164
A. Patruno and A. L. Watts
4.3.10 SAX J1748.9-2021
SAX J1748.9-2021 was discovered with the BeppoSAX/WFC on August 22, 1998
in the globular cluster NGC 6440 [148, 343]. The distance of the source is well
constrained thanks to the knowledge of the globular cluster distance of 8.5 ± 0.4
kpc [233]. RXTE/PCA and BeppoSAX/WFC monitoring revealed no pulsations, but
thermonuclear bursts were observed with no burst oscillations. The position of the
source was coincident with the position of MX 1746-20 detected by Uhuru and
OSO-7 in 1971-1972. However, given the large error box of these missions, it is not
possible to establish a secure association between the two sources. SAX J1748.92021 showed a typical average X-ray spectrum with a power-law with photon index
1.5 and a soft blackbody component at kT = 0.9 keV [148]. However, other spectral
models also gave a good fit to the data [148].
In August 2001, after 3 years of quiescence, SAX J1748.9-2021 started a
new outburst observed with the RXTE/ASM. Chandra observations allowed the
establishment of a precise position in the globular cluster and confirmed that the
1998 outburst was from the same source. Thermonuclear bursts without burst
oscillations were again observed. The beginning of the third outburst was detected
in 2005 March 7 during a routinely RXTE/PCA Galactic Bulge Scan Survey.
The source was monitored until 2005 July 21 and several pointed RXTE/PCA
observations were performed. The first pulsations reported for this source were seen
in the 2005 outburst [104, 105]. A coherent signal at 442 Hz was detected during
a flux decay reminiscent of the tail of a superburst. However, neither the temporal
profile nor the energetics of the tail were consistent with a super-burst and this lead
to the suggestion that this was a new intermittent AMXP [105]. An independent
analysis of RXTE/PCA archival data from the 2001 and 2005 outburst revealed
intermittent coherent X-ray pulsations at 442 Hz, appearing and disappearing on
timescales of few hundred seconds [2, 253]. A further outburst was observed in
December 2009 with RXTE and intermittent pulsations were observed again [261].
Observations in quiescence carried out in July 2000 and June 2003 revealed an Xray spectrum with a thermal component detected in both observations. The 2000
observation, however, required also a power law component possibly because of
residual accretion on the NS [40].
Thanks to ROSAT/HRI archival observations and a 3.5 m New Technology
Telescope (NTT) observation on 1998 August 26–27, two candidate optical counterparts, dubbed V1 and V2, were identified [343]. V2, with B 22.7 mag, has now
been recognized as the true counterpart thanks to the precise astrometric position
obtained with the 2001 Chandra observations [150]. VLA radio observations
provided stringent upper limits during the 2009 outburst and implied that the radio
emission is quenched at high X-ray luminosities [217].
A. Patruno and A. L. Watts
4.3.10 SAX J1748.9-2021
SAX J1748.9-2021 was discovered with the BeppoSAX/WFC on August 22, 1998
in the globular cluster NGC 6440 [148, 343]. The distance of the source is well
constrained thanks to the knowledge of the globular cluster distance of 8.5 ± 0.4
kpc [233]. RXTE/PCA and BeppoSAX/WFC monitoring revealed no pulsations, but
thermonuclear bursts were observed with no burst oscillations. The position of the
source was coincident with the position of MX 1746-20 detected by Uhuru and
OSO-7 in 1971-1972. However, given the large error box of these missions, it is not
possible to establish a secure association between the two sources. SAX J1748.92021 showed a typical average X-ray spectrum with a power-law with photon index
1.5 and a soft blackbody component at kT = 0.9 keV [148]. However, other spectral
models also gave a good fit to the data [148].
In August 2001, after 3 years of quiescence, SAX J1748.9-2021 started a
new outburst observed with the RXTE/ASM. Chandra observations allowed the
establishment of a precise position in the globular cluster and confirmed that the
1998 outburst was from the same source. Thermonuclear bursts without burst
oscillations were again observed. The beginning of the third outburst was detected
in 2005 March 7 during a routinely RXTE/PCA Galactic Bulge Scan Survey.
The source was monitored until 2005 July 21 and several pointed RXTE/PCA
observations were performed. The first pulsations reported for this source were seen
in the 2005 outburst [104, 105]. A coherent signal at 442 Hz was detected during
a flux decay reminiscent of the tail of a superburst. However, neither the temporal
profile nor the energetics of the tail were consistent with a super-burst and this lead
to the suggestion that this was a new intermittent AMXP [105]. An independent
analysis of RXTE/PCA archival data from the 2001 and 2005 outburst revealed
intermittent coherent X-ray pulsations at 442 Hz, appearing and disappearing on
timescales of few hundred seconds [2, 253]. A further outburst was observed in
December 2009 with RXTE and intermittent pulsations were observed again [261].
Observations in quiescence carried out in July 2000 and June 2003 revealed an Xray spectrum with a thermal component detected in both observations. The 2000
observation, however, required also a power law component possibly because of
residual accretion on the NS [40].
Thanks to ROSAT/HRI archival observations and a 3.5 m New Technology
Telescope (NTT) observation on 1998 August 26–27, two candidate optical counterparts, dubbed V1 and V2, were identified [343]. V2, with B 22.7 mag, has now
been recognized as the true counterpart thanks to the precise astrometric position
obtained with the 2001 Chandra observations [150]. VLA radio observations
provided stringent upper limits during the 2009 outburst and implied that the radio
emission is quenched at high X-ray luminosities [217].