Volume 541, May 2012
|Number of page(s)||7|
|Section||Stellar structure and evolution|
|Published online||30 April 2012|
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
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
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.
Key words: stars: neutron / X-rays: stars / stars: individual: RX J1856.5-3754
© ESO, 2012
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.