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A Strongly Heated Neutron Star in the Transient Z Source Maxi J0556-332

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A Strongly Heated Neutron Star in the Transient Z Source Maxi J0556-332 A STRONGLY HEATED NEUTRON STAR IN THE TRANSIENT Z SOURCE MAXI J0556-332 The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Homan, Jeroen, Joel K. Fridriksson, Rudy Wijnands, Edward M. Cackett, Nathalie Degenaar, Manuel Linares, Dacheng Lin, and Ronald A. Remillard. “A STRONGLY HEATED NEUTRON STAR IN THE TRANSIENT Z SOURCE MAXI J0556-332.” The Astrophysical Journal 795, no. 2 (October 22, 2014): 131. © 2014 The American Astronomical Society As Published http://dx.doi.org/10.1088/0004-637x/795/2/131 Publisher IOP Publishing Version Final published version Citable link http://hdl.handle.net/1721.1/94549 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The Astrophysical Journal, 795:131 (12pp), 2014 November 10 doi:10.1088/0004-637X/795/2/131 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. A STRONGLY HEATED NEUTRON STAR IN THE TRANSIENT Z SOURCE MAXI J0556–332 Jeroen Homan1, Joel K. Fridriksson2, Rudy Wijnands2, Edward M. Cackett3, Nathalie Degenaar4, Manuel Linares5,6, Dacheng Lin7, and Ronald A. Remillard1 1 MIT Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Avenue 37-582D, Cambridge, MA 02139, USA; [email protected] 2 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands 3 Department of Physics & Astronomy, Wayne State University, 666 W. Hancock St., Detroit, MI 48201, USA 4 Department of Astronomy, University of Michigan, 500 Church Street, Ann Arbor, MI 48109, USA 5 Instituto de Astrof´ısica de Canarias, c/ V´ıa Lactea´ s/n, E-38205 La Laguna, Tenerife, Spain 6 Universidad de La Laguna, Departamento de Astrof´ısica, E-38206 La Laguna, Tenerife, Spain 7 Space Science Center, University of New Hampshire, Durham, NH 03824, USA Received 2014 August 13; accepted 2014 September 12; published 2014 October 22 ABSTRACT We present Chandra, XMM-Newton, and Swift observations of the quiescent neutron star in the transient low-mass X-ray binary MAXI J0556–332. Observations of the source made during outburst (with the Rossi X-ray Timing Explorer) reveal tracks in its X-ray color–color and hardness–intensity diagrams that closely resemble those of the neutron-star Z sources, suggesting that MAXI J0556–332 had near- or super-Eddington luminosities for a large part of its ∼16 month outburst. A comparison of these diagrams with those of other Z sources suggests a source distance of 46 ± 15 kpc. Fits to the quiescent spectra of MAXI J0556–332 with a neutron-star atmosphere model (with or without a power-law component) result in distance estimates of 45 ± 3 kpc, for a neutron-star radius of 10 km and amassof1.4M. The spectra show the effective surface temperature of the neutron star decreasing monotonically over the first ∼500 days of quiescence, except for two observations that were likely affected by enhanced low-level ∞ = accretion. The temperatures we obtain for the fits that include a power law (kTeff 184–308 eV) are much higher than those seen for any other neutron star heated by accretion, while the inferred cooling (e-folding) timescale ∼ ∞ = ( 200 days) is similar to other sources. Fits without a power law yield higher temperatures (kTeff 190–336 eV) and a shorter e-folding time (∼160 days). Our results suggest that the heating of the neutron-star crust in MAXI J0556–332 was considerably more efficient than for other systems, possibly indicating additional or more efficient shallow heat sources in its crust. Key words: accretion, accretion disks – stars: neutron – X-rays: binaries – X-rays: individual (MAXI J0556-332) Online-only material: color figures ∼ 1. INTRODUCTION to 24 yr, and the outburst-averaged luminosities ranged from a few percent of the Eddington luminosity (LEdd)to∼LEdd.The In the past ∼15 yr the study of cooling neutron stars in (long-term) cooling seen in these systems has been modeled transiently accreting X-ray binaries has emerged as a new by a variety of cooling curves from increasingly sophisticated approach to understanding the structure of neutron stars. Brown thermal evolution codes (see, e.g., Shternin et al. 2007;Brown et al. (1998) discussed how the neutron-star crust in transiently & Cumming 2009; Page & Reddy 2012, 2013; Turlione et al. accreting systems could be driven out of thermal equilibrium 2013; Medin & Cumming 2014). Together this has resulted in with the neutron-star core. Some of the heat that is generated by valuable insights into the thermal conductivity of the neutron- non-equilibrium nuclear reactions deep in the neutron-star crust star crust, the location of heat sources in the crust, and, in some during an outburst (Haensel & Zdunik 1990) is radiated away cases, the cooling mechanisms at work in the neutron-star core in quiescence and this should result in observable cooling of the and crust (e.g., Schatz et al. 2014). neutron-star surface, especially in systems that have undergone In this paper we present the first observations in quiescence of long-duration outbursts (Wijnands et al. 2001; Rutledge et al. the cooling neutron star in MAXI J0556–332, an X-ray transient 2002a). By following the detailed evolution of the surface that was discovered in 2011 January with MAXI (Matsumura temperature during quiescence one can then extract information et al. 2011). The source had been in outburst for more than 16 on the properties of the neutron-star crust. months when it returned to quiescence in 2012 May. Although The neutron-star transient KS 1731–260 provided the first conclusive proof, such as thermonuclear bursts or pulsations, opportunity to test this when it returned to quiescence following is still missing, there is compelling evidence that the source an outburst of more than 12.5 yr (Wijnands et al. 2001). contains a neutron star. First, the optical-to-X-ray and radio- During its first year in quiescence KS 1731–260 indeed showed to-X-ray flux ratios (Russell et al. 2011; Coriat et al. 2011) significant cooling (Wijnands et al. 2002), and this continued were found to be similar to those of Galactic neutron-star low- during the next decade (Cackett et al. 2006, 2010a). Following mass X-ray binaries (NS-LMXBs). Second, Rossi X-ray Timing KS 1731–260, cooling of neutron stars heated by transient Explorer (RXTE) data of MAXI J0556–332 revealed strong accretion has been studied in a handful of systems: MXB similarities to the so-called “Z sources” (Homan et al. 2011;see 1659–29 (Wijnands et al. 2003, 2004; Cackett et al. 2006, also Sugizaki et al. 2013). The Z sources form a class of near- 2013a), XTE J1701–462 (Fridriksson et al. 2010, 2011), EXO and super-Eddington NS-LMXBs that typically trace out distinct 0748–676 (Degenaar et al. 2009, 2011b, 2014;D´ıaz Trigo et al. three-branched tracks in their X-ray color–color diagrams (CDs) 2011), and IGR J17480–2446 (Degenaar et al. 2011a, 2013). and hardness–intensity diagrams (HIDs). Combined with other These systems had outburst durations ranging from ∼11 weeks properties (e.g., correlated spectral/timing behavior, scarcity 1 The Astrophysical Journal, 795:131 (12pp), 2014 November 10 Homan et al. Table 1 Log of Quiescent Observations Used for Spectral Fitting Obs. Mission ObsID Start Time Mid Time Exposure Timeb (UT) (MJD)a (ks) 1a Swift 00032452004 2012 May 7 20:24 56054.9215 2.4 1b Swift 00032452005 2012 May 9 04:31 56056.2274 2.3 1c Swift 00032452006 2012 May 11 03:12 56058.1759 2.7 1d Swift 00032452007 2012 May 13 03:27 56060.2504 2.7 2 Chandra 14225 2012 May 21 02:23 56068.1662 9.2 3 Chandra 14226 2012 May 28 08:59 56075.4444 9.1 4 Chandra 14429 2012 Jun 5 22:08 56084.0173 13.7 5 Chandra 14433 2012 Jun 25 02:22 56103.2258 18.2 6 Chandra 14227 2012 July 29 09:32 56137.5281 18.2 7 XMM-Newton (MOS1) 0700380901 2012 Aug 17 09:45 56156.5735 28.6 XMM-Newton (MOS2) ... 2012 Aug 17 09:45 ... 28.7 XMM-Newton (pn) ... 2012 Aug 17 10:45 ... 21.5 8 XMM-Newton (MOS1) 0700381201 2012 Sep 16 23:50 56187.3194 26.2 XMM-Newton (MOS2) ... 2012 Sep 16 23:50 ... 26.8 XMM-Newton (pn) ... 2012 Sep 17 00:51 ... 17.4 9 Chandra 14434 2012 Oct 2 21:22 56203.0145 17.9 10 Chandra 14228 2013 Feb 20 18:18 56343.9210 22.7 11 XMM-Newton (MOS1) 0725220201 2013 Sep 13 23:17 56549.2302 44.9 XMM-Newton (MOS2) ... 2013 Sep 13 23:18 ... 44.5 XMM-Newton (pn) ... 2013 Sep 14 00:19 ... 34.8 Notes. a XMM-Newton mid times are from EPIC-MOS1 observations. b Live good exposure time after filtering out time intervals affected by particle flaring. of type-I X-ray bursts) these track shapes set the Z sources In this paper we report on the spectral evolution of MAXI apart from the less-luminous “atoll sources.” MAXI J0556–332 J0556–332 during its first ∼500 days in quiescence. In Section 2 was found to switch between two types of Z source behavior we provide a brief description of the observations and our data (Homan et al. 2011), making it very similar to XTE J1701–462 analysis. The outburst properties, an improved distance estimate, (Lin et al. 2009b; Homan et al. 2010), the first transient Z and fits to the quiescent spectra from the initial cooling phase source, which accreted at near/super-Eddington rates for the are presented in Section 3.
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