4 Accreting Millisecond X-ray Pulsars
159
the exception of the quasi-persistent intermittent AMXP HETE J1900.1-2455, XTE
J1807-294 had the longest AMXP outburst observed so far: ≈120 days, of which
≈109 days had sufficiently high flux to allow the detection of pulsations. Twin kHz
QPOs were detected in several observations and the separation Δν of the kHz QPOs
is consistent with the spin frequency of the AMXP [191]. No counterparts have
been reported at any wavelength during either outburst or quiescence. A candidate
optical counterpart detection was reported in 2009 [72] with V ∼22.1 and R∼21.4
along with a tentative detection in the J band. The counterpart falls within the 0 .6
Chandra (90% confidence level) error circle. No variability has been observed, with
upper limits of 0.1 mag.
4.3.5 XTE J1814-338
The fifth AMXP was found during routine Galactic center observations by the
RXTE/PCA on June 5, 2003 [202]. The pulse frequency is 314 Hz and the orbital
period 4.3 h, giving a minimum companion mass of 0.17 M . The outburst lasted
for ≈50 days and pulsations were detectable for ≈45 days. The source showed 28
Type I X-ray bursts and RXTE detected burst oscillations at the spin frequency in
all bursts. The 28th burst showed signs of PRE, allowing a determination of the
source distance of ≈8 kpc. The X-ray position of XTE J1814-338 is consistent with
the EXOSAT source EXMS B1810-337, detected in September 1984 [359]. If this
identification is correct, then XTE J1814-338 has a recurrence time of ∼20 yr.
The optical counterpart was first tentatively identified using BVRI data taken
with the Magellan 6.5 m telescope [168]. An observation of H and He emission
lines, with a double-peaked H α line typical of interacting binaries, was reported
soon after the discovery of the binary [314]. The counterpart was studied in more
detail and finally unambiguously identified [169]. The B, V and R counterparts
during the 2003 outburst had magnitudes between 18 and 19 mag, whereas the
I band was observed with an excess flux reaching about 17.4 mag. This excess
is again similar to that reported for SAX J1808.4-3658 and XTE J0929-314 and
suggests the presence of jets and/or outflows. A faint object with V ∼23.3 and
R∼22.5 was identified in ESO Very Large Telescope (VLT) archival data taken in
May 20–21, 2004 [72]. A multiband Very Large Telescope (VLT) campaign carried
in 2009 (during quiescence) shows an irradiated companion star that requires an
energy source compatible with the spin-down luminosity of a millisecond pulsar
[13]. This proves further evidence that AMXPs might turn on as radio pulsars when
in quiescence.
159
the exception of the quasi-persistent intermittent AMXP HETE J1900.1-2455, XTE
J1807-294 had the longest AMXP outburst observed so far: ≈120 days, of which
≈109 days had sufficiently high flux to allow the detection of pulsations. Twin kHz
QPOs were detected in several observations and the separation Δν of the kHz QPOs
is consistent with the spin frequency of the AMXP [191]. No counterparts have
been reported at any wavelength during either outburst or quiescence. A candidate
optical counterpart detection was reported in 2009 [72] with V ∼22.1 and R∼21.4
along with a tentative detection in the J band. The counterpart falls within the 0 .6
Chandra (90% confidence level) error circle. No variability has been observed, with
upper limits of 0.1 mag.
4.3.5 XTE J1814-338
The fifth AMXP was found during routine Galactic center observations by the
RXTE/PCA on June 5, 2003 [202]. The pulse frequency is 314 Hz and the orbital
period 4.3 h, giving a minimum companion mass of 0.17 M . The outburst lasted
for ≈50 days and pulsations were detectable for ≈45 days. The source showed 28
Type I X-ray bursts and RXTE detected burst oscillations at the spin frequency in
all bursts. The 28th burst showed signs of PRE, allowing a determination of the
source distance of ≈8 kpc. The X-ray position of XTE J1814-338 is consistent with
the EXOSAT source EXMS B1810-337, detected in September 1984 [359]. If this
identification is correct, then XTE J1814-338 has a recurrence time of ∼20 yr.
The optical counterpart was first tentatively identified using BVRI data taken
with the Magellan 6.5 m telescope [168]. An observation of H and He emission
lines, with a double-peaked H α line typical of interacting binaries, was reported
soon after the discovery of the binary [314]. The counterpart was studied in more
detail and finally unambiguously identified [169]. The B, V and R counterparts
during the 2003 outburst had magnitudes between 18 and 19 mag, whereas the
I band was observed with an excess flux reaching about 17.4 mag. This excess
is again similar to that reported for SAX J1808.4-3658 and XTE J0929-314 and
suggests the presence of jets and/or outflows. A faint object with V ∼23.3 and
R∼22.5 was identified in ESO Very Large Telescope (VLT) archival data taken in
May 20–21, 2004 [72]. A multiband Very Large Telescope (VLT) campaign carried
in 2009 (during quiescence) shows an irradiated companion star that requires an
energy source compatible with the spin-down luminosity of a millisecond pulsar
[13]. This proves further evidence that AMXPs might turn on as radio pulsars when
in quiescence.