(a) Spherically symmetric, steady mass-loss process from neuton star surface as a
result of nuclear flashes (Kato Mariko 1983)
(b) Surface properties of accreting neutron stars as a trigger of helium flash, and disk
structure in weakly magnetized neutron stars (Hoshi Reiun 1981)
(c) X-ray spectra and atmospheric structure of bursting neutron stars (Ebisuzaki
Toshikazu and Nomoto Kenichi 1986)
(d) Radial oscillation of extended envelopes of neutron stars (Shibasaki Kiyoto and
Ebisuzaki Toshikazu 1989)
(e) Thermal instability in accretion disks of neutron stars (Kakubari Ken’ichi, Hoshi
Reiun et al. 1989)
Among these, we make mention the work of Ebisuzaki and Nomoto, item (c), on
the model calculation of the stellar atmosphere for X-ray bursting neutron stars.
They calculated four cases of hydrogen-rich (HRS), helium-rich (HES), magnesiumrich (MGS), and pure-helium (HED) and constructed an X-ray luminosity (Lf)
versus X-ray color-temperature (Tc) diagram. In this diagram, they plotted the data
of three outbursts in the X-ray bursting neutron star MXB 1638–536, observed by
TENMA, and they found that the helium-rich model was generally in good agreement with observations (Ebisuzaki and Nomoto 1986).
In parallel with theoretical works, X-ray observations have been promoted by
satellites, TENMA, GINGA, and ASCA successively.
In the 1980s, Makino’s group (1988) and Leahy’s group (1985, 1989) at the ISAS
observed a massive X-ray binary GX 301–2, using TENMA, for the X-ray intensity
and spectral variations. D. A. Leahy, Nakajo Masanori, and their group found an
eclipse of a neutron star by a small-scale object in this system (Leahy et al. 1988).
GINGA observed a great number of X-ray pulsars, including some new ones.
Koyama Katsuji, Kawada Mitsunobu, and their group discovered four X-ray pulsars
and three hard X-ray sources in the 5-Kpc arm of the Galaxy, and they suggested that
this arm might be a colony of X-ray pulsars (Koyama et al. 1990). Nagase Fumiaki,
Dotani Tadayasu, and their group observed an X-ray pulsar and found two absorption features in the X-ray spectrum, which were identified as cyclotron absorption in
strong magnetic fields that are as high as 10
12 gauss. In such strong fields, electrons
were tightly spiraling around the magnetic line of force, and the electron motions
were quantized. Thus, the absorption features at 12 and 23 KeV can be interpreted as
the first and second transitions from the ground state, respectively (Nagase et al.
1991). With a γ-ray detector on board GINGA, Murakami Toshio and his group
observed several γ-ray bursts from highly magnetized neutron stars. In two cases
they discovered two absorption features, similarly interpreted as cyclotron scattering
in highly magnetized plasma (Murakami et al. 1988; Yoshida and Murakami 1990).
Based on ASCA observations, Asai Kazumi, Dotani Tadayasu, and their group
analyzed soft X-ray transients, which are most likely low-mass binary systems
containing either neutron stars (NS-LMXB) or black-holes (BH-LMXB) in the
quiescent state. The quiescent luminosity of NS-LMXB was estimated to be on the
order of 10
32
–10
33 erg s
À1 , which is much lower than the value predicted in the
current model of accretion disks. Instead, Asai’s group proposed the role of the
7.5 Stellar Physics
207
result of nuclear flashes (Kato Mariko 1983)
(b) Surface properties of accreting neutron stars as a trigger of helium flash, and disk
structure in weakly magnetized neutron stars (Hoshi Reiun 1981)
(c) X-ray spectra and atmospheric structure of bursting neutron stars (Ebisuzaki
Toshikazu and Nomoto Kenichi 1986)
(d) Radial oscillation of extended envelopes of neutron stars (Shibasaki Kiyoto and
Ebisuzaki Toshikazu 1989)
(e) Thermal instability in accretion disks of neutron stars (Kakubari Ken’ichi, Hoshi
Reiun et al. 1989)
Among these, we make mention the work of Ebisuzaki and Nomoto, item (c), on
the model calculation of the stellar atmosphere for X-ray bursting neutron stars.
They calculated four cases of hydrogen-rich (HRS), helium-rich (HES), magnesiumrich (MGS), and pure-helium (HED) and constructed an X-ray luminosity (Lf)
versus X-ray color-temperature (Tc) diagram. In this diagram, they plotted the data
of three outbursts in the X-ray bursting neutron star MXB 1638–536, observed by
TENMA, and they found that the helium-rich model was generally in good agreement with observations (Ebisuzaki and Nomoto 1986).
In parallel with theoretical works, X-ray observations have been promoted by
satellites, TENMA, GINGA, and ASCA successively.
In the 1980s, Makino’s group (1988) and Leahy’s group (1985, 1989) at the ISAS
observed a massive X-ray binary GX 301–2, using TENMA, for the X-ray intensity
and spectral variations. D. A. Leahy, Nakajo Masanori, and their group found an
eclipse of a neutron star by a small-scale object in this system (Leahy et al. 1988).
GINGA observed a great number of X-ray pulsars, including some new ones.
Koyama Katsuji, Kawada Mitsunobu, and their group discovered four X-ray pulsars
and three hard X-ray sources in the 5-Kpc arm of the Galaxy, and they suggested that
this arm might be a colony of X-ray pulsars (Koyama et al. 1990). Nagase Fumiaki,
Dotani Tadayasu, and their group observed an X-ray pulsar and found two absorption features in the X-ray spectrum, which were identified as cyclotron absorption in
strong magnetic fields that are as high as 10
12 gauss. In such strong fields, electrons
were tightly spiraling around the magnetic line of force, and the electron motions
were quantized. Thus, the absorption features at 12 and 23 KeV can be interpreted as
the first and second transitions from the ground state, respectively (Nagase et al.
1991). With a γ-ray detector on board GINGA, Murakami Toshio and his group
observed several γ-ray bursts from highly magnetized neutron stars. In two cases
they discovered two absorption features, similarly interpreted as cyclotron scattering
in highly magnetized plasma (Murakami et al. 1988; Yoshida and Murakami 1990).
Based on ASCA observations, Asai Kazumi, Dotani Tadayasu, and their group
analyzed soft X-ray transients, which are most likely low-mass binary systems
containing either neutron stars (NS-LMXB) or black-holes (BH-LMXB) in the
quiescent state. The quiescent luminosity of NS-LMXB was estimated to be on the
order of 10
32
–10
33 erg s
À1 , which is much lower than the value predicted in the
current model of accretion disks. Instead, Asai’s group proposed the role of the
7.5 Stellar Physics
207