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On The Cooling Of The Neutron Stars RX J1856-37 & IE1207.4-52

31/08/2011

A neutron star, born in a gravitational collapse with an initial temperature of ~1011K cools rapidly via neutrino emission from the core (Yakovlev & Pethick 2004). For about one minute following its birth, the star stays in a special proto-neutron star state: hot, opaque to neutrinos and larger that an ordinary neutron star (Pons et al. 2001). Later the star becomes transparent to neutrinos generated in its interior and transforms into an ordinary neutron star, where its temperature is independent of the structure (Shapiro & Teukolsky 1983).

The cooling of neutron stars is affected by many factors. Arguably the most important factor is the rate of neutrino emission from the interior of the neutron star (Yakovlev & Pethick 2004). Supplemented to this is heat capacity of the stellar interior and its associated thermal conductivity (Gnedin et al. 2001) as well as reheating mechanisms such as the frictional dissipation of rotational energy (Glen & Sutherland 1980).

Fig.1: Left ~ Theoretical considerations of a proton superfluid crust. Right ~ Temperature of neutron star surfaces as a function of age. (Credit: Yakovlev, D. G. & Pethick, C. J. 2004 Neutron Star Cooling. A&A, 42 pp.169-210.)

At a distance of 117pc RX J185635-3754 is one of the nearest neutron stars to us (Walter et al. 1996). Its identification was based on the thermal spectrum at a temperature of 57eV and the low luminosity of the extremely faint (25.7 mag) optical counterpart. Its associated spectrum can be akin to a pure blackbody spectrum at a temperature of 61eV. An altogether different object is 1E 1207.4-5209, an object located at the centre of the supernova remnant PKS 1209-52.  As opposed to RX J185635-3754, 1E 1207.4-5209 does show significant deviations from a standard black body spectrum.

So why the difference within the same class of compact objects that appear to have the same temperature? The answer probably, although this theory is untested, is quite simple. The spectrum from RX J1856-37 is the pure blackbody spectrum expected from a neutron star, whereas the light from 1E 1207.4-5209 has to escape via the gases and dust of the supernova remnant PKS 1209-52. What must also be taken into consideration is the magnetic structure, which will vary somewhat between different neutron stars (NS).

Since the age of a neutron star within a supernova remnant is sometimes well determined through the historical record these objects make good calibrators for comparison with cooling calculations (Seward & Charles 2010). Modeling of the complex atmosphere is important and, hence, there are many processes and properties that need to be taken into consideration in understanding the cooling rates of NS. In the future, as observational data supplements the wealth of theories surrounding NS structure, the cooling rates of neutron stars can be better appreciated.

Journal References:

  • Drake, J.J. et al. (2002) Is RX J1856.5-3754 A Quark Star? The Astrophysical Journal, Volume 572,(2) pp.996-1001.
  • Bignami, G. F. et al. (2004) 1E1207.4-5209 – A Unique Object. Memorie Della Società Astronomica Italiana, 75 (1) pp.448-454.
  • Yakovlev, D. G.; Pethick, C. J. (2004) Neutron Star Cooling. Astronomy & Astrophysics, 42 (1) pp.169-210.
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