Peculiar temporal and spectral features in highly obscured HMXB pulsar IGR J16320-4751 using XMM-Newton

Abstract

IGR J16320-4751 is a highly obscured HMXB source containing a very slow neutron star (𝑃𝑠𝑝𝑖𝑛 ∼ 1300 sec) orbiting its supergiant companion star with a period of ∼9 days. It shows high column density (𝑁𝐻 ∼ 2 − 5 × 1023 cm−2) in the spectrum, and a large variation in flux along the orbit despite not being an eclipsing source. We report on some peculiar timing and spectral features from archival XMM-Newton observation of this source including 8 observations taken during a single orbit. The pulsar shows large timing variability in terms of average count rate from different observations, flaring activity, sudden changes in count rate, cessation of pulsation, and variable pulse profile even from observations taken a few days apart. We note that IGR J16320- 4751 is among a small number of sources for which this temporary cessation of pulsation in the light curve has been observed. A time-resolved spectral analysis around the segment of missing pulse shows that variable absorption is deriving such a behavior in this source. Energy resolved pulse profiles in 6.2–6.6 keV band which has a partial contribution from Fe K𝛼 photons, show strong pulsation. However, a more systematic analysis reveals a flat pulse profile from the contribution of Fe K𝛼 photons in this band implying a symmetric distribution for the material responsible for this emission. Soft excess emission below 3 keV is seen in 6 out of 11 spectra of XMM-Newton observations.