Skip to main content

Astrophysical Masers

I thought I'd take care of my daily blog post business earlier today so, here's the 2 page report I made for a presentation as part of a course on Laser physics & Applications -

Astronomical Masers
by Poruri Sai Rahul


PH5814 - Laser Physics & Applications


Masers are the microwave analogues of Lasers. So far, as part of the course, we’ve been looking at optical and IR lasers. Similar to optical lasers, masers can be produced by atomic/molecular species such as Ammonia, Hydrogen, Rubidium & Free electron masers and by solid state masers such as the Ruby maser & the Fe-sapphire maser. While lasers emit in the Thz regime, masers emit in the Ghz regime.


Coming to the astronomical significance of masers, Astronomers equipped with spectroscopes started observing very strong spectral lines from astronomical sources. These sources were extremely bright in the radio spectrum while their optical counterparts looked like young stars. This emission was also discrete and not a continuum which is what caused astronomers to suggest masing of light from the background star by a medium between us and the star. Further studies revealed that such maser lines were peculiar to young type stars and were constrained mostly to star-forming regions in the galaxy. It was concurred that such maser lines are being produced by the amplification of stellar light by an expanding shell of atoms around the star, in our line of sight.


To understand the differences, let’s go back to the theory of lasers. The basic design of a laser is that of two mirrors and a lasing medium between them to provide the necessary amplification or gain. The mirrors are also highly reflective causing multiple reflections of light before it exits the cavity. We can refer to such a laser as a Multi-pass laser. Drawing this analogy, we can look at an astronomical maser as a Single-pass laser. Unlike multi-pass lasers where the pump beam is bounced back and forth multiple times through the gain medium, in single pass astronomical masers, the pump light from the background star only passes through once through the gain medium. In normal lasers, we needed multi-pass to be able to achieve meaningful gain. In astronomical masers, this is achieved using an astronomically long gain medium!


Another important thing to note is the condition of the star-forming region or the masing region. The molecules along the line of sight, which are causing the masing, should have similar velocities. This is another reason why masers are observed specifically around young stars as young stars have expanding gas shells around them, where species are collectively moving together.


Now, to get an understanding of the kind of numbers we are dealing with, let’s look at the strength of the pump, density of the gain medium, velocities of the masing species and overall size of the gain medium we’re looking at. Astronomical masers usually have a pump with a luminosity of 10^4 solar luminosity. Density of the masing medium is of the order of 10^9 per cm^3. Velocities of species are of the order 100-300km/s. Brightness of the maser lines is of the order of 10^12 K and radius of the gain medium/shell is of the 10^17 cm. Temperatures of the gain medium on the other hand should be ~160K.


NOTE : While the temperature mentioned seems absurd, it is infact the temperature of a blackbody with the intensity of the observed line at the particular frequency. This is merely a convention in Radio astronomy. But one can understand the


As you can understand by now, we overcame the inadequacies of single pass over multi-pass laser by having an extremely strong pump, a dense and cold masing medium over a very long length.


Commonly observed species that emit astronomical masers are - OH, H_2O, CH, H_2CO CH_3OH, NH_3, HC_3N and HCN.


Coming to interesting properties of such masers, as is common to lasers, line narrowing occurs in maser lines and the maser lines show high polarization. If a global magnetic field exits in the gain medium, as is the case in most star-forming regions, the magnetic field will give us an accurate quantization axis along which the species in the gain medium can emit radiation. Line narrowing is common in masers i.e the peak of the maser line goes up and the width of the line decreases, a characteristic of any maser line.


Looking at the applications of astronomical masers, they’ve been used for distance measurement, to study galactic properties such as the galactic rotation curve, the galactic magnetic field, to study interstellar scattering and to study the evolutionary schemes of late-type stars.


This small report relies largely on the review article Astronomical Masers i.e the reference [1]. As any review article on a field is, it has extensive references to other papers on topics ranging from the discovery of astronomical masers to their properties, unique properties of the different species observed and to the applications of astronomical masers. Reference [4] is a good undergraduate level introduction to the theory of lasers and basic concepts in Astronomical Masers. References [5] and [6] are also short and concise introductions to astronomical masers while references [2] and [3] can be read for a preliminary understanding of masers and astronomical masers, in general.




PS - Do get back to me if there are any corrections that need to be made or if I said something absolutely horribly wrong which needs to be corrected!

Popular posts from this blog

Animation using GNUPlot

Animation using GNUPlotI've been trying to create an animation depicting a quasar spectrum moving across the 5 SDSS pass bands with respect to redshift. It is important to visualise what emission lines are moving in and out of bands to be able to understand the color-redshift plots and the changes in it.
I've tried doing this using the animate function in matplotlib, python but i wasn't able to make it work - meaning i worked on it for a couple of days and then i gave up, not having found solutions for my problems on the internet.
And then i came across this site, where the gunn-peterson trough and the lyman alpha forest have been depicted - in a beautiful manner. And this got me interested in using js and d3 to do the animations and make it dynamic - using sliders etc.
In the meanwhile, i thought i'd look up and see if there was a way to create animations in gnuplot and whoopdedoo, what do i find but nirvana!

In the image, you see 5 static curves and one dynam…

Pandas download statistics, PyPI and Google BigQuery - Daily downloads and downloads by latest version

Inspired by this blog post : https://langui.sh/2016/12/09/data-driven-decisions/, I wanted to play around with Google BigQuery myself. And the blog post is pretty awesome because it has sample queries. I mix and matched the examples mentioned on the blog post, intent on answering two questions - 
1. How many people download the Pandas library on a daily basis? Actually, if you think about it, it's more of a question of how many times was the pandas library downloaded in a single day, because the same person could've downloaded multiple times. Or a bot could've.
This was just a fun first query/question.
2. What is the adoption rate of different versions of the Pandas library? You might have come across similar graphs which show the adoption rate of various versions of Windows.
Answering this question is actually important because the developers should have an idea of what the most popular versions are, see whether or not users are adopting new features/changes they provide…

Adaptive step size Runge-Kutta method

I am still trying to implement an adaptive step size RK routine. So far, I've been able to implement the step-halving method but not the RK-Fehlberg. I am not able to figure out how to increase the step size after reducing it initially.

To give some background on the topic, Runge-Kutta methods are used to solve ordinary differential equations, of any order. For example, in a first order differential equation, it uses the derivative of the function to predict what the function value at the next step should be. Euler's method is a rudimentary implementation of RK. Adaptive step size RK is changing the step size depending on how fastly or slowly the function is changing. If a function is rapidly rising or falling, it is in a region that we should sample carefully and therefore, we reduce the step size and if the rate of change of the function is small, we can increase the step size. I've been able to implement a way to reduce the step size depending on the rate of change of …