Proceedings of International Conference on Physics in ... - KEK

Radio Pulsar Number
250
200
150
100
50
0
0
7 8 9 10 11 12 13 14 15 16
Magnetic Field Strength [Log B(Gauss)]
Figure 3: Magnetic field distribution <strong>of</strong> normal (radio) pulsars
(blue, peak at ∼ 10 12 G) from [2], accretion-powered
pulsars (green), and magnetars (red, peak at ∼ 10 15 G) [4].
1 T = 10 4 G.
the surface magnetic field <strong>of</strong> the NS can be estimated to be
B = 3.2 × 10 15√ (P ˙
P ) T. (1)
The evaluated fields are shown in Figure 3 and Fig. 2 as
dashed lines. Typical value <strong>of</strong> normal NSs, ∼ 10 8 T, is a
basis <strong>of</strong> the current emission model from pulsars, and the
model explains the observations so far.
ULTRA-STRONG FIELD OF MAGNETARS
During a last few decades, a new subclass <strong>of</strong> isolated
NSs is emerging on the upper right corner on the P - ˙ P
diagram, mostly observed only in the X-ray band. From
their peculiar properties, the one <strong>of</strong> them are named S<strong>of</strong>t
Gamma Repeaters (SGRs) and the others are Anomalous
X-ray Pulsars (AXPs). Both <strong>of</strong> them are slowly rotating
X-ray pulsars (P ∼ 2 − 10 s), showing large period derivatives
( ˙ P ∼ 10−12 − 10−9 s s−1 ). Using the above estimation
<strong>of</strong> the magnetic field, SGRs and AXPs should have an
ultra-strong magnetic field, B ∼ 1010−11 T (Fig. 3). This
field strength is stronger than those <strong>of</strong> normal pulsars by 2–
3 orders <strong>of</strong> magnitude, and even exceeds the QED critical
field, Bcr = m2 ec3 /¯he = 4.4 × 109 T, where the Landau
level <strong>of</strong> electrons in the magnetic field becomes equivalent
with the rest mass <strong>of</strong> an electron. If SGRs and AXPs have
indeed the ultra-strong field, they are promising targets to
examine the QED physics. As shown in Fig. 3, SGRs and
AXPs have a different field distribution from those <strong>of</strong> normal
rotation powered pulsars, theay are correctively called
“magnetars” [5, 6, 7].
There are accumulated evidence that magnetars have an
ultra-strong field. First <strong>of</strong> all, bright X-ray luminosity
(Lx ∼ 1035 erg s−1 ) <strong>of</strong> magnetars cannot be explained
by their spin-down power (L ∼ 1033 erg s−1 ). This is
a prominent difference from normal pulsar, and thus, the
emission <strong>of</strong> magnetars are believed to be sustained by their
huge stored magnetic energy. Secondly, magnetars exhibit
various burst activities, which cannot be seen from normal
pulsars so far. Sporadic burst emission occurres with a
8
6
4
2
Magnetar & Accretion Pulsar Number
Figure 4: A light curve <strong>of</strong> the giant flare from SGR 1806-20
in the 20–100 keV energy range, recorded by the RHESSI
satellite on 2004 December 27. The inset shows a precursor
which was recorded 142 s before the giant flare [8].
keV 2 cm -2 s -1 keV -1
1 mCrab
1806-20 (2007 Oct.)
1900+14 (2006 Apr.)
1547-54 (2009 (2009 Jan.) Jan.)
1841-04 (2006 (2006 Apr.) Apr.)
1708-40 (2009 (2009 Aug.) Aug.)
0501+45 (2008 (2008 Aug.) Aug.)
0142+61 (2007 (2007 Aug.) Aug.)
2259+58 2259+58 (2009 (2009 (2009 (2009 (2009 May) May) May) May) May)
1 10 100
Energy (keV)
Figure 5: X-ray spectra (νFν form) <strong>of</strong> the magnetars observed
by Suzaku with abbreviated names [12]. Individual
spectra are shown with <strong>of</strong>fsets, and are arranged in order
<strong>of</strong> increasing magnetic field from bottom (weak) to top
(strong). Blue horizontal lines indicate a 1 mCrab intensity.
short time scale (a few hundread ms – a few s). One prominent
example is a giant flare from SGRs, as demonstrated in
Figure 4. A current hypothesis speculates that these bursts
are emission from trapped fire balls on the magnetar surface,
presumably related with magnetic activities (e.g., reconnections).
Interestingly, the luminosities <strong>of</strong> these burst