3,793 research outputs found
Spectral Hardening of Large Solar Flares
RHESSI observations are used to quantitatively study the hard X-ray evolution
in 5 large solar flares selected for spectral hardening in the course of the
event. The X-ray bremsstrahlung emission from non-thermal electrons is
characterized by two spectroscopically distinct phases: impulsive and gradual.
The impulsive phase usually consists of several emission spikes following a
soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as
spectral hardening while the flux slowly decreases. Both the soft-hard-soft
(impulsive) phase and the hardening (gradual) phase are well described by
piecewise linear dependence of the photon spectral index on the logarithm of
the hard X-ray flux. The different linear parts of this relation correspond to
different rise and decay phases of emission spikes. The temporal evolution of
the spectra is compared with the configuration and motion of the hard X-ray
sources in RHESSI images. These observations reveal that the two stages of
electron acceleration causing these two different behaviors are closely related
in space and time. The transition between the impulsive and gradual phase is
found to be smooth and progressive rather than abrupt. This suggests that they
arise because of a slow change in a common accelerator rather than being caused
by two independent and distinct acceleration processes. We propose that the
hardening during the decay phase is caused by continuing particle acceleration
with longer trapping in the accelerator before escape.Comment: accepted by Ap
Focusing on the extended X-ray emission in 3C 459 with a Chandra follow-up observation
6 pages, 4 figures. Reproduced with permission from Astronomy & Astrophysics. © 2019 ESO.Aims. We investigated the X-ray emission properties of the powerful radio galaxy 3C 459 revealed by a recent Chandra follow-up observation carried out in October 2014 with a 62 ks exposure. Methods. We performed an X-ray spectral analysis from a few selected regions on an image obtained from this observation and also compared the X-ray image with a 4.9 GHz VLA radio map available in the literature. Results. The dominant contribution comes from the radio core but significant X-ray emission is detected at larger angular separations from it, surrounding both radio jets and lobes. According to a scenario in which the extended X-ray emission is due to a plasma collisionally heated by jet-driven shocks and not magnetically dominated, we estimated its temperature to be ∼0.8 keV. This hot gas cocoon could be responsible for the radio depolarization observed in 3C 459, as recently proposed also for 3C 171 and 3C 305. On the other hand, our spectral analysis and the presence of an oxygen K edge, blueshifted at 1.23 keV, cannot exclude the possibility that the X-ray radiation originating from the inner regions of the radio galaxy could be intercepted by some outflow of absorbing material intervening along the line of sight, as already found in some BAL quasars.Peer reviewe
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Lyapunov functions for a non-linear model of the X-ray bursting of the microquasar GRS 1915+105
This paper introduces a biparametric family of Lyapunov functions for a
non-linear mathematical model based on the FitzHugh-Nagumo equations able to
reproduce some main features of the X-ray bursting behaviour exhibited by the
microquasar GRS 1915+105. These functions are useful to investigate the
properties of equilibrium points and allow us to demonstrate a theorem on the
global stability. The transition between bursting and stable behaviour is also
analyzed.Comment: Published on International Journal of Non-Linear Mechanics, vol. 88,
pp. 142-14
The High Energy Emission of the Crab Nebula from 20 keV to 6 MeV with INTEGRAL
The SPI spectrometer aboard the INTEGRAL mission observes regularly the Crab
Nebula since 2003. We report on observations distributed over 5.5 years and
investigate the variability of the intensity and spectral shape of this
remarkable source in the hard X-rays domain up to a few MeV. While single power
law models give a good description in the X-ray domain (mean photon index ~
2.05) and MeV domain (photon index ~ 2.23), crucial information are contained
in the evolution of the slope with energy between these two values. This study
has been carried out trough individual observations and long duration (~ 400
ks) averaged spectra. The stability of the emission is remarkable and excludes
a single power law model. The slopes measured below and above 100 keV agree
perfectly with the last values reported in the X-ray and MeV regions
respectively, but without indication of a localized break point. This suggests
a gradual softening in the emission around 100 keV and thus a continuous
evolution rather than an actual change in the mechanism parameters. In the MeV
region, no significant deviation from the proposed power law model is visible
up to 5-6 MeV. Finally, we take advantage of the spectroscopic capability of
the instrument to seek for previously reported spectral features in the covered
energy range with negative results for any significant cyclotron or
annihilation emission on 400 ks timescales. Beyond the scientific results, the
performance and reliability of the SPI instrument is explicitly demonstrated,
with some details about the most appropriate analysis method.Comment: accepted for publication in ApJ; 4 figures, 2 table
The 26 year-long X-ray light curve and the X-ray spectrum of the BL Lac Object 1E 1207.9+3945 in its brightest state
We studied the temporal and spectral evolution of the synchrotron emission
from the high energy peaked BL Lac object 1E 1207.9+3945. Two recent
observations have been performed by the XMM-Newton and Swift satellites; we
carried out X-ray spectral analysis for both of them, and photometry in
optical-ultraviolet filters for the Swift one. Combining the results thus
obtained with archival data we built the long-term X-ray light curve, spanning
a time interval of 26 years, and the Spectral Energy Distribution (SED) of this
source. The light curve shows a large flux increasing, about a factor of six,
in a time interval of a few years. After reaching its maximum in coincidence
with the XMM-Newton pointing in December 2000 the flux decreased in later
years, as revealed by Swift. The very good statistics available in the 0.5-10
keV XMM-Newton X-ray spectrum points out a highly significant deviation from a
single power law. A log-parabolic model with a best fit curvature parameter of
0.25 and a peak energy at ~1 keV describes well the spectral shape of the
synchrotron emission. The simultaneous fit of Swift UVOT and XRT data provides
a milder curvature (b~0.1) and a peak at higher energies (~15 keV), suggesting
a different state of source activity. In both cases UVOT data support the
scenario of a single synchrotron emission component extending from the
optical/UV to the X-ray band. New X-ray observations are important to monitor
the temporal and spectral evolution of the source; new generation gamma-ray
telescopes like AGILE and GLAST could for the first time detect its inverse
Compton emission.Comment: 7 pages, 6 figures, accepted for publication in A&
The complex time behaviour of the microquasar GRS 1915+105 in the \rho-class observed with BeppoSAX. III: The hard X-ray delay and limit cycle mapping
The microquasar GRS1915+105 was observed by BeppoSAX in October 2000 for
about ten days while the source was in \rho-mode, which is characterized by a
quasi-regular type I bursting activity. This paper presents a systematic
analysis of the delay of the hard and soft X-ray emission at the burst peaks.
The lag, also apparent from the comparison of the [1.7-3.4] keV light curves
with those in the [6.8-10.2] keV range, is evaluated and studied as a function
of time, spectral parameters, and flux. We apply the limit cycle mapping
technique, using as independent variables the count rate and the mean photon
rate. The results using this technique were also cross-checked using a more
standard approach with the cross-correlation methods. Data are organized in
runs, each relative to a continuous observation interval. The detected
hard-soft delay changes in the course of the pointing from about 3 s to about
10 s and presents a clear correlation with the baseline count rate.Comment: accepted for publication in A&
Log-parabolic spectra and particle acceleration in blazars. III: SSC emission in the TeV band from Mkn 501
Curved broad-band spectral distributions of non-thermal sources like blazars
are described well by a log-parabolic (LP) law where the second degree term
measures the curvature. LP energy spectra can be obtained for relativistic
electrons by means of a statistical acceleration mechanism whose probability of
acceleration depends on energy. In this paper we compute the spectra radiated
by an electron population via synchrotron (S) and Synchro-Self Compton(SSC)
processes to derive the relations between the LP parameters. These spectra were
obtained by means of an accurate numerical code. We found that the ratio
between the curvature parameters of the S spectrum to that of the electrons is
equal to about 0.2 instead of 0.25, the value foreseen in the delta
approximation. Inverse Compton spectra are also intrinsically curved and can be
approximated by a log-parabola only in limited ranges. The curvature parameter,
estimated around the SED peak, may vary from a lower value than that of the S
spectrum up to that of emitting electrons depending on whether the scattering
is in the Thomson or in the Klein-Nishina regime. We applied this analysis to
computing the SSC emission from the BL Lac object Mkn 501 during the large
flare of April 1997. We fit simultaneous BeppoSAX and CAT data and reproduced
intensities and spectral curvatures of both components with good accuracy. The
large curvature observed in the TeV range was found to be mainly intrinsic, and
therefore did not require a large pair production absorption against the
extragalactic background. We regard this finding as an indication that the
Universe is more transparent at these energies than previously assumed by
several models found in the literature. This conclusion is supported by recent
detection of two relatively high redshift blazars with H.E.S.S.Comment: Comments: 12 pages, 11 figures. Accepted for publication in the
Astronomy and Astrophysic
WATCAT: a tale of wide-angle tailed radio galaxies
We present a catalog of 47 wide-angle tailed radio galaxies (WATs), the
WATCAT; these galaxies were selected by combining observations from the
National Radio Astronomy Observatory/Very Large Array Sky Survey (NVSS), the
Faint Images of the Radio Sky at Twenty-Centimeters (FIRST), and the Sloan
Digital Sky Survey (SDSS), and mainly built including a radio morphological
classification. We included in the catalog only radio sources showing two-sided
jets with two clear "warmspots" (i.e., jet knots as bright as 20% of the
nucleus) lying on the opposite side of the radio core, and having classical
extended emission resembling a plume beyond them. The catalog is limited to
redshifts z 0.15, and lists only sources with radio emission extended
beyond 30 kpc from the host galaxy. We found that host galaxies of WATCAT
sources are all luminous (-20.5 Mr -23.7), red early-type
galaxies with black hole masses in the range M M. The spectroscopic classification indicates that they
are all low-excitation galaxies (LEGs). Comparing WAT multifrequency properties
with those of FRI and FRII radio galaxies at the same redshifts, we conclude
that WATs show multifrequency properties remarkably similar to FRI radio
galaxies, having radio power of typical FRIIs
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