166
A. Patruno and A. L. Watts
typical fast rise with a slow decay lasting for about 20 days, before entering a fast
decay phase [4, 243]. Curiously, while the source was in the fast decay phase, a very
short and faint outburst of the ultra-compact AMXP XTE J1751-305 appeared in the
same field of view (FoV) of RXTE. The mini-outburst of XTE J1751-305 lasted less
than 3 days, and pulsations were clearly visible at 435 Hz [210]. Thermonuclear
bursts were observed in IGR J17511-3057 with Swift [30], and burst oscillations
were detected in RXTE data [4, 353]. None of the bursts showed PRE, so there
are several different distance determinations in the literature. The most constraining
gives an upper limit on distance of 5 kpc [4].
Swift spectral analysis revealed a power law (Γ ∼ 2) plus a blackbody at 0.9
keV [30]. Broadband spectral data from simultaneous RXTE/PCA and Swift/XRT
observations could be well fitted with a three model component [143]: a disk with
low temperature (kT ∼ 0.24 keV), a hot black body (hot spot with kT ∼ 1 keV)
and a Comptonization component originating from the accretion shock (electron
temperature T e ∼ 30 keV; τ T ∼ 2). The spectral fitting also required a fluorescent
iron line at 6.4 keV with Compton reflection and provided an interstellar absorption
column of N H = 0.88
+0.21
−0.24 × 10 22 cm −2 . Very similar results were obtained
using simultaneous XMM-Newton and RXTE/PCA spectral data [243]. Combined
RXTE/PCA, Swift/XRT and INTEGRAL data also gave consistent results using
a thermal Comptonization model [89]. Twin kHz QPOs were observed at the
beginning and at the end of the outburst, and had Δν 120 Hz, about half the value
of the spin frequency [160]. A NIR counterpart with brightness K s = 18.0 ± 0.1
was identified with the 6.5 m Magellan Baade telescope on 2009 September 22
[335]. A second observation on October 7, while the X-ray flux was fading rapidly,
showed no counterpart with 3σ upper limits of K s > 18.8. No radio counterpart has
been found, with upper limits of 0.10 mJy [215].
4.3.13 Swift J1749.4-2807
The first observation of Swift J1749.4-2807 was made on June 2, 2006 with
the Swift/BAT telescope, which recorded a high level activity consistent with an
unidentified source [305]. This was later identified as a “burst-only” accreting NS
binary (i.e., a NS that is bright enough to be detectable only during the occurrence of
thermonuclear explosions) thanks to a spectral analysis of the Swift/BAT data [363].
The analysis was consistent with the observation of a thermonuclear burst for a
source distance of 6.7 ± 1.3 kpc. Further Swift/XRT data analysis revealed an Xray counterpart of the burst and archival XMM-Newton data showed a faint point
source coincident with the Swift/XRT position [121, 363]. Increased activity was
reported in 2010 by INTEGRAL, and was linked to the accretion powered emission
process [56, 266]. Pulsations were discovered immediately afterwards at 518 Hz
in RXTE/PCA follow-up observations [5, 6]. A very strong second harmonic was
detected at 1036 Hz [31] and orbital modulation was measured at 8.82 h, giving a
minimum donor mass of about 0.6 M [18, 320]. Swift J1749.4-2807 was finally
A. Patruno and A. L. Watts
typical fast rise with a slow decay lasting for about 20 days, before entering a fast
decay phase [4, 243]. Curiously, while the source was in the fast decay phase, a very
short and faint outburst of the ultra-compact AMXP XTE J1751-305 appeared in the
same field of view (FoV) of RXTE. The mini-outburst of XTE J1751-305 lasted less
than 3 days, and pulsations were clearly visible at 435 Hz [210]. Thermonuclear
bursts were observed in IGR J17511-3057 with Swift [30], and burst oscillations
were detected in RXTE data [4, 353]. None of the bursts showed PRE, so there
are several different distance determinations in the literature. The most constraining
gives an upper limit on distance of 5 kpc [4].
Swift spectral analysis revealed a power law (Γ ∼ 2) plus a blackbody at 0.9
keV [30]. Broadband spectral data from simultaneous RXTE/PCA and Swift/XRT
observations could be well fitted with a three model component [143]: a disk with
low temperature (kT ∼ 0.24 keV), a hot black body (hot spot with kT ∼ 1 keV)
and a Comptonization component originating from the accretion shock (electron
temperature T e ∼ 30 keV; τ T ∼ 2). The spectral fitting also required a fluorescent
iron line at 6.4 keV with Compton reflection and provided an interstellar absorption
column of N H = 0.88
+0.21
−0.24 × 10 22 cm −2 . Very similar results were obtained
using simultaneous XMM-Newton and RXTE/PCA spectral data [243]. Combined
RXTE/PCA, Swift/XRT and INTEGRAL data also gave consistent results using
a thermal Comptonization model [89]. Twin kHz QPOs were observed at the
beginning and at the end of the outburst, and had Δν 120 Hz, about half the value
of the spin frequency [160]. A NIR counterpart with brightness K s = 18.0 ± 0.1
was identified with the 6.5 m Magellan Baade telescope on 2009 September 22
[335]. A second observation on October 7, while the X-ray flux was fading rapidly,
showed no counterpart with 3σ upper limits of K s > 18.8. No radio counterpart has
been found, with upper limits of 0.10 mJy [215].
4.3.13 Swift J1749.4-2807
The first observation of Swift J1749.4-2807 was made on June 2, 2006 with
the Swift/BAT telescope, which recorded a high level activity consistent with an
unidentified source [305]. This was later identified as a “burst-only” accreting NS
binary (i.e., a NS that is bright enough to be detectable only during the occurrence of
thermonuclear explosions) thanks to a spectral analysis of the Swift/BAT data [363].
The analysis was consistent with the observation of a thermonuclear burst for a
source distance of 6.7 ± 1.3 kpc. Further Swift/XRT data analysis revealed an Xray counterpart of the burst and archival XMM-Newton data showed a faint point
source coincident with the Swift/XRT position [121, 363]. Increased activity was
reported in 2010 by INTEGRAL, and was linked to the accretion powered emission
process [56, 266]. Pulsations were discovered immediately afterwards at 518 Hz
in RXTE/PCA follow-up observations [5, 6]. A very strong second harmonic was
detected at 1036 Hz [31] and orbital modulation was measured at 8.82 h, giving a
minimum donor mass of about 0.6 M [18, 320]. Swift J1749.4-2807 was finally