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Detection of the hot component of the circumgalactic medium of the Milky Way around the sightline of Mrk 421

Presentation #107.13 in the session “ISM/Galaxies/Clusters (Poster)”.

Published onApr 01, 2022
Detection of the hot component of the circumgalactic medium of the Milky Way around the sightline of Mrk 421

The circumgalactic medium of the Milky Way has been found to contain a super-virial hot gas component at ~10 million K in addition to the warm-hot component at ~million K (Das et al. 2019a,b). Studies of the emission and absorption by these media in the soft X-ray band have been used to characterize them. In order to determine the prevalence of the super-virial component, we must look for hot gas along several sightlines. I will present archival observations made using XMM-Newton within a 5 deg circular region around the sightline of Mrk 421. The emission spectra from the CGM are modeled and fit to the observed spectra to determine its properties.

Archival observations made using the MOS camera with sufficient signal-to-noise are chosen, taking into consideration the superior spectral resolution of the instrument. The bright emission from the central blazar source is masked with a 8.75 arcmin circular region and nearby point sources are identified and masked accordingly. The diffuse emission that is left comprises the cosmic X-ray background as well as foreground sources of contamination like the local hot bubble. The contribution of these components to the spectrum is appropriately modeled. The emission from the CGM is assumed to be from a multi-temperature collisionally-ionized diffuse gas. A two component model i.e., warm-hot and hot media, is found to better fit the data compared to the single temperature warm-hot model. The best-fit temperatures of the warm-hot and the hot components are found to be log T = 6.32 ± 0.05 and log T = 6.94 ± 0.09 respectively.

To ensure that our results are not biased by any remaining contamination from Mrk 421, we look at archival observations with sightlines which lie between 2 deg and 5 deg away from the primary sightline. We find exposures from the PN camera totaling up to ~400 ks in length and proceed with similarly masking point sources and modeling the foreground contaminants. We find that the temperature estimates of the warm-hot and hot components, log T = 6.35 ± 0.06 and log T = 7.06 ± 0.08 respectively are in agreement with the estimates from the primary sightline. This result establishes a new detection of the hot component of the CGM in a sightline that is away from the galactic center.

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