Eye in the sky: Indian experts make pulsating discovery

A team of Indian researchers has recently reported the discovery of six exotic celestial bodies known as millisecond pulsars (MSPs), one of which holds the potential to join an array of pulsars that are considered to be the best clocks in the galaxy.

The discovery, made with the Giant Metrewave Radio Telescope (GMRT) in Pune, comes as a significant contribution from India in an area that has gripped astrophysicists globally since the discovery of the first MSP in 1982.

Pulsars have diametres of about 20 kilometres but the mass equals that of our Sun. MSPs are a sub-class of pulsars that spin up to several hundred times per second around their own axes. They are dense neutron stars that had ‘died’ but have returned to life by spinning themselves into incredible speeds, each full rotation taking only a few tens of milliseconds. As it spins, an MSP emits a beam of electromagnetic radiation ? similar to how a lighthouse casts a beam ? which is picked up as pulses by radio telescopes. That also explains why a pulsar that has slowed down considerably and which no longer emits radiation is considered ?dead?.

The timing of these pulses is comparable to the accuracy of atomic clocks, currently the best source of time-keeping in the world.

The discoveries of the six MSPs were made by a team from the Pune-based Inter-University Centre for Astronomy and Astrophysics (IUCAA) and the National Centre for Radio Astrophysics (NCRA) whose Giant Metrewave Radio Telescope is the world’s largest telescope operating at metre wavelengths. They had discovered these neutron stars over the past year since they joined the International Pulsar Search Consortium, a network of scientists scanning the sky for gamma-ray emitting MSPs.

The team led by Bhaswati Bhattacharyya, a post doctoral fellow at IUCAA, comprised scientists Jayanta Roy and Yashwant Gupta from NCRA, Dipankar Bhattacharyya from IUCAA and Elizabeth Ferrara from Goddard Space Flight Center. While the six celestial bodies have diverse characteristics, two of them are particularly interesting ? one for its time-keeping quality and the other for helping to explain how a `dead’ neutron star spins itself back to life using a companion star.

The researchers say that the narrow, feature-less profile of one of their millisecond pulsars makes it a good candidate for possible inclusion in the International Pulsar Timing Array, a worldwide project that aims to detect gravitational waves in the galaxy using an array of millisecond pulsars. ?If there is a gravitational wave you may find there is a change in the orbital period, that is, how much time it is taking to orbit the companion. You can detect that minute change because of the stability of the pulsar,? says Roy. ?That’s why the discovery of this MSP is also interesting because if we can contribute to the international timing array, then you can have one more point in space.?

The point he is referring to can be about 4,800 light years away from the Earth. Researchers say that an MSP will probably lose one millisecond in about 10 billion years.

?The long-term predictability of an atomic clock can be improved by comparing it with an MSP because they are really the best clocks in the world. It will help in standardising the atomic clock,? says G Srinivasan, a former professor at the Raman Research Institute in Bangalore and a specialist in condensed matter physics and astrophysics.

The GMRT team has also reported the discovery of a ?Black Widow? system, which refers to a pulsar that spins around a partner star, feeding off the matter and eventually consuming it to become an isolated star. The Black Widow pulsar helps to explain how a dead neutron star gets the energy and angular momentum to start spinning rapidly again.

From 1988 to 2008, only two Black Widow systems had been discovered in the disc of our galaxy, though in recent years scientists globally have been studying a few such systems though the research papers are yet to be published, says Bhattacharyya. The GMRT team is yet to publish its paper on the new findings.

The discoveries of MSPs have also picked up pace since 2008 when the National Aeronautics and Space Administration (Nasa) launched its Fermi Gamma Ray Space Telescope in space, leading to the discovery of 39 MSPs. ?The consortium was made to systematise the search of pulsars using positions provided by Fermi,? says Bhattacharyya.

?I would say it is really observation coming of age, new facilities coming of age. MSPs are exciting from so many points of view: the theory of general relativity, comparison with atomic clocks and as gamma ray-emitting sources, it’s quite remarkable,? says Srinivasan.

He adds that several of the explanations about MSPs, including the theory that they were spun-up objects, date back decades. ?But now that dozens of X-Ray emitting binary systems have been found, they have now been actually been caught not only in the act of spinning up, but having spun up. All these things took a long time to discover,? says Srinivasan.

With the discovery of MSPs in different stages over these past few decades, some of the key answers that researchers are trying to find out through observation include those relating to the pulsar emission mechanism, adds Roy.