2026 Theses Doctoral

A Comprehensive Study of the X-ray Point Sources in the Galactic Center and Beyond

Mandel, Shifra

The Galactic Center is home to the largest known concentration of exotic X-ray sources identified in our Galaxy. This menagerie includes a substantial number of accreting compact objects, in particular low-mass X-ray binaries (LMXBs) and cataclysmic variables (CVs). The study of these sources has important implications for a wide range of topics, including galaxy evolution, the nature of dark matter, and the rate of gravitational wave events. Despite their significance, thousands of faint Galactic Center X-ray sources have to date remained unidentified.

This work aims to comprehensively analyze the X-ray populations that inhabit the central areas of the Galactic disk, through both a statistical investigation of thousands of faint X-ray point sources, and a targeted study of MAXI J1744-294, a new Galactic Center transient. For the former, we utilize data from a groundbreaking survey by the XMM-Newton X-ray observatory to investigate the different source populations in the inner regions of the Galactic plane; for the latter, we analyze complementary data from the NuSTAR, XRISM, Swift, Chandra, XMM-Newton, NICER, and IXPE X-ray telescopes.

We identify for the first time a concentration of “hard” X-ray sources located in the Nuclear Stellar Disk (NSD). Unlike the “soft” X-ray sources, this population does not trace the stellar mass distribution, indicating an overabundance compared to the underlying stellar population. We simulate XMM-Newton spectra using physical characteristics from a variety of source types to compare their properties to those we observed in our survey data, connecting our observational results to the underlying populations. We identify MAXI J1744-294 as a candidate black hole (BH) LMXB based on its spectral and temporal properties.

This discovery provides further evidence for a cusp of BH-LMXBs in the central pc of our Galaxy, as suggested by previous observational, analytical, and theoretical work. Our study provides novel observational insights into the distribution of stellar remnants and other X-ray sources in the vicinity of the supermassive black hole at the center of our Galaxy, Sgr A∗.