89 research outputs found
Uncovering the physics behind the blazar sequence using a realistic model for jet emission
Blazar spectra are one of the most important windows into the physical
processes occurring along jets. The spectrum, composed from the different
emitting regions along the jet, allows us to constrain the physical conditions
in the jet. I present my work modelling blazar spectra using an extended
inhomogeneous jet model with an accelerating, magnetically dominated, parabolic
base transitioning to a slowly decelerating, conical section motivated by
observations, simulations and theory. We set the inner geometry of our
multi-zone model using observations of the jet in M87 which transitions from
parabolic to conical at 10^5 Schwarzschild radii. This model is able to
reproduce quiescent blazar spectra very well across all wavelengths (including
radio observations) for a sample of 42 BL Lacs and FSRQs.
Using this inhomogeneous model we are able to constrain the location at which
the synchrotron emission is brightest in these jets by fitting to the optically
thick to thin synchrotron break. We find that the radius of the jet at which
the synchrotron emission is brightest (where the jet first approaches
equipartition) scales approximately linearly with the jet power. We also find a
correlation between the length of the accelerating, parabolic section of the
jet and the maximum bulk Lorentz factor. In agreement with previous work we
find that BL Lacs are low power blazars whereas FSRQs are high power blazars.
Together with our simple jet power-radius relation this leads us to a deeper
understanding of the physics underlying the blazar sequence.Comment: 5 pages, 5 figures, to appear in "The Innermost Regions of
Relativistic Jets and Their Magnetic Fields" conference proceedings; includes
minor change
Synchrotron and inverse-Compton emission from blazar jets - III. Compton-dominant blazars
In this paper we develop the extended jet model of Potter & Cotter to model
the simultaneous multi-wavelength spectra of six Compton-dominant blazars. We
include an accelerating parabolic base transitioning to a slowly decelerating
conical jet with a geometry set by observations of M87 and consistent with
simulations and theory. We investigate several jet models and find that the
optically thick to thin synchrotron break in the radio spectrum requires the
jet to first come into equipartition at large distances along the jet (10^5
Schwarzschild radii), consistent with the observed transition from parabolic to
conical in the jet of M87. We confirm this result analytically and calculate
the expected frequency core-shift relations for the models under consideration.
We find that a parabolic jet transitioning to a ballistic conical jet, which
starts in equipartition and becomes more particle dominated at larger
distances, fits the multiwavelength data of the six blazars well, whilst an
adiabatic equipartition conical section requires very large bulk Lorentz
factors to reproduce the Compton-dominance of the blazars.
We find that all these blazars require high power, high bulk Lorentz factor
jets observed close to the line of sight as we expect from the blazar sequence
and consistent with the results from Paper II. The inverse-Compton emission in
our fits is due to inverse-Compton scattering of high-redshift CMB photons at
large distances along the jet due to the high bulk Lorentz factors of the jets.
We postulate a new interpretation of the blazar sequence based on the radius of
the transition region of the jet (where the jet is brightest in synchrotron
emission) scaling linearly with black hole mass.Comment: 13 pages, 5 figures, accepted for publication in MNRA
Vigorous star formation in a bulge-dominated ERO at z = 1.34
We present near-IR spectroscopy of three Extremely Red Objects (EROs) using
the OHS/CISCO spectrograph at Subaru telescope. One target exhibits a strong
emission line which we identify as H-alpha at z = 1.34. Using new and existing
ground-based optical and near-IR imaging, and archival HST imaging, we argue
that this target is essentially an elliptical galaxy, with an old stellar
population of around 4E11 solar masses, but having a dust-enshrouded
star-forming component with a SFR of some 50--100 solar masses per yr. There is
no evidence that the galaxy contains an AGN. Analysis of a further two targets,
which do not exhibit any features in our near-IR spectra, suggests that one is
a quiescent galaxy in the redshift range 1.2 < z < 1.6, but that the other
cannot be conclusively categorised as either star-forming or quiescent.
Even though our first target has many of the properties of an old elliptical,
the ongoing star formation means that it cannot have formed_all_ of its stellar
population at high redshift. While we cannot infer any robust values for the
star formation rate in ellipticals at z > 1 from this one object, we argue that
the presence of an object with such a high SFR in such a small sample suggests
that a non-negligible fraction of the elliptical galaxy population may have
formed a component of their stellar population at redshifts z ~ 1--2. We
suggest that this is evidence for ongoing star formation in the history of
elliptical galaxies.Comment: Accepted by MNRAS 1 April 2005. Ten pages, six figure
The Feasibility of Magnetic Reconnection Powered Blazar Flares from Synchrotron Self-Compton Emission
Order of magnitude variability has been observed in the blazar sub-class of
Active Galactic Nuclei on minute timescales. These high-energy flares are often
difficult to explain with shock acceleration models due to the small size of
the inferred emitting region, with recent particle-in-cell (PIC) simulations
showing that magnetic reconnection is a promising alternative mechanism. Here,
we present a macroscopic emission model physically motivated by PIC
simulations, where the energy for particle acceleration originates from the
reconnecting magnetic field. We track the radial growth and relative velocity
of a reconnecting plasmoid, modelling particle acceleration and radiative
losses from synchrotron and synchrotron self-Compton (SSC) emission. To test
the viability of magnetic reconnection as the mechanism behind rapid blazar
flares we simultaneously fit our model to the observed light-curve and SED from
the 2016 TeV flare of BL Lacertae. We find generally that, without considering
external photons, reconnecting plasmoids are unable to produce Compton-dominant
TeV flares and so cannot reproduce the observations due to overproduction of
synchrotron emission. Additionally, problematically large plasmoids, comparable
in size to the entire jet radius, are required to emit sufficient SSC
gamma-rays to be observable. However, our plasmoid model can reproduce the
rapid TeV lightcurve of the flare, demonstrating that reconnection is able to
produce rapid, powerful TeV flares on observed timescales. We conclude that
while reconnection can produce SSC flares on the correct timescales, the
primary source of TeV emission cannot be SSC and the size of plasmoids required
may be implausibly large.Comment: Replaced with accepted version. Contains additional figures and
considers the effect of a magnetic guide fiel
Black Holes in 4 Nearby Radio Galaxies
We study the velocity dispersion profiles of the nuclei of NGC 1326, 2685,
5273 and 5838 in the CO first overtone band. There is evidence for a black hole
(BH) in NGC 1326 and 5838. Gas is seen flowing out of the nuclear region of NGC
5273. We put upper limits on the nuclear BHs responsible for its activity and
that of NGC 2685.Comment: to appear in ApS
Cherenkov telescope array extragalactic survey discovery potential and the impact of axion-like particles and secondary gamma rays
The Cherenkov Telescope Array (CTA) is about to enter construction phase and
one of its main key science projects is to perform an unbiased survey in search
of extragalactic sources. We make use of both the latest blazar gamma--ray
luminosity function and spectral energy distribution to derive the expected
number of detectable sources for both the planned Northern and Southern arrays
of the CTA observatory. We find that a shallow, wide survey of about 0.5 hour
per field of view would lead to the highest number of blazar detections.
Furthermore, we investigate the effect of axion-like particles and secondary
gamma rays from propagating cosmic rays on the source count distribution, since
these processes predict different spectral shape from standard extragalactic
background light attenuation. We can generally expect more distant objects in
the secondary gamma-ray scenario, while axion-like particles do not
significantly alter the expected distribution. Yet, we find that, these results
strongly depend on the assumed magnetic field strength during the propagation.
We also provide source count predictions for the High Altitude Water Cherenkov
observatory (HAWC), the Large High Altitude Air Shower Observatory (LHAASO) and
a novel proposal of a hybrid detector.Comment: 16 pages, 4 figures, ApJ 2017 in pres
Gamma-ray Novae: Rare or Nearby?
Classical Novae were revealed as a surprise source of gamma-rays in Fermi LAT
observations. During the first 8 years since the LAT was launched, 6 novae in
total have been detected to > 5 sigma in gamma-rays, in contrast to the 69
discovered optically in the same period. We attempt to resolve this discrepancy
by assuming all novae are gamma-ray emitters, and assigning peak one-day fluxes
based on a flat distribution of the known emitters to a simulated population.
To determine optical parameters, the spatial distribution and magnitudes of
bulge and disc novae in M31 are scaled to the Milky Way, which we approximate
as a disc with a 20 kpc radius and elliptical bulge with semi major axis 3 kpc
and axis ratios 2:1 in the xy plane. We approximate Galactic reddening using a
double exponential disc with vertical and radial scale heights of r_d = 5 kpc
and z_d = 0.2 kpc, and demonstrate that even such a rudimentary model can
easily reproduce the observed fraction of gamma-ray novae, implying that these
apparently rare sources are in fact nearby and not intrinsically rare. We
conclude that classical novae with m_R < 12 and within ~8 kpc are likely to be
discovered in gamma-rays using the Fermi LAT.Comment: Accepted by MNRAS, 10 pages, 7 figure
Constraints on axionlike particles from a combined analysis of three flaring flat-spectrum radio quasars
Many theories beyond the Standard Model of particle physics predict the
existence of axionlike particles (ALPs) that mix with photons in the presence
of a magnetic field. Searching for the effects of ALP-photon mixing in
gamma-ray observations of blazars has provided some of the strongest
constraints on ALP parameter space so far. Previously, only individual sources
have been analysed. We perform a combined analysis on Large
Area Telescope data of three bright, flaring flat-spectrum radio quasars, with
the blazar jets themselves as the dominant mixing region. For the first time,
we include a full treatment of photon-photon dispersion within the jet, and
account for the uncertainty in our B-field model by leaving the field strength
free in the fitting. Overall, we find no evidence for ALPs, but are able to
exclude the ALP parameters neV and GeV with 95\% confidence.Comment: 18 pages, 14 figures; submitted to Physical Review
Deep optical imaging of the field of PC1643+4631A&B, II: Estimating the colours and redshifts of faint galaxies
In an investigation of the cause of the cosmic microwave background decrement
in the field of the z = 3.8 quasar pair PC1643+4631, we have carried out a
study to photometrically estimate the redshifts of galaxies in deep
multi-colour optical images of the field taken with the WHT. To examine the
possibility that a massive cluster of galaxies lies in the field, we have
attempted to recover simulated galaxies with intrinsic colours matching those
of the model galaxies used in the photometric redshift estimation. We find that
when such model galaxies are added to our images, there is considerable scatter
of the recovered galaxy redshifts away from the model value; this scatter is
larger than that expected from photometric errors and is the result of
confusion, simply due to ground-based seeing, between objects in the field. We
have also compared the likely efficiency of the photometric redshift technique
against the colour criteria used to select z>3 galaxies via the strong colour
signature of the Lyman-limit break. We find that these techniques may
significantly underestimate the true surface density of z>3, due to confusion
between the high-redshift galaxies and other objects near the line of sight. We
argue that the actual surface density of z=3 galaxies may be as much as 6 times
greater than that estimated by previous ground-based studies, and note that
this conclusion is consistent with the surface density of high-redshift objects
found in the HDF. Finally, we conclude that all ground-based deep field surveys
are inevitably affected by confusion, and note that reducing the effective
seeing in ground-based images will be of paramount importance in observing the
distant universe.Comment: 18 pages, 60 figures, submitted to MNRAS, 2 large figure avaliable at
ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure50.eps and
ftp://ftp.mrao.cam.ac.uk:/pub/PC1643/paper2.figure51.ep
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