4 Accreting Millisecond X-ray Pulsars
161
314 and XTE J1814-338. An H α line was observed in the optical spectrum of the
2004 and 2008 outbursts. Evidence was also found for an I band excess during
quiescence in an observation performed with the 4.2-m William Herschel Telescope
(WHT) on 2006 September 13 and 14 [155].
Radio observations taken on 2004 December 4 with the Ryle Telescope in
Cambridge at 15 GHz resulted in a detection at 1.1 mJy [271]. Follow up
observations taken one day later gave a non detection, with upper limits of 0.6 mJy
at 15 GHz [90]. The Westerbork Synthesis Radio Telescope (WSRT) observed IGR
J00291+5934 between 2004 December 6 and 7, and a radio counterpart was seen at
5 GHz with a flux of 0.250 ± 0.035 mJy [90]. Very Large Array (VLA) observations
performed on 2004 December 9 at 4.86 GHz confirmed the reported radio detection,
with a flux of 0.17 ± 0.05 mJy. The source was not detected in radio during the 2008
outburst, with upper limits of 0.16 mJy at 5 GHz in an observation taken between
August 15 and 16 [192].
4.3.7 HETE J1900.1-2455
The High Energy Transient Explorer (HETE-2) discovered HETE J1900.1-2455
on 2005 June 14 via an X-ray burst [342]. Follow up observations taken on June
16 by RXTE/PCA showed pulsations at 377 Hz [219]. A source distance ∼4–
5 kpc (assuming a typical NS mass of 1.4 M and a He rich NS atmosphere) was
determined using the first type I X-ray burst detected with HETE-2 [161, 325],
which showed PRE. The orbital period was determined to be 1.4 h with a minimum
companion mass of 0.016 M [158]. Unlike the other AMXPs with brown dwarf
companions discussed so far, the donor star in HETE J1900.1-2455 is most likely a
brown dwarf which does not require X-ray irradiation or a non-standard evolution
to fill its Roche lobe.
A single 882.8 Hz QPO was detected in the RXTE lightcurve during a bright flare
occurred on MJD 53,559.45 [158]. Simultaneous RXTE and INTEGRAL observations showed that the 2–300 keV spectrum is well fit by a blackbody component
peaking at 0.8 keV plus thermal Comptonization with electron temperature of 30
keV and optical depth τ T ∼2 for a slab geometry [86]. The source was detected
up to energies of 200 keV and the 0.1–200 keV luminosity was inferred to be
5 × 10 36 erg s −1 . Although the peak luminosity is not particularly high compared
to other AMXPs, the average mass accretion rate ˙
M of HETE J1900.1-2455 is
among the highest observed due to its prolonged X-ray outburst that has lasted for
8 years (and is still active as of November 2013) [97, 102]. This makes HETE
J1900.1-2455 a so called “quasi-persistent source”, i.e., an X-ray binary with a
prolonged outburst duration of several years. Despite the long baseline provided
by the outburst, pulsations were continuously detected only in the first 2 months
of the observations although sporadic pulse detections occurred over a baseline of
about 2.5 years (Sect. 4.2.1).
161
314 and XTE J1814-338. An H α line was observed in the optical spectrum of the
2004 and 2008 outbursts. Evidence was also found for an I band excess during
quiescence in an observation performed with the 4.2-m William Herschel Telescope
(WHT) on 2006 September 13 and 14 [155].
Radio observations taken on 2004 December 4 with the Ryle Telescope in
Cambridge at 15 GHz resulted in a detection at 1.1 mJy [271]. Follow up
observations taken one day later gave a non detection, with upper limits of 0.6 mJy
at 15 GHz [90]. The Westerbork Synthesis Radio Telescope (WSRT) observed IGR
J00291+5934 between 2004 December 6 and 7, and a radio counterpart was seen at
5 GHz with a flux of 0.250 ± 0.035 mJy [90]. Very Large Array (VLA) observations
performed on 2004 December 9 at 4.86 GHz confirmed the reported radio detection,
with a flux of 0.17 ± 0.05 mJy. The source was not detected in radio during the 2008
outburst, with upper limits of 0.16 mJy at 5 GHz in an observation taken between
August 15 and 16 [192].
4.3.7 HETE J1900.1-2455
The High Energy Transient Explorer (HETE-2) discovered HETE J1900.1-2455
on 2005 June 14 via an X-ray burst [342]. Follow up observations taken on June
16 by RXTE/PCA showed pulsations at 377 Hz [219]. A source distance ∼4–
5 kpc (assuming a typical NS mass of 1.4 M and a He rich NS atmosphere) was
determined using the first type I X-ray burst detected with HETE-2 [161, 325],
which showed PRE. The orbital period was determined to be 1.4 h with a minimum
companion mass of 0.016 M [158]. Unlike the other AMXPs with brown dwarf
companions discussed so far, the donor star in HETE J1900.1-2455 is most likely a
brown dwarf which does not require X-ray irradiation or a non-standard evolution
to fill its Roche lobe.
A single 882.8 Hz QPO was detected in the RXTE lightcurve during a bright flare
occurred on MJD 53,559.45 [158]. Simultaneous RXTE and INTEGRAL observations showed that the 2–300 keV spectrum is well fit by a blackbody component
peaking at 0.8 keV plus thermal Comptonization with electron temperature of 30
keV and optical depth τ T ∼2 for a slab geometry [86]. The source was detected
up to energies of 200 keV and the 0.1–200 keV luminosity was inferred to be
5 × 10 36 erg s −1 . Although the peak luminosity is not particularly high compared
to other AMXPs, the average mass accretion rate ˙
M of HETE J1900.1-2455 is
among the highest observed due to its prolonged X-ray outburst that has lasted for
8 years (and is still active as of November 2013) [97, 102]. This makes HETE
J1900.1-2455 a so called “quasi-persistent source”, i.e., an X-ray binary with a
prolonged outburst duration of several years. Despite the long baseline provided
by the outburst, pulsations were continuously detected only in the first 2 months
of the observations although sporadic pulse detections occurred over a baseline of
about 2.5 years (Sect. 4.2.1).