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Register a new userHitomi Observation of Radio Galaxy NGC 1275: The First X-ray Microcalorimeter Spectroscopy of Fe-K{alpha} Line Emission from an Active Galactic Nucleus
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The origin of the narrow Fe-K{alpha} fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In February-March 2016, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) onboard the Hitomi satellite of the Fanaroff-Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high energy resolution of ~5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-K{alpha} line with ~5.4 {sigma} significance. The velocity width is constrained to be 500-1600 km s$^{-1}$ (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus 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 from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-K{alpha} line comes from a region within ~1.6 kpc from the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-K{alpha} line from NGC 1275 is likely a low-covering fraction molecular torus or a rotating molecular disk which probably extends from a pc to hundreds pc scale in the active galactic nucleus system.
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We analyzed Suzaku/XIS data of 2006--2015 observations of a gamma-ray emitting radio galaxy NGC 1275, and brightening of the nucleus in the X-ray band was found in 2013--2015, correlating with GeV Gamma-ray brightening. This is the first evidence of variability with correlation between GeV gamma-ray and X-ray for NGC 1275. We also analyzed Swift/XRT data of NGC 1275, and found that X-ray was flaring by a factor of $sim$5 in several days in 2006, 2010, and 2013. The X-ray spectrum during the flare was featureless and somewhat steeper with a photon index of $sim$2 against $sim$1.7 in the normal state, indicating that a synchrotron component became brighter. A large Xray to GeV gamma-ray flux ratio in the flare could be explained by the shock-in-jet scenario. On the other hand, a long-term gradual brightening of radio, X-ray, and GeV gamma-ray with a larger gamma-ray amplitude could be origin of other than internal shocks, and then we discuss some possibilities.
NGC 1275 is a gamma-ray-emitting radio galaxy at the center of the Perseus cluster. Its multi-wavelength spectrum is similar to that of blazers, and thus a jet-origin of gamma-ray emissions is believed. In the optical and X-ray region, NGC 1275 also shows a bright core, but their origin has not been understood, since a disk emission is not ruled out. In fact, NGC 1275 exhibits optical broad emission lines and a X-ray Fe-K line, which are typical for Seyfert galaxies. In our precious studies of NGC 1275 with Suzaku/XIS, no X-ray time variability was found from 2006 to 2011, regardless of moderate gamma-ray variability observed by {it Fermi}-LAT~cite{Yamazaki}. We have continued monitoring observations of NGC 1275 with Suzaku/XIS. In 2013-2014, MeV/GeV gams-ray flux of NGC 1275 gradually increased and reached the maximum at the beginning of 2014. Correlated with this recent gamma-ray activity, we found that X-ray flux also increased, and this is the first evidence of X-ray variability of NGC 1275. Following these results, we discuss the emission component during the time variability, but we cannot decide the origin of X-ray variability correlating with gamma-ray. Therefore, for future observation, it is important to observe NGC 1275 by using Fermi gamma-ray, XMM-Newton, NuStar, ASTRO-H X-ray, CTA TeV gamma-ray and Kanata optical telescope.
We analyze the ultraviolet (UV) and X-ray data of NGC 1275 obtained with {it Swift}/UVOT, XRT, BAT and {it Fermi} Large Area Telescope over about 10 years to investigate the origin of the nuclear emission from NGC 1275. We confirm that the UV and soft/hard X-ray fluxes gradually increased along with the GeV gamma rays. At times, short-term variations in the UV or soft X-ray spectral regions showed rapid variations correlated with the GeV gamma-rays. However there was no significant correlation between the UV and soft X-rays. The UV spectrum had a narrow spectral shape that could be represented by single-temperature blackbody radiation. These results could possibly indicate that the long-term variability of UV and X-ray emissions is caused by the jet, while the emissions from the accretion disk contribute to the UV and X-ray bands to some extent.
Extragalactic cosmic ray populations are important diagnostic tools for tracking the distribution of energy in nuclei and for distinguishing between activity powered by star formation versus active galactic nuclei (AGNs). Here, we compare different diagnostics of the cosmic ray populations of the nuclei of Arp 220 based on radio synchrotron observations and the recent gamma-ray detection. We find the gamma-ray and radio emission to be incompatible; a joint solution requires at minimum a factor of 4 - 8 times more energy coming from supernovae and a factor of 40 - 70 more mass in molecular gas than is observed. We conclude that this excess of gamma-ray flux in comparison to all other diagnostics of star-forming activity indicates that there is an AGN present that is providing the extra cosmic rays, likely in the western nucleus.
The recent detection by the Fermi gamma-ray space telescope of high-energy gamma-rays from the radio galaxy NGC 1275 makes the observation of the very high energy (VHE: E > 100 GeV) part of its broadband spectrum particularly interesting, especially for the understanding of active galactic nuclei (AGN) with misaligned multi-structured jets. The radio galaxy NGC 1275 was recently observed by VERITAS at energies above 100 GeV for about 8 hours. No VHE gamma-ray emission was detected by VERITAS from NGC 1275. A 99% confidence level upper limit of 2.1% of the Crab Nebula flux level is obtained at the decorrelation energy of approximately 340 GeV, corresponding to 19% of the power-law extrapolation of the Fermi Large Area Telescope (LAT) result.
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