II-7. Astrophysics and Astro-Glaciology

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Broad-band spectral analysis of the gamma-ray binary system
LS 5039 and its strong MeV gamma-ray emission†
H. Yoneda,∗1 D. Khangulyan,∗2 K. Makishima,∗1,∗3,∗4 T. Enoto,∗5 K. Mine,∗3 T. Mizuno,∗6 and T. Takahashi∗4,∗3

†
∗1
∗2
∗3
∗4
∗5
∗6Condensed from the article in Astrophys. J. 917, 90 (2021)
RIKEN Nishina Center
Department of Physics, Rikkyo University
Department of Physics, University of Tokyo
Kavli IPMU, University of Tokyo
Extreme Natural Phenomena RIKEN Hakubi Research Team
Hiroshima Astrophysical Science Center, Hiroshima University- 59 -10−9
— INFC (0.45 < φ < 0.9)
— SUPC (0.0 < φ < 0.45, 0.9 < φ < 1.0)

10−10ν Fν (erg cm-2 s-1)

With the development of GeV/TeV gamma-ray observation techniques, since the 2000 s it was established that several high-mass X-ray binary systems
emit strong non-thermal emissions. They are called
gamma-ray binary systems, and so far, approximately
ten sources have been identiﬁed.1) Their spectral energy distributions (SEDs) have a peak typically beyond
1 MeV. Such non-thermal emissions indicate that eﬃcient particle acceleration takes place in them, and they
are considered a new particle accelerator in the Universe. However, the nature of compact objects and theacceleration mechanism in these systems are not wellunderstood, and this has been a long-standing question.
LS 5039 is one of the brightest known gamma-ray
binary systems in our galaxy.2) Its companion star is
an O star with a mass of 23 M , but it is not known
whether its compact object is a black hole or a neutronstar. From X-ray and TeV gamma-ray observations, itis suggested that this object accelerates TeV electrons
on the time scale of seconds, with an extremely higheﬃciency.3) Recently, by reanalysis of the COMPTEL
data, strong MeV gamma-ray emission has been foundfrom this source.4) These ﬁndings imply that it is essential to understand its broad-band spectrum from X-rays
to TeV gamma-rays. In this report, we present our results of the spectral analysis using the latest data.5)
By using 11 years of Fermi-LAT GeV gamma-ray
data and hard X-ray data of the NuSTAR observation, we updated the SED of LS 5039 with the highest
available statistics (see Fig. 1). The obtained SED andits spectral modulation along the orbital phase suggestthat LS 5039 has at least four spectral components fromthe X-ray to TeV band: (1) an X-ray component up to100 keV, (2) a spectral component that is dominant
above 100 keV up to a few tens of MeV, (3) a GeV
gamma-ray component that is almost independent of
the orbital motion, and (4) a TeV gamma-ray component. Existing theoretical models can interpret the ﬁrst
and fourth components, e.g., particle acceleration at thepulsar wind termination shock. However, we found thatthe second and third components are not explained well.
When the MeV emission is interpreted as synchrotronemission, we can constrain the acceleration eﬃciency
parameter as η < 10. This result suggests that parti-

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Energy (eV)10101011

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Fig. 1. The SED of LS 5039 with the NuSTAR and 11 yearsof Fermi LAT data. The red points represent the spectrain orbital phases around the inferior conjunction (INFC)
when the compact star is in front of the companion starfrom the observer’s perspective. The blue points denotethe spectra around the superior conjunction (SUPC)
when the compact star is behind the companion star.
See Ref. 5) for details.

cle acceleration operating in LS 5039 is diﬀerent fromthe diﬀusive shock acceleration (DSA) assumed in manymodels because DSA yields η > 10.
We have performed a broad-band spectrum analysisof the gamma-ray binary LS 5039 and summarized thefeatures of the emission components in each band. Theemission components from MeV to GeV remain uncertain, and further studies on this energy band are key tounderstanding this object’s nature. The MeV gammaray satellite COSI will be launched in 2025.6) Moreover, towards MeV/subGeV gamma-ray observation inthe 2030s, the GRAMS project with an unprecedentedly large eﬀective area has been proposed.7) We expect that these future observations will further deepen
our understanding of the mysterious gamma-ray binarysystems.
References
1) G. Dubus, Astron. Astrophys. Rev. 21, 64 (2013).
2) J. Casares et al., Mon. Not. R. Astron. Soc. 364, 899(2005).
3) T. Takahashi et al., Astrophys. J. 697, 592 (2009).
4) W. Collmar, S. Zhang, Astron. Astrophys. 565, A38(2014).
5) H. Yoneda et al., Astrophys. J. 90, 917 (2021).
6) J. Tomsick et al., Bull. Am. Astron. Soc. 51, 7 (2019).
7) T. Aramaki et al., Astrophys. J. 114, 107 (2020).

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