John’s primary work is to use X-ray and multi wavelength observations as a probe of the regions of strong gravity near accreting black holes and neutron stars. John was the chair of the RXTE Users Group, and he has been a member of the Chandra Users Committee and the INTEGRAL Users Group. He is currently a leading member of the NuSTAR Science Team, being the chair of the Galactic Binaries Working Group and the NuSTAR Users Group. John is a member of the 3G collaboration, which is developing the science case for the next generation of ground-based gravitational-wave detectors. John is also the Project Scientist for the Compton Spectrometer and Imager (COSI/COSI-X) balloon mission and is a member of the Physics of the Cosmos Program Analysis Group (PhysPAG) Executive Committee.
Between 1998 and 2019, John has been first author or co-author on 257 refereed publications, which have been cited over 8200 times. The work has included results using NuSTAR, XMM-Newton, Chandra, INTEGRAL, Swift, VLT, VLA, COSI, and more to observe accreting black holes and neutron stars, cataclysmic variables, AGN, and Sgr A*. The topics include black hole spins and warped accretion disks as well as constraints on populations of High-Mass X-ray Binaries.