plausible radii
(e.g. 8-12 km M13, Webb+07)
M28, Chandra, Servillat+12
Guillot+13 analyses
Analysed 5 objects; two give extreme values
NGC 6304
M13
NGC 6397
M28
ω Cen
Guillot+13 meta-analysis
• Assume same
radius for all
• Calculate small
(9±1km) radius,
wide range of
masses (<1.2,
>2.1)
WD Companions
• 1/3 of known periods of
cluster LMXBs are
short
• Require WD (probably
He WD) companions
• qLMXBs with WD companions likely have
He atmospheres
Bahramian+14, summary of
cluster LMXB periods
H vs. He
atmospheres
H
• Donor nature, orbital period
hard to measure
• He model can increase NS
radius up to 50%
• Should consider possibility of
He; preferable for NGC 6397
NGC 6397,
Heinke+14
He
New 47 Tuc results
2
X7
1/8 subarray
• Best constraints on NS
•
0.5
radius—11.1+-0.8 km
for 1.4 Msun (not
including systematics)
Argue high-mass NS
unlikely.
+
1
obs, removing pileup
Parameter: Mns (Msun)
• 200 ks new Chandra
1.5
min = 1.034322e+02; Levels = 1.057322e+02 1.080422e+02 1.126422e+02
7
8
9
10
11
Parameter: Rns (km)
12
13
slavko 30−Apr−2015 1
X7 mass/radius contours, 68% & 95% conf
Bogdanov+16
Combining qLMXBs
• What range of M/R curves are consistent with
current qLMXB data? Bayesian MCMC
analyses, testing EOS parametrization
• Choices are: use only H, or allow H or He for all
polytropic 3-piece EOS
EOS allowing strong phase transitions
• Ozel+15/Bogdanov+16
Work w/Steiner in progress
Ozel+16: bursts + qLMXBs
Analyses of bursts (L) and qLMXBs (R). For bursts, include systematic errors (Guver+12a,b,13).
qLMXBs assume H atm, except NGC 6397.
Ozel+16: Bayesian analysis
• Ozel+16 joint
analysis of qLMXBs,
bursters; Bogdanov+16
includes X7.
• Takes flat priors in
P at 3 densities.
• Favors R=9.9-11.2
km for 1.4 Msun.
Bogdanov+16
Steiner Bayesian analysis
• Allows H or He
• 3-piece polytropic
EOS above n0.
• 1.4 Msun radius of
11.4-13.5 km.
Steiner, CH et al. in prep
Hot spots: bias?
• Possible:
If pulsar, can irradiate poles with positrons.
If accretion channeled, poles can be heated.
No evidence yet in qLMXBs.
Hot spot will increase fitted temperature, shrink inferred radius.
If hot spot flux is large,
can give bad fit.
Elshamouty+16
Pulsations
• H atm more pulsed than BB. MonteCarlo over angles, pulsed fraction PDF.
• Chandra limits on pulses from X7, <13%.
Cen X-4 stronger limits, <6.4% (D’Angelo+15)
Elshamouty+16, simulating pulsations with H atm NS
2.0
All H
1.5
M (M )
All H;
or hotspots
2.5
1.0
• If all qLMXBs H,
0.5
then R smaller;
11-12 km.
9
10
11
12
13
14
15
16
14
15
16
R(km)
2.5
2.0
1.5
M (M )
•
If allow hotspots,
permits larger radii;
11.9-14.4 km.
0.0
1.0
0.5
Hotspots
0.0
9
10
11
12
13
R(km)
Work so far
•
X-rays allow us to study NS surface
•
Spectral lines not useful yet for NS interiors
•
X-ray bursts—2 groups use different data selection,
initially strong radius disagreement, now both ~10-13
km
•
Quiescent NSs—fewer photons, but simpler behavior. Suggest small (<11 km) radius if all H atmospheres,
but some may have He atmospheres.
Future
•
2017: NICER will constrain NS compactness,
measure M,R for ~2-4 pulsars.
•
2017-2019: addressing burst systematics,
deeper qLMXB observations. NICER pulsation constraints on qLMXBs?
•
~2028: ESA’s Athena launches; should measure
M,R to <5% (statistical) for qLMXBs. Next-generation optical telescopes should
clearly identify companions, settle H/He.
Spectral lines in bursts?