Transcript Measuring Magnetic Fields of Neutron Stars

Measuring Magnetic Fields
of Neutron Stars
Kazuo Makishima
Department of Physics,
University of Tokyo
[email protected]
“How the strong magnetic field of neutron
stars is sustained, and how it evolves.”
Jan. 6, 2003
Tokyo-Adelaide Conference
Interior of a NS
“Outer Crust”
Nuclei + electrons
“Inner Crust”
Nuclei, free neutrons,
and electrons, possibly
with “pasta” phases
“Core”
Uniform nuclear matter,
possibly an exotic phase
at the very center
Manetism provides one of the few
diagnostic tools with which we can
probe into the NS interior
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The Origin and Evolution of NS Magnetic Field
〜A scenario before the 1990s 〜
 All neutron stars are born with strong magnetic
fields (〜1012 G).
 The magnetic field is sustained by permanent
ring current, flowing possibly in the crust.
 The magnetic field decays exponentially with
time, due to Ohmic loss of the ring current.
 Radio pulsar statistics suggest a field decay
timescale of τ〜107 yr.
 The older NSs (e.g., millisecond pulsars) have
the weaker magnetic field.
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+
-
NS Populations
15
Surface Magnetic Field (G)
10
Magnetars?
14
10
10
13
10
12
10
11
Crab-like Pulsars
Binary X-ray Pulsars
Radio
Pulsars
10
10
9
10
Msec
Pulsars
0.001
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0.01
Rotation Period (sec)
0.1
1
10
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100
1000
Estimates of NS Magnetic Fields
(1) A simple-minded estimate;
flux conservation from the progenitor star
9
2
4
R
12
〜10 m, B〜10 G → R〜10 m, B〜10 G
2
(2) Assuming –d(Iω /2)/dt = mag. dipole radiation;
→ B ∝ sqrt(P dP/dt) 〜 10
11~13
G
(3) Detection of X-ray spectral features due to
(electron) cyclotron resonance ;
12
Ea = heB/2πme = 11.6 (B/10 G) keV
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An Accretion-Powered Binary X-ray Pulsar
A supersonic accretion
flow from companion
An X-ray
emitting hot
(kT~20
keV)
Aaccretion
strongly
column
magnetized NS
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A strongly magnetized
NS with a rotation period
of 0.1〜1000 sec, in a
close binary with a massdonating companion star.
A standing
shock
Electrons in the accretion column
resonantly scatter X-ray photons,
when they make transitions
between adjacent Landau levels.
→ The X-ray spectrum will bear a
strong spectral feature, called a
Cyclotron Resonance Scattering
Feature (CRSF).
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Cosmic X-ray Studies in Japan
ASCA (Advanced Satellite for ASTRO-E2 Scheduled
Cosmology & Astrophysics)
for launch in 2005
1993.2〜2000.7
Ginga (Galaxy)
1987.2〜1991.10
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Tenma (Pegasus)
1983.2〜1984.8
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M-5 launch
vehicle of ISAS
Hakucho (Cygnus)
1979.2〜1984.4
X-ray Observations of CRSFs
Before 1990, only two examples were observed
(e.g., Truemper et al. 1978, Astrophys. J. 219, L105; 1978)
of discoveries with the
Ginga Satellite
(1987-1991)
A transient X-ray
pulsar X0331+53
Counts/s/cm2/keV
A series
Makishima et al.
Astrophys. J. 365,
L59 (1990)
Er = 28 keV →
B = 2.4×1012 G
1
2
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5
10
20
Energy (keV)
50
100
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Discoveries of CRSFs with Ginga
Her X-1
Er=33
keV
4U 1538-52
Er=21
keV
X0331+53
Er=28
keV
4U 0115+63
12 &
23
keV
Cep X-4
Er=29
keV
SMC X-1
No
CRSF
Makishima et al. Astrophys. J. 525, 978 (1999)
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Observatoins with the Rossi X-ray Timing Explorer
Heindl et al.
Astrophys. J.
563, L35 (2001)
f (E) = (aE -p + bE +q)
×exp(-E /kT)
× exp(-S )
Fit residuals with exp{-S}
Fit residuals w/o exp{-S}
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S =E 2/{(E-Er)2+W 2 }
Makishima et al.
(1999)
Observatoins with BeppoSAX
Fundamental and
2nd harmonic in
4U 1909+07
4 harmonics
in 4U 0115+63
Cusmano et al.
Astron. Astrophys
338, 79 (1998)
Santangelo et al.
Astrophys. J.
523, L85 (1998)
10
20
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30
50
100
1
2
5
Energy (keV)
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10
20
50
Cyclotron Resonance Energy Er (keV)
Cyclotron Resonances and the Spectral Continuum
60
50
A0535+26
40
4U1626-67
GX301-2
30
X0331+53
Her X-1
Cep X-4
Vela X-1
Cen X-3
4U1538-52
20
Energy
4U1907+09
4U0115+63 (1991)
4U0115+63(1990)
10
6
8
10
20
30
Cutoff Energy Ec (keV)
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Ec Er
Even if CRSF is not
detected, we can
estimate the field
intensity from the Xray continuum shape
Distribution of Magnetic Field
Number
Surface magnetic fields of
log[ B /(1+z )] (Gauss)
〜15 binary X-ray pulsars
13
12
10
are tightly concentrated over
BeppoSAX
Ginga
(1-4)×1012 G.
RXTE
CRSF is yet to be detected
8
ASTRO-E2
ASCA
from the remaining 〜20
HXD
binary X-ray pulsars, but the
6
continuum shape suggests
that they have comparable
field intensities.
4
Higher-field side of the
distribution may be subject
2
to selection effects. → The
Hard X-ray Detector (HXD)
0 2
onboard ASTRO-E2 (laumch
5 10 20 50 100
in 2005) is of great value.
Cyclotron Resonance Energy (keV)
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Does the Magnetic Field Decay?
10
B (1012G)
A fast field decay
is unlikely.
1626-67
 New radio-pulsar
statistics support 1
field-non-decay
hypothesis (Itoh et
al. Astrophys. J.
455, 244; 1995)
0.10.1
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Her X-1
A0535+26
GX302-1
0331+53
Vela X-1
Cep X-4
1907-09
1538-52
0115-63
Cen X-3
Half Lifetime in the Main Sequence (yr)
1010
109
1
108
107
10
106
Companion Mass (M◎)
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105
100
The Origin and Evolution of NS Magnetic Field
〜An alternative scenario 〜
1. The field decay occurs on a very long time scale.
→ difficult to explain the weak-field NSs.
2. Strong-field and weak-field NSs are genetically
different.
3. Strong-field and weak-field objects are connected
to each other by some phase transitions.
→ Magnetic field may be a manifestation
of nuclear ferrro-magnetism.
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Ferro-magnetic and para-magnetic NSs?
Magnetic moments of neutrons may align due to exchange
interaction, which must be repulsive on the shortest range.
If all the neutrons align, we expect B〜 4×1016 G.
A small volume fraction (~10-3) is ferro-magnetic
→ strong-field NSs (1012 G) ?
Entirely para-magnetic → weak-filed NSs (＜108~9 G) ?
Phase transitions may occur depending on, e.g., age,
temperature, accretion history, etc.
A large fraction of the volume is ferro-magnetic
→ magnetars (1014~15 G) ?
The release of latent heat at the transition may explain
some soft gamma-ray repeaters?
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Magnetars?
Proton cyclotron rsonance
E = 6.3 (B/1015G) [keV]
Is this soft γ-ray
repeater a
“magnetar” with B ~
1015 G, and the burst
energy is supplied by
magnetic phase
transitions ?
SGR 1806-20
We urgently need to
Ibrahim et al., Astrophys. search for objects
14 G →
with
B~
10
J. 574, L51 (2002)
electron CRSF at
~100 keV
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The Hard X-ray Detector (HXD)
Experiment onboard ASTRO-E2
ASTRO-E2 Scheduled
for launch in 2005
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Unprecedented
sensitivity in
10~600 keV
Summary
1. X-ray observations are uncovering interesting
inference that the magnetic field of NSs is
sustained by nuclear ferro-magnetism
2. Theoretical studies of magnetic phase diagram of
nuclear matter is encouraged.
3. Search for cyclotron resonances in the ~100 keV
energy range is an important task.
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