During the past week, NuSTAR performed a Target-of-Opportunity observation of an accreting supermassive black hole powering a relativistic jet, called PKS 1725+123, for more than two days of total exposure, together with a coordinated 11-hour XMM-Newton observation. Located at a cosmic epoch in which the universe was approximately half of its current age, PKS 1725+123 was the focus of a world-wide astrophysical observing campaign across the electromagnetic spectrum during the past two weeks after the Fermi gamma-ray telescope and numerous ground-based Very High Energy gamma-ray observatories confirmed the source to be flaring in gamma-rays. PKS 1725+123 belongs to a rare class of accreting supermassive black holes, known as Very High Energy gamma-ray blazars, which are believed to be emitting powerful relativistic jets pointed towards Earth. Despite being some of the brightest objects in the gamma-ray sky and viable sources of astrophysical neutrino emission, very little is currently known about the mechanisms responsible for the extreme radiation detected from such objects. A common prediction amongst competing models of particle acceleration and radiation is that a crucial region of the spectral energy distribution for distinguishing between models lies in the high-energy X-ray band covered by NuSTAR. The corresponding spectra and variability characteristics of the outburst of PKS1725+123 provided by NuSTAR will thus be crucial in understanding the origin of some of the most energetic astrophysical flares currently known.

Authors: Peter Boorman (Caltech), Lea Marcotulli (DESY, Germany)