Updated phase coherent timing solution of the isolated neutron star RX J0720.4-3125 using recent XMM-Newton and Chandra observations

TL;DR

This paper refines the timing solution of the isolated neutron star RX J0720.4-3125 using recent XMM-Newton and Chandra data, emphasizing energy band consistency to improve phase residual analysis over a 10-16 year span.

Contribution

It provides an updated, energy band-restricted phase coherent timing solution for RX J0720.4-3125, incorporating recent observations and comparing results with previous solutions.

Findings

Timing solutions are consistent whether or not ROSAT data are included.

Restricting to the hard energy band improves data agreement across instruments.

Phase residuals remain large and variable, with recent data showing large negative values.

Abstract

Since the last phase coherent timing solution of the nearby radio-quiet isolated neutron star RX J0720.4-3125 six new XMM-Newton and three Chandra observations were carried out. The phase coherent timing solutions from previous authors were performed without restricting to a fixed energy band. However, we recently showed that the phase residuals are energy dependent, and thus phase coherent solutions must be computed referring always to the same energy band. We updated the phase coherent timing solution for RX J0720.4-3125 by including the recent XMM-Newton EPIC-pn, MOS1, MOS2 and Chandra ACIS data in the energy range 400-1000~eV. Altogether these observations cover a time span of almost 10~yrs. A further timing solution was obtained including the ROSAT pointed data. In this case, observations cover a time span of $\approx$16~yrs. To illustrate the timing differences between the soft band (120-400~eV) and the hard band (400-1000~eV) a timing solution for the soft band is also presented and the results are verified using a $\mathrm{Z_{n}^{2}}$ test. In contrast to previous work, we obtain almost identical solutions whether or not we include the ROSAT or Chandra data. Thanks to the restriction to the hard band, the data points from EPIC-pn are in better agreement with those from MOS1, MOS2 and Chandra than in previous works. In general the phase residuals are still large and vary with time. In particular, the latest XMM-Newton and Chandra data show that the phase residuals have attained relatively large and negative values.