Some neutron stars may continuously emit gravitational waves

Mumbai, Dec 15 : A study by Indian researchers suggests that a population of neutron stars can generate gravitational waves continuously, a finding that could provide an opportunity to study these waves almost permanently.

The study by Professor Sudip Bhattacharyya of the Tata Institute of Fundamental Research (TIFR), Mumbai, and Professor Deepto Chakrabarty of the Massachusetts Institute of Technology (MIT) in the US found that a population of neutron stars should spin around their axes much faster than the highest observed spin rate of any neutron star.

The observed lower spin rates are possible if these neutron stars emit gravitational waves continuously, and hence spin down, the researchers said.

Gravitational waves emitted by massive objects is a prediction of Albert Einstein's general theory of relativity, which has recently been discovered during transient phenomena of black hole mergers.

But the detection of continuous gravitational waves, which could provide an opportunity to study these waves almost permanently, is still elusive.

Neutron stars are the densest observable objects in the universe, with a fistful of stellar material outweighing a mountain on Earth.

While such stars are not bigger than a city, in size, they have more material than in the Sun crammed inside them.

A population of these stars can increase their spin rate by the transfer of matter from a normal companion star.

In fact, some of them have been observed to spin several hundred times in a second around their own axes.

In the 1970s, it was theoretically worked out how fast these neutron stars could spin, and since then this has formed the basis of studies of these stars.

But the new study -- published in the The Astrophysical Journal -- showed that for episodic mass transfer, which happens for many neutron stars, the stellar spin rate should be much higher, and the star could easily attain a spin rate more than a thousand times per second.

Since no neutron star has been observed with such a high spin rate, the team pointed out that many of these stars are likely to be slowed down by continuously emitting gravitational waves.

The new study provides a strong indication that many fast spinning neutron stars generate gravitational waves continuously, and careful observations should be made to detect such waves.