2,392 research outputs found
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 contribution of faint AGN to the hard X-ray background
Hard X-ray selection is the most efficient way to discriminate between
accretion-powered sources, such as AGN, from sources dominated by starlight.
Hard X-rays are also less affected than other bands by obscuration. We have
then carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the
largely unexplored 5-10 keV band, finding 180 sources in ~50 deg^2 of sky with
flux >5E-14 erg cm-2 s-1. After correction for the non uniform sky coverage
this corresponds to resolving about 30 % of the hard Cosmic X-ray Background
(XRB). Here we report on a first optical spectroscopic identification campaign,
finding 12 AGN out of 14 X-ray error-boxes studied. Seven AGN show evidence for
obscuration in X-ray and optical bands, a fraction higher than in previous
ROSAT or ASCA-ROSAT surveys (at a 95-99 % and 90 % confidence level
respectively), thus supporting the scenario in which a significant fraction of
the XRB is made by obscured AGN.Comment: MNRAS, revised version after minor referee comment
Swift detection of all previously undetected blazars in a micro-wave flux-limited sample of WMAP foreground sources
Almost the totality of the bright foreground sources in the WMAP CMB maps are
blazars, a class of sources that show usually also X-ray emission. However, 23
objects in a flux-limited sample of 140 blazars of the WMAP catalog (first
year) were never reported before as X-ray sources. We present here the results
of 41 Swift observations which led to the detection of all these 23 blazars in
the 0.3-10 keV band. We conclude that all micro-wave selected blazars are X-ray
emitters and that the distribution of the micro-wave to X-ray spectral slope
of LBL blazars is very narrow, confirming that the X-ray flux
of most blazars is a very good estimator of their micro-wave emission. The
X-ray spectral shape of all the objects that were observed long enough to allow
spectral analysis is flat and consistent with inverse Compton emission within
the commonly accepted view where the radiation from blazars is emitted in a
Sychrotron-Inverse-Compton scenario. We predict that all blazars and most radio
galaxies above the sensitivity limit of the WMAP and of the Planck CMB missions
are X-ray sources detectable by the present generation of X-ray satellites. An
hypothetical all-sky soft X-ray survey with sensitivity of approximately
erg/s would be crucial to locate and remove over 100,000 blazars
from CMB temperature and polarization maps and therefore accurately clean the
primordial CMB signal from the largest population of extragalactic foreground
contaminants.Comment: 13 pages, 4 figures, 5 tables, A&A in pres
The 0.1-200 keV spectrum of the blazar PKS 2005-489 during an active state
The bright BL Lac object PKS 2005-489 was observed by BeppoSAX on November
1-2, 1998, following an active X-ray state detected by RossiXTE. The source,
detected between 0.1 and 200 keV, was in a very high state with a continuum
well fitted by a steepening spectrum due to synchrotron emission only. Our
X-ray spectrum is the flattest ever observed for this source. The different
X-ray spectral slopes and fluxes, as measured by various satellites, are
consistent with relatively little changes of the peak frequency of the
synchrotron emission, always located below 10^{17} Hz. We discuss these results
in the framework of synchrotron self-Compton models. We found that for the
BeppoSAX observation, the synchrotron peak frequency is between 10^{15} and
2.5x10^{16} Hz, depending on the model assumptions.Comment: 7 pages, 4 figures, accepted for publication in A&
Spectral Evolution of PKS 2155-304 observed with BeppoSAX during an Active Gamma-ray Phase
We present the results of BeppoSAX observations of PKS 2155-304 during an
intense gamma-ray flare. The source was in a high X-ray state. A temporal
analysis of the data reveals a tendency of the amplitude of variations to
increase with energy, and the presence of a soft lag with a timescale of the
order 10^3 s. A curved continuum spectrum, with no evidence of spectral
features, extends up to ~50 keV, while there is indication of a flatter
component emerging at higher energies, consistent with the interpretation of
the broad band spectral energy distribution (SED) as due to synchrotron
self-Compton (SSC) emission from a single region. Notably, the fitting of the
SED with such a model is consistent with an interpretation of the detected soft
lag as due to radiative cooling, supporting the idea that radiation losses play
an important role in variability. The observed shifts of the SED peaks between
the lowest and highest flux levels can be accounted for by an increase of the
break energy in the relativistic particle spectrum. The model predicts emission
at TeV energies in good agreement with the recently reported detection.Comment: 36 pages (8 figures), Latex with AAS macros, etc), accepted for
publication on Astrophysical Journa
Swift observations of IBL and LBL objects
BL Lacs are an enigmatic class of active galactic nuclei (AGNs),
characterized by the non-thermal continuum typically attributed to synchrotron
and inverse Compton emission. Depending on the frequency location of the maxima
of these components, they are subdivided into three subclasses LBLs, IBLs, and
HBLs. We present the results of a set of observations of eight BL Lac objects
of LBL and IBL type performed by the XRT and UVOT detectors onboard the Swift
satellite between January 2005 and November 2006. We are mainly interested in
measuring the spectral parameters, and particularly the steepness between the
UV and the X-ray band, useful for determining the classification of these
sources. We compare the behavior of these sources with previous XMM-Newton,
BeppoSAX obser- vations and with historical data in the X-ray and in the
optical band. We are also interested in classifying the sources in our sample
on the basis of the observations and comparing them with their classification
presented in literature. We performed X-ray spectral analysis of observed BL
Lac objects using a simple powerlaw and in a few cases the log-parabolic model.
We also combined the UV emission with the low energy X-ray data to We used
observational data to classify sources in our sample and derived parameters of
their spectral energy distribution. We found that for the IBLs X-rays low
states show features of the high energy component, usually interpreted as due
to inverse Compton emission. Sources in our sample exhibit a range of temporal
UV and X-ray behaviors, some objects having clear and neat correlated UV and
X-ray variations (e.g. ON231) and other objects showing no clear (e.g. AO
0235+164) UV and X-ray correlation. Finally, we also note that our estimates of
spectral curvature are in the range of that measured for the HBLs.Comment: 10 pages, 7 figures, published in A&
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