2005ApJ...635.1217G

We describe observations of the 2003 outburst of the hard-spectrum X-ray transient 4U 1901+03 with the Rossi X-Ray Timing Explorer (RXTE). The outburst was first detected in 2003 February by the All-Sky Monitor and reached a peak 2.5-25 keV flux of 8x10^–9 ergs/cm2/s (around 240 mcrab). The only other known outburst occurred 32.2 yr earlier, likely the longest currently known recurrence time for any X-ray transient. Proportional Counter Array (PCA) observations over the 5 month duration of the 2003 outburst revealed a 2.763 s pulsar in a 22.58 days orbit. The detection of pulsations down to a flux of 3x10^–11 ergs/cm2/s (2.5-25 keV), along with the inferred long-term accretion rate of 8.1x10^–11 M_☉/yr (assuming a distance of 10 kpc), suggests that the surface magnetic field strength is below ∼5x10¹¹ G. The corresponding cyclotron energy is thus below 4 keV, consistent with the nondetection of resonance features at high energies. Although we could not unambiguously identify the optical counterpart, the lack of a bright IR candidate within the 1' RXTE error circle rules out a supergiant mass donor. The neutron star in 4U 1901+03 probably accretes from the wind of a main-sequence OB star, like most other high-mass binary X-ray pulsars. The almost circular orbit (e=0.036) confirms the system's membership in a growing class of wide, low-eccentricity systems in which the neutron stars may have received much smaller kicks as a result of their natal supernova explosions.