03/28/2024
By Sayantan Bhattacharya

The Kennedy College of Sciences, Department of Physics & Applied Physics, invites you to attend the Ph.D. Dissertation defense by Sayantan Bhattacharya titled "X-Ray Binaries in Nearby Dwarf Galaxy IC 10: Not to Mention the X-1."

Date: Friday, April 5
Time: 11:30 a.m.
Location: Perry 105 and via Zoom


Committee Members:

  • Silas G. T. Laycock, Department of Physics & Applied Physics, UMass Lowell
  • Ofer Cohen, Department of Physics & Applied Physics, UMass Lowell
  • Nishant Agarwal, Department of Physics & Applied Physics, UMass Lowell
  • Rigel Cappallo, Research Scientist, MIT Haystack Observatory

Abstract:

We present a comprehensive investigation into the stellar populations and blue supergiant high-mass X-ray binaries (BSG-HMXBs) in the dwarf star-forming galaxy IC 10. The study is motivated by two primary objectives: firstly, to conduct a detailed analysis of the blue supergiant x-ray binary population in IC 10, and secondly, to explore the properties of IC 10 X-1, the brightest source in the galaxy, which is identified as a high mass x-ray binary containing a Wolf-Rayet and black hole.

The dwarf galaxy in Cassiopeia, IC 10, located in ``close" proximity of 660 kpc, captivates astronomers with its young stellar population, abundant Wolf-Rayet stars, and the presence of massive stars in general. Leveraging a series of space-based X-ray telescope (Chandra) observations, we have identified 375 X-ray point sources. Correlating our list of x-ray sources with published optical catalogs we found those having an optical counterpart. Applying an optical color-magnitude selection to isolate blue supergiant stars, we find the final list of intriguing blue supergiants in a compact-object(x-ray) binary. The ongoing starburst in IC 10 has given rise to a diverse collection of exotic compact-object binaries, including IC 10 X-1 (a Wolf-Rayet and Blackhole high mass x-ray binary) and IC 10 X-2 (a Blue supergiant fast x-ray transient). We have also used GEMINI/GMOS optical spectroscopy to enable multi-wavelength characterization of these BSG sources.

Furthermore, our investigation into IC 10 X-1 reveals surprising findings regarding its X-ray emission and orbital dynamics. Through multi-wavelength analysis combining optical radial velocity (RV) curves and X-ray eclipse lightcurves, we uncover a quarter phase offset between these two observational features. IC 10 X-1 is a blackhole+Wolf-Rayet (BH+WR) high-mass X-ray binary with an orbital period of 34.9 hrs. The BH mass is currently unconstrained, as the BH accretion disk irradiates the WR wind and alters the ionization structure of spectral line formation regions near the WR star. This alteration of the WR wind geometry masquerades as the classic binary RV curve. To understand the nature of this complex system, we perform a detailed multi-wavelength study using optical spectroscopy (GEMINI/GMOS), X-ray phase-resolved spectroscopy (Chandra), and x-ray lightcurve analysis (Chandra+Swift+XMM), aiming to elucidate the orbital period derivative and refine our understanding of the system's binary parameters and physical nature.

Blue SG-XRBs include a major class of progenitors of double-degenerate binaries, hence their numbers are an important factor in modeling the rate of gravitational-wave sources. Identifying the nature of individual sources is a slow and difficult process, but ultimately necessary as it paves the way toward a comprehensive census of X-ray binaries (XRBs), enabling meaningful comparisons with other galaxies, for example, the Magellanic Clouds.