S. Sclavi

Roma-BZCAT: A multifrequency catalogue of Blazars

We present a new catalogue of blazars based on multi-frequency surveys and on an extensive review of the literature. Blazars are classified as BL Lacertae objects, as flat spectrum radio quas ars or as blazars of uncertain/transitional type. Each object is identified by a root name, coded as BZB, BZQ and BZU for these three subclasses respectively, and by its coordinates. This catalogue i s being built as a tool useful for the identification of the extragala ctic sources that will be detected by present and future experiments for X and gamma-ray astronomy, like Swift, AGILE, Fermi-GLAST and Simbol-X. An electronic version is available from the ASI Science Data Center web site at http://www.asdc.asi.it/bzcat.

The intra-night optical variability of the bright BL Lacertae object S5 0716+714

Aims. We address the topic of the intra-night optical variability of the BL Lac object S5 0716+714. Methods. To this purpose a long-term observational campaign was carried out, from 1996 to 2003, which allowed the collection of a very large data set, containing 10675 photometric measurements obtained in 102 nights. Results. The source brightness varied in a range of about 2 mag, although the majority of the observations were performed when it was in the range 13.0 < R < 13.75. Variability time scales were estimated from the rates of magnitude variation, which were found to have a distribution function well fitted by an exponential law with a mean value of 0.027 mag/h, corresponding to an e-folding time scale of the flux τ F = 37.6 h. The highest rates of magnitude variation were around 0.10-0.12 mag/h and lasted less than 2 h. These rates were observed only when the source had an R magnitude <13.4, but this finding cannot be considered significant because of the low statistical occurrence. The distribution of τ F has a well-defined modal value at 19 h. Assuming the recent estimate of the beaming factor δ ∼ 20, we derived a typical size of the emitting region of about 5 x 10 16 /(1 + z) cm. The possibility of searching for a possible correlation between the mean magnitude variation rate and the long-term changes in the velocity of the superluminal components in the jet is discussed.

BL Lacertae: Complex spectral variability and rapid synchrotron flare detected with BeppoSAX

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite dierent spectra: in June it was concave with a very hard component above 5{6 keV (1 1:6; 2 0:15); in December it was well tted by a single power law ( 0:6). During the rst BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3{2 keV flux doubled in20 min, while the flux above 4 keV was almost constant. This frequency{dependent event is one of the shortest ever recorded for BL Lac objects and places lower limits on the dimension and magnetic eld of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of20% in 20 min. We t the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but eective SSC + external Compton scenario, highlighting the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to -ray Spectral Energy Distributions.

THE LONG-TERM OPTICAL VARIABILITY OF THE BL LACERTAE OBJECT S5 0716+714: EVIDENCE FOR A PRECESSING JET

We present the historical 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 mag yr-1. The simultaneous occurrence of decreasing ejection velocities of superluminal moving components in the jet reported by Bach and coworkers suggests that both phenomena are related to the change of the direction of the jet to the line of sight from about 5° to 07 for an approximately constant bulk Lorentz factor of about 12. A simple explanation is a precessing relativistic jet, which should at present be close to the smallest orientation angle. One can therefore in the next 10 years expect a decrease of the mean brightness of about 1 magnitude.

Multifrequency observations of a sample of very low frequency peaked BL Lacertae objects

Context. BL Lacertae objects with an extremely low value of the peak frequency (νs ≤ 10 13 Hz) of the synchrotron component (VLBL objects) are poorly known. Aims. We selected a small sample of these candidate objects on the basis of their radio/optical flux ratio to evaluate their variability, define the low energy part of their SED and obtain a reliable estimate of their νs through a fit with a log-parabolic law. Methods. They were the object of a ground-based observational campaign in the infrared and optical bands; data from several pointings by the Swift satellite in the UV and X-ray band were also obtained for half the sample. We complemented our data with a large number of literature data. Results. These sources showed a marked flux variability, as expected for BL Lacertae objects of the low energy peaked BL Lac objects (LBL) and intermediate BL Lac objects type; all of them have a νs value around 10 13 Hz, but this is low enough only for a few sources to consider them genuine VLBL objects. We report the overall properties of individual sources and discuss some aspects of this class of rare objects, which could correspond to the transition between flat spectrum radio quasars and LBLs.

A New Fast Silicon Photomultiplier Photometer

The realization of low-cost instruments with high technical performance is a goal that deserves some efforts in an epoch of fast technological developments; indeed, such instruments can be easily reproduced and therefore allow new research programs to be opened in several observatories. We realized a fast optical photometer based on the SiPM technology, using commercially available modules. Using low-cost components, we have developed a custom electronic chain to extract the signal produced by a commercial MPPC module produced by Hamamatsu Photonics, in order to obtain submillisecond sampling of the light curve of astronomical sources (typically, pulsars). In the early 2011 February, we observed the Crab pulsar with the Cassini telescope with our prototype photometer, deriving its period and power spectrum and the shape of its light curve, in very good agreement with the results obtained in the past with other instruments.

Optical and Radio Monitoring of S5 1803+784

The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time span of nearly 6 yr are presented and discussed. The optical light curve showed an overall variation greater than 3 mag, and the largest changes occurred in three strong flares. No periodicity was found in the light curve on timescales up to a year. The variability in the radio band is very different and shows moderate oscillations around an average constant flux density rather than relevant flares, with a maximum amplitude of ~30%, without a simultaneous correspondence between optical and radio luminosity. The optical spectral energy distribution was always well fitted by a power law. The spectral index shows small variations, and there is indication of a positive correlation with the source luminosity. Possible explanations of the source behavior are discussed in the framework of current models.

The Latest Version of SiFAP: Beyond Microsecond Time Scale Photometry of Variable Objects

Technical improvements of the Silicon Fast optical Astronomical Photometer (SiFAP) allow the instrumentation to integrate photons coming from the target in time windows down to 20μs. Further hardware improvement has been implemented to tag the Time of Arrival (ToA) of each single photon. In addition, a new commercial GPS unit has replaced the older commercial unit improving time resolution. The latest version of SiFAP has been calibrated to check photometric sensitivity and linearity through observations of several standard stars. SiFAP has been also successfully tested by observing the HZ/Her X-1 Binary System estimating the spin period of the pulsar (Her X-1). Our results have been then compared to data available in literature.

A Multifrequency Blazar catalog (Roma‐BZCAT)

We present a general catalog of blazars based on an extensive review of literature data from multi‐frequency surveys. Blazars are distinguished in BL Lacertae objects. Flat spectrum radio Quasars and Blazars of uncertain/transitional type. Each object is identified by a root name, coded as BZB, BZQ and BZU for the three subclasses respectively, and by its J2000 coordinates. This catalog is being built as a tool useful for the identification of the extragalactic sources that will be detected by the GLAST LAT experiment. An electronic version is available from the ASI Science Data Center web site. The list presently includes objects between R.A.= 0h and R.A.= 12h; the full sky is expected to be available by the end of 2007, approximately when GLAST will be fully operational.

Optical follow-up of BL Lac candidates in the 2BL sample

We investigate the nature of the BL Lac candidates in the northern part of the 2BL sample (Londish et al. 2002, MNRAS, 334, 941) searching for optical variability by means of R C band photometry with the Asiago 1.8 and Loiano 1.5 m telescopes during years 2002, 2003 and 2004. We also made aperture photometry of the candidates on the plates available on the web from the Palomar and UK Schmidt telescopes. No significant variability was detected for the majority of the objects without detected radio and/or X-ray emissions, while clear variability was found for the few sources of the sample with strong radio and/or X-ray emission, which is a constant feature of the classical BL Lacs. Some considerations on the nature of the 2BL sources are made, taking into account their most recent data available in proper motions catalogues (SuperCOSMOS, USNO-B 1) and in the Sloan Digital Sky Survey.