37 research outputs found
Tracking Spectral Changes in Blazars with the Energetic Gamma Ray Experiment Telescope (EGRET)
I analyze the entire blazar data from the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO), using the skymaps that were regenerated to include the changes in performance during the mission. The sample of 98 sources consists of 66 flat spectrum radio quasars (FSRQs), 17 low-frequency peaked BL Lac objects (LBLs), 4 high-frequency peaked BL Lac objects (HBLs), 10 flat spectrum radio sources and 1 radio galaxy. I do not detect any clear pattern in the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at low fluxes in five blazars. I examine the EGRET spectral energy distribution (SED) for all the sources to identify these trends. I also observe a previously unreported spectral hysteresis in the spectral index Vs. flux space at weekly timescales, in all the three FSRQs for which data from flares lasting 3-4 weeks were available. All three sources show a counterclockwise rotation despite the widely different flux profiles. The time-averaged spectra of the HBLs are inconsistent with the predictions of the current theoretical models that have had success in describing simultaneous X-ray/TeV observations, and suggest additional components in the GeV band, as well as complex time variability. Current theoretical pictures explain the GeV emission as comptonization of the synchrotron photons in the jet, and predict hard spectra that should join smoothly with the TeV emission. The current analysis shows that the situation is more complex. The spectrum ranges from hard to soft during individual epochs, and the Mrk 421 SED shows a convex break in the aggregated data. The mission averaged EGRET spectrum for PKS~2155-304 also shows a similar (but not as pronounced) convex curvature. Simultaneous GLAST and X-ray observations of high X-ray states will address the issue of the convex curvature in the future. Such data will also explore the possibility of the steep EGRET emission originating from photons produced by electrons accelerated close to the limit of diffusive shock acceleration
Does the gamma-ray flux of the blazar 3C 454.3 vary on sub-hour timescales?
In the early days of April 2010, the blazar 3C 454.3 (z=0.859) underwent a
strong gamma-ray outburst, reaching fluxes (E > 100 MeV) in excess of 10^-5 ph
cm^-2 s^-1. The Fermi Gamma ray Space Telescope performed a 200 ks long pointed
observation starting from 5 April 2010 19:38 UTC. This allowed us to try
probing the variability of the gamma-ray emission on timescales of hours or
less. We found the variability on a few hours timescale. On sub-hour timescale
we found no evidence of significant variability, although the present
statistics is not yet conclusive and further observations are needed.Comment: 4 pages, 2 figures. Accepted for publication on MNRAS. Section 1
completely rewritten and enlarge
Does the Blazar Gamma-Ray Spectrum Harden with Increasing Flux? Analysis of 9 Years of EGRET Data
The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray
Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars
during its 9 yr lifetime. We conducted an exhaustive search of the EGRET
archives and selected all the blazars that were observed multiple times and
were bright enough to enable a spectral analysis using standard power-law
models. The sample consists of 18 flat-spectrum radio quasars(FSRQs), 6
low-frequency peaked BL Lac objects (LBLs) and 2 high-frequency peaked BL Lac
objects (HBLs). We do not detect any clear pattern in the variation of spectral
index with flux. Some of the blazars do not show any statistical evidence for
spectral variability. The spectrum hardens with increasing flux in a few cases.
There is also evidence for a flux-hardness anticorrelation at low fluxes in
five blazars. The well-observed blazars (3C 279, 3C 273, PKS 0528+134, PKS
1622-297 PKS 0208-512) do not show any overall trend in the long-term spectral
dependence on flux, but the sample shows a mixture of hard and soft states. We
observed a previously unreported spectral hysteresis at weekly timescales in
all three FSRQs for which data from flares lasting for ~(3-4) weeks were
available. All three sources show a counterclockwise rotation, despite the
widely different flux profiles. We analyze the observed spectral behavior in
the context of various inverse Compton mechanisms believed to be responsible
for emission in the EGRET energy range. Our analysis uses the EGRET skymaps
that were regenerated to include the changes in performance during the mission
Evolution of the synchrotron and inverse Compton emissions of the low energy peaked BL Lac object S5 0716+714
This paper presents a detailed analysis of the temporal and spectral
variability of the low-energy peaked BL Lac object S5 0716+714 with a long (~74
ks)X-ray observation performed by XMM-Newton on 2007 September 24-25. The
source experiences recurrent flares on timescales of hours. The soft X-ray
variations, up to a factor of ~4, are much stronger than the hard X-ray
variations. With higher energy, the variability amplitude increases in the soft
X-rays but decreases in the hard X-rays. The hard X-ray variability amplitude,
however, is effectively large. For the first time, we detect a soft lag of
~1000s between the soft and hard X-ray variations. The soft lags might become
larger with larger energy differences. The overall X-ray spectra exhibit a
softer-when-brighter trend, whereas the soft X-ray spectra appear to show a
harder-when-brighter trend. The concave X-ray spectra of the source can be
interpreted as the sum of the high-energy tail of the synchrotron emission,
dominating in the soft X rays, and the low-energy end of the inverse Compton
(IC) emission, contributing more in the hard X-rays. The synchrotron spectra
are steep (\Gamma~2.6), while the IC spectra are flat (\Gamma~1.2). The
synchrotron spectra appear to harden with larger synchrotron fluxes, while the
IC spectra seem to soften with larger IC fluxes. When the source brightens, the
synchrotron fluxes increase but the IC fluxes decrease. The synchrotron tail
exhibits larger flux variations but smaller spectral changes than the IC
component does. The crossing energies between the two components and the trough
energies of spectral energy distributions (SEDs) increase when the source
brightens. The X-ray spectral variability demonstrates that the synchrotron and
IC SED peaks of S5 0716+714 shift to higher energies when it brightens.Comment: published in Ap
Comparison of Image Restoration Methods for Lunar Epithermal Neutron Emission Mapping
Orbital measurements of neutrons by the Lunar Exploring Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter are being used to quantify the spatial distribution of near surface hydrogen (H). Inferred H concentration maps have low signal-to-noise (SN) and image restoration (IR) techniques are being studied to enhance results. A single-blind. two-phase study is described in which four teams of researchers independently developed image restoration techniques optimized for LEND data. Synthetic lunar epithermal neutron emission maps were derived from LEND simulations. These data were used as ground truth to determine the relative quantitative performance of the IR methods vs. a default denoising (smoothing) technique. We review and used factors influencing orbital remote sensing of neutrons emitted from the lunar surface to develop a database of synthetic "true" maps for performance evaluation. A prior independent training phase was implemented for each technique to assure methods were optimized before the blind trial. Method performance was determined using several regional root-mean-square error metrics specific to epithermal signals of interest. Results indicate unbiased IR methods realize only small signal gains in most of the tested metrics. This suggests other physically based modeling assumptions are required to produce appreciable signal gains in similar low SN IR applications
The Effect of Blazar Spectral Breaks on the Blazar Contribution to the Extragalactic Gamma-ray Background
The spectral shapes of the contributions of different classes of unresolved
gamma-ray emitters can provide insight into their relative contributions to the
extragalactic gamma-ray background (EGB) and the natures of their spectra at
GeV energies. We calculate the spectral shapes of the contributions to the EGB
arising from BL Lacertae type objects (BL Lacs) and flat-spectrum radio quasars
(FSRQs) assuming blazar spectra can be described as broken power laws. We fit
the resulting total blazar spectral shape to the Fermi Large Area Telescope
measurements of the EGB, finding that the best-fit shape reproduces well the
shape of the Fermi EGB for various break scenarios. We conclude that a scenario
in which the contribution of blazars is dominant cannot be excluded on spectral
grounds alone, even if spectral breaks are shown to be common among Fermi
blazars. We also find that while the observation of a featureless (within
uncertainties) power-law EGB spectrum by Fermi does not necessarily imply a
single class of contributing unresolved sources with featureless individual
spectra, such an observation and the collective spectra of the separate
contributing populations determine the ratios of their contributions. As such,
a comparison with studies including blazar gamma-ray luminosity functions could
have profound implications for the blazar contribution to the EGB, blazar
evolution, and blazar gamma-ray spectra and emission.Comment: 8 pages, emulateapj format; 5 figures; accepted for publication in
Ap
An Intriguing Convex Break in the EGRET SED of Mrk 421
Based upon analysis of the entire EGRET data from Mrk 421, it is found that the time-averaged spectra are inconsistent with the predictions of current theoretical models that have had success in describing simultaneous X-ray/TeV observations, and suggest additional components in the GeV band, as well as complex time variability. Current theoretical pictures explain the GeV emission as comptonization of the synchrotron photons in the jet, and predict hard spectra that should join smoothly with the TeV emission. Our analysis shows that the situation is more complex. The spectrum ranges from hard to soft during individual epochs, and shows a convext break in the aggregated data. We also present the mission-averaged EGRET spectrum for PKS 2155-304, which shows a similar (but not as pronounced) convex curvature. We discuss a series of possible explanations for the 10(exp 22) - 10(exp 23) HZ declining part of the EGRET nu F(sub nu), spectrum for Mrk 421, and suggest that it is synchrotron emission from the high energy tail of the electron population that produces the X-rays during the highest X-ray states. Such multi-MeV photons are produced by electrons accelerated close to the limit of diffusive shock acceleration. Simultaneous GLAST and X-ray observations of high X-ray states will address the issue of the convex curvature in the future
Components of the Extragalactic Gamma Ray Background
We present new theoretical estimates of the relative contributions of
unresolved blazars and star-forming galaxies to the extragalactic gamma-ray
background (EGB) and discuss constraints on the contributions from alternative
mechanisms such as dark matter annihilation and truly diffuse gamma-ray
production. We find that the Fermi source count data do not rule out a scenario
in which the EGB is dominated by emission from unresolved blazars, though
unresolved star-forming galaxies may also contribute significantly to the
background, within order-of-magnitude uncertainties. In addition, we find that
the spectrum of the unresolved star-forming galaxy contribution cannot explain
the EGB spectrum found by EGRET at energies between 50 and 200 MeV, whereas the
spectrum of unresolved FSRQs, when accounting for the energy-dependent effects
of source confusion, could be consistent with the combined spectrum of the
low-energy EGRET EGB measurements and the Fermi-LAT EGB measurements.Comment: version accepted for publication in the Astrophysical Journa
Chasing the heaviest black holes of jetted Active Galactic Nuclei
We investigate the physical properties of the 10 blazars at redshift greater
than 2 detected in the 3-years all sky survey performed by the Burst Alert
Telescope (BAT) onboard the Swift satellite. We find that the jets of these
blazars are among the most powerful known. Furthermore, the mass of their
central black hole, inferred from the optical-UV bump, exceeds a few billions
of solar masses, with accretion luminosities being a large fraction of the
Eddington one. We compare their properties with those of the brightest blazars
of the 3-months survey performed by the Large Area Telescope (LAT) onboard the
Fermi satellite. We find that the BAT blazars have more powerful jets, more
luminous accretion disks and larger black hole masses than LAT blazars. These
findings can be simply understood on the basis of the blazar sequence, that
suggests that the most powerful blazars have a spectral energy distribution
with a high energy peak at MeV (or even sub-MeV) energies. This implies that
the most extreme blazars can be found more efficiently in hard X-rays, rather
than in the high energy gamma-ray band. We then discuss the implications of our
findings for future missions, such as the New Hard X-ray Mission (NHXM) and
especially the Energetic X-ray Imaging Survey Telescope (EXIST) mission which,
during its planned 2 years all sky survey, is expected to detect thousands of
blazars, with a few of them at z greater than 6.Comment: Accepted for publication in MNRAS. Absorption due to Lyman-alpha
clouds calculated, and optical-UV data de-absorbed. 16 figures, 15 page
TeV BL Lac objects at the dawn of the Fermi era
We reconsider the emission properties of the BL Lac objects emitting in the
high-energy gamma-ray band exploiting the new information in the MeV-GeV band
obtained by the Large Area Telescope (LAT) onboard the Fermi Gamma-Ray Space
Telescope in its first three months of operation. To this aim we construct the
spectral energy distribution of all the BL Lacs revealed by LAT and of the
known TeV BL Lacs not detected by LAT, also including data from the Swift
satellite, and model them with a simple one-zone leptonic model. The analysis
shows that the BL Lacs detected by LAT (being or not already detected in the
TeV band) share similar physical parameters. While some of the TeV BL Lacs not
revealed by LAT have spectral energy distributions and physical parameters very
similar to the LAT BL Lacs, a group of objects displays peculiar properties
(larger electron energies and smaller magnetic fields) suggesting different
physical conditions in the emission region. Finally, we discuss possible
criteria to effectively select good new candidates for the Cherenkov telescopes
among the LAT sources, presenting a list of predicted fluxes in the very
high-energy band calculated including the effect of the absorption by the
extragalactic background light.Comment: 18 pages, 8 figures, accepted for publication in MNRA