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

sometimes exceed the Eddington luminosity (a maximum
permitted luminosity), and this excess is considered to originate
from a suppression <strong>of</strong> the Thomson cross section in
the high magnetic field. Thirdly, there is marginal evidence
<strong>of</strong> proton cyclotrons in the magnetar X-ray spectra, which
suggests B ∼ 10 15 G [9].
X-RAY EMISSION OF MAGNETARS
Persistent X-ray emission from magnetars have been extensively
observed in the ∼0.2–10 keV. In this energy band,
a thermal emission with its temperature <strong>of</strong> kT ∼ 0.5 keV is
considered to be emitted from the stellar surface. However,
through a new hard X-ray imaging with the INTEGRAL
satellite, some persistently bright magnetars were discovered
to emit a distinct hard-tail component which emerges
above ∼10 keV with an extremely hard photon index <strong>of</strong>
Γh ∼ 1 [10]. This unusual new component, though not yet
observed from all magnetars, is expected to provide a hint
to the high field physics in magnetars.
Using the Suzaku X-ray satellite [11], we performed
broad-band (0.8–70 keV) spectral analyses <strong>of</strong> the persistent
X-ray emission from 9 magnetars [12]. As shown in
Figure 5, the s<strong>of</strong>t thermal component was detected from all
<strong>of</strong> them (green lines), while the hard-tail component, dominating
above ∼10 keV, was detected at ∼1 mCrab 2 intensity
from 7 <strong>of</strong> them (red lines). In addition, as shown in this
figure, the hard-tail component has a tendency to become
stronger but s<strong>of</strong>ter towards sources with larger magnetic
fields. To quantitatively evaluate this trend, we employ the
1–60 keV fluxes <strong>of</strong> s<strong>of</strong>t-thermal and hard-tail components,
Fs and Fh, respectively, and then, calculate the hardness
ratio (HR) between these two components, ξ ≡ Fh/Fs. As
shown in Figure 6 (top), the HR is found to be tightly correlated
with the magnetic field B as
ξ = (0.09 ± 0.07) × (B/Bcr) 1.2±0.2
with a correlation coefficient <strong>of</strong> 0.873, over the range from
ξ ∼ 10 to ξ ∼ 0.1. On the other hand, as shown in Figure 6
(bottom), the photon index becomes s<strong>of</strong>ter toward stronger
field pulsars with ξ becoming larger.
Although several scenarios have been proposed [13, 14,
15, 16], the emission mechanism <strong>of</strong> the hard X-rays has
not yet been resolved. One <strong>of</strong> the biggest difficulties is
how to explain the extremely hard Γh ∼ 1 with its spectral
trend depending on B. A possible candidate <strong>of</strong> the emission
process is photon-splittings [17, 18]. In ultra-strong
magnetic fields exceeding Bcr, electron-positron pair cascades
are suppressed, while the photon splitting may be
dominant. In this case, gamma-rays from the surface may
repeatedly split into lower energy photons. This process
can also explain the differences in Γh among magnetars, in
such a way that higher fields objects will allow the photonsplitting
cascade to proceed down to lower energies, and
hence to make the continuum s<strong>of</strong>ter.
2 1 mCrab is one-thousandth <strong>of</strong> the Crab Nebula intensity, which is a
standard candle <strong>of</strong> the astronomy.
(2)
Hardness Ratio ξ = F h /F s
Hardness Ratio ξ = F h /F s
10
1
0.1
10
1
0.1
2259+58
(b)
0142+61
0501+45
(2009)
0501+45
(2008)
0142+61
1547-54
1900+14
1708-40
1841-04
10
Magnetic Field (G)
14 1015 0501+45
1806-20
1841-04
1900+14
1806-20
0 0.5 1 1.5 2
Photon index Γ h
1708-40
1547-54
Figure 6: (top) A correlation between the HR ξ and the
magnetic field B [12]. Green solid line represents the best
fit <strong>of</strong> equation (2). SGRs and AXPs are shown in red and
blue, respectively. (bottom) The HR ξ as a function <strong>of</strong> photon
indices Γh <strong>of</strong> the hard-tail component [12].
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