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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 ∼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 × 10 12   G which is consistent with the value inferred earlier under the assumption of spin equilibrium.

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