{"690523":{"#nid":"690523","#data":{"type":"event","title":"PhD Defense by Vishal Tiwari","body":[{"value":"\u003Cp\u003EIn partial fulfillment of the requirements for the degree of\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EDoctor of Philosophy in\u0026nbsp;Physics\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ESchool of Physics\u0026nbsp;Thesis\u0026nbsp;Dissertation\u0026nbsp;Defense\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EVishal Tiwari\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EDr. Tamara Bogdanovi\u0107, School of Physics, Georgia Institute of Technology (Advisor)\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ERadiation Magnetohydrodynamic Simulations of Circumbinary Disks around Massive Black Hole Binaries\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EDate: Monday, June 15, 2026\u003C\/p\u003E\u003Cp\u003ETime: 12:00 p.m.\u003C\/p\u003E\u003Cp\u003ELocation: College of Computing Building (CCB), CRA, Room:103\u003C\/p\u003E\u003Cp\u003EVirtual:\u0026nbsp;\u003Ca href=\u0022https:\/\/nam12.safelinks.protection.outlook.com\/?url=https%3A%2F%2Fgatech.zoom.us%2Fj%2F94574620706\u0026amp;data=05%7C02%7Ctm186%40gtvault.onmicrosoft.com%7Cfa51c137fae848da1cf508debc24b9b6%7C482198bbae7b4b258b7a6d7f32faa083%7C1%7C0%7C639155063115173257%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C\u0026amp;sdata=1prlM3XnsXM0f8ljAF8FHzaGHkebybffJW%2BVLB2rs0Q%3D\u0026amp;reserved=0\u0022\u003Ehttps:\/\/gatech.zoom.us\/j\/94574620706\u003C\/a\u003E\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EThesis Committee:\u003C\/strong\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EDr. David Ballantyne, School of Physics, Georgia Institute of Technology\u003C\/p\u003E\u003Cp\u003EDr. Dimitrios Psaltis, School of Physics,\u0026nbsp; Georgia Institute of Technology\u003C\/p\u003E\u003Cp\u003EDr. John Wise, School of Physics,\u0026nbsp; Georgia Institute of Technology\u003C\/p\u003E\u003Cp\u003EDr. Yan-Fei Jiang, Center for Computational Astrophysics, Flatiron Institute\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EAbstract:\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003EWe present global three-dimensional radiation magnetohydrodynamic (RMHD) simulations of sub-Eddington circumbinary disks (CBDs) around massive black hole binaries (MBHBs) to investigate the role of radiation and the binary mass ratio on accretion dynamics and electromagnetic (EM) signatures. We model binaries with a total mass of 20 million suns, on a circular orbit separated by 100 gravitational radii. The inclusion of radiation alters the disk structure compared to non-radiative magnetohydrodynamic (MHD) simulations. Due to reduced pressure support and an altered equation of state, the resulting RMHD disks are denser, thinner, and more filamentary across both equal-mass and 10:1 mass-ratio systems.\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EFor the equal-mass binary, we find that the RMHD disk features a ~\u0026nbsp;3 times lower accretion rate (~\u0026nbsp;0.15 times the Eddington rate), weaker accretion streams, and a less pronounced overdensity at the inner edge compared to its purely MHD counterpart. Driven by stronger binary torques, the equal-mass system carves out a larger cavity of ~ 200 gravitational radii\u0026nbsp;and exhibits higher disk eccentricity. In contrast, the q=0.1 unequal-mass system forms a smaller cavity of ~\u0026nbsp;170 gravitational radii, is less eccentric at the inner edge, and lacks a significant non-axisymmetric lump. Furthermore, due to its proximity to the inner edge, the secondary black hole in the 10:1 binary captures a larger fraction of the inflowing gas, accreting at a super-Eddington rate despite the overall CBD accretion remaining sub-Eddington (~\u0026nbsp;0.23 times the Eddington rate). The smaller cavity in the q=0.1 system allows the disk\u0027s photosphere to extend deeper into hot, turbulent regions. Consequently, the thermal emission from the unequal-mass binary disk is much more variable in the far-UV and soft X-ray frequencies compared to the equal-mass binary. The light curve of the q=0.1 binary exhibits a dominant periodicity corresponding to 2 binary orbits, whereas the equal-mass binary displays periodicity at 5 and 0.5 binary orbits.\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E\u003Cp\u003EThese results highlight the critical importance of radiation for the structure and observational properties of MBHB circumbinary disks, and have implications for detecting electromagnetic counterparts to LISA gravitational wave precursors and for the heavier binaries targeted by the Pulsar Timing Arrays.\u003C\/p\u003E\u003Cp\u003E\u0026nbsp;\u003C\/p\u003E","summary":"","format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003ERadiation Magnetohydrodynamic Simulations of Circumbinary Disks around Massive Black Hole Binaries\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Radiation Magnetohydrodynamic Simulations of Circumbinary Disks around Massive Black Hole Binaries"}],"uid":"27707","created_gmt":"2026-05-27 19:28:41","changed_gmt":"2026-05-27 19:29:19","author":"Tatianna Richardson","boilerplate_text":"","field_publication":"","field_article_url":"","field_event_time":{"event_time_start":"2026-06-15T12:00:49-04:00","event_time_end":"2026-06-15T14:00:00-04:00","event_time_end_last":"2026-06-15T14:00:00-04:00","gmt_time_start":"2026-06-15 16:00:49","gmt_time_end":"2026-06-15 18:00:00","gmt_time_end_last":"2026-06-15 18:00:00","rrule":null,"timezone":"America\/New_York"},"location":"College of Computing Building (CCB), CRA, Room:103","extras":[],"groups":[{"id":"221981","name":"Graduate Studies"}],"categories":[],"keywords":[{"id":"100811","name":"Phd Defense"}],"core_research_areas":[],"news_room_topics":[],"event_categories":[{"id":"1788","name":"Other\/Miscellaneous"}],"invited_audience":[{"id":"78771","name":"Public"}],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[],"email":[],"slides":[],"orientation":[],"userdata":""}}}