160
A. Patruno and A. L. Watts
4.3.6 IGR J00291+5934
IGR J00291+5934 is the fastest AMXP known, with a spin of 599 Hz. The source
was discovered by INTEGRAL on 2004 December 2 [82] and prolonged RXTE
observations lead to the discovery of pulsations [206] and an orbital period of 2.46 h,
with a minimum donor mass of about 0.04 M [207]. The 2004 outburst had a
duration of ≈14 days from the INTEGRAL discovery, with a smooth flux decay. On
2008 August 13 the source started a new outburst [52] and was observed for about
a week before reaching an X-ray flux level below the sensitivity limit of RXTE. The
peak flux observed was about half the value reached in 2004. Follow up Swift/XRT
observations on August 21 revealed a flux level below 10 −11 − 10 −12 erg s −1
and an XMM-Newton observation on August 25 provided a 2–10 keV flux of
(1.4 ± 0.3) × 10 −14 erg s −1 [189], compatible with the 10 −13 erg s −1 (0.5–10 keV)
quiescent flux level observed two months after the end of the 2004 outburst [155].
On 2008 September 21, the source re-brightened again, showing another outburst
lasting for about 14 days. The fluence of the second and third outbursts were very
similar and it is unclear whether the two 2008 outbursts were distinct [126] or part
of the same outburst. An analysis of archival RXTE/ASM data revealed possible
outbursts in 1998 November and 2001 September, suggesting a recurrence time of
3–4 yr [284].
The pulsar has shown no thermonuclear bursts despite very similar orbital
parameters to SAX J1808.4-3658. The donor star of IGR J00291+5934 is also
probably a X-ray heated brown dwarf as in SAX J1808.4-3658 [100]. Broadband
spectral observations performed during the 2004 outburst with INTEGRAL and
RXTE were well fitted with a thermal Comptonization model with an electron
temperature of 50 keV and Thomson optical depth τ T ∼ 1 in a slab geometry [88].
The RXTE data analysis of the first 12 days of the outburst revealed a power law
plus blackbody (with kT ∼1 keV) interpreted as emission from the hot spot on
the NS surface. Further simultaneous RXTE and Chandra/HETGS (High Energy
Transmission Grating Spectrometer) spectral observations collected towards the
end of the 2004 outburst revealed a power law plus a cold black body at 0.42
keV, interpreted as the cold accretion disk, with no signature of the hot spot
blackbody [236]. This is consistent with the lack of detected pulsations towards
the end of the outburst [100].
A tentative optical counterpart was identified during the 2004 outburst [95], with
an R magnitude of 17.4. Follow up spectral measurements with the 4.2-m William
Herschel Telescope on La Palma strengthened the association with the identification
of a weak HeII and H α line [292]. An optical/NIR photometric study in quiescence
was then performed in 2005 with the 3.6-m Telescopio Nazionale Galileo (TNG)
[71]. The VRIJH counterparts of IGR J00291+5934 were detected, with a strong
upper limit in the K-band. The optical light curve shows variability consistent with
the 2.46 h orbital period. The radio to X-ray Spectral Energy Distributions (SEDs)
revealed a blue component indicative of an irradiated disc [189]. The SED contained
also a transient NIR excess similar to that found in SAX J1808.4-3658, XTE J0929-
A. Patruno and A. L. Watts
4.3.6 IGR J00291+5934
IGR J00291+5934 is the fastest AMXP known, with a spin of 599 Hz. The source
was discovered by INTEGRAL on 2004 December 2 [82] and prolonged RXTE
observations lead to the discovery of pulsations [206] and an orbital period of 2.46 h,
with a minimum donor mass of about 0.04 M [207]. The 2004 outburst had a
duration of ≈14 days from the INTEGRAL discovery, with a smooth flux decay. On
2008 August 13 the source started a new outburst [52] and was observed for about
a week before reaching an X-ray flux level below the sensitivity limit of RXTE. The
peak flux observed was about half the value reached in 2004. Follow up Swift/XRT
observations on August 21 revealed a flux level below 10 −11 − 10 −12 erg s −1
and an XMM-Newton observation on August 25 provided a 2–10 keV flux of
(1.4 ± 0.3) × 10 −14 erg s −1 [189], compatible with the 10 −13 erg s −1 (0.5–10 keV)
quiescent flux level observed two months after the end of the 2004 outburst [155].
On 2008 September 21, the source re-brightened again, showing another outburst
lasting for about 14 days. The fluence of the second and third outbursts were very
similar and it is unclear whether the two 2008 outbursts were distinct [126] or part
of the same outburst. An analysis of archival RXTE/ASM data revealed possible
outbursts in 1998 November and 2001 September, suggesting a recurrence time of
3–4 yr [284].
The pulsar has shown no thermonuclear bursts despite very similar orbital
parameters to SAX J1808.4-3658. The donor star of IGR J00291+5934 is also
probably a X-ray heated brown dwarf as in SAX J1808.4-3658 [100]. Broadband
spectral observations performed during the 2004 outburst with INTEGRAL and
RXTE were well fitted with a thermal Comptonization model with an electron
temperature of 50 keV and Thomson optical depth τ T ∼ 1 in a slab geometry [88].
The RXTE data analysis of the first 12 days of the outburst revealed a power law
plus blackbody (with kT ∼1 keV) interpreted as emission from the hot spot on
the NS surface. Further simultaneous RXTE and Chandra/HETGS (High Energy
Transmission Grating Spectrometer) spectral observations collected towards the
end of the 2004 outburst revealed a power law plus a cold black body at 0.42
keV, interpreted as the cold accretion disk, with no signature of the hot spot
blackbody [236]. This is consistent with the lack of detected pulsations towards
the end of the outburst [100].
A tentative optical counterpart was identified during the 2004 outburst [95], with
an R magnitude of 17.4. Follow up spectral measurements with the 4.2-m William
Herschel Telescope on La Palma strengthened the association with the identification
of a weak HeII and H α line [292]. An optical/NIR photometric study in quiescence
was then performed in 2005 with the 3.6-m Telescopio Nazionale Galileo (TNG)
[71]. The VRIJH counterparts of IGR J00291+5934 were detected, with a strong
upper limit in the K-band. The optical light curve shows variability consistent with
the 2.46 h orbital period. The radio to X-ray Spectral Energy Distributions (SEDs)
revealed a blue component indicative of an irradiated disc [189]. The SED contained
also a transient NIR excess similar to that found in SAX J1808.4-3658, XTE J0929-