180
A. Patruno and A. L. Watts
that most X-ray timing noise may be related to a moving hot spot configuration.
Alternatively, the phase wandering might be related to complexity in the structure
of the pulsar’s magnetic field, with different multipole components dominating the
accretion process at different accretion rates [199].
Timing noise may also be due to fluctuating accretion torques [176, 177]. This
has been studied for strongly magnetized accreting pulsars like Vela X-1 [74], and it
is natural to expect something similar in AMXPs. However as discussed, this model
requires unphysical large accretion rate fluctuations to explain all timing noise or the
large jumps in phase observed for example in XTE J1814-338 (and probably also
SAX J1808.4-3658 and XTE J1807-294, [123, 239, 258, 287, 352]). Nonetheless
some X-ray timing noise, or even its entirety for AMXPs with weak timing noise
content, could in principle be related to accretion torque fluctuations.
4.5 Pulse Profiles
AMXP pulse profiles form in regions of strong gravity and encode information
about the physical properties of the emitting regions and the compactness of the
NS [185, 186, 220, 276, 278]. Light bending, aberration and relativistic Doppler
shifts all affect pulse profile shape [42, 225, 275, 277] and can be measured using
coherent timing analysis. This can be used to constrain the EoS of ultra-dense matter
(Fig. 4.7) although large uncertainties still exist: in part due to model-dependencies,
but more importantly because of the low S/N of the relativistic features.
Pulse profile shapes can also constrain the geometry of the NS magnetic field
[34, 167, 365] since multipoles generate different accretion columns and hot spots on
the NS surface (see, however, [8], for a recent discussion of the problem). The information contained in pulse profiles can be extracted by studying pulse amplitudes,
the harmonic content of the signal and the time of arrival of the pulsations. Double
peaked pulse profiles, for example, may be observed when two antipodal hot-spots
exist on the NS surface [178, 225, 277]; a high harmonic content is suggestive of a
complex field geometry [198] and the energy dependence of the pulse amplitudes
and the time lags of the pulsations provide a way to explore the angular pattern of
the radiation emitted from the NS surface [111, 277, 278]. The temporal evolution of
the pulse shapes also reveals details of the complex disk/magnetospheric interaction
responsible for magnetic channeling of the accreting plasma [142, 171].
4.5.1 Pulse Fractional Amplitudes and Phase Lags
The pulse profiles of AMXPs are sinusoidal, with only the fundamental frequency
detected in most cases (including IGR J00291+5934, XTE J0929-314, XTE J1751305, SAX J1748.9-2021 and IGR J17498-2921). In some AMXPs there is a
first overtone (e.g. XTE J1814-338, IGR J18245-2452 and NGC 6440 X-2), and
A. Patruno and A. L. Watts
that most X-ray timing noise may be related to a moving hot spot configuration.
Alternatively, the phase wandering might be related to complexity in the structure
of the pulsar’s magnetic field, with different multipole components dominating the
accretion process at different accretion rates [199].
Timing noise may also be due to fluctuating accretion torques [176, 177]. This
has been studied for strongly magnetized accreting pulsars like Vela X-1 [74], and it
is natural to expect something similar in AMXPs. However as discussed, this model
requires unphysical large accretion rate fluctuations to explain all timing noise or the
large jumps in phase observed for example in XTE J1814-338 (and probably also
SAX J1808.4-3658 and XTE J1807-294, [123, 239, 258, 287, 352]). Nonetheless
some X-ray timing noise, or even its entirety for AMXPs with weak timing noise
content, could in principle be related to accretion torque fluctuations.
4.5 Pulse Profiles
AMXP pulse profiles form in regions of strong gravity and encode information
about the physical properties of the emitting regions and the compactness of the
NS [185, 186, 220, 276, 278]. Light bending, aberration and relativistic Doppler
shifts all affect pulse profile shape [42, 225, 275, 277] and can be measured using
coherent timing analysis. This can be used to constrain the EoS of ultra-dense matter
(Fig. 4.7) although large uncertainties still exist: in part due to model-dependencies,
but more importantly because of the low S/N of the relativistic features.
Pulse profile shapes can also constrain the geometry of the NS magnetic field
[34, 167, 365] since multipoles generate different accretion columns and hot spots on
the NS surface (see, however, [8], for a recent discussion of the problem). The information contained in pulse profiles can be extracted by studying pulse amplitudes,
the harmonic content of the signal and the time of arrival of the pulsations. Double
peaked pulse profiles, for example, may be observed when two antipodal hot-spots
exist on the NS surface [178, 225, 277]; a high harmonic content is suggestive of a
complex field geometry [198] and the energy dependence of the pulse amplitudes
and the time lags of the pulsations provide a way to explore the angular pattern of
the radiation emitted from the NS surface [111, 277, 278]. The temporal evolution of
the pulse shapes also reveals details of the complex disk/magnetospheric interaction
responsible for magnetic channeling of the accreting plasma [142, 171].
4.5.1 Pulse Fractional Amplitudes and Phase Lags
The pulse profiles of AMXPs are sinusoidal, with only the fundamental frequency
detected in most cases (including IGR J00291+5934, XTE J0929-314, XTE J1751305, SAX J1748.9-2021 and IGR J17498-2921). In some AMXPs there is a
first overtone (e.g. XTE J1814-338, IGR J18245-2452 and NGC 6440 X-2), and