N. Sartore, A. Tiengo, S. Mereghetti, A. De Luca, R. Turolla, F. Haberl
Using a large set of XMM-Newton observations we searched for 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, Hohle et
al., 2010) 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 small
variations in the temperature derived with a single blackbody fit (of the order
of 1% around kT^inf \sim 61 eV). Such variations are 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
\sim 4% and \sim 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 for spectral or flux variations of RX
J1856.5-3754 from March 2005 to present-day, with limits of Delta kT^inf < 0.5%
and Delta f_X < 3% (0.15-1.2 keV), with 3sigma confidence. A slightly higher
temperature (kT^inf \sim 61.5 eV, compared to kT^\inf \sim 61 eV) was instead
measured in April 2002. If this difference is not of instrumental origin, it
implies a rate of variation \sim -0.15 eV yr^-1 between April 2002 and March
2005. The high-statistics spectrum from the selected observations is best
fitted with the sum of two blackbody models, with temperatures kT_h^inf =
62.4_{-0.4}^{+0.6} eV and kT_s^\inf = 38.9_{-2.9}^{+4.9} eV, which account for
the flux seen in the optical band. No significant spectral features are
detected, with upper limits of 6 eV on their equivalent width.
View original:
http://arxiv.org/abs/1202.2121
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