OVRO 40m Telescope

Monitoring of Fermi Blazars

In 2007, the 40 M Telescope at the Owens Valley Radio Observatory (OVRO) embarked on a new research campaign. In support of the Fermi Gamma-ray Space Telescope, launched in 2008, the OVRO 40 M Telescope is monitoring more than 1800 blazars about twice per week.

Our paper, Blazars in the Fermi Era: The OVRO 40-Meter Telescope Monitoring Program, describes our observing program in detail and presents results from 2008 and 2009. Extended analysis on the differencies of radio and gamma-ray selected samples using data between 2008 and 2011 is presented in Connecting radio variability to the characteristics of gamma-ray blazars. Other OVRO publications are listed on the OVRO 40m Papers page.

The 40 M measurements at 15 GHz are being compared to the Fermi gamma-ray measurements of the same sources. By looking for correlations in the variability, we are gaining a new understanding of the emission mechanisms at the hearts of Active Galactic Nuclei.

Reduced data for our core sample, the 1158 CGRaBS (Healey et al. 2008) north of -20° declination, are available to the public. The data can be obtained here. Use the user name guest and a blank password for access.

List of all AGN monitored at OVRO can be found here. If you wish to obtain data for a source not listed on our data page, please contact us via email.


The OVRO 40m monitoring program has led to several discoveries. Here we list a few highlights.

Symmetric Achromatic Variability in radio sources

Symmetric Achromatic Variability (SAV) is a newly discovered type of variability seen in light curves of the radio galaxies. It has been identified in 40m light curve of J1415+1320 and potentially a few other sources. This type of varibility is most likely explained by gravitational lensing, where the background quasar is lensed by an intermediate object. In this particular case the lens consists most likely of two intermediate mass range black holes. This discovery opens an entirely new opportunity to observe blazars at micro-arcsecond resolution and to probe the cosmological matter distribution at intermediate mass scales.

Caltech Press Release: Cosmic Magnifying Lens Reveals Inner Jets of Black Holes
Original publications: Vedantham et al., 2017a, Vedantham et al., 2017b

Radio Gamma-ray correlation in blazars

The origin of the Gamma-ray emission in blazars has been a long standing problem. A particular focus of the OVRO 40m monitoring campaign was to test whether the radio emission process is physically connected to the emission of Gamma-rays. Extensive correlation anaylsis between the OVRO 40m 15GHz light curves and the Fermi-LAT Gamma-ray light curves has shown that only a few dozen sources show significantly correlated variability. In these sources the radio variability lags behind the Gamma-ray emission on time scales from 30-150 days, implying that the Gamma-ray emission is produced upstream of the radio emission zone.

Original publications: Max-Moerbeck et al., 2014a, Max-Moerbeck et al., 2014b

Quasi-periodic oscillation in the blazar J1359+4011

The OVRO 40m light curve of the blazar J1359+4011 shows clear signatures of a quasi-periodic oscillation (QPO). This behaviour can be interpreted as the high-mass version of QPOs seen in microquasars and thus indicates that jet-producing X-ray binaries and blazars are similar objects on very different mass-scales.

Original publication: King et al., 2013


The OVRO 40 M Telescope Fermi Blazar Monitoring Program is supported by NASA under awards NNX08AW31G and NNX11A043G, and by the NSF under awards AST-0808050 and AST-1109911.