Research Interests

Neutron Stars:

Constraining Properties of Neutron Stars via Disk Reflection:

  • Fe lines provide a powerful tool to infer properties of neutron stars, such as their radius, magnetic field strengths, and potential boundary layers of infalling material extending from their surface. Below is an example of Fe line profiles seen in neutron star low-mass X-ray binaries that have been observed with NuSTAR with the model predicted Fe line profile overlaid in red. These lines are intrinsically narrow, but are broadened due to Newtonian motion within the disk, special relativity introducing beaming into and out of the line of sight, and gravitational redshifts due to the large potential well near the compact object. The extent of red wing indicates how close the accretion disk is to the compact object. The more extended the red wing, the closer to the neutron star and vice versa. The accretion disk must truncate at or prior to the surface of the neutron star and consequently can place an upper limit on the radius of the compact accretor. Conversely, if the disk is impeded prior to the surface then an upper limit can be placed on the strength of the magnetic field by assuming that the inner disk radius implied from the Fe line is the Alvfen radius.

Black holes:

Black hole spin determination from relativistic lines:

  • Disk reflection also occurs around black holes from the stellar to supermassive scale. At sufficiently high mass accretion rates, the disk should extend down to the innermost stable circular orbit (ISCO). The position of the ISCO in these systems is determined by the dimensionless spin parameter, which measures the mass normalized total angular momentum. Rapid rotation leads to the surrounding accretion disk extending closer to or farther away from the black hole depending on if the disk is prograde or retrograde. This is due to the black hole warping the fabric of space time. By measuring the position of the ISCO in these systems, we can determine the spin of the black hole. This is of great interest since a black hole can be completely described by three quantities: mass, angular momentum, and charge. This is known as the No-hair theorem. It is generally believed that black holes do not have a charge. This leads to just two quantities that can fully describe the most compact objects in the Universe. Measuring the spin of a black hole additionally provides insight into the growth mechanisms at the supermassive end. Rapid rotation is linked with a more coherent accretion history while a more modest spin is thought to arise from chaotic episodes of accretion or mergers.

Accretion physics around compact objects:

  • There are a number of interesting concepts that can be explored by studying the accretion emission itself. How does the ionization state of the surrounding material change as a function of radius within the disk? Is the illuminating source extended or compact in nature? These questions and more can be answered by studying multiple reflection features arising from the disk.

Publications can be found here: ADS Bumblebee Link or ORCID