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Publication Date


First Advisor

Suzan Edwards

Document Type

Honors Project

Degree Name

Bachelor of Arts




Astrophysics, X-ray astrophysics, High-energy, Pulsars, Ultraluminous x-ray sources, Be/X-ray binary


Swift J0243.6+6124 is the first known ultraluminous X-ray pulsar in our Galaxy. This system is an interacting X-ray binary, composed of a rapidly spinning and highly magnetized (≈ 1012 G) neutron star accreting material from the disk of its massive Be companion. Be/X-ray binaries are highly variable and prone to frequent X-ray outbursts. The X-ray pulsar of Swift J0243 is ultraluminous, meaning that it emits at luminosities greater than the Eddington limit for a 1.4 Msun neutron star assuming spherical accretion, Ledd = 1.8 × 1038 erg s −1 . The system reached a peak luminosity L> 1039 erg s−1 during its 2017 outburst. This luminosity is about 10× the Eddington limit for a 1.4 Msun neutron star. The Galactic location of this system allows for the study of super-Eddington accretion as an analog of distant ultraluminous X-ray sources.

I used data from the Neil Gehrels Swift X-ray Telescope to investigate the evolution of the spectral and temporal properties of this system. The goal with this dataset was to understand how accretion onto the neutron star varies in and of out of the super-Eddington regime. I searched for characteristic transitions that could reveal changes in the accretion geometry with luminosity. During the super- Eddington outburst of Swift J0243, the spin period dropped by 0.7%, the pulsed fraction nearly doubled, the hardness ratio reached its lowest point, and the pulse shape changed from single-peaked to double-peaked. Such changes may reflect a


2020 Alyssa Cassity. Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.




xii, 56 pages : illustrations (chiefly color) Includes bibliographical references (pages 50-56)