No anti-correlation between cyclotron line energy and X-ray flux in 4U 0115+634

TL;DR

This study reanalyzes the relationship between cyclotron line energy and X-ray flux in 4U 0115+63, demonstrating that previous anti-correlation findings were artifacts of continuum modeling assumptions.

Contribution

It shows that the anti-correlation between cyclotron line energy and flux is due to modeling artifacts, not an intrinsic physical property.

Findings

Anti-correlation previously reported is an artifact of continuum modeling.

Using a different continuum model, the line energy remains constant.

The apparent anti-correlation disappears with improved spectral modeling.

Abstract

We report on an outburst of the high mass X-ray binary 4U 0115+63 with a pulse period of 3.6s in 2008 March/April as observed with RXTE and INTEGRAL. During the outburst the neutron star's luminosity varied by a factor of 10 in the 3--50\,keV band. In agreement with earlier work we find evidence for five cyclotron resonance scattering features at ~10.7, 21.8, 35.5, 46.7, and 59.7keV. Previous work had found an anti-correlation between the fundamental cyclotron line energy and the X-ray flux. We show that this apparent anti-correlation is probably due to the unphysical interplay of parameters of the cyclotron line with the continuum models used previously, e.g., the negative and positive exponent power law (NPEX). For this model, we show that cyclotron line modeling erroneously leads to describing part of the exponential cutoff and the continuum variability, and not the cyclotron lines. When the X-ray continuum is modeled with a simple exponentially cutoff power law modified by a Gaussian emission feature around 10keV, the correlation between the line energy and the flux vanishes and the line parameters remain virtually constant over the outburst. We therefore conclude that the previously reported anti-correlation is an artifact of the assumptions adopted in the modeling of the continuum.