158
A. Patruno and A. L. Watts
4.3.3 XTE J0929-314
With a spin of 185 Hz and an orbital period of 43.6 min, XTE J0929-314 was the
third AMXP to be discovered, and the second in an ultra-compact binary. The source
was first detected on April 13, 2002 by the RXTE/ASM [285]. Coherent pulsations
were first detected in an observation performed on May 2, 2002 [285]. Doppler
modulation of the pulsations revealed the ultra-compact nature of the binary [99].
The very small mass function (2.7×10 −7 M ) implied a tiny donor star of minimum
mass of about 0.01 M . This requires a cold He white dwarf companion with a
possible H-rich envelope that would explain the detection of an H α line at λ =
6563 ˚ A [45] during the 2002 outburst. The 2002 outburst lasted at least 52 days, but
only 34 days were monitored by the high time resolution RXTE/PCA instrument.
The X-ray spectrum of XTE J0929-314, taken with Chandra, showed a power-law
plus blackbody similar to XTE J1751-305 [156].
XTE J0929-314 has been detected in several wavebands: radio, near IR and
optical counterparts were identified during the 2002 outburst. An optical counterpart
consistent with the position of XTE J0929-314 was detected in V, B, R and I bands
[118]. Broad band BVRI observations at the Mt Canopus 1-m telescope carried in
May 2002 revealed a modulation of ≈10% in the I band with periodicity consistent
with the orbit of the binary [114]. An anomalous I-band excess was also observed
in the first few days of the outburst and was interpreted as synchrotron radiation, in
addition to the accretion disk/irradiated donor emission. This behavior is similar
to that reported for SAX J1808.4-3658. The optical spectrum of the source (in
outburst) shows a strong emission line at 4640 ˚
A compatible with a C/O or a
He/N disc [229]. Optical spectroscopy also lead to the conclusion that the most
likely donor of XTE J0929-314 is a C/O white dwarf. A radio counterpart (during
the outburst) was detected with the VLA at 4.86 GHz [299]. A source flux of
0.31 ± 0.07 mJy (May 3) and 0.36 ± 0.05 mJy (May 7) was reported from a position
only 0.8 arcseconds from the previously detected optical position [118]. The source
has also a very faint optical counterpart (R = 27.2) in quiescence [218]. PSF V RI
photometry in quiescence [72] also suggested that the donor is irradiated by a source
which emits in excess (by a factor of ≈8) of the quiescent X-ray luminosity of XTE
J0929-314 (L X ∼4.5 × 10 31 erg s −1 ).
4.3.4 XTE J1807-294
The fourth AMXP was discovered on February 21, 2003 by RXTE [205]. The source
is the third ultra-compact AMXP, with the shortest orbital period known to date, 40.1
min, and a minimum companion mass of 0.0066 M . Simultaneous INTEGRAL,
XMM-Newton and RXTE spectral analysis [87] constrains the companion mass to
be less than 0.022 M , implying that the donor is likely a C/O white dwarf rather
than a He white dwarf, which would require unlikely a-priori low inclinations. With
A. Patruno and A. L. Watts
4.3.3 XTE J0929-314
With a spin of 185 Hz and an orbital period of 43.6 min, XTE J0929-314 was the
third AMXP to be discovered, and the second in an ultra-compact binary. The source
was first detected on April 13, 2002 by the RXTE/ASM [285]. Coherent pulsations
were first detected in an observation performed on May 2, 2002 [285]. Doppler
modulation of the pulsations revealed the ultra-compact nature of the binary [99].
The very small mass function (2.7×10 −7 M ) implied a tiny donor star of minimum
mass of about 0.01 M . This requires a cold He white dwarf companion with a
possible H-rich envelope that would explain the detection of an H α line at λ =
6563 ˚ A [45] during the 2002 outburst. The 2002 outburst lasted at least 52 days, but
only 34 days were monitored by the high time resolution RXTE/PCA instrument.
The X-ray spectrum of XTE J0929-314, taken with Chandra, showed a power-law
plus blackbody similar to XTE J1751-305 [156].
XTE J0929-314 has been detected in several wavebands: radio, near IR and
optical counterparts were identified during the 2002 outburst. An optical counterpart
consistent with the position of XTE J0929-314 was detected in V, B, R and I bands
[118]. Broad band BVRI observations at the Mt Canopus 1-m telescope carried in
May 2002 revealed a modulation of ≈10% in the I band with periodicity consistent
with the orbit of the binary [114]. An anomalous I-band excess was also observed
in the first few days of the outburst and was interpreted as synchrotron radiation, in
addition to the accretion disk/irradiated donor emission. This behavior is similar
to that reported for SAX J1808.4-3658. The optical spectrum of the source (in
outburst) shows a strong emission line at 4640 ˚
A compatible with a C/O or a
He/N disc [229]. Optical spectroscopy also lead to the conclusion that the most
likely donor of XTE J0929-314 is a C/O white dwarf. A radio counterpart (during
the outburst) was detected with the VLA at 4.86 GHz [299]. A source flux of
0.31 ± 0.07 mJy (May 3) and 0.36 ± 0.05 mJy (May 7) was reported from a position
only 0.8 arcseconds from the previously detected optical position [118]. The source
has also a very faint optical counterpart (R = 27.2) in quiescence [218]. PSF V RI
photometry in quiescence [72] also suggested that the donor is irradiated by a source
which emits in excess (by a factor of ≈8) of the quiescent X-ray luminosity of XTE
J0929-314 (L X ∼4.5 × 10 31 erg s −1 ).
4.3.4 XTE J1807-294
The fourth AMXP was discovered on February 21, 2003 by RXTE [205]. The source
is the third ultra-compact AMXP, with the shortest orbital period known to date, 40.1
min, and a minimum companion mass of 0.0066 M . Simultaneous INTEGRAL,
XMM-Newton and RXTE spectral analysis [87] constrains the companion mass to
be less than 0.022 M , implying that the donor is likely a C/O white dwarf rather
than a He white dwarf, which would require unlikely a-priori low inclinations. With