NuSTAR observation of the ongoing outburst of the accreting X-ray pulsar 2S 1553-542

ATel #14348; C. Malacaria (NASA-MSFC/USRA), K. Pottschmidt (NASA-GSFC, CRESST, UMBC), F. Fuerst (Quasar for ESA), P. Kretschmar (ESA/ESAC), J. B. Coley (Howard University, NASA-GSFC/CRESST), L. Ducci (IAAT), J. Wilms (ECAS), M. T. Wolff (NRL), R. Staubert (IAAT), I. Kreykenbohm (ECAS)
on 26 Jan 2021; 03:02 UT
Credential Certification: Christian Malacaria (cmalacaria@usra.edu)

Tweet

Based on the renewed activity from the accreting pulsar in the Be/X-ray binary system 2S 1553-542 (ATel #14301, #14308, #14335), we initiated a 30 ks Discretionary Director’s Time observation with NuSTAR, which was performed starting on January 22, 21:45:08 (UTC). The source was clearly detected through the full NuSTAR energy band (3-79 keV), with an average count rate of 45 c/s (FPMA and FPMB combined).

Following the 2015 NuSTAR observation of 2S 1553-542 analyzed by Tsygankov et al. (2016, MNRAS 457, 258), we describe the spectrum with a soft blackbody plus an absorbed cutoff power-law model modified by an Fe Kalpha line at 6.4 keV and a Gaussian absorption line to account for the Cyclotron Resonant Scattering Feature. The resulting χ²/d.o.f. is 1429/1285. The best-fit spectral parameters are (all errors at 90% c.l.):
N_H = (1.2 +/- 0.3)x10²² cm^-2, kT_BB = 1.29 +/- 0.07 keV, norm_BB = 6.9 +/- 1.3, \Gamma = -0.43 +/- 0.3,
HighECut = 5.7 +/- 0.3 keV, E_cyc = 27.0 +/- 0.3 keV, \sigma_cyc = 6.4 +/- 0.5 keV, Strength_cyc = 9.2 +/- 1.8.
These values are roughly consistent with those obtained from the NuSTAR observation in 2015. However, the currently observed flux is 1.6x10^-9 erg/cm²/s (3-79 keV), that is 60% higher than that catched by NuSTAR in 2015.

For a barycentered (not orbit corrected) lightcurve, the obtained pulse period is 9.2821(5) s. The pulse profile in the 3-79 keV energy band is dominated by a single pulse over roughly half a phase, with a hint of a second, narrower peak in the second half of the spin phase. Its pulsed fraction is about 60%.

The data analysis is ongoing. We encourage further observations at all wavelengths to cover the current outburst.

We thank the NuSTAR PI, Fiona Harrison, for approving the DDT request, and the NuSTAR SOC for rapidly carrying out the observation.