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(infinity) similar to 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 similar to 4% and similar to 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 Delta kT(infinity) < 0.5% and Delta f(X) < 3% (0.15-1.2 keV range), with 3 sigma confidence. A slightly higher temperature (kT(infinity) similar to 61.5 eV, compared to the average value of kT(infinity) similar to 61 eV) was instead measured in April 2002. If this difference is not of instrumental origin, it implies a rate of variation similar to-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)(infinity) = 62.4(-0.4)(+0.6) eV and kT(s)(infinity) = 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 similar to 6 eV on their equivalent width.