Timing analysis of 2S 1417-624 observed with NICER and Insight-HXMT

TL;DR

This paper analyzes the timing properties of the accreting pulsar 2S 1417-624 during its 2018 outburst, revealing pulse profile changes linked to accretion regime transitions and estimating the neutron star's magnetic field.

Contribution

It provides the first detailed timing analysis of 2S 1417-624 during an outburst, linking pulse profile evolution to accretion physics and estimating the magnetic field strength.

Findings

Pulse profiles change from double to triple peaks at specific luminosities.

Transition from sub-critical to super-critical accretion regime observed.

Magnetic field estimated at approximately 7×10^12 G.

Abstract

We present a study of timing properties of the accreting pulsar 2S 1417-624 observed during its 2018 outburst, based on Swift/BAT, Fermi/GBM, Insight-HXMT and NICER observations. We report a dramatic change of the pulse profiles with luminosity. The morphology of the profile in the range 0.2-10.0keV switches from double to triple peaks at $\sim2.5$ $\rm \times 10^{37}{\it D}_{10}^2\ erg\ s^{-1}$ and from triple to quadruple peaks at $\sim7$ $\rm \times 10^{37}{\it D}_{10}^2\ erg\ s^{-1}$. The profile at high energies (25-100keV) shows significant evolutions as well. We explain this phenomenon according to existing theoretical models. We argue that the first change is related to the transition from the sub to the super-critical accretion regime, while the second to the transition of the accretion disc from the gas-dominated to the radiation pressure-dominated state. Considering the spin-up as well due to the accretion torque, this interpretation allows to estimate the magnetic field self-consistently at $\sim7\times 10^{12}$G.