by S. Prasad Ganti

A recent article in Scientific American about the Celestial transients piqued my mind. To quote “Transients are astronomically sized objects that change on human timescales—in seconds, hours, days. Transients, which are astronomical objects that appear suddenly from nowhere and usually disappear soon after, contradict the standard truth that the universe changes predictably and slowly over billions of years.” I will describe some transients and the telescopes to track them.    

Examples of transients are fast radio bursts, gamma ray bursts, supernovae, core collapses, magnetars etc. Of these, supernovas are well known as they indicate a dying star which sheds all the accumulated elements due to an explosive surge as a pushback against gravity. Supernovae cook higher elements in the periodic table. A Supernovae’s brightness outshines a galaxy and lasts for a few days to months before dying. These shed elements will be inputs for formation of the next generation of stars, like our Sun.   

When a massive star collapses on itself, its electrons fuse with the nuclei of its atoms until the star is made mostly of neutrons—a neutron star—and it shrinks in the space of one second from a radius of about 6,000 kilometers to about 10 kilometers. The collapse causes a shock wave that breaks out and minutes later the star is as bright as 10 billion suns. It fades over months. This is the birth of a neutron star. Our Sun will not end up being a neutron star or a black hole. It is comparatively less massive.  

Another example of transients is Magnetars which existed only in theory until they were observed in 1998. A magnetar is a neutron star that rotates very fast in a few milliseconds. The magnetic field is very high. Our Sun’s magnetic field is about 10 gauss. Comparatively the strength of Magnetar’s magnetic field is 1000 gauss. Hence the name Magnetar. It is highly unstable and reconfigures itself quickly.  

Now that we looked at a few examples of transients, let us look at the kind of telescopes and the facilities needed.It needs a different type of telescope to look for transients. The discovery telescopes, as they are known, need to observe a large swath of the sky in one shot. A wide field camera is required to detect such images. An example is the 48 inch Samuel Oschin Telescope at the Mount Palomar observatory. It is part of a facility called the Zwicky Transient Facility, named for the famous 20th century astronomer Fritz Zwicky. 

The Zwicky Transient Facility consists of 3 telescopes – one is the 48 inch which is the discovery scope, next is the 60 inch which is used to follow up on the finds of the 48 inch. And further follow up is done using the famous 200 inch at Mount Palomar in Southern California. The 48 inch telescope is fully automated which scans wide parts of the sky every night. It is what is referred to as industrial scale operation – night after night with almost no human intervention.  

Although the 48 inch dates from last century, everything else about the telescope is modern. The wide field camera which captures 47 square degrees of the sky in one shot. And each shot lasts about 30 secs. There is a robot arm which places the filters in the path of the camera to capture a part of the spectrum. At that speed, it requires 2 nights to capture the whole northern sky. The whole electronics are maintained at a very low temperature of 100 degree kelvin using liquid nitrogen. To minimize any hardware related noise. 

Southern skies are still unknown but there are other telescopes like the new Vera Rubin telescope, located high up in the Andes mountains of Chile, which does some of the same functions. In addition, it does a lot more. 

Software then compares the images from 2 days back for the same part of the sky to detect what is different. If a difference is detected then it is likely to be a transient. But first, any spurious streaks like satellites or meteor showers should be eliminated. If genuine, an alert is generated for further follow up. Machine Learning is used for classification of events. All the data and the alerts are uploaded to Alert brokers so that different groups of scientists can filter out what is of interest to them.         

Astronomy is as much of a data science as it is just peeping into a telescope. Leading to “Time-Domain astronomy” in which the skies are photographed continuously in the night and interpretations of data are made thereof. And Python is the language used extensively in astronomy (Information Technology professionals, please note).   

Since its inception in 2017 Zwicky Transient Facility has discovered around 10,000 transients. And more importantly, its comparison software and the alert broker infrastructure formed the basis for the recent Vera Rubin telescope in Chile. This seems to be the trend for broader survey telescopes. They scan the skies and store the data so that astronomers can download and interpret them.