A BeppoSAX study of the pulsating transient X0115+63: the first X-ray spectrum with four cyclotron harmonic features

TL;DR

This study presents the first detection of four cyclotron harmonic features in the X-ray spectrum of the pulsating transient X0115+63, confirming their magnetic origin and providing insights into the neutron star's magnetic field structure.

Contribution

The paper reports the first observation of four harmonic cyclotron lines in an accreting X-ray pulsar's spectrum, advancing understanding of magnetic field effects in such systems.

Findings

Detection of four harmonic cyclotron lines at 12.74, 24.16, 35.74, and 49.5 keV.

Line energy ratios close to harmonic relations, confirming relativistic effects.

Enhanced equivalent widths of higher harmonics indicate two-photon processes in spectral formation.

Abstract

The recurrent hard pulsating X-ray transient \S was observed with \B on March 19, when the source was at a 2--10 keV flux level of $\sim$310~mCrab. We report on the high energy spectrum of the source, concentrating on cyclotron resonant scattering features. The spectrum is strongly pulse phase dependent and absorption features are detected at virtually all phases. In particular, four absorption-like features at 12.74 keV, 24.16 keV, 35.74 keV, and 49.5 keV are observed in the descending edge of the main peak of the pulse profile. The ratios between the centroid energies of the lines with respect to the first are 1:(1.9):(2.8):(3.9). These values are close to the harmonic relation expected from cyclotron resonant scattering in a strong magnetic field, when relativistic effects are taken into account. The equivalent widths of the second, third, and fourth harmonics are found to be larger than that of the first harmonic, confirming the key role of two-photon processes in the spectral formation. These results provide the first evidence for {\it four harmonically spaced lines} in the spectrum of an accreting X-ray pulsar, yielding the clearest confirmation to date of their magnetic origin.