Morphing into an all-sky Monitor
Artist’s illustration of a neutron star. Image: Casey Reed –– Penn State University/WikiMedia Commons
Artist’s illustration of a neutron star.
Image: Casey Reed –– Penn State University/WikiMedia Commons

This series illuminates how scientists calibrated CZTI, extended its energy range, and interpreted data. But that’s not all. Scientists always also attempt to see if something more can be gleaned out of the instrument –– if they can put it to use in ways other than intended.

Every astronomical telescope can gather data from a certain fraction of the entire sky. Scientists call this area the ‘field of view’ of their telescope and measure it in degrees. But is it possible for an instrument to see beyond its field of view?

As mentioned in article two, CZTI detects photons possessing energy between 20 to 380 kiloelectronvolt or keV. However, when the photons’ energy is higher than 60 keV, the instrument’s structure becomes transparent, allowing higher energy photons to enter it from all sides. It helps CZTI observe interesting astronomical sources outside its field of view.

One such category of an astronomical source is a ‘pulsar’. When stars much heavier than our Sun run out of fuel, they explode, leaving behind a core made up of neutrons. Over the lifetime of this ‘neutron star’, the core releases pulses of electromagnetic radiation; hence it is called a pulsar.

The pulsars’ electromagnetic radiation can range from radio waves to gamma rays. It originates in the magnetic field surrounding its surface and depends on the geometrical properties of the magnetic field. However, many pulsars do not emit any radiation in the radio wavelengths. So scientists rely on higher energy radiation to study such pulsars.

Crucially for astronomers, only a few telescopes exist which can study pulsars with the higher energy photons –– energy greater than 60 keV. The photons of such energies are lesser than those with lower energies. So, the detecting instrument needs to observe a particular patch of the sky for a long time, on the scale of several months, to detect them. Space instruments cannot afford to do that, as it means losing time studying other interesting astronomical sources.

The CZTI scientists leveraged CZTI’s ability to collect photons with energies higher than 60 keV from all sides to overcome this problem. It enabled the scientists to continue collecting photons from the well-studied Crab pulsar even while AstroSat was busy observing other astronomical targets. They observed the Crab pulsar for two months when it was five to 70 degrees away from the direction in which the instrument was pointing. These observations were long enough to accumulate enough photons from the pulsar. CZTI thus fills the existing gap in the study of pulsars by acting as an all-sky monitor in the hard X-ray band.

By comparing CZTI’s Crab observations with those available already from other instruments such as NASA’s Fermi Large Area Telescope, the scientists established the off-axis pulsar detection capability of CZTI in the hard X-ray band. Thus, they added another powerful tool to CZTI’s arsenal.

The scientists also extended CZTI’s capability of seeing beyond the field of view to astronomical sources that emit flux at a constant rate. While it’s easy to detect transient astronomical sources because they cause a sudden increase and decrease in the number of photon counts detected by CZTI, the same is not so easy for sources that emit a constant number of photons.

The Earth blocks the satellite from observing astronomical sources from time to time, and scientists use this to their advantage. They note the decrease in the photon number as the source goes behind the Earth and again the increase in the photon number as the source emerges from behind the Earth as seen from the satellite. An estimate of the number of photons CZTI should receive when it sees only the Earth is already available from past observations that did just that –– observe the Earth. By subtracting this number from the total photon count, the scientists calculated the number of photons emitted from the constant astronomical source of their interest.

As AstroSat completes five years of its operation, it fills us with joy and gratitude to have been able to present this series to you!.

Series edited by: Debdutta Paul

Unnati Ashar
Unnati Ashar got her MS degree in space physics from CESSI in 2019. She is currently a temporary research staff. She writes as a hobby and is passionate about sharing the wonders of astronomy with people.

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