How to Trace a Celestial Lighthouse Using a Block of Metal?

The relation between polarization angle of Crab pulsar and the phase of its rotation.

Indian astronomers announced on 6th November 2017 that they have “measured phase-resolved X-ray polarisation from the Crab pulsar using the CZT Imager on AstroSat”.

Light is an electromagnetic wave, and one of its important characteristics is the direction in which the electric and magnetic fields oscillate. This is known as polarisation. Most of the light we normally see around us has no preferred direction on the average, and is therefore unpolarised. Sometimes, though, the source of light or the medium in which it is travelling, can bias the electric and magnetic fields to oscillate in a particular direction, hence making it polarised. In astronomy, polarisation has been extensively studied in the radio and optical light for many decades now.

X-rays are light waves of very high energy, with wavelengths as small as the size of atoms. However, detecting polarisation from X-rays from celestial sources is a whole different story. The signal in X-rays is usually quite weak, and it is very difficult to build a detector that can measure X-ray polarisation easily. X-ray polarisation was measured for the first time from the Crab Nebula in 1972 and later in 1976 from satellites. However, since then no other X-ray sources have been observed to have polarization.

The Cadmium-Zinc-Telluride (CZT) Imager instrument aboard AstroSat has been designed to measure polarisation using a clever trick. An incoming X-ray photon from a celestial object interacts with atoms in a ‘pixel’ of the detector and gets absorbed. Sometimes, it can interact with a charged particle in the detector, resulting in the emission of a secondary lower energy photon (Compton Scattering). This second photon will then be absorbed in a neighbouring pixel. If the incoming photons are unpolarised, that is, have no preferred direction, then this second photon can be emitted in any direction. If we average over many such events, all angles of emission of the second photon will be equally likely to occur. However, if the incoming X-ray photon was polarised, and hence had a preferred direction, the direction of the second photon would also have a preferred direction. If we average over many such events, we will be able to measure this direction, and hence infer the polarisation angle of the original photon as well.

Astronomers used CZTI to observe the Crab Pulsar over many months. This pulsar is a neutron star that spins about its axis 30 times a second and is one of the strongest sources in the X-ray sky. Since the CZT Imager can count individual photons with an accuracy of about 20 microseconds, astronomers could now hope to study, for the very first time, how the X-ray polarisation varied over the duration of a rotation of the pulsar, which is 33 milliseconds. To measure the polarisation and ascribe to it a precise pulsar phase, astronomers needed to know the exact timing of the pulsar rotation. This was achieved with the help of the Giant Meterwave Radio Telescope as well as the Ooty Radio Telescopes.

The data had a surprise in store for them. We know that pulsars are neutron stars that act like lighthouses. As the pulsar rotates, when the lighthouse beam sweeps past Earth, we observe a pulse, hence the name ‘pulsar’. When the lighthouse beam carrying enormous energy is pointing away from us during the rotation, we do not expect to see much radiation. However, Indian astronomers saw that X-ray polarisation was high, as well as changing rapidly during this ‘off-pulse’ part of the rotation. This is unexpected, and will send theorists back to their desks, to try and modify their models of pulsar emission to explain these observations.

This marks the beginning of the field of X-ray polarimetry in astronomy. ISRO recently approved a new satellite called XPOSAT with a payload, POLIX, that is being designed at RRI, dedicated to study X-ray polarisation. This will be launched in 2019. In the meanwhile, AstroSat has opened up this new field of astronomy that will let us probe some of the most energetic and enigmatic objects in our Universe.

Click here for a press release.