Study of Supernovae using the
GROWTH-India Telescope
Supervised Learning Project
Maitreya Khandagale
Supervisor: Prof. Varun Bhalerao, IIT Bombay
Abstract
Supernovae are bright explosions of stars resulting from the final evolutionary stages of a star
or thermonuclear runaway of a white dwarf. The study of these events is extremely important,
as it helps shed light on wide ranging topics in astrophysics and cosmology. A stark example
of this is the use of Type Ia supernovae as distance probes or standard candles to measure the
acceleration of the expansion of the Universe in the Standard Cosmological Model. Supernovae
also play an essential role in the synthesis of numerous elements in the periodic table barring the
lightest. The aim of this project is to perform an analysis of young supernovae using scientific data
acquired using the fully automatic 70cm GROWTH-India Telescope located in Hanle, Ladakh.
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Contents
1 Introduction
1.1 GROWTH Collaboration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 GROWTH-India Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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2 Supernovae: Bright Explosions in the Sky
2.1 Supernova Explosion Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1 Thermal Runaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Core-Collapse Supernova . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Supernova Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Type I Supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Type II Supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Photometry
3.1 Light Curve Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Aperture Photometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Point-Spread function fitting(PSF) Photometry . . . . . . . . . . . . . . . . . . . . .
3.4 DAOPHOT: A tool for crowded field photometry . . . . . . . . . . . . . . . . . . . .
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4 Lightcurves of GIT Data
4.1 SN2019np: A type Ia SN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 SN2019gaf: A type IIb SN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5 Acknowledgements
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6 References
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1
Introduction
Time-domain astronomy deals with the study of astronomical events and their subsequent evolution
over a period of time. These events can be of various types including gamma-ray bursts, supernovae,
novae, asteroids, blazars and many more. Transients are astronomical events that are visible in the
sky for a short duration of time, ranging from a few days to a few months.
The term supernova was first coined and used in 1931 to refer to bright eruptions of stars more
powerful than novae which can be interpreted as the end of a star’s lifetime. These eruptions are several
orders of magnitudes higher than the Sun making them ideal targets for optical telescopes all across
the globe. Studies revealed that our own Galaxy was a host of many such events recorded in history
such as the Crab Nebula (SN1054 AD), Tychos Supernova (SN1572 AD), and Keplers Supernova
(SN1604 AD), among many others discovered more recently with modern detection methods. Study
on explosion phenomenon in various types of supernovae has led to the discovery that the members
of a particular class the Type Ia supernovae explode in such a way as to be considered a standard
candle. The brightness of these objects, visible in galaxies at vast distances, means that they can be
used to determine the size and the expansion rate of the Universe.
Figure 1: SN1994D(bright spot on the lower left) within its host galaxy NGC 45261
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http://www.spacetelescope.org/images/opo9919i/
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1.1
GROWTH Collaboration
GROWTH-Global Relay of Observatories Watching Transients Happen is an international collaboration of various institutions and observatories across the world, led by Caltech, dedicated to study
fast-changing events in the cosmos like supernovae, neutron stars or black hole mergers and near-earth
asteroids. The main aim of the collaboration is to coordinate all the telescopes to efficiently follow up
astronomical transients which is huge factor when it comes to time-domain astronomy.
Figure 2: Telescopes part of the GROWTH collaboration2
1.2
GROWTH-India Telescope
A crucial branch of the GROWTH collaboration is the fully robotic, remotely controlled 0.7 meter
optical telescope known as the GROWTH-India Telescope(GIT). The telescope is located at the Indian
Astronomical Observatory in Ladakh, India. Situated at 4500 meters above mean sea level, one of
the highest observatory sites in world which makes it an ideal location for astronomical observations.
GIT is a crucial node in the GROWTH collaboration for time-domain astronomy. It is controlled via
a satellite connection to the Centre For Research and Education in Science and Technology(CREST),
Indian Institute of Astrophysics, Bangalore operated jointly by IIA, Bangalore and IIT Bombay, and
saw its first light in June, 2018.
Figure 3: GROWTH India Telescope3
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http://growth.caltech.edu/observatories.html
https://sites.google.com/view/growthindia/
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As a member of the IIT Bombay team that operates the GIT, I have been a part of the team that
has been involved in the remote observations of supernovae, gamma-ray bursts and electro-magnetic
counterparts to gravitational waves. I also contributed to the scanning of compact binary mergers
detected by the Zwicky Transient Facility. Listed below are the Gamma-ray Coordinates Network
Circulars(GCNs) submitted by the GROWTH-India team and the GROWTH Collaboration to which
I have contributed in:
• 24351 : LIGO/Virgo S190426c: GROWTH India follow-up of GRAWITA transient
• 24745 : GRB190530A: Photometric follow-up with GROWTH-India Telescope
• 24748 : GRB190604B: GROWTH-India Telescope followup-no optical counterpart found
• 25616, 25656 : LIGO/Virgo S190901ap: Candidates from the Zwicky Transient Facility
• 25632 : LIGO/Virgo S190901ap: GROWTH-India follow-up of ZTF19abvionh
• 25706 : LIGO/Virgo S190910d: Candidates from the Zwicky Transient Facility
• 25722, 25727 : LIGO/Virgo S190910h: Candidates from the Zwicky Transient Facility
• 25899 : LIGO/Virgo S190930t: Candidates from the Zwicky Transient Facility
However my major science goal this semester has been the study of young supernovae and the corresponding data analysis. I have been working under the guidance of two PhD students, Anirban Dutta
and Avinash Singh, from the Indian Institute of Astrophysics, Bangalore. In the subsequent sections
I have presented the theoretical aspects of supernovae and related data analysis techniques I learnt
over the duration of the semester.
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2
Supernovae: Bright Explosions in the Sky
Supernovae are stellar explosions which release massive amounts of energy in various forms including
radiant energy such as neutrinos, gravitational waves and the entire electromagnetic spectrum, kinetic
energy of ejecta, high energy cosmic rays. The energy released by a supernova produces astronomical phenomena that depends on the type of progenitor star, and its circumstellar and interstellar
environment including whether the progenitor had a binary companion.
(a)
2.1
(b)
Supernova Explosion Models
A star can go supernova in either of the following two ways:
2.1.1
Thermal Runaway
A white dwarf is a very dense star composed mainly of Carbon and Oxygen. These are stars that
are supported only by electron degeneracy pressure. In 1930, S. Chandrashekhar showed that such
configurations can only exist if the star had a certain range of mass with an upper limit of 1.44 M ,
which is famously known as the Chandrashekhar Limit. Any white dwarf having mass beyond this
limit will collapse and appear as a supernova. The collapse is caused by either the accretion of matter
from a nearby star, a binary companion, or from the merger of two white dwarfs to produce a new
star that is over the Chandrashekhar Limit. Fusion of Carbon and Oxygen nuclei releases energy
and raises the temperature of the star. The degeneracy pressure being independent of temperature,
a runaway thermonuclear reaction ensues leading to the collapse of the star. No compact remnant is
left as result of this mechanism.
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2.1.2
Core-Collapse Supernova
Over their lifetime, most massive stars generally evolve in such a way that their resulting core is
consisting of heavy elements, typically Fe, but for the least massive stars in this range, a mixture of
O/Ne/Mg/Si is observed. The core is surrounded by an envelope whose properties depend on various
factors of the progenitor star like its binarity, mass-loss history, metallicity. Such stars which have their
masses in the range of 8 to 140 M produce a core-collapse supernova. Core-collapse can be a result of
various processes including the core, which is essentially a white dwarf, exceeding the Chandrashekhar
Limit; electron capture by Mg followed by explosive O fusion; pair-instability or photodisintegration.
Core-collapse supernovae can leave a neutron star, a black hole as a stellar remnant or no remnant in
some cases.
2.2
Supernova Classification
Supernovae are primarily classified into two types depending on the presence of Hydrogen lines in
the supernova spectrum. Further sub-classification is made on the basis of varying properties of the
spectrum and the observed lightcurves of the supernovae. Type I SN do not show any Hydrogen
features in their spectra whereas Type II do.
2.2.1
Type I Supernovae
• Type Ia: These supernovae are the most commonly observed class of supernovae and the most
luminous resulting from explosions of white dwarfs pushed over the Chandrashekhar Limit. They
are characterized by strong ionised silicon absorption line at 6150 Å.
• Type Ib: Supernovae of these type are a result of core-collapse. SNe Ib are defined by the
presence of prominent He I absorption lines at 5876 Å, 6678 Å, 7065 Å. Strong evidence suggests
that this class has massive star progenitors which seem to have retained only a small fraction of
their original hydrogen envelope.
• Type Ic: SNe Ic are almost similar to Ib but do not show He I features. They often show a
prominent Si II feature at 6150 Åand a strong O I at 7774 Å. Virtually all Gamma-ray Bursts
and X-Ray Flashes are accompanied by a SNe Ic-BL, a subclass which shows the existence of
broader emission lines.
Figure 4: Lightcurve Comparison 4
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https://upload.wikimedia.org/wikipedia/commons/e/e0/Comparatives upernovat ypel ightc urves.png
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2.2.2
Type II Supernovae
• Type II-P: Normal Type II SN showing broad Hydrogen lines for the lifetime of decline. These
SN show a distinctive plateau after attaining peak luminosity which stays relatively constant for
a few months before the decline resumes.
• Type II-L: These are Normal Type II SN which lack a distinct plateau. Here, the ’L’ stands
for Linear although the decline might not necessarily be linear.
• Type IIn: This class of SNe has relatively narrow features in their spectra hence the ’n’. Observations showing emission line spectra for >10 days is usually required for a IIn classification.
Gal-Yam et al. 2014; Khazov et al. 2016
• Type IIb: These SNe show Hydrogen emission lines at early times but later are dominated by
Helium lines. They can be said to be a combination of normal Type II and Ib.
Figure 5: SN II-P and SN II-L features5
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https://upload.wikimedia.org/wikipedia/commons/0/0c/SNIIcurva.svg
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Photometry
Brightness of stars in astronomical images is the most important detail in optical astronomy. The
brightness is measured in terms of a logarithmic scale where the units are called as magnitudes. The
apparent magnitude(m) is the brightness of an object as visible in the sky and is calculated as
m − mref = −2.5log(
F
)
Fref
where mref and Fref are the apparent magnitude and the observed flux of the reference star respectively, whose magnitude we know. F is the observed flux of the target that we are interested in.
However in practice we know the number of counts in the pixels of the camera used. This number of
counts is directly proportional to the incoming flux of the star where the constant of proportionality
changes with the exposure time i.e higher exposure time gives more number of counts resulting in a
higher flux. Zero point is defined as the magnitude of the star that gives one count in the image. The
true apparent magnitude is given by,
m = −2.5log(counts) + ZP
The process of finding the Zero Point of an image and thus measuring the corresponding magnitude
of stars is termed as photometry.
3.1
Light Curve Analysis
A light curve of any astronomical transient serves a crucial purpose in its study. It is a graph showing
the varying luminosity of the target as a function of time. The light curve analysis of a supernova
over a significant amount of time helps us in identifying its properties.
3.2
Aperture Photometry
Aperture photometry is all about calculating the photometry of a target using the right aperture
size. For instance, if we choose an aperture of smaller size than the target, then we can say that a
significant portion of the light coming from the target will be left out of the aperture and the resulting
flux calculation will be inaccurate. Similarly, if we choose a large aperture, light from various other
source will be included and will result in a higher total flux and we won’t be able to do photometry
on the desired target. Typically an aperture size of twice the FWHM of the target is chosen, followed
by choosing a larger size for the annulus. The annulus starts at the end of the aperture and extends
upto the given size. The purpose of choosing an annulus is to estimate the background counts and to
remove them, so as to get accurate magnitudes.
F = S − sky ∗ npix
where F is the real flux, S is the total counts in the aperture, sky is the estimated sky background
counts and npix is the number of pixels in the aperture. Then the magnitude is calculated using the
previous equation,
m = −2.5log(counts) + ZP
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3.3
Point-Spread function fitting(PSF) Photometry
All point sources imaged by a detector can be represented by a point-spread function. If this function
is not spatially invariant, we assume that it can be modelled. Given that the shape of a source on
a focal plane is known, the amplitude scales linearly with the brightness of the the star forming the
image. Thus, for any image, we must ’fit’ the PSF template to the observed image, keeping in mind
that the source sits on some specific value of sky background. In principle, the process is straightforward: Identify the stars in our image, determine the background level, use ’isolated’ stars to derive
the PSF template, and then fit this template to all the other stars in the frame. In practice, this
process must be re-iterated several times as one develops an improved PSF model and background
level.
3.4
DAOPHOT: A tool for crowded field photometry
We also used an additional package in IRAF called DAOphot to make measurements. DAOphot
determines and mathematically fits the point spread function introduced on the stellar images using a
numerical deconvolution. We selected several stars on each frame and images of the light profiles are
shown in sequence. We then approved or declined each star based on whether or not it looked like a
good candidate to determine the point spread function. A good candidate looks roughly Gaussian. We
threw out candidates that did not have clearly defined PSFs or that looked saturated. We also did not
include the galactic nuclei in this process because they are extended images with non-gaussian profiles.
Figures 3.4 through 3.6 show examples of these three profiles respectively as they appear in the IRAF
program using DAOphot. Once we had selected at least six candidate stars, the software package
determined a sort of average PSF and then deconvoluted each object before performing photometry
on them. This process was the most time consuming portion of all the data reduction as we had to
set several parameters for each different frame. Specifically, we had to find the gain and the read
noise for each individual frame. We also had to determine the pixel radius for the average star in
each individual frame as well as the pixel radius where the stars luminosity profile fades into the
background noise. We made photometric measurements on the same objects for this process as we
did for the regular aperture photometry. Following are the surface plots of a good star(to be used for
PSF), a saturated star and background:
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Lightcurves of GIT Data
4.1
SN2019np: A type Ia SN
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4.2
SN2019gaf: A type IIb SN
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Acknowledgements
I sincerely thank Prof. Varun Bhalerao and GROWTH for giving me this opportunity to learn and
work with the GROWTH-India Telescope. I am indebted to Anirban Dutta and Avinash Singh for
supervising me throughout my learning on this project, and the GROWTH-India team for helping me
with various aspects of this project.
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References
• ”Handbook of Supernovae” (Springer Reference) Athem W. Alsabti, Paul Murdin
• ”A User’s Guide to Stellar CCD Photometry with IRAF” Phillip Masey, Lindsey E. Davis
• ”Point-Spread Function Fitting Photometry” Heasley, J.N
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