94 research outputs found
Compton Dominance and the Blazar Sequence
Does the "blazar sequence" exist, or is it a result of a selection effect,
due to the difficulty in measuring the redshifts of blazars with both high
synchrotron peak frequencies (\gtrsim 10^{15} Hz) and luminosities (\gtrsim
10^{46} erg s^{-1})? We explore this question with a sample of blazars from the
Second Catalog of Active Galactic Nuclei (AGN) from the Fermi Large Area
Telescope (LAT). The Compton dominance, the ratio of the peak of the Compton to
the synchrotron peak luminosities, is essentially a redshift-independent
quantity, and thus crucial to answering this question. We find that a
correlation exists between Compton dominance and the peak frequency of the
synchrotron component for all blazars in the sample, including ones with
unknown redshift. We then construct a simple model to explain the blazar
properties in our sample, where the difference between sources is due to only
the magnetic field of the blazar jet emitting region, the external radiation
field energy density, and the jet angle to the line of sight, with the magnetic
field strength and external energy density being correlated. This model can
reproduce the trends of the blazars in the sample, and predicts blazars may be
discovered in the future with high synchrotron peak frequencies and
luminosities. At the same time the simple model reproduces the lack of
high-synchrotron peaked blazars with high Compton dominances (\gtrsim 1).Comment: 12 pages, 9 figures, 3 tables. Accepted by Ap
Modeling Fermi Large Area Telescope and Multiwavelength Data from Blazars
Blazars are active galactic nuclei with relativistic jets pointed at the
Earth, making them extremely bright at essentially all wavelengths, from radio
to gamma rays. I review the modeling of this broadband spectral energy
distributions of these objects, and what we have learned, with a focus on gamma
rays.Comment: 21 pages, 3 figures. To appear in the Proceedings of the 3rd Annual
Conference on High Energy Astrophysics in Southern Africa (HEASA2015), 18-20
June 2015, Johannesburg, South Africa, Eds. M. Boettcher, D. Buckley, S.
Colafrancesco, P. Meintjes and S. Razzaque. arXiv admin note: text overlap
with arXiv:1303.509
Blazars in Context in the Fermi Era
Blazars are the most plentiful gamma-ray source at GeV energies, and despite
detailed study, there is much that is not known about these sources. In this
review I explore some recent results on blazars, including the controversy of
the "blazar sequence", the curvature in the LAT spectra, and the location along
the jet of the gamma-ray emitting region. I conclude with a discussion of
alternative modeling possibilities.Comment: 7 pages, 4 figures. 2012 Fermi Symposium proceedings - eConf C12102
The Gamma-Ray View of the Extragalactic Background Light
The Extragalactic Background Light (EBL) from the infrared (IR) through the
ultraviolet (UV) is dominated by emission from stars, either directly or
through absorption and reradiation by dust. It can thus give information on the
star formation history of the universe. However, it is difficult to measure
directly due to foreground radiation fields from the Galaxy and solar system.
Gamma-rays from extragalactic sources at cosmological distances (blazars and
gamma-ray bursts) interact with EBL photons creating electron-positron pairs,
absorbing the gamma-rays. Given the intrinsic gamma-ray spectrum of a source
and its redshift, the EBL can in principle be measured. However, the intrinsic
gamma-ray spectra of blazars and GRBs can vary considerably from source to
source and the from the same source over short timescales. A maximum intrinsic
spectrum can be assumed from theoretical grounds, to give upper limits on the
EBL absorption from blazars at low redshift with very high energy (VHE)
gamma-ray observations with ground-based Atmospheric Cherenkov telescopes. The
Fermi-LAT observations of blazars and GRBs can probe EBL absorption at higher
redshifts. The lower energy portion of the LAT spectrum of these sources is
unattenuated by the EBL, so that extrapolating this to higher energies can give
the maximum intrinsic spectrum for a source. Comparing this to the observed
higher energy LAT spectrum will then give upper limits on the EBL absorption.
For blazars which have been detected by both the Fermi-LAT and at higher
energies by Cherenkov telescopes, combined LAT-VHE observations can put more
stringent constraints on the low redshift EBL. These procedures above can also
be reversed: for sources with an unknown redshift, a given EBL model and
gamma-ray spectrum can lead to an upper limit on the source's redshift.Comment: 4 page, 1 figure. Proceedings of Recontres de Moriond (cosmology),
March 13-20 201
External Compton Scattering in Blazar Jets and the Location of the Gamma-Ray Emitting Region
I study the location of the -ray emission in blazar jets by creating
a Compton-scattering approximation valid for all anisotropic radiation fields
in the Thomson through Klein-Nishina regimes, which is highly accurate and can
speed up numerical calculations by up to a factor . I apply this
approximation to synchrotron self-Compton, and external Compton-scattering of
photons from the accretion disk, broad-line region (BLR), and dust torus. I use
a stratified BLR model and include detailed Compton-scattering calculations of
a spherical and flattened BLR. I create two dust torus models, one where the
torus is an annulus, and one where it is an extended disk. I present detailed
calculations of the photoabsorption optical depth using my detailed BLR and
dust torus models, including the full angle dependence. I apply these
calculations to the emission from a relativistically moving blob traveling
through these radiation fields. The ratio of -ray to optical flux
produces a predictable pattern that could help locate the -ray emission
region. I show that the bright flare from 3C 454.3 in 2010 November detected by
the Fermi Large Area Telescope is unlikely to originate from a single blob
inside the BLR since it moves outside the BLR in a time shorter than the flare
duration, although emission by multiple blobs inside the BLR is possible; and
-rays are unlikely to originate from outside the BLR from scattering of
photons from an extended dust torus, since then the cooling timescale would be
too long to explain the observed short variability.Comment: Accepted by ApJ. 22 pages, 19 Figures, 5 Table
The Binary Black Hole Merger Rate from Ultraluminous X-ray Source Progenitors
Ultraluminous X-ray sources (ULXs) exceed the Eddington luminosity for a
black hole. The recent detection of black hole mergers by
the gravitational wave detector ALIGO indicates that black holes with masses do indeed exist. Motivated by this, we explore a scenario where
ULXs consist of black holes formed by the collapse of high-mass,
low-metallicity stars, and that these ULXs become binary black holes (BBHs)
that eventually merge. We use empirical relations between the number of ULXs
and the star formation rate and host galaxy metallicity to estimate the ULX
formation rate and the BBH merger rate at all redshifts. This assumes the ULX
rate is directly proportional to the star formation rate for a given
metallicity, and that the black hole accretion rate is distributed as a
log-normal distribution. We include an enhancement in the ULX formation rate at
earlier epochs due to lower mean metallicities. With simplified assumptions,
our model is able to reproduce both the rate and mass distribution of BBH
mergers in the nearby universe inferred from the detection of GW 150914, LVT
151012, GW 151226, and GW 170104 by ALIGO if the peak accretion rate of ULXs is
a factor 1 --- 300 greater than the Eddington rate. Our predictions of
the BBH merger rate, mass distribution, and redshift evolution can be tested by
ALIGO in the near future, which in turn can be used to explore connections
between the ULX formation and BBH merger rates over cosmic time.Comment: 10 pages, 9 figures. Accepted by MNRA
Cosmic Ray Electron Evolution in the Supernova Remnant RX J1713.7-3946
A simple formalism to describe nonthermal electron acceleration, evolution,
and radiation in supernova remnants (SNRs) is presented. The electron
continuity equation is analytically solved assuming that the nonthermal
electron injection power is proportional to the rate at which the kinetic
energy of matter swept up in an adiabatically expanding SNR shell. We apply
this model to \fermi\ and HESS data from the SNR \rxj, and find that a one-zone
leptonic model with Compton-scattered cosmic microwave background (CMB) and
interstellar infrared photons has difficulty providing a good fit to its
spectral energy distribution, provided the source is at a distance $\sim 1\
\kpc from the Earth. However, the inclusion of multiple zones, as hinted at by
recent {\em Chandra} observations, does provide a good fit, but requires a
second zone of compact knots with magnetic fields B\sim 16\ \mu$G, comparable
to shock-compressed fields found in the bulk of the remnant.Comment: 13 pages, 10 figures, 2 tables (emulateapj). Accepted by Ap
On the Spectral Break in the Fermi-LAT Spectrum of 3C 454.3
Fermi Gamma ray Space Telescope observations of the flat spectrum radio
quasar 3C~454.3 show a spectral-index change
at break energy GeV. Such a sharp break is
inconsistent with a cooling electron distribution and is poorly fit with a
synchrotron self-Compton model. We show that a combination of two components,
namely the Compton-scattered disk and broad-line region (BLR) radiations,
explains this spectral break and gives a good fit to the quasi-simultaneous
radio, optical/UV, X-ray, and -ray spectral energy distribution
observed in 2008 August. A sharp break can be produced independent of the
emitting region's distance from the central black hole if the BLR has a
gradient in density , consistent with a wind model for the BLR.Comment: 13 pages, 2 figures, 1 table. Accepted by ApJ letters
Fourier Analysis of Blazar Variability: Klein-Nishina Effects and the Jet Scattering Environment
The strong variability of blazars can be characterized by power spectral
densities (PSDs) and Fourier frequency-dependent time lags. In previous work,
we created a new theoretical formalism for describing the PSDs and time lags
produced via a combination of stochastic particle injection and emission via
the synchrotron, synchrotron self-Compton, and external Compton (EC) processes.
This formalism used the Thomson cross section and simple -function
approximations to model the synchrotron and Compton emissivities. Here we
expand upon this work, using the full Compton cross section and detailed and
accurate emissivities. Our results indicate good agreement between the PSDs
computed using the -function approximations and those computed using
the accurate expressions, provided the observed photons are produced primarily
by electrons with energies exceeding the lower limit of the injected particle
population. Breaks are found in the PSDs at frequencies corresponding to the
cooling timescales of the electrons primarily responsible for the observed
emission, and the associated time lags are related to the difference in
electron cooling timescales between the two energy channels, as expected. If
the electron cooling timescales can be determined from the observed time lags
and/or the observed EC PSDs, then one could in principle use the method
developed here to determine the energy of the external seed photon source for
EC, which is an important unsolved problem in blazar physics.Comment: 10 pages, 5 figures. Accepted by Ap
Fourier Analysis of Blazar Variability
Blazars display strong variability on multiple timescales and in multiple
radiation bands. Their variability is often characterized by power spectral
densities (PSDs) and time lags plotted as functions of the Fourier frequency.
We develop a new theoretical model based on the analysis of the electron
transport (continuity) equation, carried out in the Fourier domain. The
continuity equation includes electron cooling and escape, and a derivation of
the emission properties includes light travel time effects associated with a
radiating blob in a relativistic jet. The model successfully reproduces the
general shapes of the observed PSDs and predicts specific PSD and time lag
behaviors associated with variability in the synchrotron, synchrotron
self-Compton (SSC), and external Compton (EC) emission components, from sub-mm
to gamma-rays. We discuss applications to BL Lacertae objects and to
flat-spectrum radio quasars (FSRQs), where there are hints that some of the
predicted features have already been observed. We also find that FSRQs should
have steeper PSD power-law indices than BL Lac objects at Fourier frequencies <
10^{-4} Hz, in qualitative agreement with previously reported observations by
the Fermi Large Area Telescope.Comment: 17 pages, 8 figures. Accepted by Ap
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