Hitomi Observation of Radio Galaxy NGC 1275: The First X-ray Microcalorimeter Spectroscopy of Fe-K{\alpha} Line Emission from an Active Galactic Nucleus

摘要

The origin of the narrow Fe-K{\alpha} fluorescence line at 6.4 keV fromactive galactic nuclei has long been under debate; some of the possible sitesare the outer accretion disk, the broad line region, a molecular torus, orinterstellar/intracluster media. In February-March 2016, we performed the firstX-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS)onboard the Hitomi satellite of the Fanaroff-Riley type I radio galaxy NGC 1275at the center of the Perseus cluster of galaxies. With the high energyresolution of ~5 eV at 6 keV achieved by Hitomi/SXS, we detected theFe-K{\alpha} line with ~5.4 {\sigma} significance. The velocity width isconstrained to be 500-1600 km s$^{-1}$ (FWHM for Gaussian models) at 90%confidence. The SXS also constrains the continuum level from the NGC 1275nucleus up to ~20 keV, giving an equivalent width ~20 eV of the 6.4 keV line.Because the velocity width is narrower than that of broad H{\alpha} line of~2750 km s$^{-1}$, we can exclude a large contribution to the line flux fromthe accretion disk and the broad line region. Furthermore, we performed pixelmap analyses on the Hitomi/SXS data and image analyses on the Chandra archivaldata, and revealed that the Fe-K{\alpha} line comes from a region within ~1.6kpc from the NGC 1275 core, where an active galactic nucleus emissiondominates, rather than that from intracluster media. Therefore, we suggest thatthe source of the Fe-K{\alpha} line from NGC 1275 is likely a low-coveringfraction molecular torus or a rotating molecular disk which probably extendsfrom a pc to hundreds pc scale in the active galactic nucleus system.