tokens: Acknowledgments This work is supported by the Scientific and Technological Research Council of Turkey (TBTAK) under the grant 113F354. Preliminary results of this paper were presented at NewCompStar 2015 Annual Meeting held in Budapest, Hungary, which was supported by COST action MP1304. I am grateful to M. Ali Alpar for fruitful discussions on this and related topics over the years. I wish to express my special thanks to Peter S. Shternin for useful remarks on the cooling of neutron stars. I acknowledge the referee for insightful comments that lead to clarification of some points.
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