Calibrator Sources for GMRT and their Spectral Properties

A Thesis submitted to the

UNIVERSITY OF PUNE

for the degree of

Master of Science

(Partly by Papers and Partly by Research)

(in Physics)

by

JANGAM MANISHA SADASHIV

Giant Meterwave Radio Telescope,
National Center for Radio Astrophysics,
TIFR,
Pune $-$ 411 007.

October, 2006

DECLARATION

This thesis 'Calibrator Sources for GMRT and their Spectral Properties' is a presentation of my original research work. Wherever contributions of others are involved, every effort is made to indicate this clearly, with due reference to the literature, and acknowledgment of collaborative research and discussion.

The work was done under the supervision of Sr. Prof. S. Ananthkrishnan at the GMRT, National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune.

Jangam Manisha S.

In my capacity as supervisor of Ms. Manisha Jangam, I certify that the above statements are true to the best of my knowledge.

Sr. Prof. S. Ananthkrishnan Thesis Supervisor

Place : Pune Date : October 17$^{th}$, 2006.

Acknowledgment

No work can go single handedly. I would like to thank a lot to 'Lord Krishna' for His divine blessings. I'm quite grateful to my guides Prof. A.P. Rao and Prof. S. Ananthkrishnan for their valuable time and guidance at every stage of the project.

I thank to Prof. A.P. Rao who acted as the co-guide and helped very significantly in the thesis work. I thank him for teaching me AIPS (data analysis software) procedures and techniques of low frequency data analysis. I also thank to him for following up the project work, his very useful comments during thesis writing, his encouragement and understanding.

I would like to thank Prof. S. Ananthkrishnan for being official guide for the thesis work and for his useful comments during thesis writing.

I thank to Prof. Rajaram Nityananda and Prof. A.P. Rao for allotting telescope time for making observations of the project work. I also thank Prof. Rajaram Nityananda for the courses he gave on Synthesis Imaging. I thank to Dr. Ishwara Chandra, Dr. Ramana Athreya and Shri. S. Sabhapathy for their help and comments during this project work. I would also like to thank Dr. Sabyasachi Pal for helping me with the data analysis of about 50 sources at 235 MHz. I would like to give my special thanks to Dr. Dharam Vir Lal for clearing my fundamentals and his help regarding data analysis and most importantly for moral support throughout the project work.

I thank all my colleagues in the GMRT control room, Aba, Mangesh, Jitendra, Sanjay, Jayprakash, Manish, Santaji, Deepak and Sachin for helping me during the observations and project work. Thanks to Santaji and Sanjay for their useful programs which I used for this project work. Thanks to Jitendra for his useful comments regarding AIPS procedures. And thanks to all for providing a nice and encouraging environment in the control room.

I would like extend my gratitude to all my friends, Ayesha, Babita, Jyoti, Nutan, Manisha, Sangita, Neeraj, Anand, Jayanta, Ramchandra, Sachin P, Sachin S, Vishal, Sanjay, Yogesh, Koyel, Sandhya, Kalindi, Aparna and Swapna for providing very cheerful and nice environment at GMRT, Khodad and NCRA, Pune.

Finally, I deeply indebted to my husband, Santosh for his understanding and support throughout the work. I dedicate this report to him and to my parents.

SYNOPSIS

GMRT is the largest telescope in the world operating at low radio frequencies, viz., 150 MHz, 235 MHz, 325 MHz, 610 MHz and 1000-1450 MHz. To observe any source in the sky at these frequencies we need to observe a phase and a flux density (also called as the amplitude) calibrator for correcting the phase and the amplitude variations introduced in the source signal due to the observing instrument as well as the ionosphere. These calibrator sources should preferably be point sources at these observing frequencies. Alternatively, non-point source can also be used, provided it has sufficient flux density on all baselines and its structure (i.e. the UV model) is also known.

The main purpose of this project is to image large number of radio sources and identify good calibrator sources at both 610 and 235 MHz, using the GMRT. At GMRT, simultaneous observations at 610 and 235 MHz can be made and therefore it is very useful to have a set of calibrator sources which can be used at both these observing frequencies.

Another key motivation is to study the spectra of radio sources. The variation of the flux density ($S$) with the frequency ($\nu$) is expressed by the proportionality,

$$S \propto \nu^{-\alpha},$$

where, $\alpha$ is the spectral index.

The spectra of radio sources is the main source of information regarding the physical processes inside them. All the GMRT calibrator sources emit non-thermal radiation, which is due to synchrotron emission from high energy electrons spiraling in magnetic fields. Such sources often show anomalous behavior at low radio frequencies due to several effects, e.g., synchrotron self absorption, free-free absorption, Razin-Tsytovich effect, spectral steepening due to surrounding diffuse emission, etc. We will also use our GMRT observations for large number of calibrator sources to investigate these effects.