# Observational constraints on the magnetic field of the bright transient Be/X-ray pulsar SXP 4.78

## TL;DR

This study analyzes spectral and timing data from multiple observatories to constrain the magnetic field of the Be/X-ray pulsar SXP 4.78, finding it to be relatively weak with an upper limit of less than 6 x 10^{11} G.

## Contribution

The paper provides the first comprehensive magnetic field constraints for SXP 4.78 using multi-instrument spectral and timing analysis during an outburst.

## Key findings

- Magnetic field B < 6 x 10^{11} G based on absence of cyclotron lines.
- Variability power spectrum evolved rapidly during outburst rise.
- Multiple low-frequency QPOs observed at different outburst stages.

## Abstract

We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasi-periodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field $B \lesssim 2 \times10^{12}$ G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field $B < 6 \times 10^{11}$ G.