NICER measurement of the period derivative of magnetar Swift J1555.2-5402
ATel #14684; M. Ng (MIT), C.-P. Hu (NCUE, RIKEN), T. Enoto (RIKEN), T. Guver (Istanbul Univ.), K. Bansal (JPL), K. C. Gendreau, Z. Arzoumanian, T. E. Strohmayer, Z. Wadiasingh (NASA/GSFC), C. Kouveliotou (GWU), D. Chakrabarty (MIT), E. Gogus (Sabanci Univ.), G. Younes (GWU), A. B. Pearlman (McGill University, McGill Space Institute, Caltech), W. A. Majid (JPL, Caltech), on behalf of the NICER team
on 8 Jun 2021; 03:28 UT
Credential Certification: Mason Ng (masonng@mit.edu)
Subjects: X-ray, Neutron Star, Transient, Pulsar, Magnetar
Referred to by ATel #: 14685
On 2021 June 03, Swift/BAT discovered short duration X-ray bursts from Swift J1555.2-5402, and NICER subsequently discovered its 3.86 s pulsations (ATel #14674, GCN #30120, #30121). NICER has since continued monitoring Swift J1555.2-5402 from 2021 June 03 12:35 UTC to 2021 June 07 13:42 UTC (GCN #30126), resulting in 15.5 ks of accumulated exposure. The 2-8 keV count rate has been steady at roughly 4 c/s (including a background of ~0.3 c/s). The event times were barycenter-corrected using the coordinates reported by Swift/XRT in GCN #30122.
We have performed timing analyses with 38 pulse times of arrival (TOAs) based on a uniform dataset from a single instrument and energy band (2-8 keV). Here we report spin period P = 3.8610189(4) s and a significant spin period derivative of Pdot = (2.2 +/- 0.7) x 10-11 s/s at barycentric epoch T0 = MJD 59370.55 (TDB). These are also expressed as F0 = 0.258998994(26) Hz and F1 = (-1.5 +/- 0.4) x 10-12 Hz/s. Our Pdot value is over 2 sigma smaller than that reported in ATel #14679.
These timing parameters imply a characteristic age of (2.8 +/- 0.8) x 103 years for the magnetar assuming a rotating magnetic dipole and a braking index of 3. The derived surface magnetic field strength is (2.9 +/- 0.4) x 1014 G, and the spin-down luminosity is (1.5 +/- 0.5) x 1034 erg/s, assuming a neutron star moment of inertia of 1045 cm2 g. All quoted uncertainties are 1 sigma. Our Pdot value is roughly a factor of 3 smaller than that reported in ATel #14679, implying an older magnetar and a weaker magnetic field.
The long term 2-8 keV light curve of the source does not show any trend of a decline and the inferred spectral parameters from all of the observations obtained so far are in line with previous results. The 2-8 keV spectrum is well-described by a blackbody with a temperature and apparent emitting radius at 10 kpc of kT = 1.24 +/- 0.03 keV and R = 1.71 +/- 0.07 km, respectively. The inferred hydrogen column density assuming ISM abundance is NH = (7.70 +/- 0.25) x 1022 cm-2. These results indicate that the absorbed and unabsorbed fluxes of the source in the 1-10 keV band are (4.41 +/- 0.09) x 10-11 erg/s/cm-2 and (6.94 +/- 0.12) x 10-11 erg/s/cm-2, respectively.
We have also detected four additional bursts at a 5-sigma significance level or greater, on top of the four already reported in GCN #30126. Continued X-ray and radio monitoring is encouraged.
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.
