4 Accreting Millisecond X-ray Pulsars
145
relatively weak magnetic field (B∼10 8 G). Two independent papers published in
1982 [1, 281] proposed transfer of angular momentum through accretion as the
mechanism responsible for the spin-up of pulsars. This is known as the recycling
scenario (see [312] for an excellent review). This name originates from the fact
that radio pulsars switch off their pulsed radio emission after entering the so-called
“pulsar graveyard”. If this happens while the NS is in a binary with a non-collapsed
low or intermediate mass stellar companion, binary evolution of the system [23, 328]
can bring the companion into Roche lobe contact and trigger a prolonged epoch of
mass transfer from the companion (donor) towards the NS (accretor). The mass
is transferred with large specific angular momentum and the NS is spun-up by the
resulting accretion torques. Once the mass transfer episode terminates, the NS might
eventually switch on again as a “recycled” millisecond radio pulsar.
The first AMXP (SAX J1808.4-3658), found in 1998 with the Rossi X-ray Timing
Explorer (RXTE) [360] provided a beautiful confirmation of the recycling scenario.
Fourteen more AMXPs have since been found, with spin frequencies from 182 to
599 Hz. Another important milestone came with the discovery (in 2007) of a binary
radio millisecond pulsar (PSR J1023+0038) for which archival optical observations,
taken ∼7 years before the radio pulsar discovery, showed evidence for an accretion
disk 1 [9]. This is the first NS observed to have switched on as a radio pulsar
after being an X-ray binary and another system, XSS J12270-4859 [78, 79], has
been recently discovered to behave in a very similar way [16, 29, 297]. A final
confirmation that indeed AMXPs and radio pulsars are related has recently come
with the discovery of the system IGR J18245-2452 which has shown both an AMXP
and a radio millisecond pulsar phase (see [247] and Sect. 4.3.15 for a detailed
discussion).
The RXTE observatory has played an extraordinary role by discovering many
systems of this kind and by collecting extensive data records of each outburst
detected during its 15 year lifetime. The excellent timing capabilities of RXTE
have brought new means to study NSs with coherent X-ray timing, and helped to
constrain the long term properties of many AMXPs over a baseline of more than
a decade. Observation of the orbital Doppler shift of the AMXP pulse frequency
contains information on the orbital parameters of the binary and their evolution in
time. Binary evolution has benefited from the study of AMXPs [25, 76, 230] which
are now known to include ultra-compact systems (orbital period P b 80 min)
with white dwarf companions, compact systems (P b 1.5–3 h) with brown dwarf
donors and wider systems (P b 3.5–20 h) with main sequence stars. Other Xray and gamma ray space missions like XMM-Newton, INTEGRAL, Chandra, Swift
and HETE have also played an important role in discovering and understanding
the spectral and timing properties of these objects. Multiwavelength observations
covering radio, infrared, optical and UV wavelengths have also illuminated different
aspects of these fascinating systems. Several optical and infrared counterparts have
1 A new transition from a radio pulsar to a LMXB has happened and is currently ongoing at the
moment of writing this review [265, 313].
145
relatively weak magnetic field (B∼10 8 G). Two independent papers published in
1982 [1, 281] proposed transfer of angular momentum through accretion as the
mechanism responsible for the spin-up of pulsars. This is known as the recycling
scenario (see [312] for an excellent review). This name originates from the fact
that radio pulsars switch off their pulsed radio emission after entering the so-called
“pulsar graveyard”. If this happens while the NS is in a binary with a non-collapsed
low or intermediate mass stellar companion, binary evolution of the system [23, 328]
can bring the companion into Roche lobe contact and trigger a prolonged epoch of
mass transfer from the companion (donor) towards the NS (accretor). The mass
is transferred with large specific angular momentum and the NS is spun-up by the
resulting accretion torques. Once the mass transfer episode terminates, the NS might
eventually switch on again as a “recycled” millisecond radio pulsar.
The first AMXP (SAX J1808.4-3658), found in 1998 with the Rossi X-ray Timing
Explorer (RXTE) [360] provided a beautiful confirmation of the recycling scenario.
Fourteen more AMXPs have since been found, with spin frequencies from 182 to
599 Hz. Another important milestone came with the discovery (in 2007) of a binary
radio millisecond pulsar (PSR J1023+0038) for which archival optical observations,
taken ∼7 years before the radio pulsar discovery, showed evidence for an accretion
disk 1 [9]. This is the first NS observed to have switched on as a radio pulsar
after being an X-ray binary and another system, XSS J12270-4859 [78, 79], has
been recently discovered to behave in a very similar way [16, 29, 297]. A final
confirmation that indeed AMXPs and radio pulsars are related has recently come
with the discovery of the system IGR J18245-2452 which has shown both an AMXP
and a radio millisecond pulsar phase (see [247] and Sect. 4.3.15 for a detailed
discussion).
The RXTE observatory has played an extraordinary role by discovering many
systems of this kind and by collecting extensive data records of each outburst
detected during its 15 year lifetime. The excellent timing capabilities of RXTE
have brought new means to study NSs with coherent X-ray timing, and helped to
constrain the long term properties of many AMXPs over a baseline of more than
a decade. Observation of the orbital Doppler shift of the AMXP pulse frequency
contains information on the orbital parameters of the binary and their evolution in
time. Binary evolution has benefited from the study of AMXPs [25, 76, 230] which
are now known to include ultra-compact systems (orbital period P b 80 min)
with white dwarf companions, compact systems (P b 1.5–3 h) with brown dwarf
donors and wider systems (P b 3.5–20 h) with main sequence stars. Other Xray and gamma ray space missions like XMM-Newton, INTEGRAL, Chandra, Swift
and HETE have also played an important role in discovering and understanding
the spectral and timing properties of these objects. Multiwavelength observations
covering radio, infrared, optical and UV wavelengths have also illuminated different
aspects of these fascinating systems. Several optical and infrared counterparts have
1 A new transition from a radio pulsar to a LMXB has happened and is currently ongoing at the
moment of writing this review [265, 313].