57 research outputs found
Inverse Compton X-ray Emissions from TeV blazar Mrk421 during a Historical Low-Flux State Observed with NuSTAR
We report on the detection of excess hard X-ray emission from the TeV BL Lac
object Mrk421 during the historical low-flux state of the source in January
2013. NuSTAR observations were conducted four times between MJD56294 and
MJD56312 with a total exposure of 80.9 ksec. The source flux in the 3-40 keV
range was nearly constant except for MJD56307, when the average flux level
increased by a factor of three. Throughout the exposure, the X-ray spectra of
Mrk421 were well represented by a steep power-law model with a photon index of
3.1, although a significant excess was noted above 20 keV in the MJD56302 data
when the source was in its faintest state. Moreover, Mrk421 was detected at
more than the 4-sigma level in the 40-79 keV count maps for both MJD56307 and
MJD56302 but not during the remaining two observations. The detected excess
hard X-ray emissions connect smoothly with the extrapolation of the high-energy
gamma-ray continuum of the blazar constrained by Fermi-LAT during the source
quiescence. These findings indicate that, while the overall X-ray spectrum of
Mrk421 is dominated by the highest-energy tail of the synchrotron continuum,
the variable excess hard X-ray emission above 20 keV (on the timescale of a
week) is related to the inverse Compton emission component. We discuss the
resulting constraints on the variability and spectral properties of the
low-energy segment of the electron energy distribution in the source.Comment: 6 pages, 4 figures, accepted for publication in Ap
A Novel Method for Estimating the Ambient Medium Density Around Distant Radio Sources from Their Observed Radio Spectra
The dynamical evolution and radiative properties of luminous radio galaxies
and quasars of the FRII type, are well understood. As a result, through the use
of detailed modeling of the observed radio emission of such sources, one can
estimate various physical parameters of the systems, including the density of
the ambient medium into which the radio structure evolves. This, however,
requires rather comprehensive observational information, i.e. sampling the
broad-band radio continua of the targets at several frequencies, and imaging
their radio structures with high resolution. Such observations are, on the
other hand, not always available, especially for high-redshift objects. Here we
analyze the best-fit values of the source physical parameters, derived from an
extensive modeling of the largest currently available sample of FRII radio
sources, for which good-quality multi-wavelength radio flux measurements could
be collected. In the analyzed dataset, we notice a significant and non-obvious
correlation between the spectral index of the non-thermal radio emission
continuum, and density of the ambient medium. We derive the corresponding
correlation parameters, and quantify the intrinsic scatter by means of Bayesian
analysis. We propose that the discovered correlation could be used as a
cosmological tool to estimate the density of ambient medium for large samples
of distant radio galaxies. Our method does not require any detailed modeling of
individual sources, and relies on limited observational information, namely the
slope of the radio continuum between the rest-frame frequencies 0.4GHz and
5GHz, possibly combined with the total linear size of the radio structure.Comment: Accepted for publication in the Astrophysical Journa
Multi-wavelength variability study of the classical BL Lac object PKS 0735+178 on timescales ranging from decades to minutes
We present the results of our power spectral analysis for the BL Lac object
PKS 0735+178 utilizing the Fermi-LAT survey at high-energy -rays,
several ground-based optical telescopes, and single-dish radio telescopes
operating at GHz frequencies. The novelty of our approach is that, by combining
long-term and densely sampled intra-night light curves in the optical regime,
we were able to construct for the first time the optical power spectrum of the
blazar for a time domain extending from 23 years down to minutes. Our analysis
reveals that: (i) the optical variability is consistent with a pure red noise,
for which the power spectral density can well be approximated by a single
power-law throughout the entire time domain probed; (ii) the slope of power
spectral density at high-energy -rays (), is significantly
flatter than that found at radio and optical frequencies () within the
corresponding time variability range; (iii) for the derived power spectra we
did not detect any low-frequency flattening, nor do we see any evidence for
cut-offs at the highest frequencies down to the noise floor levels due to
measurement uncertainties. We interpret our findings in terms of a model where
the blazar variability is generated by the underlying single stochastic process
(at radio and optical frequencies), or a linear superposition of such processes
(in the -ray regime). Along with the detailed PSD analysis, we also
present the results of our extended (1998-2015) intra-night optical monitoring
program and newly acquired optical photo-polarimetric data for the source.Comment: The Astrophysical Journal, in pres
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