Multi-messenger astronomy:
Monitoring of AGN in search for the origin of Neutrinos

Throughout the history of modern astronomy the introduction of new theories and new technologies has led to giant leaps in our understanding not only of astronomy but also of physics and cosmology. The story began with the Copernican revolution, and continued with Galileo's introduction of the optical telescope, the building of the 100-inch telescope on Mount Wilson, the successive openings of the radio, X-ray, gamma-ray, and, most recently and spectacularly, the gravitational wave windows on the Universe. The opening of the neutrino and highest energy cosmic-ray window is the last frontier in this tradition.

Active galactic nuclei (AGN) have long been considered prime candidates for neutrino emission. The 40m Telescope of the Owens Valley Radio Observatory (OVRO) is a worldwide unique facility able to test the hypothesis whether Neutrinos detected with the IceCube experiment originate from radio-loud AGN. A recent study using OVRO 40m data is suggesting first evidence in support of this hypothesis, but a dedicated monitoring campaign of a large sample of AGN required to reach a confident conclusion. The new proposed OVRO AGN multi-messenger monitoring program is hoping to embark on a campaign of monitoring more than 5500 AGN in support of both multi-messenger astrophysics - in collarobarion with the IceCube experiment - and the physics of relativistic jets.

This new proposed program will build on the more than a decade long expertise gained through the OVRO 40m Telescope Monitoring Program, a monitoring monitoring program begun in 2008 and discontinued in 2020 due to lack of funding. The monitoring program has led to numerous publications listed on this site, about one publication per month.

A list of all objects monitored at OVRO can be found here. If you wish to obtain data for a source in this sample please contact us via email.

Discoveries

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

PKS 2131-021: A unique super-massive black hole binary candidate

The light curve of PKS 2131-021 covering 45 years combining data from OVRO, UMRAO, and Haystack Observatory exhibits unique periodic radio flux density variations, which strongly suggest an Super-Massive Black Hole Binary (SMBHB) with an orbital separation of ~0.001-0.01 pc. The periodic variation is remarkably sinusoidal. The thorough statistical analyses demonstrate conclusively, at the 4.6-sigma significance level, that the periodicity in this object is not due to random fluctuations in flux density. The sine-wave modulation in the flux density can be easily explained with the orbital motion of a SMBHB, combined with the strong Doppler boosting of the approaching relativistic jet.

Caltech Press Release: Colossal Black Holes Locked in Dance at Heart of Galaxy
Original publications: O'Neill et al., 2022

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.