Spectral monitoring of RX J1856.5-3754 with XMM-Newton

Analysis of EPIC-pn data

¹ INAF – Istituto di Fisica Spaziale e Fisica Cosmica, via E. Bassini 15, 20133 Milano, Italy
e-mail: sartore@iasf-milano.inaf.it
² IUSS – Istituto Universitario di Studi Superiori, viale Lungo Ticino Sforza 56, 27100 Pavia, Italy
³ INFN – Istituto Nazionale di Fisica Nucleare, sezione di Pavia, via A. Bassi 6, 27100 Pavia, Italy
⁴ Dipartimento di Fisica e Astronomia, Università di Padova, via Marzolo 8, 35131 Padova, Italy
⁵ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
⁶ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei München, Germany

Received: 20 November 2011
Accepted: 8 February 2012

Abstract

Using a large set of XMM-Newton observations, we searched for the long-term spectral and flux variability of the isolated neutron star RX J1856.5-3754 in the time interval from April 2002 to October 2011. This is the brightest and most extensively observed source of a small group of nearby, thermally emitting isolated neutron stars, of which at least one member (RX J0720.4-3125) has shown long-term variability. A detailed analysis of the data obtained with the EPIC-pn camera in the 0.15−1.2   keV energy range reveals only small variations in the temperature derived with a single blackbody fit (of about 1% around an average value of kT^∞ ~ 61 eV). These variations appear to be correlated with the position of the source on the detector and can be ascribed to an instrumental effect, most likely a spatial dependence of the channel-to-energy relation. For the sampled instrumental coordinates, we quantify this effect as variations of  ~4% and  ~15 eV in the gain slope and offset, respectively. Selecting only a homogeneous subset of observations, with the source imaged at the same detector position, we find no evidence of either spectral or flux variations of RX J1856.5-3754 from March 2005 to the present-day, with limits of ΔkT^∞ < 0.5% and Δf_X < 3% (0.15−1.2   keV range), with 3σ confidence. A slightly higher temperature (kT^∞ ~ 61.5  eV, compared to the average value of kT^∞ ~ 61 eV) was instead measured in April 2002. If this difference is not of instrumental origin, it implies a rate of variation  ~ –0.15 eV yr^-1 between April 2002 and March 2005. The high-quality-statistics source spectrum from the selected observations is best fitted with the sum of two blackbody models, with temperatures of kT_h^∞ = 62.4_-0.4^+0.6 eV and kT_s^∞ = 38.9_-2.9^+4.9 eV, which can also account for the flux seen in the optical band. No significant narrow or broad spectral features are detected, with upper limits of  ~6 eV on their equivalent width.