For the first time, the possible appearance of ultra-high energy photons from merging compact objects such as black holes was tested. Constraints on the photon fluence could be placed from data of the Pierre Auger Observatory. This is a unique contribution to the new field of "multimessenger astronomy".
The breakthrough discovery of gravitational waves gave access to a new type of transient astronomical event: the merging process of two compact stellar-mass objects. Meanwhile, almost 100 mergers were registered in the first runs of the Advanced LIGO and Virgo detectors. Most of them were binary black holes, but also mergers including neutron stars were seen.
To better understand these exciting events, information from further signal messengers is essential (multimessenger astronomy). In fact, coincident signals could be observed over a large range of the electromagnetic spectrum at rather low energies and searches for neutrinos were conducted, too. Only neutral particles (neutrinos, photons) can help here, since charged cosmic rays would be deflected on the way from the source towards Earth, losing any signal coincidence. Ultra-high energy photons with energies above 1019 eV have so far not been tested for a possible coincident arrival from such mergers. As a new "test laboratory", these mergers may also offer surprises including unexpected discoveries.
A selection of 10 mergers was used to look for coincident air showers in the data set of the Pierre Auger Observatory. The analysis focused on this subset, since most mergers are very far away or had only a very uncertain sky localization. Using two time windows (1000 sec and 1 day after the merger), no air showers of time-directional coincidence were observed, and limits on the photon fluence were placed. For the closest event, the binary neutron star merger GW 170817 at about 40 Mpc distance, this means that no more than 20% of its total merger energy was released as photons at highest energies.
The number of merger events measured with the upgraded gravitational wave detectors will grow significantly, and a particular exciting case would be mergers even closer than GW 170817. The field of multimessenger astronomy is still young and might offer further discoveries and surprises.
Upper limits on the fluence (energy per area) of ultra-high energy photons from the selected merger events for the long (a) and the short (b) time window.
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