Daniel Amoils

General Relativity (GR) is our current best theory of gravity, but it is believed that the theory breaks down in extreme scenarios, such as inside a black hole. A black hole is the remnant of a star, a region of space where gravity is so strong that nothing, not even light, can escape. In GR, the space around a black hole is described by the Kerr metric. My project attempts to measure potential deviations from the Kerr metric around the black hole known as MAXI J1820+070 to test GR. These deviations are quantified using the 13 deformation parameter. When the value of this parameter is zero, the observation matches the prediction of the Kerr metric from GR, and when it doesn’t equal zero, the prediction and observations differ.  

To measure the deviations, I used the X-ray light emitted from the accretion disk, a disk of hot gas around the black hole. I used the models nkbb and relxill_nk, developed by my mentor, to constrain 13. Nkbb models the light emitted from the heat of the gas in the disk, and relxill_nk models the light that is reflected off the disk. I selected two observations from the NuSTAR telescope and then extracted spectral data from the results. I then fit these spectra to nkbb and relxill_nk and found the parameters that best matched the data. 

My results show that the constraints from one of the observations used were stronger than those in previous studies but did not agree with the value predicted by GR. This result was likely due to the models not being well suited to the observation rather than evidence against GR. The constraints from the other observation were not as strong due to there not being strong enough data to distinguish the value of the parameter. The strong constraints from one of the observations suggest that other observations from this black hole may provide both strong and accurate constraints.