Sounding rocket observation of the southern galactic bulge with the X-ray Quantum Calorimeter (XQC)

The galactic center soft X-ray bulge, clearly seen in the ROSAT R45 map, is one of the brightest diffuse X-ray sources in the night sky. The spatial extent and the spectral signature of the bulge can be fit rather well to an adiabatic polytrope filling the gravitational potential of the Galaxy supported against collapse by its own pressure. Other theories for the origin of the hot gas in the bulge include outflows from star-forming regions along the inner spiral arm of the Galaxy which has a similar extent. Both the models mentioned above predict the presence of multiple phases at different temperatures, which is confirmed by Suzaku and XMM-Newton observations from the region showing evidence of components ranging from 1.5 - 4 million Kelvin. Disentangling these observations to determine the physical state and relative abundances in the different components requires better spectral resolution than exists in current CCD detectors. X-ray quantum calorimeter (XQC) is a sounding rocket X-ray telescope with silicon thermistor microcalorimeters operated at 50 mK. With a detector area of 1.44 cm² mechanically collimated to a 60 degree field of view, XQC has a high throughput (~ 1 cm² sr) along with an energy resolution of ~ 8 eV FWHM below 1 keV. Since the galactic center is too far south to observe from NASA's standard launch site at White Sands Missile Range (WSMR), XQC was part of NASA's long-awaited campaign to Australia. We were the first payload to launch and had a successful flight observing the galactic center soft X-ray bulge. We present a high-resolution spectrum of the southern galactic bulge, with clearly resolved lines particularly from multiple ionization states of iron and oxygen, discuss the characteristics of the spectrum and comment on potential origins for the hot gas in the center of our Galaxy.