Are pulsars having a (gravitational) wobble?

Title: Re-visiting gravitational wave events via pulsars
Authors: Minati Biswal, Shreyansh S. Dave, and Ajit M. Srivastava
First author’s institution: Institute of Physics, Bhubaneswar 751005, India
Status: Open access on arXiv

Gravitational waves (GW) are ripples in space-time caused by some of the most energetic processes in the universe. Typically, these waves occur when large, dense bodies like black holes or neutron stars begin accelerating towards each other. The decreasing orbit (inspiral) followed by the collision (merger) releases energy in the form of these waves, which propagate outwards from the site of the event at exactly the speed of light (a prediction of general relativity and later confirmed by the LIGO detector).

 It is difficult to get a sense of how energetic these events are. The first ever detected black hole merger event released 3 solar masses worth of gravitational energy (we can equate mass to energy using this equation), peaking at a rate of roughly 4 x 10^49 watts. This is more than the combined power of all light radiated by all the stars in the observable universe put together.

Although we can readily detect gravitational wave events, there are outstanding issues with accurately pinpointing their origin on the sky. There are a couple of reasons for this:

1) Gravitational wave detectors, such as LIGO and Virgo, are not pointed at the sky like conventional telescopes. Instead, they detect distortions in the path of light beams within the instrument, using a technique known as interferometry.  This makes them very sensitive to the presence of GW events, but not their location. There are ways to localise GW signals using a network of multiple detectors but these are presently not very precise.

2) Most GW events so far have been from black hole collisions. Black holes, by definition, do not emit light, which makes it impossible for telescopes to find accompanying electromagnetic signals associated with such events.

Concerns with 2) changed fairly recently when a binary neutron star merger led to the first observation of an electromagnetic counterpart to a GW event, in the form of high-energy gamma and X-ray emission (see this Astrobite for more on this famous event). Today’s authors are now pointing at another observable tracer for gravitational wave events of the past and future: pulsars.

Pulsars are a class of neutron stars, which emit beams of high-energy radiation, visible only when the beam is pointed in the direction of the observer. Neutron stars have the additional property of having short, regular rotational periods, thus they appear to pulse. These periods can be milliseconds to seconds, based on the size of the pulsar, and can be measured extremely precisely. 

In today’s paper, the authors suggest that gravitational waves passing through pulsars can cause them to wobble, affecting the pulsar’s moment of inertia, which in turn affects its spin rate. In other words, the speed at which a pulsar is observed to be spinning can be changed by an amount that is detectable by telescopes, and can therefore be used as a sign for a GW event. Interestingly, pulsars might themselves be able to produce a certain kind of gravitational wave, which you can find out about in this Astrobite. The bite you are currently reading, however, focuses only on the use of pulsars as GW tracers.

An additional property called resonance could result in the pulsar ‘ringing’ (i.e. spinning at an altered rate for up to 10 minutes after the gravitational wave event, which itself lasts milliseconds), allowing for an even clearer signal to be detected, provided the pulsar is located somewhat close (a thousand light years) to the GW event.

Figure 1: Schematic highlighting the arrival of a GW from a GW source along path C. However, since GW propagate radially (in a circle around a point), they will also traverse path B, encounter a pulsar and distort its spin rate. The resulting distorted pulses can be observed by us via path A. Figure 1 in paper.

The authors suggest that for GW events which have been detected already by LIGO and Virgo such as the binary neutron star merger, astronomers should look for specific pulsars which might have been affected and observe their current and future spin rates.

More interestingly however, is that these pulsar distortions can be used to pick up on gravitational wave events we might have missed in the past. Supernovae, the bright and energetic explosions at the end of stellar life, have long been an exciting candidate for gravitational waves. However, no gravitational waves associated with supernovae have been observed so far. As the core of a supernova collapses, the resulting dense proto-neutron star collects material around the explosion. During this turbulent accretion process, the neutron star vibrates, sending off gravitational waves. The only problem is that a supernova would have to go off fairly close (i.e. within our galaxy) for the LIGO detectors to detect it, and there is no way of precisely predicting when the next Milky Way supernova will be (at a rough ~2 per century).

The authors expect supernovae events to have a strong effect on galactic pulsars. It may be possible to identify elusive GW signals from supernovae using future observations of pulsars  or by analysing recorded pulse data from the past. What is needed now is an analysis of nearby and distant pulsar spin rates for distortions, in order to determine if they reveal any missed gravitational wave events of the past or future.

About Sunayana Bhargava

I'm a 3rd year PhD student at the University of Sussex, looking at X-ray observations of galaxy clusters to learn more about dark matter and large-scale structure.

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