Introduction

The Whole Earth Blazar Telescope (WEBT) is an international collaboration of optical, near-infrared, and radio observers who in concert have the capability to obtain continuous, high-temporal-density monitoring of blazars.

Because of the longitude diversity of participating telescopes, it is feasible to obtain 24-hour continuous optical observations, with observing activity moving from east to west around the world as the Earth rotates.

The multifrequency WEBT data are extremely useful for understanding the highly variable emission of blazars. These sources are active galactic nuclei, whose central engine is a billion solar mass black hole fed by an accretion disc, while two relativistic plasma jets are launched from the polar regions. The peculiarity of blazars is that one of the two jets is oriented towards the Earth, so that its emission is Doppler boosted. As a consequence, the observed flux is enhanced, the emission is blue-shifted in frequency, and the variability time-scales are contracted. The jet emission can be observed at all wavelengths, from the radio to the gamma rays, and varies on time-scales ranging from minutes to years.

The optical-to-radio light curves obtained by the WEBT are often studied in conjunction with observations at higher frequencies (ultraviolet, X- and 𝛾-rays), acquired by satellites and ground-based TeV telescopes, to have a complete broad-band view of the source emission.

A brief outline of WEBT history

In the years from 1991 to 2000 the Compton Gamma Ray Observatory (CGRO) satellite discovered that the extragalactic sky is full of blazars.

In 1997, John Mattox (Boston University, USA) founded the Whole Earth Blazar Telescope (WEBT), as a collaboration of optical observers whose goal was to support the high-energy observations with continuous optical monitoring.

In 2000, the Presidency of the WEBT passed to Massimo Villata (Osservatorio Astronomico di Torino, Italy), who extended the collaboration to radio and near-IR observers. Claudia M. Raiteri (Osservatorio Astronomico di Torino, Italy) covered the role of WEBT Executive Officer, supporting the President in the WEBT management.

The WEBT has organized tens of multiwavelength campaigns on specific objects, with different time extensions.

In 2007, the GLAST-AGILE Support Program (GASP) was started, in view of the launch of the AGILE and Fermi (formerly GLAST) 𝛾-ray satellites. Its aim is to provide long-term low-energy support to the space observations. The list of GASP blazars to be monitored in a continuous way (see Target List) now includes 15 BL Lac-type objects + 14 flat-spectrum radio quasars (FSRQs).

The participation in the WEBT has been increasing in time; about 200 observers have contributed to the WEBT activities, using more than 100 telescopes  (see Archive). Also advanced amateur astronomers have joined the monitoring efforts.

WEBT deliverables

• Multiband light curves + polarimetry + spectroscopy 
• Theoretical models for the interpretation of the observations
• Data Archive, where published data are stored and are available upon request one year after publication
• Papers on main international refereed journals (see Publications)
• Proceedings of conferences where the WEBT results have been presented (see Publications)