62
M. Burgay et al.
times of arrival, range from 0.1 s to 7.7 s, with a median of 1.5 s, which is much
higher than that of normal (non recycled) pulsars (0.6 s) (from PSRcat [1])
For 20 RRATs, it has been possible to also measure the first derivative of the
spin period [22] and it was found to be positive. This, in addition to the detection of
RRAT J1819−1458 [23] in the X-rays, where it behaves like a quite normal pulsar,
allowed us to establish that RRATs are indeed a subclass of the rotational-powered
NSs.
The mechanism responsible for the transient behaviour of RRATs is not established yet. Several hypotheses have been proposed, none of which, however, is
able to satisfactorily account for all of RRATs’ characteristics (see e.g. [24] for a
brief overview). Whatever they are, whether they are really only emitting occasional
bursts or their steady pulsed emission is just too faint to be seen, RRATs possibly
represent a very significant fraction of the entire NS population: the sporadicity of
their detectable emission, in fact, results in a substantially increased estimate of
the total number of Galactic active radio-emitting neutron stars, with RRATs being
probably as many as 4 times the number of normal radio pulsars (but see [25] for a
thorough discussion of RRAT and NS populations)!
2.3 Relativistic Binary Pulsars
As mentioned in Sect. 2.1, some radio pulsars—most notably some of the recycled
radio pulsars—have a high potential for studying many physical and astrophysical
phenomena. Thanks to their periodic signals, they can be used as precision cosmic
clocks, with a stability that is only slightly worse than that of the last-generation
laboratory clocks [26]; they have the capability of providing that stability over time
intervals longer than those tested for the best atomic clocks so far. When such a
precision clock is placed in a tight orbit around a compact object, such as a white
dwarf (WD) or a second neutron star, the times of arrival of the pulses can be
significantly affected by the deformed space-time around the compact companion,
and relativistic effects become measurable.
2.3.1 Basic Evolution
Recycled pulsars, of which relativistic binaries are a sub-class, are thought to be
created in binary systems in which a companion star transfers mass and angular
momentum onto the NS surface (recycling model) [27]. Through stellar winds or
Roche lobe overflow, the NS can hence be accelerated to spin periods of few or few
tens of milliseconds, depending on the amount of mass accreted, hence on the initial
mass of the companion star.
During the mass-transfer phase, the NS is seen as a bright, pulsating X-ray
source, if the magnetic field is high enough to channel the accreted matter onto
M. Burgay et al.
times of arrival, range from 0.1 s to 7.7 s, with a median of 1.5 s, which is much
higher than that of normal (non recycled) pulsars (0.6 s) (from PSRcat [1])
For 20 RRATs, it has been possible to also measure the first derivative of the
spin period [22] and it was found to be positive. This, in addition to the detection of
RRAT J1819−1458 [23] in the X-rays, where it behaves like a quite normal pulsar,
allowed us to establish that RRATs are indeed a subclass of the rotational-powered
NSs.
The mechanism responsible for the transient behaviour of RRATs is not established yet. Several hypotheses have been proposed, none of which, however, is
able to satisfactorily account for all of RRATs’ characteristics (see e.g. [24] for a
brief overview). Whatever they are, whether they are really only emitting occasional
bursts or their steady pulsed emission is just too faint to be seen, RRATs possibly
represent a very significant fraction of the entire NS population: the sporadicity of
their detectable emission, in fact, results in a substantially increased estimate of
the total number of Galactic active radio-emitting neutron stars, with RRATs being
probably as many as 4 times the number of normal radio pulsars (but see [25] for a
thorough discussion of RRAT and NS populations)!
2.3 Relativistic Binary Pulsars
As mentioned in Sect. 2.1, some radio pulsars—most notably some of the recycled
radio pulsars—have a high potential for studying many physical and astrophysical
phenomena. Thanks to their periodic signals, they can be used as precision cosmic
clocks, with a stability that is only slightly worse than that of the last-generation
laboratory clocks [26]; they have the capability of providing that stability over time
intervals longer than those tested for the best atomic clocks so far. When such a
precision clock is placed in a tight orbit around a compact object, such as a white
dwarf (WD) or a second neutron star, the times of arrival of the pulses can be
significantly affected by the deformed space-time around the compact companion,
and relativistic effects become measurable.
2.3.1 Basic Evolution
Recycled pulsars, of which relativistic binaries are a sub-class, are thought to be
created in binary systems in which a companion star transfers mass and angular
momentum onto the NS surface (recycling model) [27]. Through stellar winds or
Roche lobe overflow, the NS can hence be accelerated to spin periods of few or few
tens of milliseconds, depending on the amount of mass accreted, hence on the initial
mass of the companion star.
During the mass-transfer phase, the NS is seen as a bright, pulsating X-ray
source, if the magnetic field is high enough to channel the accreted matter onto