NuSTARとXMM-Newtonを⽤いたマグネターSGR 1900+14のX線観測
The Astrophysical Journal Supplement Series, 231:8 (21pp), 2017 July

Enoto

丹波翼、⾺場彩、⼩⾼裕和 (東京⼤学)、榎⼾輝揚 (京都⼤学)
Published in PASJ. Tamba et al. (2019) doi: 10.1093/pasj/psz073 arxiv: 1906.04406

1. イントロダクション

Figure 1. Known galactic SGRs and AXPs on the Galactic coordinate, where transient sources (circle) and persistently bright objects (triangle) are classiﬁed
L164
ENOTO
Table 1. The red ﬁlled symbols are sources observed with Suzaku. The spiral armsof our Galaxy are also indicated.

マグネターBlackbody Power-law
1 mCrabØ 研究対象: マグネター)*+)*,)*
̇𝑃 = 2– 11 s、𝑃 = 10 – 10s s の中性⼦星パルサー→ 強⼒な双極⼦磁場 (∝ 𝑃𝑃)̇ 〜 𝟏𝟎𝟏𝟒 –𝟏𝟎𝟏𝟓 G (図1)
The Astrophysical Journal Supplement Series,23
231:8 (21pp),24
2017 July
Enoto et al.
)**2
極めて強いX線光度10 – 10 erg s は、超強磁場(≳ 10 G)に伴う磁気エネルギーの解放で説明されうる磁気エネルギーの解放機構の解明には、放射帯域であるX線での観測が重要。

--

keV2 cm-2 s-1 keV-1

𝑃̇ (s s-1)-

Ø 観測的着⽬点: マグネターの硬X線⾮熱的放射-

マグネターの典型的なスペクトル: Blackbody (熱的) + Power-law (⾮熱的) (図2)
→ ⾮熱的成分の起源は謎。
Power-law成分の光⼦指数は磁場の強さと正の相関を持つ。(Enoto+ 2010)
𝑃 (s) (Enoto+ 2017)→ 強磁場中の光⼦分裂で説明可能？ (図3) (Barding & Harding, 2001)
図1 パルサーの分布図。

-光⼦指数 Γ1.5

soft

1.0

4.2. The Reality of Bd𝛾
Different from the magnetar model, an alternative scenario to𝛾 from P andexplain SGRs and AXPs assumes that Bd calculated
弱磁場Ṗ neither represents the𝛾 true dipole ﬁeld strength near a stellar

0.0
hard
*2

10
10*4Figure 11. (Top) Comparisonof representative X-ray
双極⼦磁場
𝐵=spectra of magnetars
10低エネルギー光⼦があまり⽣成されず、hardなスペクトルに

surface nor exceeds the critical ﬁeld BQED . In this case, X-ray
radiation is not powered by magnetic energy but by the𝛾accretion of a fallback disk that is left over from a supernova
explosion of the progenitor (Chatterjee et al. 2000; Alpar 2001;
Benli et al. 2013). The bulk-motion Comptonization of theaccretion column is attributed to the HXC radiation (Trümperet al. 2010, 2013; Kylaﬁs et al. 2014).
The accretion scenario still has difﬁculties explaining thebroadband X-ray observations. (1) First, as already statedabove, the accretion-powered pulsars do not show two distinct
spectral components even in the low X-ray luminosity. (2) The
power-law HXC extend up to ∼100 keV with the hard Gh
without any absorption nor cutoff features which are usually
expected from the electron cyclotron resonance of the ordinary*3 the spectral
accretion-powered neutron stars. (3) Finally,(Olausen & Kaspi 2014)=
scalings implies that the Bd values
play an important role in
the emission mechanism, especially in the HXC. The nominal(Olausen & Kaspi 2014)
value Bd derived from P and Ṗ is, therefore, considered to be atrue poloidal component near the stellar surface.
The present work suggested that Bd is one main control
parameter of the HXC radiation. Some theoretically motivatedemission models have already been developed so far in themagnetar scheme; these include thermal bremsstrahlung(Thompson & Beloborodov 2005), synchrotron radiation (Heyl& Hernquist 2005; Thompson & Beloborodov 2005), resonant
scattering (Baring & Harding 2007; Nobili et al. 2008; Viganòet al. 2012; Beloborodov 2013), and down-cascade due tophoton splitting (Paper I). Although the mechanism is stillobservationally poorly understood, the appropriate radiation
scenario would include the physics in the strong magnetic ﬁeld
to explain the scaling. For example, the hard photon index Gh in
our sample is correlated with Bd. This cannot be explained onlyby the difference of viewing angles, and should be further
compared with models (e.g., Beloborodov 2013).

*3after correcting the data for their distances. (Bottom) The photon index Gh ofthe HXC as a function of the magnetic ﬁeld Bd. Symbols are the same as inFigure 10.

we call this soft radiation below 10 keV the “Soft X-rayComponent (SXC).”
Recent observations have revealed a new distinctive “Hard
X-ray Component (HXC)” dominating above 10 keV. First
detected with INTEGRAL from persistently bright sources(Kuiper et al. 2006; den Hartog et al. 2008a, 2008b), the HXC
was later reconﬁrmed by Suzaku and NuSTAR (Morii et al.
2010; Enoto et al. 2011; An et al. 2013). This HXC extends upto 100 keV or more with a hard photon index Gh ~ 1, butmust cut off at some energy because of an upper limit by theCGRO/COMPTEL at 1 MeV. This power-law HXC is
now believed to be an optically thin emission presumably
from a pulsar magnetosphere in the magnetar scheme (e.g.,Beloborodov 2013).
There have also been subsequent discoveries of transientobjects, mainly discovered through burst activities, e.g.,

頻繁な光⼦分裂により低エネルギー光⼦が多く⽣成され、softなスペクトルに𝛾𝛾0.5-0.5Figure 2. SGRs and AXPs on the P–Ṗ diagram, together with the grids of Bd,
tc , and L sd. The ATNF pulsar catalog is used for other pulsars (Manchesteret al. 2005). Star and square symbols are objects showing X-ray outbursts andassociation with supernova remnants, respectively. The pulsar death line (Chen& Ruderman 1993; Zhang et al. 2000), BQED , and photon splitting line (Baring& Harding 1998) are also indicated.

Figure 13. Same as Figure 7, but the observed
𝛾 ﬂux was converted to the𝛾unabsorbed 1–20 keV luminosity,and the outburst decay was ﬁtted with𝛾
強磁場Equation (12).
𝛾

図3 双極⼦磁場と光⼦指数の相関 (Enoto+ 2010)。

2. マグネター SGR 1900+14の同時広帯域観測
Ø SGR 1900+14)*+)*
̇𝑃~5 s、𝑃~10s s → マグネターでも⼤きな𝐵 ~7×10 G
特性年齢𝜏A ~0.9 kyr と⽐較的若い。
暗い天体で、先⾏研究による観測ではパラメータ決定精度が不⼗分。

-

Ø 同時広帯域観測-

軟X線に優れたXMM-Newtonと硬X線に優れたNuSTARを⽤いて1–78 keVを観測。

SGR 0501+4516 (e.g., Enoto et al. 2009; Rea et al. 20.)
7 Octsurges
Such transient sources occasionallycausein persi
1806-20 (200X-rays of a few orders of magnitude, followed by a gradual d(Rea & Esposito 2011). During these “outburst” states, spor
short bursts with short timescale durations.) s) havepr(∼1A
60024 ( et al. 2008). Althou
detected (Nakagawa et al. 2007;
1900+1Israel
complete picture has yet to be developed, bursts are thoughoriginate from magnetic reconnection (Lyutikov 2003) orJan.)90
cracking of the crust with starquakes(Thompsonet al. 2002
02(447-551The SXC and HXC match ideally with the simultan
0.2–600 keV broadband coverage of the Suzaku sate
(Mitsuda et al. 2007). Our previous study of nine SGRs
Apr.)hereafter Pap
600AXPs utilizing Suzaku (Enotoetal.
2010a,2(1841-04
suggested that (1) phase-averaged X-ray radiation of SGAXPs commonly consists of the SXC below 10 keV andHXC above 10 keV in both quiescent states.) and transguA
900 , and (3) their wide-b
outbursts, (2) Gh depends on17B0d8-and
40 (t2ch
spectral properties are tightly correlated with Bd and tch.
As the detailed description following Paper I, this pug.) of SGRs
Aprovides a summary of Suzaku observations
805 (204+10AXPs, combined with a systematicspectral study of all Su
05sources and the X-ray decaying behavior of transient sour+61ug.)
A70(2042 Data Reduction
2. Observation01and

2.1. Suzaku Observations
May)

2009
(85259+ or Transient Sources
2.1.1. Persistently 2Bright1 1 summarizes all 10Table
SGRs and AXPs 100
that Suzaku
1keV
10keV
100keV(keV)In this table, theobserved as of 2013 Energy
December.
“transources”
exhibit prominent
soft X-rayof twoofto tFigure
1. Background-subtractedνFν spectraof the increases
persistent emission
the magnetars,
after eliminating
instrumentaldecays
responses.on
Interstellar
orders ofshown
magnitudes
and thesubsequent
timescaleabsorption
isincluded.
Individual
spectraareshown
withoffsets
andaremonthstoyears,while
the“persistentlybrightones”
arerelatiarranged in order of increasing characteristic age from top (young)
35 to bottom−1stablewith theirlines
X-ray
luminosities
around Green,
10 erg(old).
Blue horizontal
indicate
a 1 mCrab intensity.
red,s and. Th
blueillustratedlines represent
soft component,
the hard-tail X-ray
component,
thein theFigure
3 by long-term
ﬂuxandrecord
SNRrepresentativecontamination (includinglineemission)
inthe1E
1841−045
spectrum,sources. Since this “persistent” and “transirespectively. If a source was observed more than once, one observation is shown.
is somehow
phenomenologicalwithout
The classiﬁcation
GSO data of 1E 1547.0−5408
are included
after Enoto et al. (2010a).
The a(e.g., Pons& shown
Rea 2012),
we classiﬁedin thisfull consensus
names of the sources,
which arehere as abbreviations,are listed
in pa footnote
to Table1.
relatively
variablesources as “transients.”

図2 様々なマグネターのスペクトル。

Our Suzaku sample in Table 1 includes 16 pointings
persistently
brightsources
and 10 withtargettheof previous
opportunity
15–60
keV range,
which
is consistentre- (T
sultobservations
from INTEGRAL(Figurein Kuiperobjects.
et al. 2006).
to follow
up10
6 transient
ThisThese
covers 1Suzakuspectra
INTEGRAL
all the∼29reconfirm
sourcesthe
orprevious
candidates
knowndetections
to date.11of Du
the operational
hard-tail component
fromwesomepersistent
includconstraints,
wereunable sources,
to observe
the reing transients
4U 0142+61,1RXS
J170849.0−400910,
1E
1841−045,
SGR 0418+5729 and SGR 1745−29. In the fol
SGR
1900+14 (Mereghettiet al. 2005;
ing1806−20,analyses,andweSGR
reprocessed
all the publisheddata w

Kuiper et al. 2006; Götz et al. 2006; den Hartog et al. 2007,2008).
11 The absorbed 2–10 keV and 15–60 keV X-ray fluxes are
The latest magnetars and candidates are listed in http://www.phsummarizedin Table 1. Thus, the hard-bandflux
is comparable
mcgill.ca/∼pulsar/magnetar/main.html(see alsoOlausen& Kaspi 2014
to, or sometimes even higher than, that in the softer band.
The unified presentation in Figure 1 yields an intriguing
2inference that the hard-tail component becomes weaker for olderobjects. To quantify this property, we simultaneously fitted theXIS and the HXD spectra of each object with a common spectral
model. For the hard-tail component, we employed a single
power law for all the sources. The soft component of younger
magnetars was reproduced by a single blackbody (Model A),
but older sources required some modification to this model

表1 SGR 1900+14の同時観測の詳細。
NuSTAR
XMM-Newton
衛星(3–78 keV)
(0.1–15 keV)
MOS1, MOS2, PN
検出器
FPMA
ターゲット
SGR 1900+14
⽇付
2016-10-20
露光時間
123 ks
23 ks

3. 結果: ⾃転位相によって異なるスペクトル
Figure 12. Surface temperature of magnetars measured with Suzaku in theCBB model (ﬁlled symbols) as a function of the surface dipole magnetic ﬁeld
Bd. For comparison, isolated neutron stars in quiescence are shown (opensymbols) from Viganò et al. (2013); SGRs/AXPs (open circles), high-Bpulsars (open squares), X-ray isolated neutron stars (open diamonds), androtation-powered pulsars (open triangles). Data from Haberl (2007); Pons et al.
(2007) are also compared. Two slopes are indicated with T∝B1 (a = 2 ) andkT µ B 0.5 (a = 0 , exponential decay of magnetic ﬁeld).

/dof = 1.18(444/378)<latexit sha1_base64="c6P4VII2ceg24QbgG2U1Auw5K6Q=">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</latexit>

1010)2 Blackbody

XMM-Newton

1010)422(data−model)/error

1.0

NuSTAR−5−2-2111

スペクトルを5つの位相でそれぞれ調べる

0.800
2255

1010Energy (keV)
Energy (keV)

20205050

図3 SGR 1900+14のスペクトル。

1.01
0.8
0.8
1.5
2
2.5
1.52−10 keV Flux (10 erg
2.02.5s cm )2–10 keV Unabsorbed Power-law Flux)*,)*),(10erg s cm )−120.81010)3−4

Norm Intens Ser 1−31.2

1.2

Power-law

0.010.01

𝜒Folded period: 5.22669000000000 s

Normalized Intensity

normalized counts s−1 keV−10.10.1

Photon Index2

光⼦指数 Γ

- SGR 1900+14のスペクトルはBlackbody + Power-lawでよくフィットできる (図3)。
- ⾃転周期5.22669 sで畳み込んだパルスプロファイル(図4)をもとに、⾃転位相を5分割してそれぞれのスペクトルをフィットしたと
PN, FPMA
ころ、光⼦指数が1.02–1.44と有意に変化し、光⼦指数が⾮熱的成分のフラックスと正の相関を持った(図5上)。
1.6
14→ フラックスの⼤きい部分＝磁場の⼤きい領域と考えると、
soft「磁場の強い部分でsoftなスペクトルを作る」という
1.4
1.4
多天体間の傾向を1天体の回転位相によるスペクトルの違いのみで再現(図5下)。
MOS1, MOS2, PN, NuSTAR FPMA
1.21.2hard
3−78 keV

0.50.5

Start Time 17681 17:03:11:759

磁極付近: 強磁場磁極1.51.5

マグネター

Stop Time 17684 12:01:14:558

図4 SGR 1900+14のパルスプロファイルと⾃転位相による5分割の⽅法。

4. MeVガンマ線天⽂学への展望

γγ11Phase
Phase

−2

回転軸

1周期00−1光子分裂が頻繁
低エネルギー光子が多くソフトなスペクトル

B=1014-15 G

図5 光⼦指数とフラックスの相関(上)、対応する物理的描像(下)。

マグネターの⾮熱的放射のべき成分はどのエネルギー帯域までのびているかはっきりしていない。
⾮熱的放射の起源は電⼦・陽電⼦対消滅に伴う511 keVの光⼦の分裂の他に磁気球からのコンプトン散乱という仮説もあり、MeV帯域でのマグネターの観測が実現すれば、511 keV付近のスペクトルの折れ曲がりを⾒ることで2つの仮説の切り分けが可能となる。

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