4 Accreting Millisecond X-ray Pulsars
165
4.3.11 NGC 6440 X-2
On 2009 June 28, the X-ray transient NGC 6440 X-2 was discovered [129] in
a Chandra observation of the globular cluster NGC 6440 (the same cluster that
harbors the AMXP SAX J1748.9-2021). Coherent pulsations at 206 Hz were
detected in a subsequent outburst in RXTE/PCA observations on 2009 August
30 [3]. The orbital period is 0.95 h, making this the fifth ultra-compact AMXP
discovered. One of the most striking features of this pulsar is the very low long-term
average mass accretion rate ˙
M < 2 × 10 −12 M yr −1 , which could be determined
thanks to the well-defined distance to the globular cluster [131]. Peak flux is one
of the faintest observed from the AMXPs (L 2−10 keV < 1.5 × 10 36 erg s −1 ) and the
outburst length is extremely short, with an duration of ≈3–5 days. It should be noted
that the RXTE/PCA Galactic Bulge Scan flux limit corresponds to a luminosity
∼10 35 erg s −1 [3] so that the presence of a longer and very faint outburst cannot
be ruled out, although archival Chandra observations did not reveal any counterpart
in quiescence with an upper limit on the X-ray luminosity of L X 10 31 erg s −1 .
This raises the question of how many faint sources of this kind might have been
missed, particularly important given that AMXPs seem to be associated with faint
LMXBs, and systems of this type may therefore contain undiscovered AMXPs.
The recurrence time of the outburst is also the shortest among AMXPs, ∼1
month. An RXTE/PCA and Swift/XRT monitoring campaign revealed the occurrence of 10 outbursts between 2009 and 2011 [131, 252]). In November 2009,
visibility constraints and a new outburst of the other AMXP SAX J1748.9-2021
prevented further observations of NGC 6440 X-2. A strong (∼50% rms) 1 Hz
modulation was observed in the light-curve of at least 6 outbursts [252], which
strongly resembled the 1 Hz modulation seen in SAX J1808.4-3658 [255, 341].
Despite the precise astrometric X-ray position of the source, the optical counterpart of NGC 6440 X-2 has not yet been identified, with B > 22 and V > 21
from archival Hubble Space Telescope (HST) imaging of the globular cluster when
NGC 6440 X-2 was in quiescence. During the 2009 August outburst, Gemini-South
observations did not reveal any counterpart with g > 22. Observations carried
with the CTIO 4-m telescope during the 2009 July outburst also did not reveal any
counterpart with J > 18.5 and K > 17 [131].
4.3.12 IGR J17511-3057
The X-ray binary IGR J17511-3057 was discovered during Galactic Bulge Monitoring by the INTEGRAL satellite in 2009 September 12 [14]. Coherent pulsations
at 245 Hz and an orbital modulation of 3.5 h were measured with follow up
observations by the RXTE/PCA [211]. Given the X-ray mass function, the minimum
donor star mass is 0.13 M . The X-ray light-curve was also extensively monitored
by Swift/XRT, XMM-Newton, RXTE/PCA and Chandra. The light-curve showed a
165
4.3.11 NGC 6440 X-2
On 2009 June 28, the X-ray transient NGC 6440 X-2 was discovered [129] in
a Chandra observation of the globular cluster NGC 6440 (the same cluster that
harbors the AMXP SAX J1748.9-2021). Coherent pulsations at 206 Hz were
detected in a subsequent outburst in RXTE/PCA observations on 2009 August
30 [3]. The orbital period is 0.95 h, making this the fifth ultra-compact AMXP
discovered. One of the most striking features of this pulsar is the very low long-term
average mass accretion rate ˙
M < 2 × 10 −12 M yr −1 , which could be determined
thanks to the well-defined distance to the globular cluster [131]. Peak flux is one
of the faintest observed from the AMXPs (L 2−10 keV < 1.5 × 10 36 erg s −1 ) and the
outburst length is extremely short, with an duration of ≈3–5 days. It should be noted
that the RXTE/PCA Galactic Bulge Scan flux limit corresponds to a luminosity
∼10 35 erg s −1 [3] so that the presence of a longer and very faint outburst cannot
be ruled out, although archival Chandra observations did not reveal any counterpart
in quiescence with an upper limit on the X-ray luminosity of L X 10 31 erg s −1 .
This raises the question of how many faint sources of this kind might have been
missed, particularly important given that AMXPs seem to be associated with faint
LMXBs, and systems of this type may therefore contain undiscovered AMXPs.
The recurrence time of the outburst is also the shortest among AMXPs, ∼1
month. An RXTE/PCA and Swift/XRT monitoring campaign revealed the occurrence of 10 outbursts between 2009 and 2011 [131, 252]). In November 2009,
visibility constraints and a new outburst of the other AMXP SAX J1748.9-2021
prevented further observations of NGC 6440 X-2. A strong (∼50% rms) 1 Hz
modulation was observed in the light-curve of at least 6 outbursts [252], which
strongly resembled the 1 Hz modulation seen in SAX J1808.4-3658 [255, 341].
Despite the precise astrometric X-ray position of the source, the optical counterpart of NGC 6440 X-2 has not yet been identified, with B > 22 and V > 21
from archival Hubble Space Telescope (HST) imaging of the globular cluster when
NGC 6440 X-2 was in quiescence. During the 2009 August outburst, Gemini-South
observations did not reveal any counterpart with g > 22. Observations carried
with the CTIO 4-m telescope during the 2009 July outburst also did not reveal any
counterpart with J > 18.5 and K > 17 [131].
4.3.12 IGR J17511-3057
The X-ray binary IGR J17511-3057 was discovered during Galactic Bulge Monitoring by the INTEGRAL satellite in 2009 September 12 [14]. Coherent pulsations
at 245 Hz and an orbital modulation of 3.5 h were measured with follow up
observations by the RXTE/PCA [211]. Given the X-ray mass function, the minimum
donor star mass is 0.13 M . The X-ray light-curve was also extensively monitored
by Swift/XRT, XMM-Newton, RXTE/PCA and Chandra. The light-curve showed a