Rapid Cooling of the Neutron Star in the Quiescent Super-Eddington Transient XTE J1701-462

arXiv:1003.3460v2 [astro-ph.HE] 12 Apr 2010 The Astrophysical Journal, 714:270–286, 2010 May 1 Preprint typeset using LATEX style emulateapj v. 2/16/10 RAPID COOLING OF THE NEUTRON STAR IN THE QUIESCENT SUPER-EDDINGTON TRANSIENT XTE J1701–462 Joel K. Fridriksson1,2, Jeroen Homan2, Rudy Wijnands3, Mariano M´endez4, Diego Altamirano3, Edward M. Cackett5, Edward F. Brown6, Tomaso M. Belloni7, Nathalie Degenaar3, and Walter H. G. Lewin1,2 ABSTRACT We present Rossi X-Ray Timing Explorer and Swift observations made during the final three weeks of the 2006–2007 outburst of the super-Eddington neutron star (NS) transient XTE J1701–462, as well as Chandra and XMM-Newton observations covering the first ≃800 days of the subsequent quiescent phase. The source transitioned quickly from active accretion to quiescence, with the luminosity dropping by over 3 orders of magnitude in ≃13 days. The spectra obtained during quiescence exhibit both a thermal component, presumed to originate in emission from the NS surface, and a non-thermal component of uncertain origin, which has shown large and irregular variability. We interpret the observed decay of the inferred effective surface temperature of the NS in quiescence as the cooling of the NS crust after having been heated and brought out of thermal equilibrium with the core during the outburst. The interpretation of the data is complicated by an apparent temporary increase in temperature ≃220 days into quiescence, possibly due to an additional spurt of accretion. We derive an exponential decay timescale of ≃120+30 −20 days for the inferred temperature (excluding observations affected by the temporary increase). This short timescale indicates a highly conductive NS crust. Further observations are needed to confirm whether the crust is still slowly cooling or has already reached thermal equilibrium with the core at a surface temperature of ≃125 eV. The latter would imply a high equilibrium bolometric thermal luminosity of ≃5 × 1033 erg s−1 for an assumed distance of 8.8 kpc. Subject headings: accretion, accretion disks – stars: neutron – X-rays: binaries – X-rays: individual (XTE J1701–462) 1. INTRODUCTION Understanding the internal properties of neutron stars (NSs) remains one of the major unresolved problems in as- trophysics. Efforts to constrain these properties most com- monly focus on narrowing down the allowed regions in NS mass–radius diagrams, to thereby rule out some of the pro- posed equations of state for the matter inside the stars. An alternative approach is to observe the cooling of NSs (for a review see Yakovlev & Pethick 2004). Initially, this ap- proach focused mainly on the long-term cooling of isolated NSs over the first ∼106 years after their birth, but it has in the past decade been extended to NSs reheated by transient accretion. As matter from a binary companion is accreted onto the surface of a NS, matter already present is compressed further down into the crust to higher densities. This leads to heating due to nuclear reactions, so-called deep crustal heating (Brown et al. 1998; Rutledge et al. 2002; Haensel & Zdunik 2008). Most of the heat is produced by 1 Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139; joelkf@mit.edu. 2 MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139. 3 Astronomical Institute “Anton Pannekoek,” University of Ams- terdam, Science Park 904, 1098 XH, Amsterdam, The Netherlands. 4 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands. 5 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109. 6 Department of Physics and Astronomy, National Superconduct- ing Cyclotron Laboratory, and the Joint Institute for Nuclear As- trophysics, Michigan State University, East Lansing, MI 48824. 7 INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy. pycnonuclear reactions hundreds of meters below the sur- face and is spread throughout the star by heat conduction. Cooling takes place via neutrino emission from the interior and photon emission from the surface. In ∼104 years the NS enters a limit cycle in which heating during accretion episodes is on average balanced by cooling during and be- tween outbursts (Colpi et al. 2001). The temperature of the NS core is not expected to change appreciably after this, and its value depends on the long-term time-averaged mass accretion rate as well as on the efficiency of the cooling mechanisms at work, which in turn depend sensitively on the properties of the material inside the star. High-mass NSs are thought to potentially have much more powerful neutrino emission mechanisms active in their cores, com- pared to their low-mass counterparts (Yakovlev et al. 2003; Yakovlev & Pethick 2004); this is referred to as enhanced cooling, in contrast to the so-called standard cooling of the low-mass NSs

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