NuSTAR view of Be/X-ray binary pulsar 2S 1417-624 during 2018 giant outburst

TL;DR

This study presents detailed timing and spectral analysis of the Be/X-ray binary pulsar 2S 1417-624 during its 2018 giant outburst, revealing complex pulse profile evolution, spectral characteristics, and insights into its emission geometry.

Contribution

The paper provides the first detailed timing and spectral analysis of 2S 1417-624 during a giant outburst using NuSTAR and Swift data, including pulse profile evolution and spectral modeling.

Findings

Detection of 17.475 s pulsations up to 79 keV.

Complex energy-dependent pulse profile evolution.

No cyclotron line detected in the spectrum.

Abstract

We report the results obtained from a detailed timing and spectral studies of Be/X-ray binary pulsar 2S 1417-624 using data from Swift and NuSTAR observatories. The observations were carried out at the peak of a giant outburst of the pulsar in 2018. X-ray pulsations at 17.475 s were detected in the source light curves up to ~79 keV. The evolution of the pulse profiles with energy was found to be complex. A four-peaked profile at lower energies gradually evolved into a double-peak structure at higher energies. The pulsed fraction of the pulsar, calculated from the NuSTAR observation was found to follow an anti-correlation trend with luminosity as observed during previous giant X-ray outburst studies in 2009. The broadband spectrum of the pulsar is well described by a composite model consisting of a cutoff power law model modified with the interstellar absorption, a thermal blackbody component with a temperature of $\approx$1 keV and a Gaussian function for the 6.4 keV iron emission line. Though the pulsar was observed at the peak of the giant outburst, there was no signature of presence of any cyclotron line feature in the spectrum. The radius of the blackbody emitting region was estimated to be $\approx$2 km, suggesting that the most probable site of its origin is the stellar surface of the neutron star. Physical models were also explored to understand the emission geometry of the pulsar and are discussed in the paper.