Irradiation of a standard accretion disk by a compact x-RAY source cannot explain the observed x-RAY/UV lags in NGC 4151 despite UV/optical lags which follow predictions, suggesting an extended…

Wayne State University, located in Detroit, Michigan, was awarded a $38,500 research grant effective January 1, 2025, with completion targeted for December 31, 2026. This civilian agency grant supports a four-month Neutron Imaging Collaborative Research Effort (NICER) campaign investigating the X-ray corona and inner accretion disk structure of the active galactic nucleus NGC 4151. The project addresses a fundamental astronomical puzzle: standard accretion disk irradiation models cannot adequately explain observed X-ray and ultraviolet lags detected in the source, despite accurate predictions for UV/optical timing relationships. The research will utilize daily-cadence X-ray spectroscopy with sufficient signal-to-noise ratio to obtain time-resolved measurements of intrinsic X-ray fluxes across multiple energy bands, accounting for the source's highly variable obscuration that may complicate corona size estimates.

The award delivers coordinated multi-wavelength observations correlating the NICER X-ray data with complementary ultraviolet and optical measurements to map the spatial structure of the X-ray corona and map inner disk properties. This comparative spectral-timing analysis will test whether an extended X-ray corona, an additional reprocessor between the X-rays and disk, or other physical mechanisms explain the observed X-ray/UV timing discrepancies. The contract carries no set-aside designation and was funded through a civilian federal agency's Shared Services Center. Wayne State University's Sponsored Project Administration Division will execute the work from its Detroit, Michigan location, leveraging the institution's established expertise in advanced scientific research and federal grant administration across multiple domains.

Irradiation of a standard accretion disk by a compact x-RAY source cannot explain the observed x-RAY/UV lags in NGC 4151 despite UV/optical lags which follow predictions, suggesting an extended x-RAY corona or an additional reprocessor between the x-rays and the disk. However, the source is affected by highly variable obscuration which may distort size estimates of the soft x-RAY corona using observed count rates. Our 4-month nicer campaign will deliver x-RAY spectra with a daily cadence and sufficient s/n to obtain timeresolved measurements of the intrinsic x-RAY fluxes in multiple bands, which will be tested for correlation with coordinated UV/optical observations to map the x-RAY corona and inner disk.