Brightness constraint for cooling models of young neutron stars

Abstract

We study the systematics of neutron star cooling curves with three representative masses from the most populated interval of the estimated mass distribution for compact objects. The cooling simulations are made in the framework of the nuclear medium cooling (NMC) scenario using different combinations of possible nucleon-nucleon pairing gaps. Possible heating or enhanced cooling mechanisms in the crust are not considered. We define a constraint on the highest possible temperatures for a given age of young neutron stars and show that this limits the freedom of modeling pairing gaps and crust properties.Typeset using REVT E X 1 Studies of neutron star cooling evolution become very actual due to the presently known surface temperature and age data provided by X-ray observatories such as CHANDRA, XMM Newton and from the ROSAT catalogue [1]. These new data open a wide perspective for nuclear astrophysics for which up to now the knowledge of internal structure of the compact stars and the properties of stellar matter under extreme conditions remain central problems. Theoretical models and hypotheses about the equation of state of high density matter provide different alternatives for the explanation of the same set of observational temperature -age (TA) data points, when additional constraints are not provided. In this work we point out an existing correlation between the crust model and cooling behaviour of light neutron stars, which has a selective power in combination of TA data with the mass spectrum of neutron stars.In our recent investigations of the cooling evolution of neutron stars (NS) we have adopted the so called nuclear medium cooling (NMC) scenario [2], which goes beyond the minimal cooling scenario [3], where in-medium modifications of cooling regulators by definition have been disregarded. Both approaches agree in the philosophy that such very effective cooling mechanism like the direct Urca process should not occur in typical NS. In these approaches the main cooling process is the modified Urca process, which in our NMC scenario also includes the in-medium softening of the pion propagator [4]. Earlier investigations within this cooling scenario [5,6] have chosen the crust model as a simplified Tsuruta law T Tsur s = (10 T in ) 2/3 . Although it is shown in Ref. [7] that the cooling evolution could be essentially affected by the inclusion of internal heating, nevertheless the latter is expected to be important for late time evolution and will not affect the results of this paper. It has been omitted in the present cooling scenario. For more recent reviews on the cooling scenarios see [8][9][10].The cooling simulations presented in this work are based on a code with a number of improved inputs concerning the heat conductivity, the nucleon-nucleon pairing gaps and a new model of the neutron star crust and envelope. These models are basically taken from the recent calculations of Ref. [11], where the amount of light elements in the crust and the 2 influence of the magnetic field have been taken int...

Physics of Neutron Star Crusts

“…Grigorian [176] recently argued that cooling models predicting neutron stars with an age between about 10 3 − 10 4 years to be hotter than those already observed, should be rejected since if such stars existed in our galaxy, they would have already been detected. This brightness constraint puts restrictions on the T s − T b relationship hence on crust models.…”

Nuclear medium cooling scenario in light of new Cas A cooling data and the2M⊙pulsar mass measurements

“…The nuclear medium cooling scenario worked out in Refs. [17,[27][28][29][30] has been successfully applied to the description of the body of known surface temperature-age data of neutron stars [11,[31][32][33][34]. It exploits a strong dependence of the main cooling mechanisms on the density and thus on the neutron star mass.…”

The Zoo of neutron stars

“…Two important additions to these tests are so-called brightness constraint [50] and mass constraint [51]. In [50] it was proposed to take into account the fact that despite many observational efforts very hot NSs with ages ∼ 10 3 -10 4 yrs have not been discovered.…”