Date of Submission

Spring 2023

Academic Program

Physics

Project Advisor 1

Shuo Zhang

Abstract/Artist's Statement

Abstract (for Astronomy-Lovers)

Supermassive blackholes exist in almost all the galaxies with a bulge component. Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, exhibits unique flaring activities on a daily basis. During the flares, Sgr A*’s X-ray luminosity increases by a factor of ∼ 10 − 400. Using new and archival X-ray observations obtained by NuSTAR from 2016 to 2022, totalling 1.33 Ms exposure time, we studied Sgr A* X-ray variability and searched for flare events. As a result, we detected a total of seven bright flares, doubling the previously detected bright Sgr A* flares reported in the literature. The flare durations range from ∼ 800s to ∼ 5600s with absorbed flux between 1.0−3.0 e−11 erg cm−2 s−1 at 3-79keV energy band. A notable flare detected in 2018 demonstrates a double-peak feature separated by 15 minutes, suggesting fast recovery of physical conditions such as magnetic field strength. We also saw a trend suggesting that brighter flares have harder spectrum at 2σ confidence level. These newly detected flares in the broad X-ray band will allow us to investigate flare spectral dependence on luminosity, and to constrain the radiation mechanism as well.

Abstract (for Math-Lovers)

Bayesian Block Analysis (BBA) is a core flare-searching math method used in this study. First described by Jeffrey D. Scargle in 1998, it has provided astronomers a standard criterion for determining localized features (bursts or flares), revealing the shape of a pulse, and characterizing overall data variability. We studied both the math deduction and the computational application of BBA, including topics in Bayesian Statistics, Poisson Process, and so on. In order to search for Sagittarius A* flares, we implemented BBA on all observational data obtained by the NuSTAR space telescope between 2016 and 2022, and determined the precise start and stop times of the seven new bright flares that we found. This allows us to separate the flare signals from the quiescence of the supermassive black hole, and therefore largely increases the precision of the following spectral studies in quest for knowledge about the supermassive black hole at the center of our galaxy.

Senior Project submitted to The Division of Science, Mathematics and Computing of Bard College.

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