Energy Dependence of Quasi-periodic Oscillations in Accreting X-ray Pulsars

We present the results from an investigation of the energy dependence ofQuasi-Periodic Oscillations (QPOs) exhibited by accreting X-ray pulsars usingdata from archival XMM-Newton, NuSTAR, RXTE, andNICER observations. In a search for the presence of QPOs in 99XMM-Newton and NuSTAR observations, we detected QPOs ineleven observations of five sources, viz., 4U 1626–67 (48 mHz), IGRJ19294+1816 (30 mHz), V 0332+53 (2, 18 and 40 mHz), Cen X–3 (30 mHz), and XTEJ1858+034 (180 mHz). A positive correlation of the QPO rms amplitude withenergy is exhibited by 4U 1626–67, IGR J19294+1816, Cen X–3 and XTEJ1858+034, while no energy dependence is observed in V 0332+53. We alsoanalysed the energy spectrum to decouple thermal (soft-excess) from non-thermalemission and determine if the soft-excess has different QPO properties. Wefound no evidence for different QPO characteristics of the soft excess. TheNuSTAR observations of V 0332+53 during the Type-I outburst in 2016show the presence of twin QPOs at 2.5 mHz and 18 mHz, while theXMM-Newton and NuSTAR observations during the Type-IIoutburst in 2015 show a QPO at 40 mHz. We review the observed QPO properties inthe context of QPOs found in other types of accreting sources and the modelsusually used to explain the QPOs in accreting X-ray pulsars.