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Neutron star: A laboratory in the sky

Sudip Bhattacharyya

23 February 2012

Apart from black holes, neutron stars are the densest known objects in the universe. Moreover, while light from a black hole cannot escape, a neutron star can be seen directly by the light it emits. This makes these stars immensely important for probing some extreme aspects of the universe and for testing fundamental laws of nature, which cannot be done by experiments in terrestrial laboratories. For example, the density of a cold neutron star core is several times the nuclear density, and hence exotic matter (such as deconfined quark) may exist there. This is a fundamental problem of particle physics, and the only way to solve this problem is to measure the mass and radius of a neutron star using astronomical techniques. Another fundamental problem of physics is to test Einstein's general theory of relativity, the most accepted law of gravitation, in the strong gravity regime. X-rays originated from a region very close to the neutron star can be useful to test a law of gravitation. Other fundamental aspects of nature can be probed using extreme properties of neutron stars, such as nuclear burning in a very strong gravity, a surface magnetic field between 100 to 10 billion times the highest magnetic field ever produced in a laboratory, etc. Therefore, neutron stars are nature-provided laboratories in the sky.

At TIFR, these extreme aspects of neutron stars are studied in order to probe the fundamental physics, which is inaccessible by experiments. Two X-ray observational aspects, among others, from these stars are used as tools for such probing. These aspects are thermonuclear X-ray bursts and relativistic spectral emission lines. The former one, which originates from the neutron star surface, is a truly multi-disciplinary field, involving astrophysics, nuclear physics and fluid dynamics, and has implications for particle physics and laws of gravitation. At TIFR, a recent discovery (reference 1) of an unsuspected correlation between two properties of these bursts made them a more reliable tool to measure the neutron star radius. Note that, such a measurement, which is essential to probe the high-density degenerate matter, is extremely difficult. The second observational aspect, i.e., the spectral line, originates from the accretion disc close to a neutron star. This line, which is affected by special relativistic and gravitational effects, can provide useful ways to measure neutron star parameters and to test the general theory of relativity. The discovery of such lines by a current TIFR member in 2007 opened up these new ways (reference 2).

References:

1. Sudip Bhattacharyya, M. Coleman Miller and Duncan K. Galloway, "Systematic variation in the apparent burning area of thermonuclear bursts and its implication for neutron star radius measurement", Monthly Notices of the Royal Astronomical Society, 401, 2, 2010.

2. Sudip Bhattacharyya and Tod E. Strohmayer, "Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-Ray Binary Serpens X-1", Astrophysical Journal Letters, 664, L103, 2007.

Q & A

Tomin : Has the findings helped experimental astrophysicists in finding the radius of a neutron star? What prediction of general theory of relativity does neutron stars validate.

Sudip Bhattacharyya : Astrophysicists are working on thermonuclear X-ray bursts in order to measure the radii of neutron stars. Future X-ray satellites, such as Astrosat and LOFT, will be useful for this purpose. Studies on neutron stars have the potential to validate the predictions of general theory of relativity, such as existence of frame-dragging and innermost stable circular orbit.

Tomin : Does neutron stars have uniform density through out? Does every neutron star pulsate? How does neutron stars form accretion disks?                     

Sudip Bhattacharyya : The density of a neutron star decreases by about 14 orders of magnitude from the centre to the surface. No, every neutron star does not pulsate. When a neutron star accretes matter from a companion star, this matter can form a disk because of its large initial angular momentum and the fluid dynamical reasons.

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