NICER observation of the transient high mass X-ray binary pulsar 4U 1901+03

G. K. Jaisawal, K. Pottschmidt, M. T. Wolff, M. T. Wolff, C. Malacaria, J. B. Coley, K. C. Gendreau, Z. Arzoumanian, P. Kretschmar, J. Wilms

Research output: Other contributionNet publication - Internet publicationResearch

Abstract

Referred to by ATel #: 12519, 12554, 12556, 12560 Tweet Following the recent MAXI alert of X-ray activity from the binary X-ray pulsar 4U 1901+03 (ATel #12498), the Neutron Star Interior Composition Explorer (NICER) performed dedicated observations on 2019 February 15 at 15:05:00 UTC for an exposure time of ~1.4 ks. MAXI and Swift-BAT show that the rising phase of the outburst continued through the NICER snapshot. The outburst might have reached its peak at about 170 mCrab (15-50 keV) around 2019 February 18, see the BeXRB monitor at http://integral.esac.esa.int/bexrbmonitor/Plots/sim_plot_4U1901+03.html. An initial timing analysis of the NICER data detected pulsations with a barycentric period of 2.762 s in the 0.3-12 keV band. This is consistent with the pulsar's known spin period and with Fermi/GBM observations during the last outburst in 2011 (ATel #3839). The non-sinusoidal pulse profile in the 0.3-12 keV band can be described either as double-peaked or as a single asymmetric peak with a slow decline interrupted by a ``notch''. The profile observed with NICER is energy dependent and its pulse fraction increases with energy. We note that RXTE observations during the 2003 outburst showed a strong luminosity dependence of the profile (Galloway et al., 2005, ApJ 635, 1217). We extracted the energy spectrum from the early NICER data. The preliminary 0.5-10 keV spectrum can be approximated with an absorbed power-law model with a thermal blackbody component. The spectral parameters are: hydrogen column density N_H = (2.36 +/-0.04) x10^22 cm^-2, black body temperature kT = 1.43 +/-0.03 keV, and photon index Gamma = -0.6 +/-0.4. An iron line signature is also detected in the 6-7 keV range. The line parameters are: energy 6.6 +/- 0.1 keV, width 0.8 +/- 0.2 keV. The best-fit reduced chi^2 is 1.24 for 905 degrees of freedom. The uncertainties are given at the 90% confidence level. The continuum parameters should be regarded as preliminary due to the strong iron line and the presence of a ``10 keV feature'' in the hard X-rays, reported in ATel #12514. After the initial NICER observation a revised source position and a possible UKIDS counterpart were reported in ATel #12514. We note that the revised position is also consistent with a 2MASS source that is a Be star candidate, star ``417'' of Reig and Milonaki (2016, A&A 594, 45). We further note that the initial NICER observation was performed using the RXTE position reported by Galloway et al. (2003a, IAUC 8081 ). Since the offset was only around 1 arcmin we do not expect a significant effect on the measured NICER count rate and flux. We obtained a 0.2-12 keV count rate of 237 counts/s, a 0.5-10 keV absorbed flux of (2.58 +/-0.02) x10^-9 erg/s/cm^2, and a 0.5-10 keV unabsorbed flux of (2.98+/-0.03) x10^-9 erg/s/cm^2. NICER is continuing to observe 4U 1901+03. 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.
Original languageEnglish
Publication date1 Feb 2019
Publication statusPublished - 1 Feb 2019
SeriesThe Astronomer's telegram
NumberATel #12515

Keywords

  • Binary
  • Neutron Star
  • Transient

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