162
A. Patruno and A. L. Watts
An optical counterpart was identified with the Robotic Palomar 60-inch Telescope on June 18 [94]. The source had a brightness R∼18.4. Near IR observations
taken with the 1.3 m robotic PAIRITEL telescope on Mt. Hopkins indicated a
counterpart with J = 17.6 ± 0.2, with no variability between observations taken
on June 16 and 18 [315]. The counterpart was observed again with the FLWO
1.2-m telescope at Mt. Hopkins with V = 18.09 ± 0.03, R = 18.02 ± 0.03 and
I = 17.88±0.02 [316]. A spectral analysis of the optical counterpart showed a HeII
4686 ˚
A, suggesting correct identification of the binary’s optical counterpart. Further
optical observations taken with the 8.4-m Large Binocular Telescope (LBT) on Mt.
Graham, Arizona, showed a brightness of R = 18.64 ± 0.02 and 18.54 ± 0.02 on
2007 June 13, after an initial decline of 1.7 mag in optical, and a non-detection in Xrays which occurred two weeks prior [75, 333]. Phase resolved optical photometry
and spectroscopy obtained in September 2006 at the 2.3-m ATT, 3.5-m WIYN
and 8.4-m LBT [84], revealed a sinusoidal modulation in the R-band, close to the
orbital period. However, further analysis showed that the modulation is probably
due to a super-hump of a precessing accretion disk, and not irradiation of the donor
star. A broad H α emission line in a 10-m Keck I spectrum taken by the same
authors confirmed this interpretation. No radio counterpart was identified in VLA
observations on 2005 June 19 and 24 at 8.46 GHz [302].
4.3.8 Swift J1756.9-2508
Swift J1756.9-2508 was discovered by the Burst Alert Telescope (BAT) aboard Swift
on June 7, 2007 [170]. High time resolution data from the RXTE/PCA, showed that
the source was a 182 Hz AMXP [209]. This was the fourth ultra-compact AMXP
discovered, with an orbital period of 54.7 min. The minimum donor star mass, given
by the X-ray mass function, is 0.0067 M . It has been proposed that Swift J1756.92508 harbors a He rich white dwarf which is irradiated by X-ray flux from the
accretor (see Fig. 4.4 and [170]). The spectrum of the source, taken with Swift/XRT,
BAT and RXTE/PCA, is well fit by a single absorbed power law with spectral index
Γ ∼ 2 [170]. The 2007 outburst length was 13 days, with no prior outbursts seen
in archival data. A second outburst was detected on 2009 July 8 by Swift/BAT and
RXTE/PCA [256]. This second outburst showed a very similar length and shape as
the 2007 outburst. No thermonuclear bursts were observed. There is evidence for
a fluorescent Fe line in the X-ray spectrum, although the poor spectral resolution
of RXTE prevented to constrain the physical parameters of the system [260]. A
possible NIR K s counterpart was identified [38] by inspecting the variability of 70
objects within the 3.5 arcsec error circle of the Swift/XRT position. The candidate
counterpart exhibited variability of about 1.3 mag in two observations taken close
to the end of the outburst (2007 June 19) and during quiescence (2007 July 1). No
radio counterpart has been identified [134, 273] and Chandra observations taken on
2007 July 7 detected one single photon in a 11 ks long observations [240].
A. Patruno and A. L. Watts
An optical counterpart was identified with the Robotic Palomar 60-inch Telescope on June 18 [94]. The source had a brightness R∼18.4. Near IR observations
taken with the 1.3 m robotic PAIRITEL telescope on Mt. Hopkins indicated a
counterpart with J = 17.6 ± 0.2, with no variability between observations taken
on June 16 and 18 [315]. The counterpart was observed again with the FLWO
1.2-m telescope at Mt. Hopkins with V = 18.09 ± 0.03, R = 18.02 ± 0.03 and
I = 17.88±0.02 [316]. A spectral analysis of the optical counterpart showed a HeII
4686 ˚
A, suggesting correct identification of the binary’s optical counterpart. Further
optical observations taken with the 8.4-m Large Binocular Telescope (LBT) on Mt.
Graham, Arizona, showed a brightness of R = 18.64 ± 0.02 and 18.54 ± 0.02 on
2007 June 13, after an initial decline of 1.7 mag in optical, and a non-detection in Xrays which occurred two weeks prior [75, 333]. Phase resolved optical photometry
and spectroscopy obtained in September 2006 at the 2.3-m ATT, 3.5-m WIYN
and 8.4-m LBT [84], revealed a sinusoidal modulation in the R-band, close to the
orbital period. However, further analysis showed that the modulation is probably
due to a super-hump of a precessing accretion disk, and not irradiation of the donor
star. A broad H α emission line in a 10-m Keck I spectrum taken by the same
authors confirmed this interpretation. No radio counterpart was identified in VLA
observations on 2005 June 19 and 24 at 8.46 GHz [302].
4.3.8 Swift J1756.9-2508
Swift J1756.9-2508 was discovered by the Burst Alert Telescope (BAT) aboard Swift
on June 7, 2007 [170]. High time resolution data from the RXTE/PCA, showed that
the source was a 182 Hz AMXP [209]. This was the fourth ultra-compact AMXP
discovered, with an orbital period of 54.7 min. The minimum donor star mass, given
by the X-ray mass function, is 0.0067 M . It has been proposed that Swift J1756.92508 harbors a He rich white dwarf which is irradiated by X-ray flux from the
accretor (see Fig. 4.4 and [170]). The spectrum of the source, taken with Swift/XRT,
BAT and RXTE/PCA, is well fit by a single absorbed power law with spectral index
Γ ∼ 2 [170]. The 2007 outburst length was 13 days, with no prior outbursts seen
in archival data. A second outburst was detected on 2009 July 8 by Swift/BAT and
RXTE/PCA [256]. This second outburst showed a very similar length and shape as
the 2007 outburst. No thermonuclear bursts were observed. There is evidence for
a fluorescent Fe line in the X-ray spectrum, although the poor spectral resolution
of RXTE prevented to constrain the physical parameters of the system [260]. A
possible NIR K s counterpart was identified [38] by inspecting the variability of 70
objects within the 3.5 arcsec error circle of the Swift/XRT position. The candidate
counterpart exhibited variability of about 1.3 mag in two observations taken close
to the end of the outburst (2007 June 19) and during quiescence (2007 July 1). No
radio counterpart has been identified [134, 273] and Chandra observations taken on
2007 July 7 detected one single photon in a 11 ks long observations [240].