Spectral monitoring of RX J1856.5-3754 with XMM-Newton. Analysis of EPIC-pn data

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 ΔfX < 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 kTh∞ = 62.4-0.4+0.6 eV and kTs∞ = 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.