Flares, broadening of the pulse-frequency peak and quasi-periodic oscillations in the transient X-ray pulsar 4U 1901+03

Abstract

After a long quiescence of three decades, the transient X-ray pulsar 4U 1901+03 became highly active in 2003 February. From the analysis of a large number of Rossi X-ray Timing Explorer/ Proportional Counter Array (RXTE/PCA) observations of this source, we report here the detection of X-ray flares, a broadening of the pulse frequency peak and Quasi Periodic Oscillations (QPOs). The X-ray flares showed spectral changes, had a duration of 100 s - 300 s, and were more frequent and stronger during the peak of the outburst. In most of the observations during the outburst we also detected a broadening of the pulse frequency peak. We have also found intensity dependent changes in the pulse profile at very short timescales. This reveals a coupling between the periodic and the low frequency aperiodic variabilities. In addition, near the end of the outburst we have detected a strong QPO feature centered at ~ 0.135 Hz. The QPO feature is broad with a quality factor of 3.3 and with an rms value of 18.5+/-3.1%. Using the QPO frequency and the X-ray luminosity during the QPO detection period we estimated the magnetic field strength of the neutron star as 0.31*10^12 G which is consistent with the value inferred earlier under the assumption of spin equilibrium.