Whole Earth Blazar Telescope
The Whole Earth Blazar Telescope (WEBT) is an international consortium of astronomers created in 1997, with the aim to study a particular category of Active Galactic Nuclei (AGN) called blazars, which are characterized by strong and fast brightness variability, on time scales down to hours or less.
This collaboration involves many telescopes observing at optical, near-infrared, and radio (millimetric and centimetric) wavelengths. Thanks to their different geographic location all around the world, the emission variations of the pointed source can be monitored 24 hours a day, with the observing task moving from east to west as the Earth rotates.
WEBT observations are often carried out in conjunction with observations at higher frequencies, from ultraviolet to gamma rays, performed by both space and ground-based telescopes. In this way, information on blazar emission over almost the whole electromagnetic spectrum can be obtained.
The multi-wavelength studies performed by the WEBT have the purpose of understanding the physical mechanisms that rule the variable emission of these celestial objects. This emission mainly comes from a plasma jet pointing closely to the line of sight, and originating from a supermassive black hole located in the core of the host galaxy.
Three years after, in 2000, the leadership was committed to Massimo Villata, from the Observatory of Turin. A constitution was issued, defining purposes and management of the organization. Soon after, also radio and near-infrared observers joined the consortium.
Until February 2009, the WEBT has organised 24 observing campaigns, with the participation of more than one hundred telescopes. Each campaign is devoted to a specific source, and is led by a Campaign Manager appointed by the President. The Campaign Manager is responsible for the observing strategy, data collection, analysis and interpretation, and finally takes care of the publication of the results.
This is the list of the blazars that have been targets of WEBT campaigns:
- AO 0235+16 
- Markarian 421 
- S5 0716+71 
- BL Lacertae 
- Markarian 501
- 3C 66A 
- OJ 287 
- 3C 454.3 
- 3C 279 
On September 4, 2007, the WEBT started a new project: the GLAST-AGILE Support Program (GASP). Its aim is to provide observing support at longer wavelengths to the observations by the gamma-ray satellites GLAST (Gamma-ray Large Area Space Telescope, later renamed Fermi Gamma-ray Space Telescope in honor of the famous Italian physicist Enrico Fermi), and AGILE (Astro-rivelatore Gamma a Immagini LEggero). The GASP strategy is a long-term monitoring of selected targets, with periodic data gathering and analysis.
The list of the GASP monitored blazars includes 28 bright objects: 3C 66A, AO 0235+16, PKS 0420−01, PKS 0528+134, S5 0716+71, PKS 0735+17, OJ 248, OJ 49, 4C 71.07, OJ 287, S4 0954+65, Markarian 421, 4C 29.45, ON 231, 3C 273, 3C 279, PKS 1510−08, DA 406, 4C 38.41, 3C 345, Markarian 501, 4C 51.37, 3C 371, PKS 2155−304, BL Lacertae, CTA 102, 3C 454.3 and 1ES 2344+514.
- "WEBT First Light - a study of AO 0235+16". Astronomy & Astrophysics. 377: 396–412. Bibcode:2001A&A...377..396R. arXiv: . doi:10.1051/0004-6361:20011112.
- "The WEBT campaign to observe AO 0235+16 in the 2003-2004 observing season. Results from radio-to-optical monitoring and XMM-Newton observations".
- "Multifrequency variability of the blazar AO 0235+164. The WEBT campaign in 2004-2005 and long-term SED analysis".
- "Radio-to-UV monitoring of AO 0235+164 by the WEBT and Swift during the 2006-2007 outburst". Astronomy and Astrophysics. 480: 339–347. Bibcode:2008A&A...480..339R. arXiv: . doi:10.1051/0004-6361:20079044.
- "Optical monitoring of the blazar Mk421 during the TeV outburst".
- "The 0716+714 WEBT campaign of February 1999". Bibcode:2000A&A...363..108V.
- "Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. I. Simultaneous broadband observations during November 2003".
- "Testing the inverse-Compton catastrophe scenario in the intra-day variable blazar S5 0716+71. II. A search for intra-day variability at millimetre wavelengths with the IRAM 30 m telescope".
- "The WEBT BL Lacertae Campaign 2000".
- "The WEBT BL Lacertae Campaign 2001 and its extension. Optical light curves and colour analysis 1994-2002".
- "The WEBT campaigns on BL Lacertae. Time and cross-correlation analysis of optical and radio light curves 1968-2003".
- "Coordinated Multiwavelength Observation of 3C 66A during the WEBT Campaign of 2003-2004".
- "The Whole Earth Blazar Telescope Campaign on the Intermediate BL Lac Object 3C 66A in 2007-2008". The Astrophysical Journal. 694: 174–182. Bibcode:2009ApJ...694..174B. arXiv: . doi:10.1088/0004-637X/694/1/174.
- "Prominent activity of the blazar OJ 287 in 2005. XMM-Newton and multiwavelength observations".
- "The unprecedented optical outburst of the quasar 3C 454.3. The WEBT campaign of 2004-2005". Astronomy and Astrophysics. 453: 817–822. Bibcode:2006A&A...453..817V. arXiv: . doi:10.1051/0004-6361:20064817.
- "The radio delay of the exceptional 3C 454.3 outburst. Follow-up WEBT observations in 2005-2006".
- "WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase. Detection of the little and big blue bumps".
- "The WEBT Campaign on the Blazar 3C 279 in 2006". The Astrophysical Journal. 670: 968–977. Bibcode:2007ApJ...670..968B. arXiv: . doi:10.1086/522583.
- "Results of WEBT, VLBA and RXTE monitoring of 3C 279 during 2006-2007". Astronomy and Astrophysics. 492: 389–400. Bibcode:2008A&A...492..389L. arXiv: . doi:10.1051/0004-6361:200810937.
- "WEBT publications".