Support ATel At Patreon

[ Previous | Next | ADS ]

Further NICER observations of the accreting millisecond pulsar Swift J1756.9-2508

ATel #11581; P. M. Bult, K. C. Gendreau (NASA/GSFC), P. S. Ray (NRL), D. Altamirano (Univ. of Southampton), Z. Arzoumanian, T. E. Strohmayer (NASA/GSFC), J. Homan (Eureka Scientific & SRON), D. Chakrabarty (MIT) for the NICER Team
on 27 Apr 2018; 02:30 UT
Credential Certification: Peter Bult (p.m.bult@nasa.gov)

Subjects: X-ray, Neutron Star, Transient, Pulsar

Referred to by ATel #: 11603

The accreting millisecond X-ray pulsar Swift J1756.9-2508 has been in outburst since 2018 April 1 (ATel #11497, #11502, #11505, #11523, #11566) and has been subject to regular monitoring with NICER (ATel #11502). Our preliminary results show that the source flux has been decaying since the first NICER observation obtained on 2018 April 3 15:18 UTC.

The most recent positive NICER detection of this source was during an observation starting on April 11 01:16 UTC, for 5 ks. At that time, the spectrum was well described (red. chi^2 = 1.09; 468 dof) by an absorbed power-law model with N_H = (4.8 +/- 0.1)e22 cm^(-2) and power-law photon index Gamma=(2.3 +/- 0.1), yielding a 1-10 keV unabsorbed flux of F=(9.5+/-0.4)e-11 erg/s/cm^2.

NICERs latest observation was performed on 2018 April 25 20:16 UTC, for a total of 1.5 ks. We found that the 1-10 keV light curve was consistent with background levels. Applying the above model to the April 25 data, we obtain a 95%-confidence upper limit on the 1-10 keV unabsorbed source flux of F=1.9e-11 erg/s/cm^2. (This limit does not account for systematic uncertainties in our background model.) Our upper limit is about 5 times fainter than the previously detected flux. Periodicity searches also find no evidence of pulsed X-ray emission in this observation. The lack of a pulsed detection likely indicates that this transient source has returned to X-ray quiescence.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.