Old Posts

Title: Gamma-ray emission in the Seyfert galaxy NGC 4151: Investigating the role of jet and coronal activities.
Authors: Yoshiyuki Inoue, Dmitry Khangulyan

NGC 4151, a nearby Seyfert galaxy, has recently been reported to emit gamma rays in the GeV range, posing an intriguing astrophysical mystery. The star formation rate of NGC 4151 is too low to explain the observed GeV flux, but the galaxy is known for its coronal activity in X-ray and jet activity in radio. We propose that either the combination of these two activities or the jet activity alone can account for the gamma-ray spectrum. An energy-dependent variability search will allow one to distinguish between the two scenarios, as the coronal component can only contribute at energies of ≲1 GeV. Our analysis also indicates that it might still be difficult to see coronal neutrinos from the apparently X-ray brightest Seyfert NGC 4151 with current-generation neutrino observatories.

Title: High energy emission component, population, and contribution to the extragalactic gamma-ray background of gamma-ray emitting radio galaxies.
Authors: Ellis R. Owen, Kinwah Wu, Yoshiyuki Inoue, H.-Y. Karen Yang, Alison M. W. Mitchell

Galaxy evolution is an important topic, and our physical understanding must be complete to establish a correct picture. This includes a thorough treatment of feedback. The effects of thermal-mechanical and radiative feedback have been widely considered, however cosmic rays (CRs) are also powerful energy carriers in galactic ecosystems. Resolving the capability of CRs to operate as a feedback agent is therefore essential to advance our understanding of the processes regulating galaxies. The effects of CRs are yet to be fully understood, and their complex multi-channel feedback mechanisms operating across the hierarchy of galaxy structures pose a significant technical challenge. This review examines the role of CRs in galaxies, from the scale of molecular clouds to the circum-galactic medium. An overview of their interaction processes, their implications for galaxy evolution, and their observable signatures is provided and their capability to modify the thermal and hydrodynamic configuration of galactic ecosystems is discussed. We present recent advancements in our understanding of CR processes and interpretation of their signatures, and highlight where technical challenges and unresolved questions persist. We discuss how these may be addressed with upcoming opportunities.

Title: The centimeter-to-submillimeter broad-band radio spectrum of the central compact component in a nearby type-II Seyfert galaxy NGC 1068.
Authors: Tomonari Michiyama, Yoshiyuki Inoue, Akihiro Doi

We analyze all the available Atacama Large Millimeter / submillimeter Array archival data of the nearby Type-II Seyfert galaxy NGC 1068, including new 100 GHz data with the angular resolution of 0\farcs05, which was not included in previous continuum spectral analysis. By combining with the literature data based on the Very Large Array, we investigate the broadband radio continuum spectrum of the central ≲7 pc region of NGC 1068. We found that the flux density is between ≈10-20 mJy at 5-700 GHz. Due to the inability of the model in previous studies to account for the newly added 100 GHz data point, we proceeded to update the models and make the necessary adjustments to the parameters. One possible interpretation of this broadband radio spectrum is a combination of emission from the jet base, the dusty torus, and the compact X-raying corona with the magnetic field strength of ≈20 G on scales of ≈30 Schwarzschild radii from the central black hole. In order to firmly identify the compact corona by omitting any other possible extended components (e.g., free-free emission from ionized gas around), high-resolution/sensitivity observations achieved by next-generation interferometers will be necessary.

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Title: On the Zeeman Effect in Magnetically-Arrested Disks.
Authors: Yoshiyuki Inoue

Magnetically arrested disk (MAD) has been argued as the key accretion phase to realize the formation of relativistic jets. However, due to the lack of magnetic field measurements of accreting systems, MAD has not been observationally confirmed yet. Here we propose that a strong magnetic field accompanied by MAD would induce the Zeeman splitting of relativistically broadened Fe Kα fluorescence lines in X-ray binaries and active galactic nuclei, where we consider a two-phase medium in the inner accretion disk, magnetically dominated hot corona and cold reflector. Such a geometrical configuration is suggested from X-ray observations and recently confirmed by numerical simulations. Although turbulence in accretion flows would broaden the split lines, future X-ray high-energy resolution satellites, XRISM and Athena, would be capable of seeing the Zeeman effect on the Fe lines in X-ray binaries in the case with the MAD configuration. The signature of the Zeeman split lines would provide observational evidence for MAD.

We are pleasure to hear that two excellent undergraduate students in our group defended their bachelor thesis presentations. Katsunori Kusakabe looked into the stellar magnetic field condition to realize the magnetically arrested disk in X-ray binaries. Tatsuki Fujiwara worked on 26Al gamma-ray line signals from solar bodies as well as the estimation of past cosmic-ray activity using comets. Congratulations to all of them!

Title: MeV Gamma-Ray Source Contribution to the Inner Galactic Diffuse Emission .
Authors: Naomi Tsuji, Yoshiyuki Inoue, Hiroki Yoneda, Reshmi Mukherjee, Hirokazu Odaka

The origin of the inner Galactic emission, measured by COMPTEL with a flux of ∼10−2 MeV cm−2 s−1 sr−1 in the 1-30 MeV energy range from the inner Galactic region, has remained unsettled since its discovery. In this paper, we elaborate on a model of individual MeV gamma-ray sources unresolved by COMPTEL. This is conducted for sources crossmatched between the Swift-BAT and Fermi-LAT catalogs by interpolating the energy spectra in the hard X-ray and GeV gamma-ray ranges, as well as unmatched sources between the two catalogs. We find that the source contribution to the COMPTEL emission would be at least ~20%. Combined with the Galactic diffuse emission, which is not well constrained, the COMPTEL emission can be roughly reproduced in some cases.

Title: ALMA Detection of Parsec-scale Blobs at the Head of a Kiloparsec-scale Jet in the Nearby Seyfert Galaxy NGC 1068.
Authors: Tomonari Michiyama, Yoshiyuki Inoue, Akihiro Doi, Dmitry Khangulyan We present Atacama Large Millimeter/submillimeter Array observations at ≈100 GHz with 0.05 arcsec (3 pc) resolution of the kiloparsec-scale jet seen in the nearby Seyfert galaxy NGC 1068, and we report the presence of parsec-scale blobs at the head of the jet. The combination of the detected radio flux (≈0.8 mJy), spectral index (≈0.5), and the blob size (≈10 pc) suggests a strong magnetic field of B≈240μG. Such a strong magnetic field most likely implies magnetic field amplification by streaming cosmic rays. The estimated cosmic-ray power by the jet may exceed the limit set by the star formation activity in this galaxy. This result suggests that even modest-power jets can increase the galactic cosmic-ray content while propagating through the galactic bulge.