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

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.