The NANOGrav 15-year (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nanohertz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black-hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this paper, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant-power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index gamma. Concurrently, it better approximates physically realistic GWB spectra -- especially those of cosmological origin -- than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.) that are ultimately combined in a Bayesian model average. 

Biography

Amit Bhoonah is an Associate Member of the NANOGrav Collaboration working primarily with the New Physics Working Group. He completed his PhD at Queen's University, where most of his research focused on astrophysical probes of dark matter, particularly galactic center gas clouds. He was then a postdoctoral fellow at Colorado State University and the University of Pittsburgh, where his work focused on using high-precision atomic spectroscopy for dark matter detection, neutrino mass model building, and low-scale baryon and lepton number violation.

Inviter: Ning-Qiang Song