4 Accreting Millisecond X-ray Pulsars
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quiescence, although the upper limits are unconstraining due to the poor photon
flux. The shortest outburst recurrence time is one month for the globular cluster
source NGC 6440 X-2, with an outburst duration of less than 4–5 days, whereas the
longest outburst, from HETE J1900.1-2455, has so far lasted for almost 7 years (see
Sect. 4.3). The small group of AMXPs therefore form a rather heterogeneous class
of objects. There are, however, some common features shared by all AMXPs:
• Outburst luminosities are usually faint, suggesting low time averaged mass
accretion rates (Sect. 4.3).
• Spin frequencies appear to be uniformly distributed with an abrupt cutoff
at about ∼700 Hz (Sect. 4.6).
• Ultra-compact binaries are rather common, comprising about 40% of the
total AMXP population.
• Very small donors are preferred, with masses almost always below 0.2 M .
• The orbital periods are always relatively short, with P b < 1 day. Therefore
the known AMXPs are probably not the progenitors of wide orbit binary
millisecond radio pulsars.
4.2.1 Intermittency
Until 2007 it was believed that AMXPs showed X-ray pulsations throughout outbursts. The seventh AMXP (HETE J1900.1-2455), however, showed an unexpected
new behaviour [101]. For the first ≈20 days of its outburst (which started in
2005) it showed typical AMXP pulsations. The pulsations then became intermittent, appearing and disappearing on different timescales for the next ≈2.5 years.
Pulsation disappeared and never reappeared again after MJD 54,499 [250], with the
most stringent upper limits of 0.05% rms on the fractional pulse amplitude (i.e., a
sinusoidal fractional amplitude of 0.07%, see Eq. (4.15)).
This discovery was exciting, since it may help to bridge the gap between nonpulsating LMXBs and AMXPs. It also became immediately clear that other LMXBs
might show sporadic episodes of pulsations during their outbursts. This has now
been found to be the case for two other sources: Aql X-1 and SAX J1748.9-2021.
Aql X-1 is perhaps the most striking case recorded so far: coherent pulsations were
discovered in 1998 RXTE archival data and appeared in only one 120s data segment
out of a total exposure time of 1.5 Ms from more than 10 years of observations.
The extremely short pulse episode has raised discussions about whether it really
originated from magnetic channeled accretion, and whether Aql X-1 can truly be
considered an AMXP. However, the high coherence of the signal leaves little doubt
149
quiescence, although the upper limits are unconstraining due to the poor photon
flux. The shortest outburst recurrence time is one month for the globular cluster
source NGC 6440 X-2, with an outburst duration of less than 4–5 days, whereas the
longest outburst, from HETE J1900.1-2455, has so far lasted for almost 7 years (see
Sect. 4.3). The small group of AMXPs therefore form a rather heterogeneous class
of objects. There are, however, some common features shared by all AMXPs:
• Outburst luminosities are usually faint, suggesting low time averaged mass
accretion rates (Sect. 4.3).
• Spin frequencies appear to be uniformly distributed with an abrupt cutoff
at about ∼700 Hz (Sect. 4.6).
• Ultra-compact binaries are rather common, comprising about 40% of the
total AMXP population.
• Very small donors are preferred, with masses almost always below 0.2 M .
• The orbital periods are always relatively short, with P b < 1 day. Therefore
the known AMXPs are probably not the progenitors of wide orbit binary
millisecond radio pulsars.
4.2.1 Intermittency
Until 2007 it was believed that AMXPs showed X-ray pulsations throughout outbursts. The seventh AMXP (HETE J1900.1-2455), however, showed an unexpected
new behaviour [101]. For the first ≈20 days of its outburst (which started in
2005) it showed typical AMXP pulsations. The pulsations then became intermittent, appearing and disappearing on different timescales for the next ≈2.5 years.
Pulsation disappeared and never reappeared again after MJD 54,499 [250], with the
most stringent upper limits of 0.05% rms on the fractional pulse amplitude (i.e., a
sinusoidal fractional amplitude of 0.07%, see Eq. (4.15)).
This discovery was exciting, since it may help to bridge the gap between nonpulsating LMXBs and AMXPs. It also became immediately clear that other LMXBs
might show sporadic episodes of pulsations during their outbursts. This has now
been found to be the case for two other sources: Aql X-1 and SAX J1748.9-2021.
Aql X-1 is perhaps the most striking case recorded so far: coherent pulsations were
discovered in 1998 RXTE archival data and appeared in only one 120s data segment
out of a total exposure time of 1.5 Ms from more than 10 years of observations.
The extremely short pulse episode has raised discussions about whether it really
originated from magnetic channeled accretion, and whether Aql X-1 can truly be
considered an AMXP. However, the high coherence of the signal leaves little doubt