Energy-Dependent Harmonic Ratios of the Cyclotron Features of X0331+53 in the 2004-2005 Outburst

TL;DR

This study analyzes how the cyclotron resonance energies of X0331+53 vary with luminosity during its 2004-2005 outburst, revealing anti-correlated energy shifts and harmonic ratio changes that suggest different emission heights in the accretion column.

Contribution

It provides the first detailed analysis of the luminosity-dependent changes in cyclotron harmonic energies for X0331+53 using RXTE data, highlighting the variable resonance ratio.

Findings

Fundamental resonance energy varies from ~22 to ~27 keV with luminosity.

Second harmonic energy varies from ~49 to ~54 keV, with weaker fractional change.

Resonance energy ratio shifts from ~2.2 to 2.0, indicating different emission heights.

Abstract

We report on changes of the cyclotron resonance energies of the recurrent transient pulsar, X0331+53 (V0332+53). The whole RXTE data acquired in the 2004-2005 outburst were utilized. The 3-80 keV source luminosity varied between 1.7x10^36 and 3.5x10^38 ers/s, assuming a distance of 7 kpc. We confirmed that the fundamental cyclotron resonance energy changed from ~22 to ~27 keV in a clear anti-correlation to the source luminosity, and without any hysteresis effects between the rising and declining phases of the outburst. In contrast, the second harmonic energy changed from ~49 to ~54 keV, implying a weaker fractional change as a function of the luminosity. As a result, the observed resonance energy ratio between the second harmonic and the fundamental was ~2.2 when the source was most luminous, whereas the ratio decreased to the nominal value of 2.0 at the least luminous state. Although the significance of this effect is model dependent, these results suggest that the fundamental and second harmonic resonances represent different heights in the accretion column, depending on the mass accretion rate.