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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 410; no. 3; pp. 1489 - 1495
• Main Authors: James, Marykutty, Paul, Biswajit, Devasia, Jincy, Indulekha, Kavila
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.01.2011; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Luminosity; Magnetic fields; Pulsars; pulsars: individual: 4U 1901+03; stars: individual: 4U 1901+03; stars: neutron; X-ray astronomy; X-rays: binaries; X-rays: stars; X-rays: stars; X-rays: binaries; pulsars: individual: 4U 1901+03; stars: neutron; stars: individual: 4U 1901+03; RXTE satellite; Neutron stars; Intensity; Variability; Binary X ray source; Cosmic x-ray sources; Luminosity; Quasi periodic variation; Quiescence; stars: individual: 4U 1901+03-stars: neutron-pulsars: individual: 4U 1901+03; Transient X ray source; X-ray binary stars; Quality factor; Pulsars; Magnetic fields; Pulse shape
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2010.17543.x

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 × 1012 G which is consistent with the value inferred earlier under the assumption of spin equilibrium.