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A. Patruno and A. L. Watts
4.3.2 XTE J1751-305
The AMXP XTE J1751-305 was discovered by RXTE on April 3, 2002 and coherent
pulsations at 435 Hz were observed immediately [204]. The orbital period of 42 min
makes this AMXP an ultra-compact binary with a heated He or C/O white dwarf of
minimum donor mass 0.013–0.017 M
2 (depending on the NS mass) [76]. Archival
observations taken with the All-Sky-Monitor (ASM) aboard RXTE showed that a
faint outburst had already occurred in 1998. Three other outbursts were observed
in 2005, 2007 and 2009. The first two had very faint peak luminosities of about
10–20% the value reached in 2002. The 2005 outburst in particular was observed by
INTEGRAL [117] and lasted only 2 days, with only one follow-up RXTE observation
[326]. No high resolution timing data were taken, and the association of the outburst
with XTE J1751-305 remains dubious given the large source position error of the
two X-ray observatories. On April 5, 2007, XTE J1751-305 was observed with
RXTE during routine galactic bulge monitoring observations [203]. No pulsations
were seen because of the dim flux (10 mCrab in the 2–10 keV) and the short
bulge scan exposure (∼50 s). However, a follow-up Swift observation made the
identification of the source certain thanks to the high angular resolution of the XRT
telescope [208]. The outburst had a fluence of about 10 −4 erg cm −2 , comparable
to the already low 2002 outburst fluence of 2.5 × 10 −3 erg cm −2 [204]. In 2009
INTEGRAL recorded a new outburst [55] and RXTE observed it extensively, with
pulsations immediately detected[210]. This outburst has a duration and brightness
similar to the 2002 event.
XTE J1751-305 shows no Type I X-ray bursts and its distance is therefore
difficult to determine. The X-ray spectrum taken with XMM-Newton in 2002 showed
a power law with spectral index 1.44 ± 0.01 contributing 83% of the total 0.5–10.0
keV flux and a blackbody at temperature kT = 1.05 ± 0.01 keV [214]. A broadband
spectrum analysis of the 2002 outburst was performed by combining RXTE (PCA
and HEXTE) and XMM-Newton (EPIC-pn) data [111]. The analysis revealed the
presence of three components, two blackbodies and a hard component, which were
interpreted as a thermal emission from a cold accretion disk (kT ∼0.6 keV), a hotter
(kT ∼1 keV) hot spot on the NS surface and a thermal Comptonization in plasma
of temperature kT ∼40 keV (see Fig. 4.3). These findings were very similar to the
spectral modeling described above for SAX J1808.4-3658. Unsuccessful searches
for an optical/near IR counterpart were carried out during the 2002 outburst and in
quiescence [154]. However the upper limits are not particularly constraining if one
assumes that the source is close to the Galactic center (d = 8.5 kpc).
2 Hydrogen rich donor stars never reach orbital periods below ∼80 min [282].
A. Patruno and A. L. Watts
4.3.2 XTE J1751-305
The AMXP XTE J1751-305 was discovered by RXTE on April 3, 2002 and coherent
pulsations at 435 Hz were observed immediately [204]. The orbital period of 42 min
makes this AMXP an ultra-compact binary with a heated He or C/O white dwarf of
minimum donor mass 0.013–0.017 M
2 (depending on the NS mass) [76]. Archival
observations taken with the All-Sky-Monitor (ASM) aboard RXTE showed that a
faint outburst had already occurred in 1998. Three other outbursts were observed
in 2005, 2007 and 2009. The first two had very faint peak luminosities of about
10–20% the value reached in 2002. The 2005 outburst in particular was observed by
INTEGRAL [117] and lasted only 2 days, with only one follow-up RXTE observation
[326]. No high resolution timing data were taken, and the association of the outburst
with XTE J1751-305 remains dubious given the large source position error of the
two X-ray observatories. On April 5, 2007, XTE J1751-305 was observed with
RXTE during routine galactic bulge monitoring observations [203]. No pulsations
were seen because of the dim flux (10 mCrab in the 2–10 keV) and the short
bulge scan exposure (∼50 s). However, a follow-up Swift observation made the
identification of the source certain thanks to the high angular resolution of the XRT
telescope [208]. The outburst had a fluence of about 10 −4 erg cm −2 , comparable
to the already low 2002 outburst fluence of 2.5 × 10 −3 erg cm −2 [204]. In 2009
INTEGRAL recorded a new outburst [55] and RXTE observed it extensively, with
pulsations immediately detected[210]. This outburst has a duration and brightness
similar to the 2002 event.
XTE J1751-305 shows no Type I X-ray bursts and its distance is therefore
difficult to determine. The X-ray spectrum taken with XMM-Newton in 2002 showed
a power law with spectral index 1.44 ± 0.01 contributing 83% of the total 0.5–10.0
keV flux and a blackbody at temperature kT = 1.05 ± 0.01 keV [214]. A broadband
spectrum analysis of the 2002 outburst was performed by combining RXTE (PCA
and HEXTE) and XMM-Newton (EPIC-pn) data [111]. The analysis revealed the
presence of three components, two blackbodies and a hard component, which were
interpreted as a thermal emission from a cold accretion disk (kT ∼0.6 keV), a hotter
(kT ∼1 keV) hot spot on the NS surface and a thermal Comptonization in plasma
of temperature kT ∼40 keV (see Fig. 4.3). These findings were very similar to the
spectral modeling described above for SAX J1808.4-3658. Unsuccessful searches
for an optical/near IR counterpart were carried out during the 2002 outburst and in
quiescence [154]. However the upper limits are not particularly constraining if one
assumes that the source is close to the Galactic center (d = 8.5 kpc).
2 Hydrogen rich donor stars never reach orbital periods below ∼80 min [282].