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

A Sample-oriented Catalogue of BL Lacertae Objects

We present a catalogue of 233 BL Lacertae objects compiled through an extensive bibliographic search updated to mid-1995. A large fraction of the sources listed in the catalogue belongs to well-defined samples and can be used for statistical purposes. A smaller fraction consists of miscellaneous (but confirmed) BL Lacs and of objects classified as BL Lac candidates. We discuss the selection criteria of the different samples, report the discovery of two previously unnoticed BL Lacs in the Palomar-Green survey, and comment on the possible association of some of the still unidentified high galactic latitude gamma-ray (EGRET) sources with BL Lacs. Some statistical properties of the catalogue are also briefly discussed.Comment: To appear in MNRAS, 7 pages, uses mn.tex. Two figures, table in TeX and ASCII format, and macro available at http://itovf2.roma2.infn.it/padovani/catalogue.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 ($f_{41GHz} \ge 1$ Jy, $|b_{\rm II}| > 15^\circ$) 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

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 $\alpha_{\rm x}\sim 1.4$. 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 ($\alpha_{\rm x}\sim 1.5$) well correlated with $\alpha_{\rm ox}$, 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 ($\alpha_{\rm x}\sim 1.1$) 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

Are the X-ray Spectra of Flat-spectrum Radio Quasars and BL Lacertae Objects Different?

We study the X-ray spectra of 114 flat-spectrum radio quasars (FSRQ) using the hardness ratios as given in the WGA catalogue of ROSAT sources. This sample includes all WGA FSRQ with high-quality data and comprises about 20 per cent of presently known such objects, which makes this the largest FSRQ sample ever studied in the X-ray band. We find that FSRQ have a distribution of energy spectral indices ranging between 0 and 3 with a mean value alpha_x ~ 1. This is consistent with that of low-energy cutoff BL Lacs (LBL; alpha_x ~ 1.1), generally found in radio surveys, but significantly different from that of high-energy cutoff BL Lacs (HBL), normally selected in the X-ray band, which display steeper X-ray spectra (alpha_x ~ 1.5). The shape of the optical-to-X-ray continuum is concave (that is alpha_x < alpha_ox) for the majority of FSRQ, as found for LBL, supporting a dominance of inverse Compton emission in the X-ray band in most objects. Our results are at odds with previous studies of the X-ray spectra of FSRQ, which were however plagued by low spectral resolution and/or small number statistics and selection effects, and have important implications for the proposed connections between FSRQ and BL Lacs.Comment: 8 pages, 5 figures, TeX file, 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/diff.htm