OBSERVING THE VARIABILITY OF AGN: APERTURE PHOTOMETRY VS. PSF FITTING by Eric C ...

50 Pages·2007·1.69 MB·English

OBSERVING THE VARIABILITY OF AGN:
APERTURE PHOTOMETRY VS. PSF FITTING
by
Eric C. Allan
A senior thesis submitted to the faculty of
Brigham Young University
in partial fulfillment of the requirements for the degree of
Bachelor of Science
Department of Physics and Astronomy
Brigham Young University
August 2007
Copyright (cid:13)c 2007 Eric C. Allan
All Rights Reserved
BRIGHAM YOUNG UNIVERSITY
DEPARTMENT APPROVAL
of a senior thesis submitted by
Eric C. Allan
This thesis has been reviewed by the research advisor, research coordinator,
and department chair and has been found to be satisfactory.
Date J. Ward Moody, Advisor
Date Eric Hintz, Research Coordinator
Date Ross Spencer, Chair
ABSTRACT
OBSERVING THE VARIABILITY OF AGN:
APERTURE PHOTOMETRY VS. PSF FITTING
Eric C. Allan
Department of Physics and Astronomy
Bachelor of Science
Photometry of galactic nuclei is adifficult task due mainly to extreme obscura-
tion and light contamination from the nuclear bulge. To reach accuracy levels
that are of interest astrophysically (on the order of 0.05 to 0.001 magnitudes)
requires careful observing techniques and special reduction algorithms. In this
paperwemakeacomparisonbetweenstandardaperturephotometryandDAO
photometry–a point spread fitting technique–to illustrate the pros and cons of
using these techniques to obtain brightnesses of galactic nuclei. We will show
that the DAO method seems to be more effective in all cases, but its accuracy
is inconclusive. We will provide some data for a further comparison between
these two techniques and a third bulge fitting technique.
ACKNOWLEDGMENTS
I would like to thank my research partner Adam Johanson for helping me
along the way. I would also like to thank my research mentor Dr. Moody for
helping me learn what it’s like to be a scientist. I would also like to thank the
guys at tech support for always keeping the Linux machines up to snuff.
I would like to thank Tenagra Observatories for sending us these images.
We couldn’t have done it without them.
I would also like to thank my wife who supported me as I worked on this
project.
Contents
Table of Contents vi
List of Figures vii
1 Introduction 1
1.1 Significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 Observations 4
2.1 Tenagra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Data 7
3.1 Data Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1 Aperture Photometry . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.2 DAO Photometry . . . . . . . . . . . . . . . . . . . . . . . . . 11
4 Analysis 14
4.1 M81 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 M101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3 M51 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4 The Three Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5 Conclusion 19
Bibliography 21
A Light Curves For 6-Pixel Aperture 22
B Light Curves For 3-Pixel Aperture 28
C Light Curves For DAOphot 34
D Tables of Error Values 40
D.1 M81 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
D.2 M101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
D.3 M101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
vi
List of Figures
2.1 Tenagra II Telescope . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 M81 and Comparison Stars . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 M101 and Comparison Stars . . . . . . . . . . . . . . . . . . . . . . . 10
3.3 M51 and Comparison Stars . . . . . . . . . . . . . . . . . . . . . . . 10
3.4 Light Profile of a Star . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.5 Light Profile of a Saturated Star . . . . . . . . . . . . . . . . . . . . . 12
3.6 Light Profile of a Galaxian Nucleus . . . . . . . . . . . . . . . . . . . 13
vii
Chapter 1
Introduction
Active Galactic Nuclei (AGN) are characterized by several factors including a bright
un-resolved source, highly variable brightness and emission by non-thermal radiation
which is usually Inverse Compton Scattering or synchrotron radiation. They are
extremely luminous. The variability range can cover several magnitudes while the
varability rate is as short as a few days or even several hours. Light travel time
arguments on the variability constrain the varying region to be on the order of the
size of the solar system.
The small size of AGN coupled with their rapid variability constrain the possible
causes of this heavily studied phenomenon. The standard model for AGN assumes
that their luminosity stems from supermassive black holes (of order 106 - 109 times
solar) at the centers of the galaxies. [Carrol et al. (1996)] The black holes form
accretion disks as gravity-captured matter spirals into them. The matter undergoes
a huge loss of potential energy as it approaches the black hole. This energy loss is
converted to electromagnetic radiation, which interacts with the halo of gas and dust
surrounding the black hole.
Recent studies also indicate that black holes are probably present at the center
1
1.1 Significance 2
of every galaxy. [Van der Marel (1999)] This evidence comes from observing veloc-
ity dispersion profiles of the galaxian bulges. That is, nearly all galaxies show an
increase in stellar velocities in their centers that is consistent with the presence of a
supermassive black hole. If this is true, then we should be able to observe the same
characteristics of AGN in every normal galaxy as well, though perhaps on a relatively
small level.
1.1 Significance
Theamountofvariabilityinanormalgalacticnucleusishardtopredict. Butitissafe
to assume that it would be smaller in magnitude range than a typical AGN. There-
fore in order to observe these fluctuations in a normal galaxy, we need to determine
the most accurate method of photometry for extended objects like galaxies. Most
of the methods currently used were developed for point sources like stars. Galaxy
nuclei may often be stellar-like sources but they are surrounded by resolved bulges
containing hundreds of millions of stars. The bulge light creates problems with both
aperture photometry and with point-spread function (PSF) fitting techniques such
as DAOphot. My research partner’s previous results were inconclusive as to which of
these two methods was more accurate. [Johanson (2007)] We will attempt to clarify
this ambiguity.
A new method, proposed by professor Stephen McNeil from BYU Idaho as part of
his thesis work [McNeil (2004)], uses bulge light as a reference against which nuclear
light can be compared. The light coming from the bulge originates from billions of
stars, and should be stable down to millimagnitudes. This bulge referencing method
carefully maps the bulge light using concentric apertures then subtracts them from
their neighbors to find the amount of light in each ring.
1.1 Significance 3
We intend to perform the best aperture photometry on galaxy nuclei that could
possibly be done. We will do these preliminary measurements to provide data for
a comparison with Professor McNeil’s method. This will hopefully give credibility
to the new bulge fitting technique and pave the way for an extensive observation of
galactic nuclei. In the end, this study will provide insights into the obscuring bulges
of galactic nuclei and allow us to probe around the black holes. If this method can
be perfected it will give additional evidence for the existence of black holes in every
galaxy and will possibly allow us to understand a little more about galactic evolution.