Evidence for a resonant cyclotron line in IGR J16493-4348 from the Swift-BAT hard X-ray survey

Abstract

Context. Resonant absorption cyclotron features are a key diagnostic tool to directly measure the strength of the magnetic field of accreting neutron stars. However, typical values of cyclotron features lie in the high-energy part of the spectrum between 20 keV and 50 keV, where detection is often damped by the low quality statistics of results derived from single pointed observations. Aims: We show that long-term monitoring campaign performed with Swift-BAT of persistently, but faint, accreting high-mass X-ray binaries is able to reveal in their spectra the presence of cyclotron features. Methods: We extracted the average Swift-BAT 15-150 keV spectrum from the 54 months long Swift-BAT survey of the high-mass X-ray source IGR J16493-4348. To constrain the broadband spectrum, we used soft X-ray spectra from Swift-XRT and Suzaku pointed observations. Results: We model the spectra using a set of phenomenological models usually adopted to describe the energy spectrum of accreting high-mass X-ray binaries. Irrespective of the models we used, we found significant improvements in the spectral fits adding to the models a broad (10 keV width) absorption feature, with best-fitting energy estimate between 30 and 33 keV, that we interpret as evidence of a resonant cyclotron absorption feature. We also discuss possible instrumental biases related to the use of Swift-BAT for this kind of study and the statistical method to weight the confidence level of this detection. Correcting for the gravitational redshift of a 1.4 Msun neutron star, the inferred surface magnetic field is Bsurf ~ 3.7 × 10^12 gauss. The spectral parameters of IGR J16493-4348 fit well with empirical correlations observed when the whole sample of high-mass binaries with detected cyclotron features is considered.