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Probing the Geometry & Physics of the Emission Region of AGN using Hard X-ray
and Gamma -ray Observations
Natasha Woods
Mentors: Marco Ajello & Masaaki Hayashida
How will you be doing the comparisons?
An active galactic nucleus (AGN) is a relatively small region of space at the center of a
galaxy that is very luminous over all or part of the electromagnetic spectrum1. This
radiation is theorized to be a product of massive black holes at the center of all AGN.
AGN are divided into two classes: radio-quiet and radio-loud. In the latter, jets from
AGN dominate the luminosity at radio wavelengths and possibly other wavelengths as
well. On the other hand, radio-quiet galaxies have negligible jet-related emission.
Unified Models for AGNs attempt to prove that all AGN are intrinsically the same and
that the differences observed depend on the viewing angle. Unifying radio-quiet AGNs
requires unifying Seyfert galaxies. These galaxies mainly emit gamma-rays up to ~100
keV(2). Seyfert galaxies are further classified by the wavelengths of their brightest
emissions and the width of their spectra lines. Seyfert 1 galaxies (Sy1) are brightest at
UV and X-ray wavelengths while Seyfert 2 galaxies (Sy2) are brightest at infrared
wavelengths(4). Sy1 galaxies have broad spectra lines due to cold material orbiting the
black hole at high speeds and therefore radiating photons at varying Doppler shifts while
and Sy2 galaxies have narrow spectra lines due to the presence of cold material further
from the black hole causing the photons emitted to move a slower speeds(6).
Another explanation of the observed differences between Sy1 and Sy2 galaxies is that
Sy1 galaxies are viewed from above the accretion disk whereas Sy2 are viewed from a
more edge-on view with respect to the accretion disk. If this is the case the reflection
component (X-ray emission from the hot corona) which is scattered by the accretion disk
or the inside walls of the molecular torus should be more intense in Sy1 than in Sy2.
Furthermore, the X-ray spectrum measured by Swift/BAT2 should be steeper for Sy1. To
determine if this is true a sample of ~200 AGN will be statistically analyzed using a
distribution of power law slopes (intensity as a function of frequency) to see if there is an
intrinsic difference between Sy1 and Sy2s. If one is found it will be quantified by
determining the average covering factor of the reflection medium of Sy1 and Sy2s.
Additionally, stacked spectra will be calculated to determine if there is a high energy cutoff for the radiation from these two classes of AGN.
Most of these analysis techniques will be completed using xspec11, Root, and other
programs in the HEASOFT package (Are you learning the software).
Finally, Fermi-LAT3 has detected gamma-ray emission exceeding 100 MeV from a
couple hundred AGNs. This emission is believed to be caused by jets perpendicular to the
1
AGN are probably responsible for ~100% of the cosmic X-ray background (CXB).
is part of an international mission to study Gamma-ray bursts. The Burst Alert Telescope(BAT) is
designed to identify Gamma-ray bursts' location and brightness (7).
3Large Area Telescope(LAT) is a high-energy gamma-ray instrument on the Fermi Gamma Ray
Telescope(8).
2Swift
accretion disks of radio-loud AGNs. Other theories predict that this radiation is due to
radio-quiet AGNs. Therefore, high energy emission of radio-quiet AGN will also be
examined (what do you mean by examined?) to see if Fermi has detected any such AGN.
REFERENCES
(1) An introduction to AGN by Bradley M. Peterson (1997)
(2)http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html
(3)http://praxis.pha.jhu.edu/papers/papers/afdscirev_b/node9.html
(4)http://scienceworld.wolfram.com/astronomy/Seyfert1Galaxy.html
(5)Galaxies in Turmoil by Chris Kitchin (2007)
(6)http://www.astro.isas.jaxa.jp/~takahasi/English-Version/swift-e.html
(7)http://www-glast.stanford.edu/