Submillimetre observations of RX J1856.5--3754

arXiv:1003.4501v1 [astro-ph.SR] 23 Mar 2010 Mon. Not. R. Astron. Soc. 000, 1–6 (2010) Printed 2 November 2021 (MN LATEX style file v2.2) Submillimetre observations of RX J1856.5–3754⋆ B. Posselt1†, K. Schreyer2, R. Perna3, M.W. Sommer4, B. Klein5, P. Slane1 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, 02138 Cambridge, MA, USA 2 Astrophysikalisches Institut und Universit¨ats-Sternwarte Jena, Schillerg¨asschen 2-3, D-07745 Jena, Germany 3 440 UCB, U. of Colorado, Boulder, 80309, USA 4 Argelander-Institut f¨ur Astronomie, Auf dem H¨ugel 71, D-53121 Bonn, Germany 5 Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69,D-53121 Bonn,Germany 2 November 2021 ABSTRACT We report on submillimetre bolometer observations of the isolated neutron star RX J1856.5–3754 using the LABOCA bolometer array on the Atacama Pathfinder Experiment (APEX) Telescope. No cold dust continuum emission peak at the po- sition of RX J1856.5–3754 was detected. The 3σ flux density upper limit of 5 mJy translates into a cold dust mass limit of a few earth masses. We use the new sub- millimetre limit, together with a previously obtained H-band limit, to constrain the presence of a gaseous, circumpulsar disc. Adopting a simple irradiated-disc model, we obtain a mass accretion limit of ˙M ∼< 1014 g s−1, and a maximum outer disc radius of ∼1014 cm. By examining the projected proper motion of RX J1856.5–3754, we specu- late about a possible encounter of the neutron star with a dense fragment of the CrA molecular cloud a few thousand years ago. Key words: submillimetre- stars: neutron- X-rays: individual: RX J1856.5–3754 1 INTRODUCTION Since the discovery of planets around the pulsar PSR 1257+12 by Wolszczan & Frail (1992), dusty discs around pulsars have become interesting to observers, who have been trying to detect them in the infrared (IR) or at (sub-)millimeter wavelengths. Most comprehensive are the searches by L¨ohmer et al. (2004) and Greaves & Holland (2000). Previous searches had concentrated mainly on recycled, thus formerly accreting, radio pulsars as the most likely objects to be surrounded by dusty discs. Neither of the above mentioned surveys detected dust emission around the planet-hosting PSR 1257+12, nor did the recent search with Spitzer at 24 µm and at 70 µm by Bryden et al. (2006). However, dusty discs could also be present around non-recycled neutron stars (NSs). Following the supernova explosion, which creates the NS, some of the explosion ejecta may fail to escape and remain bound - forming a fallback disc. Such fallback discs are a general prediction of current supernova models (Michel & Dessler 1981; Chevalier 1989), and have been invoked by a number of authors to explain a variety of phenomena related to NSs (e.g., Chatterjee et al. 2000; Alpar 2001; Menou et al. ⋆Based on ESO observations made with the APEX-telescope at Llano de Chajnantor under programme 81.D-0777 † E-mail: bposselt@cfa.harvard.edu 2001; Blackman & Perna 2004). Recently, Wang et al. (2006) reported the discovery of mid-infrared emission from a cool disc around the Anomalous X-ray pulsar AXP 4U 0142+61. Wang et al. (2006) interpreted their detection as a passive disc, while Ertan et al. (2007) argued that it could also originate from an actively accreting disc. To date, AXP 4U 0142+61 is the only isolated neutron star for which a fallback disc is believed to have been detected. Such discs appear to be rare. According to Ek¸s˙I & Alpar (2005) fallback disc are rare because they are likely to be disrupted when the newly born NS spins rapidly through the propeller stage, at which in-flowing matter, instead of being accreted, would be expelled. The fallback discs can survive if the initial NS spin is slow enough (⩾40 ms at a magnetic moment of µ = 1030 G cm3). Jones (2007) studied the effect of pulsar wind induced ablation of fallback discs. He concluded that long-lived discs could be present in many pulsars without exceeding published limits on IR luminosity. In the following, we report on submillimetre observations of RX J1856.5–3754, which is the brightest and closest member of a class of NSs neglected so far in the search for circumstellar discs, the X-ray thermal isolated neutron stars. They are peculiar because they show pure thermal soft X-ray spectra without any (confirmed) non-thermal emission, especially no confirmed radio emission; for re- views, see Kaplan (2008) or Haberl (2007). These objects 2 B. Posselt et al. have periods in the range of 4 to 12 s. Thus, they are much slower than the bulk of radio pulsars. Their X-ray pulse periods and period derivatives are similar to those of the AXPs and soft gamma-ray repeaters, SGRs, (see, e.g., Kaplan & van Kerkwijk 2009). This has led to discussions about whether they may be related to those objects. For example, Alpar (2001, 2007) suggested that the X-ray thermal isolated NSs may simply have accretion discs with smaller masses than those of the AXPs. We note that it is currently no

…(Full text truncated)…

This content is AI-processed based on ArXiv data.