4 Accreting Millisecond X-ray Pulsars
151
4.3 Observations of the AMXPs
In this section we discuss the main characteristics of each AMXP, including number
of outbursts observed, luminosity, distance, orbital parameters and (at the end of
each subsection) radio/IR/optical counterparts. We devote Sects. 4.4 and 4.6 to
the discussion of the results on the pulsar rotational parameters and their secular
evolution, respectively. We briefly mention results relating to X-ray bursts, burst
oscillations and aperiodic variability but refer to Sects. 4.7 and 4.8 for a general
discussion of these topics. The astrometric position of each AMXP is given in
Table 4.2.
4.3.1 SAX J1808.4-3658
The source SAX J1808.4-3658 was discovered with the X-ray satellite BeppoSAX
during an outburst in September 1996 [147]. The source showed via three type
I X-ray bursts that the compact object in the binary is a NS. The 2–28 keV
peak luminosity of 3 × 10 36 erg s −1 [149] is rather faint and below the average
peak luminosity reached by other LMXBs. X-ray pulsations were not detected
during this outburst, with poorly constraining upper limits of 20% on the pulsed
fraction. Coherent pulsations at 401 Hz were discovered instead in 1998, during the
second observed outburst [360], thanks to the better sensitivity of the Proportional
Counter Array (PCA) on RXTE. An orbital modulation of 2.01 h was detected using
Doppler delays in the coherent timing data [50] and it was suggested that the
companion of the pulsar is a heated brown dwarf with mass of ≈0.05 M [27]. No
thermonuclear X-ray bursts were observed during the 1998 outburst, but re-analysis
of the 1996 data provided marginal evidence for burst oscillations at the NS spin
frequency [149].
SAX J1808.4-3658 went into outburst again in 2000, 2002, 2005, 2008 and
2011, with an approximate recurrence time of about 1.6–3.3 yr, and is the best
sampled and studied of all AMXPs. All of the outbursts showed faint luminosities,
with peaks always below 10 37 erg s −1 , even when considering broad energy bands.
The 2000 outburst was poorly sampled due to solar constraints, and was observed
only during the “flaring tail”, a peculiar outburst phase displayed by only a very
small number of X-ray sources. The typical SAX J1808.4-3658 outburst can be
split in five phases: a fast rise, with a steep increase in luminosity lasting only a
few days, a peak, a slow decay stage, a fast decay phase and the flaring tail. The
first four phases are typical of several X-ray binaries and dwarf novae and can
in principle be partially explained with the disk instability model. The flaring tail
instead shows bumps, called “reflares”, with a quasi-oscillatory behaviour of a few
days and a variation in luminosity of up to three orders of magnitude on timescales
∼1–2 days [255, 358, 361]. The flaring tail has no clear explanation within the disk
instability model [181], but has been observed in all outbursts [98, 255]. Pulsations
151
4.3 Observations of the AMXPs
In this section we discuss the main characteristics of each AMXP, including number
of outbursts observed, luminosity, distance, orbital parameters and (at the end of
each subsection) radio/IR/optical counterparts. We devote Sects. 4.4 and 4.6 to
the discussion of the results on the pulsar rotational parameters and their secular
evolution, respectively. We briefly mention results relating to X-ray bursts, burst
oscillations and aperiodic variability but refer to Sects. 4.7 and 4.8 for a general
discussion of these topics. The astrometric position of each AMXP is given in
Table 4.2.
4.3.1 SAX J1808.4-3658
The source SAX J1808.4-3658 was discovered with the X-ray satellite BeppoSAX
during an outburst in September 1996 [147]. The source showed via three type
I X-ray bursts that the compact object in the binary is a NS. The 2–28 keV
peak luminosity of 3 × 10 36 erg s −1 [149] is rather faint and below the average
peak luminosity reached by other LMXBs. X-ray pulsations were not detected
during this outburst, with poorly constraining upper limits of 20% on the pulsed
fraction. Coherent pulsations at 401 Hz were discovered instead in 1998, during the
second observed outburst [360], thanks to the better sensitivity of the Proportional
Counter Array (PCA) on RXTE. An orbital modulation of 2.01 h was detected using
Doppler delays in the coherent timing data [50] and it was suggested that the
companion of the pulsar is a heated brown dwarf with mass of ≈0.05 M [27]. No
thermonuclear X-ray bursts were observed during the 1998 outburst, but re-analysis
of the 1996 data provided marginal evidence for burst oscillations at the NS spin
frequency [149].
SAX J1808.4-3658 went into outburst again in 2000, 2002, 2005, 2008 and
2011, with an approximate recurrence time of about 1.6–3.3 yr, and is the best
sampled and studied of all AMXPs. All of the outbursts showed faint luminosities,
with peaks always below 10 37 erg s −1 , even when considering broad energy bands.
The 2000 outburst was poorly sampled due to solar constraints, and was observed
only during the “flaring tail”, a peculiar outburst phase displayed by only a very
small number of X-ray sources. The typical SAX J1808.4-3658 outburst can be
split in five phases: a fast rise, with a steep increase in luminosity lasting only a
few days, a peak, a slow decay stage, a fast decay phase and the flaring tail. The
first four phases are typical of several X-ray binaries and dwarf novae and can
in principle be partially explained with the disk instability model. The flaring tail
instead shows bumps, called “reflares”, with a quasi-oscillatory behaviour of a few
days and a variation in luminosity of up to three orders of magnitude on timescales
∼1–2 days [255, 358, 361]. The flaring tail has no clear explanation within the disk
instability model [181], but has been observed in all outbursts [98, 255]. Pulsations