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Keywords:

  • stars: neutron;
  • pulsars: individual: RX J0720.4−3125

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 7 or 14 yr period) or possibly a glitch that occurred around MJD = 528 66 ± 73 d.

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 MJD ≈ 530 00 d the spectra indicate a very slow cooling by ∼2 eV over 7 yr.