The continued spectral and temporal evolution of RX J0720.4-3125

TL;DR

This study analyzes recent X-ray observations of RX J0720.4-3125, revealing that its spectral and timing variations are more consistent with a sudden glitch event than free precession, indicating slow cooling over years.

Contribution

The paper provides new observational evidence supporting a glitch as the cause of spectral and timing variations in RX J0720.4-3125, challenging previous free precession models.

Findings

Timing behavior suggests a sudden glitch event.

Spectral parameters changed significantly around the glitch time.

The neutron star exhibits slow cooling over 7 years.

Abstract

RX J0720.4-3125 is the most peculiar object among a group of seven isolated X-ray pulsars (the so-called "Magnificent Seven"), since it shows long-term variations of its spectral and temporal properties on time scales of years. This behaviour was explained by different authors either by free precession (with a seven or fourteen years period) or possibly a glitch that occurred around $\mathrm{MJD=52866\pm73 days}$. We analysed our most recent XMM-Newton and Chandra observations in order to further monitor the behaviour of this neutron star. With the new data sets, the timing behaviour of RX J0720.4-3125 suggests a single (sudden) event (e.g. a glitch) rather than a cyclic pattern as expected by free precession. The spectral parameters changed significantly around the proposed glitch time, but more gradual variations occurred already before the (putative) event. Since $\mathrm{MJD\approx53000 days}$ the spectra indicate a very slow cooling by $\sim$2 eV over 7 years.