Speaker: Conjeepuram V Ambarish, Physics PhD Graduate Student
Abstract: The galactic center soft X-ray bulge, clearly seen in the ROSAT all-sky survey map in the 0.44-1.2 keV band, is one of the brightest diffuse X-ray features in the sky in this energy range. The extended nature of the emission suggests that the source is likely hot gas shock heated by supernovae. Deep shadows from molecular clouds at 2-4 kpc show that the soft X-ray bulge is not a local feature and is likely associated with either the galactic center or an inner spiral arm. The soft X-ray bulge has been studied previously with ROSAT and Suzaku, but with their energy resolution of ~ 300 eV and 60 eV respectively, they are only able to separate bulge emission from at least four foreground and background emission components without resorting to highly oversimplified models.
The X-ray quantum calorimeter (XQC) is a sounding rocket instrument 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) and an energy resolution of ~ 8 eV FWHM below 1 keV. Since the soft X-ray bulge is too far south to observe from NASA’s usual launch site at White Sands Missile Range (WSMR), XQC was part of NASA’s long-awaited campaign to Australia, where we had a successful flight to observe the bulge.
I present here a high resolution spectrum with clearly resolved lines from multiple ionization states of carbon, nitrogen, oxygen, neon and iron. A model-independent analysis of ratios of fluxes from these lines requires emission from a minimum of three thermal components at ~ 1,2, and 8 MK. I show that our spectrum is consistent with emission from either an adiabatic polytrope filling the inner galaxy or an isothermal halo at ~ 2 MK, both with additional X-ray emission from unresolved stars.
An alternative explanation for the soft X-ray bulge could be a series of blowouts from star-forming regions in the inner spiral arm. The relative abundance estimates from the observation, particularly the Fe/O ratio, are used to distinguish between a galactic bulge or a spiral arm origin for the hot gas since these two regions should be dominated by different types of supernovae, type Ia and type II respectively. The metal yields of the two are distinct with Type Ia producing mostly iron and Type II producing oxygen and other alpha elements. Our fits indicate an Fe/O ratio of about twice the solar value, favoring enrichment by Type Ia supernova and a galactic bulge origin for the hot gas.