2.4 Investigations into Radio Pulsars
57
radiant energy, but also accumulates the rotating energy. And thus, the rotations
of neutron stars are be speeded up. In millisecond pulsars evolving, maybe they
have ever undergone these rotating accelerations. After the companions evolve into
supernovas, the neutron stars stop radiating X-rays due to being unable to capture
matter from the companions, and then gradually evolve into the millisecond pulsars
with radio radiations.
The rotating periods of millisecond pulsars are very stable and the periodic noises
also smaller. And thus, the statuses and properties of the celestial objects near the
millisecond pulsars might indirectly be investigated by observing the periodic disturbances of the pulsars. The millisecond pulsars become effective probes to investigate
the characteristics of extra-solar celestial objects. In 1999, Wolszczan reported variations in the time of arrival of pulses from PSR B1937+21, and the companion having
a mass almost equal to Cereus, located at 2.71 AU away from the pulsar [3]. More
recent observations have not detected any regular periodic signal associated with the
companion. Nevertheless, it is argued that the slight variations in pulse arrival-time
are consistent with an asteroid belt having a total mass less than 0.05 earth-masses.
Of course, to confirm the possible asteroid belt, the detection of periodicity in pulse
timing variations associated with individual asteroids is still necessary [4]. At present,
it has been discovered in the millisecond pulsar binary systems that the companions
may be planets, optical stars, white dwarfs or neutron stars. The millisecond pulsars
provide good space experimental conditions to detect gravitational waves and other
celestial objects, which is also very advantageous to investigating the long-term
evolutions of stars.
Furthermore, the periodic change rates of millisecond pulsars get to 10
–19 –10
–21
s/s, and their long-term stabilities have been even better than atomic clocks. And
hence, the millisecond pulsars are ideal signal sources to study the pulsar timing
models. The stable pulsar timing system is built as the datum of the atomic time and
the astronomical timescale. A radio pulsar timing system was built at the Arecibo
observatory in 1984. Two millisecond pulsars, PSR B1937+21 and PSR B1855+09,
have been observed by using the Arecibo radio telescope for long term, and a large
amount of data collected. As a result, the radio pulsar timing accuracy has reached
the levels of 1 microsecond and even 0.2 microseconds.
In general, many factors to affect pulsar timing stability are unable to be separated
effectively by using the individual pulsar timing, unless creating a Pulsar Timing
Array (PTA), a set of millisecond pulsars which is analyzed to search for correlated
signatures in the pulse arrival-time. That is to say, an ensemble pulsar time can be
created by utilizing many millisecond pulsars with smaller periodic noises and from
various directions. There are many applications for the PTAs. The most well known is
to use an array of millisecond pulsars to detect and analyze gravitational waves. Such
detection would result from a detailed investigation into the correlation between the
arrival-times of pulses emitted by millisecond pulsars as a function of the pulsars’
angular separations. Currently, there are three active pulsar timing array projects: the
first is the Parkes Pulsar Timing Array (PPTA) at the Parkes radio telescope, which
has been collecting data since 2005 and the second is the European Pulsar Timing
Array (EPTA), using data from the Lovell Telescope, Westerbork Synthesis Radio
57
radiant energy, but also accumulates the rotating energy. And thus, the rotations
of neutron stars are be speeded up. In millisecond pulsars evolving, maybe they
have ever undergone these rotating accelerations. After the companions evolve into
supernovas, the neutron stars stop radiating X-rays due to being unable to capture
matter from the companions, and then gradually evolve into the millisecond pulsars
with radio radiations.
The rotating periods of millisecond pulsars are very stable and the periodic noises
also smaller. And thus, the statuses and properties of the celestial objects near the
millisecond pulsars might indirectly be investigated by observing the periodic disturbances of the pulsars. The millisecond pulsars become effective probes to investigate
the characteristics of extra-solar celestial objects. In 1999, Wolszczan reported variations in the time of arrival of pulses from PSR B1937+21, and the companion having
a mass almost equal to Cereus, located at 2.71 AU away from the pulsar [3]. More
recent observations have not detected any regular periodic signal associated with the
companion. Nevertheless, it is argued that the slight variations in pulse arrival-time
are consistent with an asteroid belt having a total mass less than 0.05 earth-masses.
Of course, to confirm the possible asteroid belt, the detection of periodicity in pulse
timing variations associated with individual asteroids is still necessary [4]. At present,
it has been discovered in the millisecond pulsar binary systems that the companions
may be planets, optical stars, white dwarfs or neutron stars. The millisecond pulsars
provide good space experimental conditions to detect gravitational waves and other
celestial objects, which is also very advantageous to investigating the long-term
evolutions of stars.
Furthermore, the periodic change rates of millisecond pulsars get to 10
–19 –10
–21
s/s, and their long-term stabilities have been even better than atomic clocks. And
hence, the millisecond pulsars are ideal signal sources to study the pulsar timing
models. The stable pulsar timing system is built as the datum of the atomic time and
the astronomical timescale. A radio pulsar timing system was built at the Arecibo
observatory in 1984. Two millisecond pulsars, PSR B1937+21 and PSR B1855+09,
have been observed by using the Arecibo radio telescope for long term, and a large
amount of data collected. As a result, the radio pulsar timing accuracy has reached
the levels of 1 microsecond and even 0.2 microseconds.
In general, many factors to affect pulsar timing stability are unable to be separated
effectively by using the individual pulsar timing, unless creating a Pulsar Timing
Array (PTA), a set of millisecond pulsars which is analyzed to search for correlated
signatures in the pulse arrival-time. That is to say, an ensemble pulsar time can be
created by utilizing many millisecond pulsars with smaller periodic noises and from
various directions. There are many applications for the PTAs. The most well known is
to use an array of millisecond pulsars to detect and analyze gravitational waves. Such
detection would result from a detailed investigation into the correlation between the
arrival-times of pulses emitted by millisecond pulsars as a function of the pulsars’
angular separations. Currently, there are three active pulsar timing array projects: the
first is the Parkes Pulsar Timing Array (PPTA) at the Parkes radio telescope, which
has been collecting data since 2005 and the second is the European Pulsar Timing
Array (EPTA), using data from the Lovell Telescope, Westerbork Synthesis Radio