Variable optical/infrared counterpart to the transient gamma-ray source J0109+6134

Context. We investigate the optical/infrared counterpart to the flaring gamma-ray source J0109+6134, which is believed to be a blazar seen through the Galactic plane. Aims. The original aim of this work was to confirm the previously proposed optical counterpart by means of studying its temporal behaviour. The study was later extended to infrared wavelengths as new data became available. Methods. We conducted a long-term differential CCD photometry campaign using the robotic Liverpool telescope. In addition, we used infrared satellite observations to also explore the source variability at these longer wavelengths. Results. Evidence of variability well correlated with gamma-ray flares has been observed so far only in the infrared domain. This fact strongly supports that the proposed optical/infrared counterpart identification is correct. Moreover, our optical photometric campaign revealed an intense optical flare with 1.7 mag amplitude that occurs on time-scales of weeks. This optical event was observed to evolve without a counterpart in the nearly simultaneous gamma-ray monitoring by the Fermi satellite. Gamma-ray orphan optical flares have rarely been observed in other blazars, and J0109+6134 appears to be an interesting additional example for future studies. © 2014 ESO.