The first gravitational lens seen in Gamma-rays

The sky seen by the Fermi Large Area Telescope (LAT). The gamma-ray sky is dominated by blazars and diffuse radiation from our galaxy. Credit: NASA/DOE/Fermi LAT Collaboration

The Fermi satellite was launched in 2008 and since then it has continuously monitored the sky at gamma-ray energies above 100 MeV. Most of the sources detected at these energies are blazars, Active Galactic Nuclei in which the accretion onto a supermassive black hole also leads to the launching of two opposite relativistic jets. If a jet is pointing close to our line of sight we will see intense high energy emission due to strong Doppler boosting.

Fermi has so far detected well over 1000 blazars. One of these is B0218+357, which is known from optical and radio observations to be gravitationally lensed by a foreground spiral galaxy. The lens forms two closely spaced images of the blazar. The Fermi Large Area Telescope (LAT) can not spatially separate the two images so it can only measure the sum of both. However,
with timing analysis it is still possible to separate the signal of the individual components. This is because the path length from the blazar to us is different for the two images so we measure all blazar variability twice, with some time separation.
It is an advantage to do this for gamma-rays since blazar variability is more rapid at higher energies.

Illustration of the gravitational lens. Credit: NASA's Goddard Space Flight Center

A great opportunity to study these effects occurred in Autumn 2012 when B0218+357 went through an outburst, where it was much brighter than usual. The Fermi team analysing the data, was able to make an accurate determination of the time delay between the two images. This was found to be 11.46 +/-0.16 days.
These results can be used both to make an independent estimate of the Hubble constant and to study the structure of the blazar jet by comparing the lens properties at different wavelengths.
A paper describing the study has been accepted for publication by The Astrophysical Journal Letters and is also available at
http://arxiv.org/abs/1401.0548.

See also NASA’s Fermi Makes First Gamma-ray Study of a Gravitational Lens

and (in Swedish)
Gravitationslins studerad i gamma-strålning

– Stefan Larsson, researcher at the Oskar Klein Centre
stefan@astro.su.se

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