On Timing and Spectral Characteristics of the X-ray Pulsar 4U 0115+63: Evolution of the Pulsation Period and the Cyclotron Line Energy

TL;DR

This paper reviews 40 years of observations of the X-ray pulsar 4U 0115+63, analyzing its spectral and timing properties, including cyclotron line harmonics and their relation to luminosity, highlighting the need for improved physical models.

Contribution

It provides a comprehensive analysis of the pulsar's spectral evolution and the complexities in interpreting cyclotron line energy variations with luminosity.

Findings

Detection of multiple cyclotron harmonics up to 60.7 keV.

Confirmation that continuum model choice affects observed anticorrelation.

Open question on the evolution of cyclotron line energy with luminosity.

Abstract

An overview of the results of observations for the transient X-ray pulsar 4U 0115+63, a member of a binary system with a Be star, since its discovery to the present day (~40 years) based on data from more than dozen observatories and instruments is presented. A overall light curve and the history of change in the spin frequency of the neutron star over the entire history of its observations, which also includes the results of recent measurements made by the INTEGRAL observatory during the 2004, 2008, and 2011 outbursts, are provided. The source's energy spectra have also been constructed from the INTEGRAL data obtained during the 2011 outburst for a dynamic range of its luminosities (~10^{37} - 7 x 10^{37} erg/s). We show that apart from the fundamental harmonic of the cyclotron absorption line at energy ~11 keV, its four higher harmonics at energies ~24, 35.6, 48.8, and 60.7 keV are detected in the spectrum. We have performed a detailed analysis of the source's spectra in the 4-28 keV energy band based on all of the available RXTE archival data obtained during bright outbursts in 1995-2011. We have confirmed that modifying the source's continuum model can lead to the disappearance of the observed anticorrelation between the energy of the fundamental harmonic of the cyclotron absorption line and the source's luminosity. Thus, the question about the evolution of the cyclotron absorption line energy with the luminosity of the X-ray pulsar 4U 0115+63 remains open and a physically justified radiation model for X-ray pulsars is needed to answer it.