Accreting on the edge: a luminosity-dependent cyclotron line in the Be/X-ray Binary 2S 1553-542 accompanied by accretion regimes transition

TL;DR

This study analyzes a Be/X-ray binary during an outburst, revealing a luminosity-dependent cyclotron line and evidence of accretion regime transition from sub-critical to super-critical states based on spectral and timing observations.

Contribution

First detection of a luminosity-dependent cyclotron line in 2S 1553-542, linking spectral features to accretion regime changes during an outburst.

Findings

Discovery of a luminosity-dependent cyclotron line energy.

Identification of accretion regime transition at L_crit ≈ 4×10^{37} erg/s.

Variability in pulse profiles and pulsed fraction along the outburst.

Abstract

Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the luminosity-dependent spectral and timing features of the neutron star's emission can be analyzed in detail, thus shedding light on the accretion regime at work. We took advantage of a monitoring campaign performed with NuSTAR, Swift/XRT, AstroSat and NICER, to follow the Be/X-ray Binary 2S 1553-542 along one of its rare outbursts and trace its spectral and timing evolution. We report the discovery of a luminosity-dependent cyclotron line energy for the first time in this source. The pulse profiles and pulsed fraction also show variability along the outburst, consistently with the interpretation that the source transitions from the sub-critical to the super-critical accretion regime, separated by a critical luminosity of L$_{crit}\approx4\times10^{37}$ erg/s.