Measuring A Truncated Disk in Aquila X-1

Ashley L. King, John A. Tomsick, Jon M. Miller, Jerome Chenevez, Didier Barret, Steven E. Boggs, Deepto Chakrabarty, Finn E. Christensen, William W. Craig, Felix Furst, Charles J. Hailey, Fiona A. Harrison, Michael L. Parker, Daniel Stern, Patrizia Romano, Dominic J. Walton, William W. Zhang

Published 2016-02-24Version 1

We present NuSTAR and Swift observations of the neutron star Aquila X-1 during the peak of its July 2014 outburst. The spectrum is soft with strong evidence for a broad Fe K\alpha line. Modeled with a relativistically broadened reflection model, we find that the inner disk is truncated with an inner radius of 15+/-3 R_G. The disk is likely truncated by either the boundary layer and/or a magnetic field. Associating the truncated inner disk with pressure from a magnetic field gives an upper limit of B<5+/-2x10^8G. Although the radius is truncated far from the stellar surface, material is still reaching the neutron star surface as evidenced by the X-ray burst present in the t NuSTAR observation.