a r X i v :a s t r o -p h /0509109v 1 6 S e p 2005D RAFT VERSION F EBRUARY 5,2008Preprint typeset using L A T E X style emulateapj v.6/22/04THE LOW QUIESCENT X-RAY LUMINOSITY OF THE TRANSIENT X-RAY BURSTER EXO 1747–214J OHN A.T OMSICK 1,D AWN M.G ELINO 2,P HILIP K AARET 3Draft version February 5,2008ABSTRACTWe report on X-ray and optical observations of the X-ray burster EXO 1747–214.This source is an X-ray transient,and its only known outburst was observed in 1984–1985by the EXOSAT satellite.We re-analyzed the EXOSAT data to derive the source position,column density,and a distance upper limit using its peak X-ray burst flux.We observed the EXO 1747–214field in 2003July with the Chandra X-ray Observatory to search for the quiescent counterpart.We found one possible candidate just outside the EXOSAT error circle,but we cannot rule out the possibility that the source is unrelated to EXO 1747–214.Our conclusion is that the upper limit on the unabsorbed 0.3–8keV luminosity is L <7×1031erg s −1,making EXO 1747–214one of the faintest neutron star transients in quiescence.We compare this luminosity upper limit to the quiescent luminosities of 19neutron star and 14black hole systems and discuss the results in the context of the differences between neutron stars and black holes.Based on the theory of deep crustal heating by Brown and coworkers,the luminosity implies an outburst recurrence time of >1300yr unless some form of enhanced cooling occurs within the neutron star.The position of the possible X-ray counterpart is consistent with three blended optical/IR sources with R -magnitudes between 19.4and 19.8and J -magnitudes between 17.2and 17.6.One of these sources could be the quiescent optical/IR counterpart of EXO 1747–214.Subject headings:accretion,accretion disks —stars:neutron —stars:individual (EXO 1747–214)—X-rays:stars —X-rays:general1.INTRODUCTIONObservations of transient X-ray binary systems in quies-cence have greatly advanced studies of compact objects over the past decade.While both black hole and neutron star sys-tems can approach or possibly even exceed their Eddington luminosities during outbursts,their quiescent X-ray luminosi-ties can be factors of >107lower than their peak luminosities.Quiescent optical observations allow for the measurement of compact object masses as the optical companion’s radial ve-locity curves can be measured as well as the binary inclination via studies of ellipsoidal modulations (Charles &Coe 2003,and references therein).Such measurements have been espe-cially important for confirming that there is a population of compact objects with masses that are too high (>3M ⊙)to be neutron stars so that these objects are very likely black holes.In the future,mass measurements of neutron star transients may lead to constraints on the equation of state for matter at high densities (Lattimer &Prakash 2004).Quiescent X-ray observations are also interesting for neu-tron star and black hole systems.For many neutron star sys-tems,a thermal component is seen in the quiescent X-ray spectrum that is likely blackbody emission from the neutron star surface.Measurements of the evolution of this compo-nent allow for constraints on the thermal properties of the neutron star crust and core (Wijnands 2004).The core tem-perature is thought to be set by the accretion history over a time period of 10,000years (Brown,Bildsten &Rutledge 1998;Colpi et al.2001).Quiescent X-ray spectra of black hole systems do not appear to show a blackbody component,which is one reason they tend to be fainter than neutron star1Center for Astrophysics and Space Sciences,Code 0424,University of California at San Diego,La Jolla,CA,92093(e-mail:jtomsick@ucsd.edu)2Michelson Science Center,California Institute of Technology,770South Wilson Avenue,MS 100-22,Pasadena,CA 911253Department of Physics and Astronomy,University of Iowa,Iowa City,IA 52242systems.It has also been argued that,in quiescence,black holes are radiatively less efficient than neutron star systems,which has been taken as evidence that a large fraction of the accreted matter is advected across the black hole event hori-zon (Narayan,Garcia &McClintock 1997;McClintock et al.2003).With sensitive X-ray missions like the Chandra X-ray Observatory and the X-ray Multi-Mirror Mission (XMM-Newton),it has been possible to observe many more neutron star and black hole systems in quiescence.As more systems are observed,it has been found that many of the neutron star systems are fainter than the 1032−34erg s −1range usually con-sidered as the typical range for neutron stars in quiescence (Campana et al.2002;Wijnands et al.2005b;Tomsick et al.2004;Jonker,Wijnands &van der Klis 2004).Thus,it is im-portant to obtain the largest samples possible to avoid any selection biases when comparing the quiescent properties of black holes and neutron stars.In this paper,we use archival data as well as our re-cent observations to study the neutron star X-ray transient EXO 1747–214.This system was discovered during the EXOSAT Galactic plane scans in 1984June (Warwick et al.1988;Parmar et al.1985)and was detected again by EXOSAT in 1985April at about the same flux level (Magnier et al.1989;Parmar et al.1985).These are the only two known de-tections of the source,so that it has apparently been in qui-escence for 20years.An X-ray burst was detected in 1985,demonstrating that the system definitely harbors a neutron star (Magnier et al.1989).Although EXOSAT provided a rela-tively good 15′′position for the source,and the extinction along the line of sight to the source (l =7◦.00,b =+2◦.95)is fairly low,the source has not been optically identified.The primary goal of this work is to use a Chandra observation of moderate length to detect the source in quiescence and to search for an optical counterpart.We report on our search be-low,including a constraint on the quiescent luminosity and a comparison between the Eddington-scaled luminosity of
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