The giant outburst of EXO 2030+375 I: Spectral and pulse profile evolution

TL;DR

This study analyzes the spectral and pulse profile evolution of the Be X-ray binary EXO 2030+375 during its third giant outburst, revealing luminosity-dependent changes and modeling emission patterns with relativistic ray-tracing.

Contribution

It provides detailed spectral and timing analysis of the outburst and introduces a relativistic ray-tracing model to explain pulse profile changes with luminosity.

Findings

Pulse profiles depend on luminosity with a sharp transition at 2x10^36 erg/s.

Spectral shape remains stable during rise and decline phases.

Relativistic ray-tracing explains observed pulse profile variations.

Abstract

The Be X-ray binary EXO 2030+375 went through its third recorded giant outburst from June 2021 to early 2022. We present the results of both spectral and timing analysis based on NICER monitoring, covering the 2-10 keV flux range from 20 to 310 mCrab. Dense monitoring with observations carried out about every second day and a total exposure time of 160 ks allowed us to closely track the source evolution over the outburst. Changes in spectral shape and pulse profiles showed a stable luminosity dependence during the rise and decline. The same type of dependence has been seen in past outbursts. The pulse profile is characterized by several distinct peaks and dips. The profiles show a clear dependence on luminosity with a stark transition at a luminosity of 2x10^36 erg/s, indicating a change in the emission pattern. Using relativistic ray-tracing, we demonstrate how anisotropic beaming of emission from an accretion channel with constant geometrical configuration can give rise to the observed pulse profiles over a range of luminosities.