Radio emission from gamma-ray flare sources discovered by fermi-lat
Date
2016-05-10
Authors
Van Zyl, Pfesesani Victoria
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Abstract
Quasi-simultaneous observations of a flaring blazar source at multiple frequencies, offer an unprecedented
view of the region surrounding a supermassive black hole during a large energy outburst.
Blazars are active galaxies that host a super massive black hole releasing large amounts of energy
through narrow jets of highly relativistic plasma located along the polar axes. Within these jets,
electrons and protons move at relativistic speeds creating interactions that generate radio waves and
gamma-rays that travel down the jet towards the observer. Based on the angle of inclination of the
source towards the observer ( < 20◦), we can study relativistically boosted emission to peer into
regions where high-energy particles (gamma-rays) are thought to be generated. Using high cadence
monitoring campaigns, both the slow and fast variations in the source flux can be traced in detail
revealing spatial and temporal information about the source state and activity.
In this dissertation I studied the physics behind the variable behavior of the bright blazar
PKS 1424-418, also known as J1427-4206. PKS 1424-418 is a Southern Hemisphere blazar that
recently underwent a number of flaring events detected by FERMI-LAT. The study was specifically
concerned with the behavior of PKS 1424-418 during the outburst/flaring events that occurred between
19 October 2012 and 9 October 2013. PKS 1424-418’s daily gamma-ray flux reached an
average of 1.4 ± 0.2 x 10−6 ph cm−2 s−1 for E > 100 MeV, triggering radio follow up observations
with the Hartebeesthoek Radio Astronomy Observatory 26 m radio telescope at 2.3-GHz, 4.8-GHz,
8.4-GHz and 12.2-GHz frequencies. The objective was to examine the nature of the relationship
between the high-energy gamma rays detected by FERMI-LAT and the low-energy radio waves detected
by the Hart26m radio telescope. In the study we investigated the relationship between the
two energy regimes using Discrete cross-correlation functions to estimate the time-lags between two
corresponding frequencies. We also studied the spectral index variation to establish the source behavior
over the observing period at multiple epochs. A Lomb-Scargle periodicity search was also
performed to investigate whether some periodic modulation was present in the gamma-ray data as it
varied quite dramatically on shorter time-scales. Observations in gamma-rays and radio frequencies
were done using the All-Sky mode and drift scan technique respectively at the different frequencies.
Results indicated the existence of a strong correlation between the gamma-ray and radio data,
with the gamma rays leading the radio. With each gamma-ray flaring event the radio spectra
indicated some spectral hardening and the possibility of an 86 day gamma-ray period in the shorter
term flares was also established in the study.
This study however only shows the large scale relationship between time-series over the entire
observing period. On smaller scales, each gamma-ray and radio flare is unique and as such requires
individual analysis for each respective component. to successfully achieve this, more data is needed
to confirm the individual radio flaring periods. Observations at VLBI scales are extremely useful in
this kind of work and instrumental in studying the source structure behavior during flaring and will
form part of the future work planned for studying blazar source variability.
Description
Degree of Master of Science by research only:
A dissertation submitted to the faculty of science, University of the Witwatersrand, Johannesburg,
in fulfillment of the requirements for the degree of Master of Science.
Department of Physics
Faculty of Science
University of the Witwatersrand
January 4, 2016