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

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 feature and quasi-periodic oscillations (QPOs). The X-ray flares showed spectral changes, had a duration of 100-300 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 time-scales. 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 centred at similar to 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 per cent. 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 x 1012 G which is consistent with the value inferred earlier under the assumption of spin equilibrium.