3,113 research outputs found
Multi-frequency, multi-messenger astrophysics with Swift. The case of blazars
During its first 10 years of orbital operations Swift dedicated approximately
11% of its observing time to blazars, carrying out more than 12,000
observations of ~1,600 different objects, for a total exposure time of over 25
million seconds. In this paper I briefly discuss the impact that Swift is
having on blazar multi-frequency and time-domain astrophysics, as well as how
it is contributing to the opening of the era of multi-messenger astronomy.
Finally, I present some preliminary results from a systematic analysis of a
very large number of Swift XRT observations of blazars. All the "science ready"
data products that are being generated by this project will be publicly
released. Specifically, deconvolved X-ray spectra and best fit spectral
parameters will be available through the ASDC "SED builder" tool
(https://tools.asdc.asi.it/SED) and by means of interactive tables
(http://www.asdc.asi.it/xrtspectra). Innovative data visualisation methods (see
e.g. http://youtu.be/nAZYcXcUGW8) are also being developed at ASDC to better
exploit this remarkable and rapidly growing data set.Comment: 8 pages 10 figures, revised manuscript for the Journal of High Energy
Astrophysics for the dedicated issue: "Swift: Ten Years of Discovery
The discovery of high power - high synchrotron peak blazars
We study the quasi-simultaneous near-IR, optical, UV, and X-ray photometry of
eleven gamma-ray selected blazars for which redshift estimates larger than 1.2
have been recently provided. Four of these objects turn out to be high-power
blazars with the peak of their synchrotron emission between ~ 3 x 10^15 and ~
10^16 Hz, and therefore of a kind predicted to exist but never seen before.
This discovery has important implications for our understanding of physical
processes in blazars, including the so-called "blazar sequence", and might also
help constraining the extragalactic background light through gamma-ray
absorption since two sources are strongly detected even in the 10 - 100 GeV
Fermi-LAT band. Based on our previous work and their high powers, these sources
are very likely high-redshift flat-spectrum radio quasars with their emission
lines swamped by the non-thermal continuum.Comment: 5 pages, 6 colour figures, accepted for publication in the Monthly
Notices of the Royal Astronomical Societ
Evidence for a Significant Blazar Contamination in CMB Anisotropy Maps
The analysis of the recent WMAP source catalog shows that the vast majority
of bright foreground extragalactic sources detected in CMB maps are Blazars. In
this paper we calculate the contamination of CMB anisotropy maps by this type
of flat-spectrum, strongly variable and polarized extragalactic radio sources
using up-to-date results from recent deep multi-frequency surveys. We found
that more than 50 known Blazars (or Blazar candidates) are included in the
90/150 GHz BOOMERANG anisotropy maps, a factor > 15 larger than previously
reported. Using a recent derivation of the Blazar radio LogN-LogS we calculate
that these sources induce an average sky brightness of 0.2 Jy/deg^2,
corresponding to an average temperature of ~3-5 muK. Moreover, we find that the
associated level of fluctuations is of the order of C_{l, Blazar}= 1.3 10^{-2}
mu K^2 sr at 41 GHz. Taking into account both Blazar variability and the many
steep-spectrum radio sources that flatten at high frequencies, as well as the
contribution of radio-galaxies, we find that the level of residual fluctuation
due to discrete extragalactic foreground sources could be factor of ~2 - 3
higher than the above estimate. We show that the Blazar induced fluctuations
contaminate the CMB spectrum at the level of ~ 20-50 % at l = 500 and 50-100 %
at l = 800. Careful cleaning for Blazar contamination of high sensitivity/high
resolution CMB maps is therefore necessary before firm conclusions about weak
features, like secondary high-l peaks of the CMB power spectrum or very weak
signals like CMB polarization measurements, can be achieved.Comment: 10 pages, 6 Postscript figures, 1 GIF figure (Fig.3). Better version
of Fig.3 and a full list of Blazar's SED found at
http://www.asdc.asi.it/boomerang/. A&A, submitte
The ROSAT X-ray Spectra of BL Lacertae Objects
We study the X-ray spectra of 85 BL Lacertae objects using the hardness
ratios as given in the WGA catalogue of {\it ROSAT} sources. This sample
includes all WGA BL Lacs with high-quality data and comprises about 50 per cent
of presently known BL Lacs. We find that BL Lacs have energy power-law spectral
indices between 0 and 3 with a mean value . Significant
differences, however, are present between high-energy cutoff BL Lacs (HBLs),
normally selected in the X-ray band, and low-energy cutoff BL Lacs (LBLs),
generally found in radio surveys. HBLs have steeper X-ray spectral slopes
() well correlated with , and
anti-correlated with the X-ray-to-radio flux ratio and cutoff frequency, with
convex overall broad-band spectra. LBLs, on the other hand, have flatter X-ray
spectra () and concave optical-X-ray continuum. We
interpret these results in terms of different mechanisms being responsible for
the X-ray emission in the two classes, namely synchrotron and inverse Compton
for HBLs and LBLs respectively. The observed differences are consistent with
the hypothesis that HBLs and LBLs are powered by essentially the same
non-thermal engines differing mainly in their synchrotron cutoff energy.Comment: Uuencoded compressed tar file containing 9 page TeX file and 8
postscript figures. Uses mn.tex and psfig.tex. To appear in MNRAS. TeX and
postscript files also available at
http://itovf2.roma2.infn.it/padovani/xray_spectra/bllacs.htm
The number counts, luminosity functions and evolution of microwave-selected (WMAP) blazars and radio galaxies
(Abridged) We carried out an extensive search to identify the counterparts of
all the sources listed in the WMAP 3-yr catalogue using literature and archival
data. Our work led to the identification of 309 WMAP sources, 98% of which are
blazars, radio quasars or radio galaxies. At present, 15 objects still remain
without identification due to the lack of optical spectroscopic data or a clear
radio counterpart. Our results allow us to define a flux limited sample of 203
high Galactic latitude microwave sources ( Jy, ) which is virtually completely identified (99%). The microwave band
is ideally suited for blazar statistical studies since this is the part of the
em spectrum that is least affected by the superposition of spectral components
of different origin. Using this data-set we derived number counts, luminosity
functions and cosmological evolution of blazars and radio galaxies at microwave
frequencies. Our results are in good agreement with those found at radio
frequencies. The 5 GHz bivariate blazar luminosity functions are similar to
those derived from the DXRBS survey, which shows that this sample is
representative of the blazar population at 41 GHz. Microwave selected broad-
lined quasars are about 6 times more abundant than BL Lacs, a ratio that is
similar to, or larger than, that seen at radio and gamma-ray frequencies, once
spectral selection effects are taken into account. This strongly suggests that
the mechanism responsible for the generation of gamma-rays is, at first order,
the same in all blazar types. Our results confirm the findings of Giommi &
Colafrancesco (2004, 2006) that blazars and radio galaxies are the largest
contaminants of the CMB anisotropy maps. We predict that these sources are also
bright gamma-ray sources, most of which will be detected by AGILE and FERMI.Comment: 18 pages, 11 figures, 3 tables. A&A in pres
Radio Luminosities and Classificatory Criteria of BL Lacertae Objects
Using the sample of radio selected BL Lacertae objects (RBLs) and X-ray
selected BL Lacertae objects (XBLs) presented by Sambruna et al. (1996), we
calculated the luminosities of radio, optical and X-ray of each source and made
the statistical analysis among the luminosities at different wave-bands,
broad-band spectral indices from radio to X-ray () and peak
frequencies (). Our results are as follows: (i) there is a positive
correlation between radio luminosity and and a
negative correlation between and . High-energy peak BL Lacs
(HBLs) and low-energy peak BL Lacs (LBLs) can be distinguished very well, the
dividing lines are probably those of (erg/sec) and
(or )0.75 for - plot and
those of (erg/sec) and for the
- plot; (ii) there is a weak positive correlation between
optical luminosity and and a negatively weak
correlation between and ; (iii) there is no correlation
between X-ray luminosity and or between and
. From our analysis, we find that synchrotron radiation is the main
X-ray radiation mechanism for HBLs while inverse Compton scattering for LBLs.Comment: 9 pages, 3 figures. Submitted to A&
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