Swift observations of the accreting millisecond pulsar IGR J17498-2921: from outburst to quiescence
ATel #3661; M. Linares (MIT), E. Bozzo (ISDC), D. Altamirano, N. Degenaar, R. Wijnands (Amsterdam), P. Soleri (Groningen), T. Belloni (INAF-OAB), T. di Salvo, A. D'Ai (Palermo), A. Papitto, A. Riggio, L. Burderi (Cagliari)
on 26 Sep 2011; 15:17 UT
Credential Certification: Manuel Linares (linares@mit.edu)
Subjects: X-ray, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 15996
Swift has been monitoring the accreting millisecond pulsar IGR
J17498-2921 since the start of its outburst in 2011 August 12 (ATels
#3551, #3555, #3556). We detected two X-ray bursts on Aug. 18 and
28. During the first ~12 days the average persistent XRT count rate
remained approximately constant at 3-7 c/s. Around Aug. 24 the source
count rate started to decrease slowly, down to ~1 c/s on
Sep. 11.
The source continued to decay and was detected by the Swift-XRT until
Sep. 18. During two observations taken on Sep. 17 and 18, for a total
exposure of 3.8ksec, the source spectrum was soft, and could be fitted
with an absorbed blackbody model with temperature 0.6+/-0.1 keV (or an
absorbed power law model with photon index 3.2+/-0.3; for a column
density fixed at 3e22 cm^-2, ATel #3555). The corresponding 0.5-10 keV
absorbed flux was (1.7+/-0.2)E-12 erg/cm2/s, and the luminosity about
3E34 erg/s (at a distance of 7.6kpc; ATel #3568).
IGR J17498-2921 was not detected during five Swift-XRT observations on
Sep. 19, 20, 22, 24 and 26, for a total exposure of
10.6ksec. Combining the five observations we estimate a 95% upper
limit on the 0.5-10 keV luminosity of 3.6E33 erg/s (at 7.6 kpc, and
using a kT=0.3 keV blackbody model). This is consistent with quiescent
luminosities of other accreting millisecond pulsars, and we conclude
that IGR J17498-2921 has returned to quiescence after a ~40 days long
outburst.
We thank Neil Gehrels, Caryl Gronwall and the full Swift team for
their help in promptly scheduling the observations. This work made use
of data supplied by the UK Swift Science Data Centre at the University
of Leicester.
