Neutron Stars:
Insights into their Formation, Evolution
& Structure from their
Masses and Radii
Supernovae and Gamma Ray Bursts in
Kyoto
Feryal Ozel
University of Arizona
In collaboration with T. Guver, M. Baubock, L. Camarota, P. Wroblewski,
A. Santos Villarreal; G. Baym, D. Psaltis, R. Narayan, J. McClintock
Neutron Star Masses
 Understand stellar evolution & supernova explosions
 Find maximum neutron star mass
 Dense Matter EoS
 GR tests
 GW signals
Neutron Star Masses
Rely on pulsars/neutron stars in
binaries
Group by
Data Quality: Number of
measurements, type of
errors
Source type: Double NS,
Recycled NS, NS with
High Mass Companion
Total of 6 pairs of double
neutron stars (12) and
9 NS+WD systems with
precisely measured masses
31 more neutron stars with
reasonably well determined
masses
NS Mass Measurements
Özel et al. 2012
Current Record Holders: M= 1.97±0.04 M Demorest et al. 2010
M= 2.01±0.04 M Antoniadis et al. 2013
NS Mass Distributions
Özel et al. 2012
NS Mass Distributions
I. Lifetime of accretion/recycling shifts the mean 0.2 M up
II. There is no evidence for the effect of the maximum mass
on the distribution
III. Double Neutron Star mass distribution is peculiarly
narrow
Why is the DNS distribution so narrow?
Black Hole Masses
Determine velocity amplitude K, orbital period P, mass function
f
3
3
+ Varying levels of
data on inclination
and mass ratio
Radial Velocity (km s-1)
K Porb (m1 sini)
f (M ) =
=
2p G (m1 + m2 )2
4U 1543-47
Time (HJD-2,450,600+)
from Orosz et al. 1998
Masses
of
Stellar
Black
Holes
Özel, Psaltis, Narayan,
& McClintock 2010
Parameters of the Distribution
• Cutoff mass
≥ 5 M
• Fast decay at
high mass end
• Not dominated
by a particular
group of sources
Özel et al. 2010
See also
Bailyn et al. 1998
Farr et al. 2011
Neutron Stars and Black Holes
Özel et al. 2012
Failed Supernovae?
PROGENITOR MASS
< 15 M
Successful SNe
No fallback
NS remnant
~16-25 M
> 25 M
Significant pre-SN
Failed SNe
mass loss
Direct collapse
Eject H envelope
BH Mass = He core mass
Kochanek 2013
Woosley & Heger 2012
Lovegrove & Woosley 2013
NS Radii – What is the Appeal?
The Physics of Cold
Ultradense Matter
NS/BHs division
Supernova mechanism
GRB durations
Gravitational waves
Image credit:
Chandra X-ray Observatory
Mass-Radius Relation
P
EoS
ρ
Özel & Psaltis 2009, PRD, 80,103003
Read et al. 2009, PRD
The pressure at three fiducial densities capture the
characteristics of all equations of state
This reduces ~infinite parameter problem to 3 parameters
Mass-Radius Measurement to EoS:
a formal inversion

Data simulated
using the
FPS EoS
≥ 3 Radius measurements achieve a faithful recovery of the EoS
Özel & Psaltis 2009, PRD
Measuring Neutron Star
Radii
Complications:
1. The radius and mass measurements are coupled
2. Need sources where we see the neutron star surface,
the whole neutron star surface, and nothing but the
neutron star surface
Low Mass X-ray Binaries
ASM Counts s-1
Two windows onto the neutron star surface during
periods of quiescence and bursts
Modified Julian Date - 50000
• Low magnetic fields (B<109 G)
• Expectation for uniform emission from surface
Radii from Quiescent LMXBs
in Globular Clusters
Five Chandra observations of U24
in NGC 6397
Guillot et al. 2011
Heinke et al. 2006; Webb & Barret 2007; Guillot et al. 2011
Evolution of Thermonuclear Bursts
Constant, Reproducible Apparent Radii
4U 1728-34
Level of systematic uncertainty < 5% in apparent radii
Two Other Measurements:
Distances and Eddington Limit
Frad
Fgrav
Time (s)
Measuring the Eddington Limit
4U 1820-30
Guver, Wroblewski, Camarota, & Ozel 2010, ApJ
Pinning Down NS Radii
Globular cluster source EXO 1745-248
Özel et al. 2009, ApJ, 693, 1775
Current Radius
Measurements Remarkable
agreement in radii
between different
spectroscopic
measurements
R ~ 9-12 km
Majority of the
10 radii smaller
than vanilla
nuclear EoS AP4
predictions
Can already
constrain the
neutron star EoS
The Pressure of Cold Ultradense
Matter
Özel, Baym, & Guver 2010, PRD, 82, 101301
Conclusions
• Nuclear EoS that fit low-density data too stiff
at high densities
• Indication for new degrees of freedom in NS
matter
• NS-BH mass gap and narrow DNS distribution
point to new aspects of supernova mechanism
Additional Slides
The Future
a NASA Explorer
an ESA M3 mission
Is the low-mass gap due to a
selection effect?
Transient
black holes
Follow-up criterion:
1 Crab in outburst
If L ~ M, could lead
to a low-mass gap
But it is not a selection effect…
Brighter sources
are nearby ones
Persistent Sources
• Bowen emission line blend technique, @ 4640 A
• Applied mostly to neutron star binaries, which are
persistent (Steeghs & Casares 2002)
Steeghs & Casares 2002
Persistent Sources
• Bowen emission line blend technique
• Applied so far to neutron star binaries, which are
persistent
• Can help address if sample of transients introduces a
selection effect
Highest Mass Neutron Star
Measurement of the
Shapiro delay in
PSR J1614-2230
with the GBT
Demorest et al. 2010
Highest Mass Neutron Star
M= 1.97±0.04 M
SAX J1748.9-2021
GR Effects at Moderate Spins
Baubock et al. 2012
Neutron Star Surface Emission
• Low magnetic fields
• Plane parallel
atmospheres
• Radiative equilibrium
• Non-coherent scattering
• Possible heavy elements
from Madej et al. 2004
Majczyna et al 2005
Ozel et al. 2009
Suleimanov et al. 2011
Effects of Pile-up on X7
spectrum
Analysis of the Burst Spectra
4U 1636-536
26 d.o.f.
1712 spectra
Spectra are well-described by
Comptonized atmosphere models
Is There A Stiff EoS in 4U 1724The source used307?
by Suleimanov et al. 2011
Redshift Measurement
M/R from spectral lines:
E = E0 ( 1
2M
R
)
Cottam et al. 2003, Nature
These lines do not come from the stellar surface
Lin, Ozel, Chakrabarty, Psaltis 2010, ApJ