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

Abstract

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 for 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 statistics from single pointed observations. 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. 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. 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 for a resonant cyclotron absorption feature. We also discuss instrumental issues related to the use of Swift-BAT for this kind of studies and the statistical method to weight the confidence level of this detection. Correcting for the gravitational redshift of a 1.4 M$_{\sun}$ neutron star, the inferred surface magnetic field is Bsurf 3.7 x 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.

arXiv commentPublished in Astronomy & Astrophysics, 2011, 532, A73

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• NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619
• Magnetic fields of neutron stars in X-ray binaries
• On the possibility of disk-fed formation in supergiant high-mass X-ray binaries
• Catalog of the Galactic population of X-ray pulsars in High-mass X-ray binary systems
• A Study of the 20 Day Superorbital Modulation in the High-Mass X-ray Binary IGR J16493-4348
• The Orbital Parameters of the Eclipsing High-Mass X-ray Binary Pulsar IGR J16493-4348 from Pulsar Timing
• Swift discovery of the orbital period of the HMXB IGR J015712-7259 in the Small Magellanic Cloud
• Swift unveils the orbital period of IGR J18214-1318