4 Accreting Millisecond X-ray Pulsars
167
identified as the first eclipsing AMXP [201] and the inclination of the system was
constrained to be i ∼ 74–78 ◦ [6, 201]. The binary showed three eclipses with a
duration of 2172 ± 13 s, allowing the first attempted detection of Shapiro delay
effects in X-ray timing data for an object outside the Solar System [201].
To fit the 0.5–40 keV spectrum, an absorption column density of N H = 3.0 ×
10 22 cm −2 and a power-law with spectral index Γ 1.7 are required [91]. The
absorption column is about 3 times larger than the expected Galactic column density
in the direction of the source. The X-ray light-curve showed an exponential decay
with a Swift/XRT non detection after 11 days since its first 2010 observation. Due
to the crowded field (the source is on the Galactic plane), no single counterpart has
been identified in NIR counterpart searches [73]. More than forty counterparts were
identified in the X-ray error circle with ESO’s Very Large Telescope (VLT).
4.3.14 IGR J17498-2921
IGR J17498-2921 was discovered by INTEGRAL on 2011 August 11 [109]. Soon
after, coherent X-ray pulsations were discovered in RXTE/PCA data at 401 Hz with
an orbital period modulation of 3.8 h [244]. Type I X-ray bursts were observed
with INTEGRAL [92] and RXTE. Burst oscillations were detected [49, 194] and
evidence of PRE placed the source at a distance of 7.6 kpc [194]. Swift/XRT
observed the source returning to quiescence on 2011 September 19 [195]. A
candidate NIR counterpart of IGR J17498-2921 was first detected in archival data
taken with the 4-m VISTA telescope at the Paranal observatory [120]. The optical
counterpart was detected on 2011 August 25 and 26 with the 2-m Faulkes Telescope
South [304]. The position detected was consistent with both the Chandra and the
NIR counterparts. However, further observations suggested that the NIR/optical
counterpart is a foreground star aligned by chance with the X-ray source [339]. This
counterpart was confirmed to be a foreground source with observations taken at the
2.5-m Irenee du Pont telescope. The analysis suggested that, given the distance and
extinction of IGR J17498-2921, the expected magnitudes are R ∼ 28 and J ∼ 23.3,
both well above the limit reached by the optical/NIR observations [336].
4.3.15 IGR J18245-2452
The most recent addition to the AMXP family was discovered by INTEGRAL
on 2013 March 28 [83] during an X-ray outburst (20–100 keV luminosity of
3 × 10 36 erg s −1 ) and is perhaps the most spectacular of all the AMXPs. The source,
at a distance of 5.5. kpc is located in the core of the globular cluster M28 [132].
XMM-Newton observations revealed an AMXP whose spin (254 Hz) and orbital
parameters were identical to those of a previously known radio millisecond pulsar
located in the same region (PSR J1824-2452I, a.k.a. M28I) [247]. This makes this
source the first known radio millisecond pulsar that has switched to an AMXP
state. The 0.5–10 keV luminosity shows a very peculiar X-ray flickering with flux
167
identified as the first eclipsing AMXP [201] and the inclination of the system was
constrained to be i ∼ 74–78 ◦ [6, 201]. The binary showed three eclipses with a
duration of 2172 ± 13 s, allowing the first attempted detection of Shapiro delay
effects in X-ray timing data for an object outside the Solar System [201].
To fit the 0.5–40 keV spectrum, an absorption column density of N H = 3.0 ×
10 22 cm −2 and a power-law with spectral index Γ 1.7 are required [91]. The
absorption column is about 3 times larger than the expected Galactic column density
in the direction of the source. The X-ray light-curve showed an exponential decay
with a Swift/XRT non detection after 11 days since its first 2010 observation. Due
to the crowded field (the source is on the Galactic plane), no single counterpart has
been identified in NIR counterpart searches [73]. More than forty counterparts were
identified in the X-ray error circle with ESO’s Very Large Telescope (VLT).
4.3.14 IGR J17498-2921
IGR J17498-2921 was discovered by INTEGRAL on 2011 August 11 [109]. Soon
after, coherent X-ray pulsations were discovered in RXTE/PCA data at 401 Hz with
an orbital period modulation of 3.8 h [244]. Type I X-ray bursts were observed
with INTEGRAL [92] and RXTE. Burst oscillations were detected [49, 194] and
evidence of PRE placed the source at a distance of 7.6 kpc [194]. Swift/XRT
observed the source returning to quiescence on 2011 September 19 [195]. A
candidate NIR counterpart of IGR J17498-2921 was first detected in archival data
taken with the 4-m VISTA telescope at the Paranal observatory [120]. The optical
counterpart was detected on 2011 August 25 and 26 with the 2-m Faulkes Telescope
South [304]. The position detected was consistent with both the Chandra and the
NIR counterparts. However, further observations suggested that the NIR/optical
counterpart is a foreground star aligned by chance with the X-ray source [339]. This
counterpart was confirmed to be a foreground source with observations taken at the
2.5-m Irenee du Pont telescope. The analysis suggested that, given the distance and
extinction of IGR J17498-2921, the expected magnitudes are R ∼ 28 and J ∼ 23.3,
both well above the limit reached by the optical/NIR observations [336].
4.3.15 IGR J18245-2452
The most recent addition to the AMXP family was discovered by INTEGRAL
on 2013 March 28 [83] during an X-ray outburst (20–100 keV luminosity of
3 × 10 36 erg s −1 ) and is perhaps the most spectacular of all the AMXPs. The source,
at a distance of 5.5. kpc is located in the core of the globular cluster M28 [132].
XMM-Newton observations revealed an AMXP whose spin (254 Hz) and orbital
parameters were identical to those of a previously known radio millisecond pulsar
located in the same region (PSR J1824-2452I, a.k.a. M28I) [247]. This makes this
source the first known radio millisecond pulsar that has switched to an AMXP
state. The 0.5–10 keV luminosity shows a very peculiar X-ray flickering with flux