You are an expert at summarizing long articles. Proceed to summarize the following text: 
in the past @xmath015 yr the study of cooling neutron stars in transiently accreting x - ray binaries has emerged as a new approach to understanding the structure of neutron stars . @xcite discussed how the neutron - star crust in transiently accreting systems could be driven out of thermal equilibrium with the neutron - star core . some of the heat that is generated by non - equilibrium nuclear reactions deep in the neutron - star crust during an outburst @xcite is radiated away in quiescence and this should result in observable cooling of the neutron - star surface , especially in systems that have undergone long - duration outbursts @xcite . by following the detailed evolution of the surface temperature during quiescence one can then extract information on the properties of the neutron - star crust .    the neutron - star transient ks 1731260  provided the first opportunity to test this when it returned to quiescence following an outburst of more than 12.5 yr @xcite . during its first year in quiescence ks 1731260  indeed showed significant cooling @xcite , and this continued during the next decade @xcite . following ks 1731260 , cooling of neutron stars heated by transient accretion has been studied in a handful of systems : mxb 165929 @xcite , xte j1701462  @xcite , exo 0748676 @xcite , and igr j174802446 @xcite . these systems had outburst durations ranging from @xmath011 weeks to @xmath024 yr , and the outburst - averaged luminosities ranged from a few percent of the eddington luminosity ( @xmath5 ) to @xmath0@xmath5 . the ( long - term ) cooling seen in these systems has been modeled by a variety of cooling curves from increasingly sophisticated thermal evolution codes ( see , e.g. , * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? together this has resulted in valuable insights into the thermal conductivity of the neutron - star crust , the location of heat sources in the crust , and , in some cases , the cooling mechanisms at work in the neutron - star core and crust ( e.g. , * ? ? ? * ) . the temperatures of the neutron star in maxi j0556332  during its first @xmath0500 days in quiescence are extraordinarily high compared to those measured for other cooling neutron stars ( see figure [ fig : sources ] ) . even at 500 days into quiescence its temperature is higher than the maximum quiescent temperature seen in the other five cooling systems . it likely is also hotter than the neutron star in sax j1750.82900 , which was claimed by @xcite to be the hottest quiescent neutron star in an x - ray binary system , although maxi j0556332  is expected to continue cooling for a while . the obtained temperatures for maxi j0556332  are sensitive to the assumed distance ( see table [ tab : fits ] ) , but even for distances well below our estimated distance range ( e.g. , 20 kpc ) we find temperatures ( 134218 ev for model i and 131195 ev for model ii ) that are substantially higher than those observed in xte j1701462  during its first @xmath0500 days ( 125163 ev ) . the short cooling timescale observed in maxi j0556332  implies a high thermal conductivity of the crust , similar to the other cooling neutron stars that have been studied . given the similarities between the outbursts of maxi j0556332  and xte j1701462 , it is interesting to compare these two systems in more detail , as it may help us understand what causes the neutron - star crust in maxi j0556332  to be so hot . maxi j0556332  was in outburst for @xmath0480 days with a time - averaged luminosity of @xmath01.7@xmath9 ergs@xmath10 , while xte j1701462  was in outburst for @xmath0585 days with a time - averaged luminosity @xmath02.0@xmath11 ergs@xmath10  @xcite . the total radiated energies of the maxi j0556332  and xte j1701462  outbursts are therefore 7.1@xmath12 erg and 1.0@xmath13 erg , respectively . despite the fact that the radiated energies and time - averaged luminosities of the two systems are comparable , the initial luminosity of the thermal component ( which reflects the temperature at shallow depths in the crust at the end of the outburst ) is an order of magnitude higher in maxi j0556332  than in xte j1701462 . this suggests the presence of additional shallow heat sources in the crust of maxi j0556332  and/or that the shallow heat sources in maxi j0556332  were more efficient per accreted nucleon . the high observed temperatures are difficult to explain with current crustal heating models . bringing the initial temperatures down to those seen in xte j1701462  requires a distance of @xmath01015 kpc ( depending on the assumed model ) . such distances are problematic for several reasons . first it implies that z source behavior in maxi j0556332  is observed at much lower luminosities ( by factors of nine or more ) than in other z sources . second , fits to the quiescent spectra with such a small distance are of poor quality . finally , a smaller distance does not solve the fact that crustal heating appears to have been much more efficient per accreted nucleon than in other sources . a reduction in distance by a factor of three results in a reduction in luminosity and presumably then , by extension , the total mass accreted onto the neutron star and total heat injected into the crust by a factor of nine . given that we inferred @xmath030% less mass accreted onto the neutron star in maxi j0556332  during its outburst than in xte j1701462  for our preferred distance of @xmath045 kpc , this would mean @xmath012 times less mass accreted onto maxi j0556332  than xte j1701462 yet similar initial temperatures . the nsa model that we used to fit the thermal emission from the neutron star in maxi j0556332  did not allow us to explore values of the neutron - star parameters other than @xmath14 and @xmath15 km , as these parameters are advised to remain fixed at those values @xcite . while other neutron - star atmosphere models allow for changes in @xmath16 and @xmath17 , none of the available models are able to handle the high temperatures observed during the first @xmath0200 days of quiescence . it is , of course , possible that the properties of the neutron star in maxi j0556332  are significantly different from those in the other cooling neutron - star transients that have been studied . lower temperatures would be measured if one assumed a lower @xmath16 and/or a larger @xmath17 . to estimate the effects of changes in neutron - star parameters we used the nsatmos model to fit the spectrum of observation 11 , initially assuming @xmath14 and @xmath1810 km . while keeping the distance from this fit fixed , and changing @xmath16 to 1.2@xmath2 and @xmath17 to 13 km ( values that are still reasonable ) , the measured temperature was reduced by only @xmath010% . such changes are not large enough to reconcile the temperatures measured in maxi j0556332  with those of the other sources .    an alternative explanation for the high inferred temperatures could be that part of the quiescent thermal emission is caused by low - level accretion . indications for low - level accretion in early quiescence were seen in the form of small flares in the _ swift_/xrt light curve ( figure [ fig : curves]b ; see below for a more detailed discussion ) . the level of accretion in between those flares , where most of our observations were done , is unclear however . @xcite suggest that low - level accretion onto neutron stars can result in thermal emission , although it would be hard to distinguish it from that of a cooling neutron star @xcite . an additional hard x - ray component could be produced by emission from a radiatively inefficient ( advection - dominated ) accretion flow @xcite and/or by energetic protons illuminating the neutron - star surface @xcite . as shown in section [ sec : spectra ] , we find that the addition of a power - law to our spectral model ( to fit the hard , non - thermal emission ) presents a minor , but statistically significant improvement to our fits of the quiescent spectra of maxi j0556332 . we also find that , when adding a power - law , the fluxes of the power - law and thermal components are strongly correlated ( see table [ tab : bestfit2 ] ) , which one would expect if low - level accretion played a significant role . however , the index of the power - law is unusually low and when fixing it to commonly observed values ( @xmath19=1.52 ) the addition of a power - law no longer presents a statistical improvement . it is possible that the nsa model simply does not present a good - enough description of the thermal emission from a non - accreting neutron star and that the ( shallow ) power law is merely fitting the resulting discrepancies ; this would likely also result in correlated behavior between the nsa and power - law fluxes , as observed . @xcite showed that xte j1701462  likely had not yet reached a steady - state temperature profile at the end of its 19-month outburst . as a result of this , the crust temperature profile had a dip at intermediate densities , leading to a temporary plateau in the temperature decay followed by late - time cooling , both of which have now been observed in xte j1701462  ( j.k . fridriksson et al . , in preparation ) . given the outburst similarities between xte j1701462  and maxi j0556332 , we expect the cooling curve of maxi j0556332  to eventually show similar features , although their onset may occur at different times than in xte j1701462 .    in observations 4 and 6 large increases in the neutron - star temperature and luminosity and power - law luminosity were seen with respect to the overall trend . this behavior is similar to that of two flares seen during the quiescent phase of xte j1701462  @xcite . additional flares can be seen in the _ swift _  light curves shown in figure [ fig : curves]b . significant flare - like variations in the quiescent luminosity of other ns - lmxbs have been reported as well : e.g. , a strong @xmath2016 d flare in sax j1750.82900 @xcite , a similarly strong ( @xmath03@xmath21 ergs@xmath10 ) @xmath204 day flare in ks 1741293 @xcite , and a @xmath01 week @xmath09@xmath21 ergs@xmath10 flare in grs j17412853 @xcite . variations on timescales of hours to years have also been observed in the quiescent emission of cen x-4 @xcite , aql x-1 @xcite , and sax j1808.43658 @xcite . the origin of these flares and variations are almost certainly accretion related . the decay timescale of the second flare observed in xte j1701462  is similar to that of the main outburst decay @xcite , suggesting a common ( possibly viscous ) timescale . the decay of the first of the flares seen in maxi j0556332  with _ swift _ , a @xmath010 d flare which ended just before our first _ chandra _  observation ( see figure [ fig : curves]a ) , is reasonably well resolved with _ swift _  and has a decay timescale of a few days , also similar to that of the outburst decay . moreover , the power - law flux increases strongly during the flares and such strongly variable power - law components are not expected from a cooling neutron star , whereas they are commonly associated with low - level accretion , both onto neutron stars and black holes . finally , the fact that the thermal and non - thermal component are correlated during the flares suggests that the accretion flow is reaching the neutron - star surface . for this reason , the flare observations are not considered in our modeling of the cooling curves . in addition to the early flaring activity , maxi j0556332  showed a much stronger rebrightening event , starting around day @xmath0170 of quiescence ; it lasted for @xmath060 days , reached a luminosity of @xmath08@xmath22 ergs@xmath10 , and had an average luminosity of @xmath04.4@xmath23 ergs@xmath10 . although the fluence of this event was a factor of @xmath033 smaller than that of the main outburst , it is only @xmath030% smaller than , e.g. , the 2010 outburst of igr j174802446 , in which moderate heating occurred @xcite . however , our observations after the large rebrightening event did not suggest strong deviations from the ongoing cooling trend . the distance to maxi j0556332  remains an uncertain factor in our fits to the quiescent spectra and modeling of the cooling curve , and this complicates the comparison with other sources . we have employed two methods to estimate the distance to the source : one was a comparison of the cd / hid tracks with those of other z sources and the other the quiescent spectral fits themselves . in the other cooling neutron - star transients that have been studied , quiescent spectral fits suffer in general from a high degree of degeneracy between the normalization ( which depends on the distance and neutron - star radius ) and the temperature of the neutron - star atmosphere spectral component . due to the low effective temperature , the small effective area of the detectors at the lowest energies , and the effects of interstellar absorption , it is primarily the high - energy tail of the thermal component that is being fitted , and usually fits of similar quality can be obtained for a rather wide range of distance ( and/or radius ) values . this degeneracy is broken to a significant extent in maxi j0556332  due to the very low absorption towards the source and the ( presumably ) much higher temperatures exhibited by it . the fits are quite sensitive to the value of the normalization and it was not possible to achieve fits of adequate quality except for large distance values . our confidence in such a large value for the distance to the system is strengthened by the fact that the fits with these large distance values also give @xmath24  values that are consistent with other measurements , whereas significantly smaller distance values do not , and the fact that the best - fit distance ( @xmath045@xmath13 kpc for @xmath17 = 10 km ) is nicely consistent with the range we estimate from the cd / hid tracks of the source during outburst ( @xmath046@xmath115 kpc ) . our distance estimates put maxi j0556332  far into the galactic halo . while such a location is not common for x - ray binaries , there are several other systems whose properties also suggest very large distances : e.g. , gs 135464 has a distance greater than 27 kpc @xcite and igr j170913624 could be as distant as 35 kpc if its mass and luminosity are similar to grs 1915 + 105 @xcite , although both sources lie close to the galactic plane . an optical spectrum of the companion star in quiescence might provide a better handle on the distance to maxi j0556332  than the above methods . we are grateful to harvey tananbaum , belinda wilkes , and the _ chandra _ team for allowing us to use the remaining time of a previously triggered target of opportunity program . we also thank norbert schartel for approving two _ xmm - newton _  director s discretionary time observations , and neil gehrels and his team for the flexible scheduling of our _ swift _  observations . j.h .  would like to thank the members of sron utrecht , where part of this work was done , for their hospitality . this research has made use of data , software , and/or web tools obtained from nasa s high energy astrophysics science archive research center ( heasarc ) , a service of goddard space flight center and the smithsonian astrophysical observatory . support for this work was provided by the national aeronautics and space administration through _ chandra _  award number go213052x issued by the _ chandra _ x - ray observatory center , which is operated by the smithsonian astrophysical observatory for and on behalf of the national aeronautics space administration under contract nas8 - 03060 , and through _ swift _  award number nnx12ad56 g . r.w .  was supported by an european research council starting grant . finally , we thank the referee for his / her constructive comments .
we present _ chandra _ , _ xmm - newton _ , and _ swift _  observations of the quiescent neutron star in the transient low - mass x - ray binary maxi j0556332 . 
 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 j0556332  had near- or super - eddington luminosities for a large part of its @xmath016 month outburst . 
 a comparison of these diagrams with those of other z sources suggests a source distance of 46@xmath115 kpc . 
 fits to the quiescent spectra of maxi j0556332  with a neutron - star atmosphere model ( with or without a power - law component ) result in distance estimates of 45@xmath13 kpc , for a neutron - star radius of 10 km and a mass of 1.4 @xmath2 . 
 the spectra show the effective surface temperature of the neutron star decreasing monotonically over the first @xmath0500 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 ( @xmath3=184308 ev ) are much higher than those seen for any other neutron star heated by accretion , while the inferred cooling ( @xmath4-folding ) timescale ( @xmath0200 days ) is similar to other sources . 
 fits without a power - law yield higher temperatures ( @xmath3=190336 ev ) and a shorter @xmath4-folding time ( @xmath0160 days ) . 
 our results suggest that the heating of the neutron - star crust in maxi j0556332  was considerably more efficient than for other systems , possibly indicating additional or more efficient shallow heat sources in its crust .