Variations in the Cyclotron Resonant Scattering Features during 2011 outburst of 4U 0115+63

TL;DR

This study analyzes the 2011 outburst of 4U 0115+63, revealing two independent cyclotron lines at ~11 keV and ~15 keV, and their variations with luminosity, challenging previous models that treated them as a single line.

Contribution

It provides the first evidence for two distinct CRSF sets in 4U 0115+63 and explains their luminosity-dependent behavior, refining the understanding of magnetic field structures.

Findings

Detected two independent CRSFs at ~11 keV and ~15 keV.

Observed that the 11 keV line weakens while the 15 keV line strengthens with decreasing luminosity.

Proposed that earlier anti-correlation results from modeling both lines as one.

Abstract

We study the variations in the Cyclotron Resonant Scattering Feature (CRSF) during 2011 outburst of the high mass X-ray binary 4U 0115+63 using observations performed with Suzaku, RXTE, Swift and INTEGRAL satellites. The wide-band spectral data with low energy coverage allowed us to characterize the broadband continuum and detect the CRSFs. We find that the broadband continuum is adequately described by a combination of a low temperature (kT ~ 0.8 keV) blackbody and a power-law with high energy cutoff (Ecut ~ 5.4 keV) without the need for a broad Gaussian at ~ 10 keV as used in some earlier studies. Though winds from the companion can affect the emission from the neutron star at low energies (< 3 keV), the blackbody component shows a significant presence in our continuum model. We report evidence for the possible presence of two independent sets of CRSFs with fundamentals at ~ 11 keV and ~ 15 keV. These two sets of CRSFs could arise from spatially distinct emitting regions. We also find evidence for variations in the line equivalent widths, with the 11 keV CRSF weakening and the 15 keV line strengthening with decreasing luminosity. Finally, we propose that the reason for the earlier observed anti-correlation of line energy with luminosity could be due to modelling of these two independent line sets (~ 11 keV and ~ 15 keV) as a single CRSF.