S8-3
Cosmic X-ray Study of the Physics in Strong Magnetic Fieldand Expectation for Ground Experiments
宇宙Ｘ線の観測による天体の超強磁場現象と地上実験への期待
Wataru Iwakiri1 and Teruaki Enoto1,2
岩切渉, 榎戸輝揚
1High Energy Astrophysics Laboratory, RIKEN Nishina Center 2-1, Hirosawa, Wako, Saitama 351-0198 Japan理研仁科センター高エネルギー宇宙物理研究室 〒351-0198 埼玉県和光市広沢2-1
2
NASA Goddard Space Flight Center, Astrophysics Science Devision, Code 662, Greenbelt, MD 20771, USA

Cosmic X-ray observation toward neutron star is a useful method to study the plasma physics in anultra-strong magnetic field. Accretion-powered X-ray pulsars are now believed to have 108 T fieldaround their magnetic pole which field strength is measured via cyclotron resonance scattering featuresappearing a (quasi) thermal X-ray spectrum. Moreover, X-ray astronomy recently has accumulated
evidence that some neutron stars exhibit an extremely strong field of ~1011 T. In this talk, we will provide
recent observational results and their interpretations.

1. Introduction
Astronomy has provided powerful tools tostudy the extreme physics since there are many
extreme environments in universe, such as astrong gravitational field and high temperature. In
particular, X-ray observation of Neutron Stars(NSs) is good plasma diagnosis in strong
magnetic fields, exceeding Mega Tesla.
NSs are born after a gravitational collapse of a
massive star and believed that their degeneratepressure by neutrons counteracts the force of theirgravity. Therefore the radius is very small (~10 km)
while the mass is close to Sun. NSs emit
electromagnetic radiation in various wavelengths.
The radiation has been observed as pulsed emission
with their stellar rotation (mainly 0.1 – 10 s). Figure
1 shows the diagram of the rotation period and theirderivatives. If we assume the loss of their rotationenergy is converted into a magnetic dipole radiationaccording to a standard pulsar model [2], themagnetic field strength of the NSs can be estimatedby the following equation,𝑃𝑃̇

B = 1.0 × 108 √(1 s) (10−15 s/s) T.

Fig.1. A period and their derivative (𝑃 − 𝑃̇) diagramof neutron stars [1](http://www.atnf.csiro.au/research/pulsar/psrcat/).

(1)

The evaluated fields are shown as blue dashed lines
in Figure 1.
2. Accretion Powered Pulsar
An accretion powered pulsar is a magnetized NSs(～108 T) orbiting with a normal stellar companion.
Figure 2 shows a schematic view of such accretion
powered pulsar system. The accreting matter from anormal star is funneled along the magnetic field toFig.2. A schematic view of accretion powered pulsar.

the NS poles. Since the gravitational potential of
NS is very large, the kinetic energy of the accretion
matter is eventually converted to thermal energy(the electron temperature reaches several keV). As aresult, the accretion column of thermal plasma isformed around the magnetic pole and emits X-rays
photons which are modulated with their spin. The
typical spectrum is approximated as power-law inthe 5 - 20 keV with a quasi-exponential cutoff atenergy ~20 keV. In such an environment, where thestrength of the field is greater than the thermalenergy of the plasma, a magnetic field effects aredominant in there radiative transfer. For suchoccasion, there is a clear spectral feature in X-rayspectra called Cyclotron Resonance Scattering
Features (CRSFs). CRSF appears at energy of

E𝑎 = ℏ𝑒𝐵⁄𝑚𝑒 = 11.6(𝐵⁄108 T) keV. (2)This feature has been observed as a absorption atseveral ten keV band from ~20 sources [3,4], which
number is about 20% of known accretion powered
pulsar. CRSF has appeared sometimes in multiple
harmonics (the record is five harmonics from 4U0115+63) [5]. Using this CRSF diagnosis, accretion
powered pulsar is considered as ideal laboratory forplasma physics in a strong magnetic field.
However, there is still some contradiction between
observational results and theoretical prediction in
profile of CRSFs. For example, many theories
predict that there should a fine structure of emission
wings caused by incoherent scattering around theabsorption feature [6,7], but such a structure has notbeen observed yet.
To obtain high quality data of the profile, we tried
to a deep observation of 4U 1626-67 using Suzaku
X-ray satellite [8]. Figure 3 shows the results ofpulse-resolved spectroscopy. The results show that
the CRSF feature at the bright phase is well fitted
by typical absorption model. On the other hand, inthe dim phase, we find that the feature is better
described in terms of an emission rather thanabsorption. Moreover, the continuum spectrum alsochanged between the bright and the dim phase [9].
We speculated that the detected emission-line likefeature is an emission wing of the cyclotron
resonance scattering and that the variation of thecontinuum is caused by magnetic field effects as theoptical depth changes. However, these effects arestrongly depend on the geometry of accretion
column, density and thermal gradient, and thus theorigin is still under discussion.
3.Magnetar
Recent X-ray observations have suggested thatsome isolated neutron stars are appearing at theupper right corner in Fig.1. Following theestimation of the magnetic field described in theequation (1), they are considered to have extremely
strong magnetic field in the 1010-11 T. It means thatthe estimated field strength exceeds the QED
critical field strength ( Bcr = 𝑚𝑒 2 𝑐 3 ⁄ℏ𝑒 = 4.4 ×
109 T ). The sources are called magnetar in
distinction from normal NSs. Evidence for theirFig.3. The phase resolved spectra of 4U 1626-67
obtained by Suzaku [9].

ultra-strong field also can be seen in different
observational properties (bright X-ray luminosityand bursting activity) [10]. The observed persistent
X-ray emission from magnetars is composed ofthermal emission with its temperature of ~ 0.5 keVand the hard non-thermal tail component emerging
above ~10 keV with a hard photon index Γ~1[11-12]. One of the interpretations of the hardcomponent is photon-splitting caused by QEDeffects [10]. Moreover, CRSF-like absorption
feature has been clearly detected from a one
magnetar recently [13]. If the line is a “proton”
cyclotron scattering feature, the result would bestrong evidence that magnetar is an ideal laboratory
to examine the plasma physics in ultra-strong field.
4. Expectation for ground experiments
So far we have outlined the potential of a cosmic
X-ray observation for studying plasma physics inextreme magnetic field. However, we would like toemphasize that there is some uncertainties, such asa distance of sources, geometry of the plasma andfield. Therefore, if possible, the confirmation of theobservational results on a ground experiment isvery important to validate theoretical models andunderstand the extreme field physics including
effects of higher-order term of QED.
References[1]

Manchester, R. N., et al., 2005, VizieR Online Data Catalog.

[2]Goldreich, P., & Julian, W. H. 1969, Astrophysical Journal, 157, 869.

[3]

Mihara, T. 1995, PhD thesis, , Dept. of Physics, Univ. of Tokyo.

[4]

Coburn, W., et al. 2002, Astrophysical Journal, 580, 394.

[5]Santangelo, A., et al. 1999, ApJL, 523, L85[6]

Nishimura, O. 2008, Astrophysical Journal, 672, 1127

[7]

Schönherr, G., Wilms, J., Kretschmar, P., et al. 2007, A&A, 472, 353[8]

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