An
impression of the gamma-ray binary system LS 5039. A neutron star
(left) and its massive, companion star (right). The research team
suggests that the neutron star at the heart of LS 5039 has an
ultra-strong magnetic field, and is arguably a magnetar. The field
accelerates high-energy particles inside the bow-shaped region, thereby
emitting gamma-rays that characterize the gamma-ray binary system.
(Credit: Kavli IPMU) A team of researchers led by members of the Kavli
Institute for the Physics and Mathematics of the Universe (Kavli IPMU)
has analyzed previously collected data to infer the true nature of a
compact object—found to be a rotating magnetar, a type of neutron star
with an extremely strong magnetic field—orbiting within LS 5039, the
brightest gamma-ray binary system in the Galaxy. Including former graduate student Hiroki Yoneda,
Senior Scientist Kazuo Makishima and Principal Investigator Tadayuki
Takahashi at the Kavli IMPU, the team also suggest that the particle
acceleration process known to occur within LS 5039 is caused by
interactions between the dense stellar winds of its primary massive
star, and ultra-strong magnetic fields of the rotating magnetar. Gamma-ray binaries are a system of massive,
high-energy stars and compact stars. They were discovered only recently,
in 2004, when observations of very-high-energy gamma-rays in the
teraelectronvolt (TeV) band from large enough regions of the sky became
possible. When viewed with visible light, gamma-ray binaries appear as
bright bluish-white stars, and are indistinguishable from any other
binary system hosting a massive star. However, when observed with X-rays
and gamma-rays, their properties are dramatically different from those
of other binaries. In these energy bands, ordinary binary systems are
completely invisible, but gamma-ray binaries produce intense non-thermal
emission, and their intensity appears to increase and decrease
according to their orbital periods of several days to several years. Once the gamma-ray binaries were established as a
new astrophysical class, it was quickly recognized that an extremely
efficient acceleration mechanism should operate in them. While the
acceleration of TeV particles requires tens of years in supernova
remnants, which are renowned cosmic accelerators, gamma-ray binaries
boost electron energy beyond 1 TeV in just tens of seconds. Gamma-ray
binaries can thus be considered one of the most efficient particle
accelerators in the Universe. In addition, some gamma-ray binaries are known to
emit strong gamma-rays with energies of several megaelectron volts
(MeV). Gamma-rays in this band are currently difficult to observe; they
were detected from only around 30 celestial bodies in the whole sky. But
the fact that such binaries emit strong radiation even in this energy
band greatly adds to the mystery surrounding them, and indicates an
extremely effective particle acceleration process going on within them. Around 10 gamma-ray binaries have been found in
the Galaxy thus far—compared to more than 300 X-ray binaries that are
known to exist. Why gamma-ray binaries are so rare is unknown, and,
indeed, what the true nature of their acceleration mechanism is, has
been a mystery—until now. Through previous studies, it was already clear
that a gamma-ray binary is generally made of a massive primary star
that weighs 20-30 times the mass of the Sun, and a companion star that
must be a compact star, but it was not clear, in many cases, whether the
compact star is a black hole or a neutron star. The research team
started their attempt by figuring out which is generally the case. One of the most direct pieces of evidence for the
presence of a neutron star is the detection of periodic fast pulsations,
which are related to the neutron star rotation. Detection of such
pulsation from a gamma-ray binary almost undoubtedly discards the black
hole scenario. In this project, the team focused on LS 5039,
which was discovered in 2005, and still keep its position as the
brightest gamma-ray binary in the X-rays and gamma-ray range. Indeed,
this gamma-ray binary was thought to contain a neutron star because of
its stable X-ray and TeV gamma-ray radiation. However, until now,
attempts to detect such pulses had been conducted with radio waves and
soft X-rays—and because radio waves and soft X-rays are affected by the
primary star’s stellar winds, detection of such periodical pulses had
not been successful. This time, for the first time, the team focused on
the hard X-ray band (>10 keV) and observation data from LS 5039
gathered by the hard X-ray detector (HXD) on board the space-based
telescopes Suzaku (between September 9 and 15, 2007) and NuSTAR (between
September 1 and 5, 2016)—indeed, the six-day Suzaku observation period
was the longest yet using hard X-rays. Both observations, while separated by nine years,
provided evidence of a neutron star at the core of LS 5039: the periodic
signal from Suzaku with a period of about 9 seconds. The probability
that this signal arises from statistical fluctuations is only 0.1
percent. NuSTAR also showed a very similar pulse signal, though the
pulse significance was lower—the NuSTAR data, for instance, was only
tentative. By combining these results, it was also inferred that the
spin period is increasing by 0.001 s every year. Based on the derived spin period and the rate of
its increase, the team ruled out the rotation-powered and
accretion-powered scenarios, and found that the magnetic energy of the
neutron star is the sole energy source that can power LS 5039. The
required magnetic field reaches 1011 T, which is 3 orders of
magnitude higher than those of typical neutron stars. This value is
found among so-called magnetars, a subclass of neutron stars which have
such an extremely strong magnetic field. The pulse period of 9 seconds
is typical of magnetars, and this strong magnetic field prevents the
stellar wind of the primary star from being captured by a neutron star,
which can explain why LS 5039 does not exhibit properties similar to
X-ray pulsars (X-ray pulsars usually occur in X-ray binary systems,
where the stellar winds are captured by its companion star). Interestingly, the 30 magnetars that have been
found so far have all been found as isolated stars, so their existence
in gamma-ray binaries was not considered a mainstream idea. Besides this
new hypothesis, the team suggests a source that powers the non-thermal
emission inside LS 5039—they propose that the emission is caused by an
interaction between the magnetar’s magnetic fields and dense stellar
winds. Indeed, their calculations suggest that gamma-rays with energies
of several megaelectronvolts, which has been unclear, can be strongly
emitted if they are produced in a region of an extremely strong magnetic
field, close to a magnetar. Journal: Physical Review Letter
Authors: Hiroki Yoneda (1,2,3), Kazuo Makishima (2,1), Teruaki Enoto
(4), Dmitry Khangulyan (5), Takahiro Matsumoto (1), Tadayuki Takahashi
(2,1)
Author affiliation:
1. Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
2. Kavli Institute for the Physics and Mathematics of the Universe
(WPI), The University of Tokyo Institutes for Advanced Study, The
University of Tokyo, 5-1-5 Kashiwa-no-ha, Kashiwa, Chiba 277-8583, Japan
3. RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan DOI: https://doi.org/10.1103/PhysRevLett.125.111103 (Posted on September 8, 2020)
Media contact: John Amari Source: Kavli Institute for the Physics and Mathematics of the Universe
These results potentially settle the
mystery as to the nature of the compact object within LS 5039, and the
underlying mechanism powering the binary system. However, further
observations and refining of their research is needed to shed new light
on their findings.
Paper details
Paper title: Sign of hard X-ray pulsation from the gamma-ray binary system LS 5039
4. Extreme Natural Phenomena RIKEN Hakubi Research Team, Cluster for
Pioneering Research, RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan
5. Department of Physics, Rikkyo University, 3-34-1 Nishi Ikebukuro, Toshima, Tokyo 171-8501, Japan
Abstract of the dissertation
Preprint (arXiv.org page)
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