72 research outputs found
Thermal emission signatures in non-thermal blazars
Blazars, a subclass of active galactic nuclei with powerful relativistic plasma jets, are
among the most luminous and violently variable objects in the universe. They emit
radiation across the entire electromagnetic spectrum, and often change in brightness
over the course of hours or days. Different emission mechanisms are necessary in order
to explain the observed flux in different frequency ranges. In the ultraviolet-optical-
infrared regime these include components that arise from: 1) polarized synchrotron
radiation emanating from a powerful parsec-scale jet flowing from near the central
accreting black hole, 2) a multi-temperature accretion disk emitting thermal radia-
tion, and 3) an optically thick dusty torus located several parsecs from the central
engine that absorbs and re-emits, at infrared wavelengths, radiation originating in
the accretion disk. The goal of this study is to determine the relative importance
of these spectral components in the spectra of blazars. I use data from the Spitzer
Space Telescope in order to search for the presence of the dusty torus surrounding
four blazars, as well as to determine its luminosity and temperature. In two of the
observed sources, 1222+216 and CTA102, I determine that the torus can be modeled
as a 1200 K blackbody emitting at nearly 10 46 erg s −1 . Furthermore, I determine
the relative variability of the accretion disk of a sample of blazars by using spec-
tropolarimetry observations to separate the optical-UV spectrum into a polarized
viiicomponent, consisting of radiation described by a power-law F ν ∝ ν −α , and an ac-
cretion disk which consists of a thin disk described by the power-law F disk ∝ ν 1/3
plus a hot-spot of variable temperature. The spectra of several blazars are explained
by a version of this model in which the thin disk component is held constant, while
the blackbody varies on timescales of approximately years resulting with a flux of
the blackbody component comparable to the power-law disk component. I find that
variations in the emission from the hot-spot occurs approximately within 100 days
of γ-ray variations
A method for localizing energy dissipation in blazars using Fermi variability
The distance of the Fermi-detected blazar gamma-ray emission site from the
supermassive black hole is a matter of active debate. Here we present a method
for testing if the GeV emission of powerful blazars is produced within the
sub-pc scale broad line region (BLR) or farther out in the pc-scale molecular
torus (MT) environment. If the GeV emission takes place within the BLR, the
inverse Compton (IC) scattering of the BLR ultraviolet (UV) seed photons that
produces the gamma-rays takes place at the onset of the Klein-Nishina regime.
This causes the electron cooling time to become practically energy independent
and the variation of the gamma-ray emission to be almost achromatic. If on the
other hand the gamma-ray emission is produced farther out in the pc-scale MT,
the IC scattering of the infrared (IR) MT seed photons that produces the
gamma-rays takes place in the Thomson regime, resulting to energy-dependent
electron cooling times, manifested as faster cooling times for higher Fermi
energies. We demonstrate these characteristics and discuss the applicability
and limitations of our method.Comment: Accepted by Astrophysical Journal Letter
The Lack of Torus Emission from BL Lacertae Objects: An Infrared View of Unification with WISE
We use data from the Wide-Field Infrared Survey Explorer (WISE) to perform a
statistical study on the mid-infrared (IR) properties of a large number
() of BL Lac objects --- low-luminosity Active Galactic Nuclei (AGN)
with a jet beamed toward the Earth. As expected, many BL Lac objects are so
highly beamed that their jet synchrotron emission dominates their IR spectral
energy distributions. In other BL Lac objects, however, the jet is not strong
enough to completely dilute the rest of the AGN emission. We do not see
observational signatures of the dusty torus from these weakly beamed BL Lac
objects. The lack of observable torus emission is consistent with suggestions
that BL Lac objects are fed by radiatively inefficient accretion disks.
Implications for the "nature vs. nurture" debate for FR I and FR II radio
galaxies are briefly discussed. Our study supports the notion that, beyond
orientation, accretion rate plays an important role in AGN unification.Comment: 6 Pages, 3 Figures, accepted for publication in ApJ Letter
Emission from Hot Dust in the Infrared Spectra of Gamma-ray Bright Blazars
A possible source of -ray photons observed from the jets of blazars
is inverse Compton scattering by relativistic electrons of infrared seed
photons from a hot, dusty torus in the nucleus. We use observations from the
Spitzer Space Telescope to search for signatures of such dust in the infrared
spectra of four -ray bright blazars, the quasars 4C 21.35, CTA102, and
PKS 1510089, and the BL Lacertae object ON231. The spectral energy
distribution (SED) of 4C 21.35 contains a prominent infrared excess indicative
of dust emission. After subtracting a non-thermal component with a power-law
spectrum, we fit a dust model to the residual SED. The model consists of a
blackbody with temperature K, plus a much weaker optically thin
component at K. The total luminosity of the thermal dust emission is
erg s. If the dust lies in an equatorial
torus, the density of IR photons from the torus is sufficient to explain the
-ray flux from 4C 21.35 as long as the scattering occurs within a few
parsecs of the central engine. We also report a tentative detection of dust in
the quasar CTA102, in which the luminosity of the infrared excess is erg s. However, in CTA102 the far-IR spectra are too
noisy to detect the m silicate feature. Upper limits to the luminosity
from thermal emission from dust in PKS 1510-089, and ON231, are,
, and erg s, respectively. These
upper limits do not rule out the possibility of inverse Compton up-scattering
of IR photons to -ray energies in these two sources. The estimated
covering factor of the hot dust in 4C 21.35, 22%, is similar to that of
non-blazar quasars; however, 4C 21.35 is deficient in cooler dust.Comment: 23 Pages, 5 Figures, 2 Tables, 1 Machine Readable Table. Accepted to
Ap
Multiwavelength Variations of 3C 454.3 during the November 2010 to January 2011 Outburst
We present multiwavelength data of the blazar 3C 454.3 obtained during an
extremely bright outburst from November 2010 through January 2011. These
include flux density measurements with the Herschel Space Observatory at five
submillimeter-wave and far-infrared bands, the Fermi Large Area Telescope at
gamma-ray energies, Swift at X-ray, ultraviolet (UV), and optical frequencies,
and the Submillimeter Array at 1.3 mm. From this dataset, we form a series of
52 spectral energy distributions (SEDs) spanning nearly two months that are
unprecedented in time coverage and breadth of frequency. Discrete correlation
anlaysis of the millimeter, far-infrared, and gamma-ray light curves show that
the variations were essentially simultaneous, indicative of co-spatiality of
the emission, at these wavebands. In contrast, differences in short-term
fluctuations at various wavelengths imply the presence of inhomegeneities in
physical conditions across the source. We locate the site of the outburst in
the parsec-scale core, whose flux density as measured on 7 mm Very Long
Baseline Array images increased by 70 percent during the first five weeks of
the outburst. Based on these considerations and guided by the SEDs, we propose
a model in which turbulent plasma crosses a conical standing shock in the
parsec-scale region of the jet. Here, the high-energy emission in the model is
produced by inverse Compton scattering of seed photons supplied by either
nonthermal radiation from a Mach disk, thermal emission from hot dust, or (for
X-rays) synchrotron radiation from plasma that crosses the standing shock. For
the two dates on which we fitted the model SED to the data, the model
corresponds very well to the observations at all bands except at X-ray
energies, where the spectrum is flatter than observed.Comment: Accepted for publication in Astrophysical Journal. 82 pages, 13
figure
Variable Gamma-ray Emission Induced by Ultra-High Energy Neutral Beams: Application to 4C +21.35
The flat spectrum radio quasar (FSRQ) 4C +21.35 (PKS 1222+216) displays
prominent nuclear infrared emission from ~1200 K dust. A 70 -- 400 GeV flare
with ~10 min variations during half an hour of observations was found by the
MAGIC telescopes, and GeV variability was observed on sub-day timescales with
the Large Area Telescope on Fermi. We examine 4C +21.35, assuming that it is a
source of ultra-high energy cosmic rays (UHECRs). UHECR proton acceleration in
the inner jet powers a neutral beam of neutrinos, neutrons and gamma rays from
photopion production. The radiative efficiency and production spectra of
neutrals formed through photohadronic processes with isotropic external target
photons of the broad line region and torus are calculated. Secondary radiations
made by this process have a beaming factor ~\delta^5, where \delta is the
Doppler factor. The pair-production optical depth for gamma rays and the
photopion efficiency for UHECR neutrons as they pass through external isotropic
radiation fields are calculated. If target photons come from the broad line
region and dust torus, large Doppler factors, \delta >~100 are required to
produce rapidly variable secondary radiation with isotropic luminosity >~1e47
erg/s at the pc scale. The \gamma-ray spectra from leptonic secondaries are
calculated from cascades initiated by the UHECR neutron beam at the pc-scale
region and fit to the flaring spectrum of 4C +21.35. Detection of >~100 TeV
neutrinos from 4C +21.35 or other VHE blazars with IceCube or KM3NeT would
confirm this scenario.Comment: 21 pages, 13 figures; replaced 4 figures to show neutron and neutrino
production from combined infrared and broad-line region radiation fields;
added references and improvements; ApJ, in pres
Energetic Constraints on a Rapid Gamma-Ray Flare in PKS 1222+216
We study theoretical implications of a rapid Very-High-Energy (VHE) flare
detected by MAGIC in the Flat-Spectrum Radio Quasar PKS 1222+216. The minimum
distance from the jet origin at which this flare could be produced is 0.5 pc. A
moderate Doppler factor of the VHE source, D_{VHE} ~ 20, is allowed by all
opacity constraints. The concurrent High-Energy (HE) emission observed by Fermi
provides estimates of the total jet power and the jet magnetic field strength.
Energetic constraints for the VHE flare are extremely tight: for an isotropic
particle distribution they require a huge co-moving energy density in the
emitting region and a very efficient radiative process. We disfavor hadronic
processes due to their low radiative efficiency, as well as the synchrotron
scenario recently proposed for the case of HE flares in the Crab Nebula, since
the parameters needed to overcome the radiative losses are quite extreme. The
VHE emission can be explained by the Synchrotron Self-Compton (SSC) mechanism
for D_{VHE} ~ 20 or by the External Radiation Compton (ERC) mechanism involving
the infrared radiation of the dusty torus for D_{VHE} ~ 50. After discussing
several alternative scenarios, we propose that the extreme energy density
constraint can be satisfied when the emission comes from highly anisotropic
short-lived bunches of particles formed by the kinetic beaming mechanism in
magnetic reconnection sites. By focusing the emitting particles into very
narrow beams, this mechanism allows one to relax the causality constraint on
the source size, decreasing the required energy density by 4 orders of
magnitude.Comment: 12 pages, 2 figures, accepted for publication in MNRA
On leptonic models for blazars in the Fermi era
Some questions raised by Fermi-LAT data about blazars are summarized, along
with attempts at solutions within the context of leptonic models. These include
both spectral and statistical questions, including the origin of the GeV breaks
in low-synchrotron peaked blazars, the location of the gamma-ray emission
sites, the correlations in the spectral energy distributions with luminosity,
and the difficulty of synchrotron/SSC models to fit the spectra of some TeV
blazars.Comment: 9 pages, 1 figure, in "Beamed and Unbeamed Gamma Rays from Galaxies,"
Muonio, Finland, 11-15 April, 2011, ed. R. Wagner, L. Maraschi, A. Sillanpaa,
to appear in Journal of Physics: Conference Serie
- …