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

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85741 Garching, Germany e-mail: mhohle@astro.uni-jena.de
² Astrophysikalisches Institut und Universitäts-Sternwarte Jena, Schillergässchen 2-3, 07745 Jena, Germany
³ University Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands
⁴ Department of Physics, University of Padua, via Marzolo 8, 35131 Padova, Italy
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5, 6NT, UK
⁶ SRON, Netherlands Institute of Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 13 November 2009
Accepted: 6 July 2010

Abstract

Aims. 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.

Methods. 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 ≈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 Z_n² test.

Results. 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. Using this and previous timing solutions, the residuals indicate a cyclic behaviour with a period ≈7–9 yrs if the variations follow a sinusoid, or twice this value in case the residuals are modulated by an abs(sine) probably approaching a new minimum around MJD = 55 240 days (February 2010). As an alternative interpretation, the phase residuals can be fitted with a glitch that occured around MJD = 53 000 days.