Ⅱ-7. Astrophysics and Astro-Glaciology
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Polarized X-rays emitted from a magnetar†
K. Uchiyama,∗1,∗2 R. Taverna,∗3 R. Turolla,∗3,∗6 F. Muleri,∗4 J. Heyl,∗5 S. Zane,∗6 L. Baldini,∗7,∗8
D. González-Caniulef,∗5 M. Bachetti,∗9 J. Rankin,∗4 I. Caiazzo,∗10 N. D. Lalla,∗11 V. Doroshenko,∗12M. Errando,∗13 E. Gau,∗13 D. Kırmızıbayrak,∗5 H. Krawczynski,∗13 M. Negro,∗14,∗15 M. Ng,∗16 N. Omodei,∗11A. Possenti,∗9 T. Tamagawa,∗1,∗2,∗17 and M. C. Weisskopf∗18 on behalf of the IXPE magnetar WG
N 0°
30°

larizationangle

Magnetars are neutron stars with ultra-strong magnetic ﬁelds. Magnetars exhibit steady X-ray pulsed
emissions with a luminosity of approximately 1033 –1035 erg s−1 , spin period P of approximately 0.1–12 s,and large spin-down rates Ṗ of approximately 10−14 –10−10 s s−1 . This translates into magnetic ﬁelds of up
to B approximately 1015 G, assuming a conventional
spin-down model. The external magnetic ﬁeld of magnetars is expected to have a toroidal component thattwists the ﬁeld lines. Because charged particles must
ﬂow along the closed magnetic ﬁeld lines to sustain theﬁeld, the star magnetosphere becomes optically-thickduring resonant Compton scattering (RCS).1) Becauseof its strong magnetic ﬁeld, magnetar X-ray emission
is linearly polarized in two normal modes, namely, theordinary (O) and extraordinary (X) modes, and upon
reprocessing via RCS, X-mode photons become dominant.
4U 0142 + 61 is the brightest persistent magnetar
with an unabsorbed ﬂux of 6 × 10−11 erg s−1 cm−2in the 2–10 keV range. Here, we report on the polarimetric observation of 4U 0142 + 61 conducted by
the Imaging X-ray Polarimetry Explorer (IXPE) fromJanuary 31, 2022 to February 27, 2022 for a total onsource time of 840 ks.
Results are shown in Fig. 1 in the form of a polar plot
where the polarization degree (PD) is the radial coordinate and polarization angle (PA) the azimuth. The
measured PD is 15 ± 1% at low energies (2–4 keV).
At 4–5 keV PD becomes consistent with zero and thenincreases to 35 ± 7% at 5.5–8 keV. The PA is approxi-

5.5-8.0 keV

Po60°

E 90°

-30°

90 deg rotation-60°

2.0-3.0 keV 4.0-4.8 keV
4.8-5.5 keV

3.0-4.0 keV
1020304050-90° W

Polarization degree [deg]Fig. 1. Polar plot showing the energy dependence of themeasured PD and PA. Contours enclose the 68.3% conﬁdence level regions. Stars indicate the corresponding
PD and PA calculated using the RCS model.

mately 50◦ East of North at energies below 4 keV andswings by 90◦ , settling at 40◦ West of North, above
5 keV.
The polarization pattern as a function of energy,
with a minimum of PD and swing of PA by 90◦ at
approximately 4–5 keV, suggests that the X-ray emission from 4U 0142 + 61 is characterized by two distinct
components polarized in two diﬀerent normal modes.
The low-energy component is thought to be producedby thermal surface emission while the high-energy
component by photons up-scattered in the magnetosphere. The measured polarization fraction at highenergies (35% at 5.5–8 keV) is indeed compatible withthe theoretical expectations of the RCS model andsuggests that X-mode photons dominate. Although
the measured polarization fraction at low energies (2–4 keV) diﬀers from the predictions of published theoretical models,2) we found that the radiation emitted
from a condensed3) equatorial belt results in a predominance of O-mode photons with PD of approximately
15%.
In conclusion, the positive detection of polarized
emission from 4U 0142 + 61 by IXPE further supports the magnetar model. In particular, the distinctive energy-dependent pattern can be explained by assuming emission from the bare condensed surface reprocessed by RCS in the twisted magnetosphere.

†Condensed from the article in Science 378, 646 (2022)
RIKEN Nishina Center∗2 Department of Physics, Tokyo University of Science∗3 Department of Physics and Astronomy, University of Padova∗4 Istituto di Astroﬁsica e Planetologia Spaziali∗5 Department of Physics and Astronomy, University of BritishColumbia∗6 Mullard Space Science Lab., University College London∗7 Dipartimento di Fisica Enrico Fermi, Università di Pisa
∗8 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa
∗9 Osservatorio Astronomico di Cagliari, INAF
∗10 TAPIR at Caltech∗11 Department of Physics, Stanford University∗12 Institut für Astronomie und Astrophysik, UniversitätTübingen∗13 Physics Department and McDonnell Center for the SpaceSciences, Washington University∗14 University of Maryland, Baltimore Country∗15 NASA Goddard Space Flight Center∗16 Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology∗17 RIKEN Cluster for Pioneering Research∗18 NASA Marshall Space Flight Center∗1References
1) C. Thompson et al., Astrophys. J. 574, 332 (2002).
2) R. Taverna et al., Mon. Not. R. Astron. Soc. 492, 5057
(2020).
3) A. Y. Potekhin et al., Astron. Astrophys. 546, A121
(2012).

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