The X-/γ-Ray Correlation in NGC 4945 and the Nature of Its γ-Ray Source

Abstract

We report hints for the correlation between the X-ray and γ-ray emission in the nearby galaxy NGC 4945, which harbors both an active galactic nucleus and a nuclear starburst region. We have divided the Fermi/LAT observations of NGC 4945 into two datasets, comprising events detected during the low (L) and high (H) level of X-ray emission from the active nucleus of this galaxy, determined using the Swift/BAT light curve. Both datasets contain an equal amount of 3.8 years of LAT data and NGC 4945 is detected with a similar statistical significance of ∼ 15σ in L and 14σ in H. However, the slope of the γ-ray spectrum hardens with the increase of the X-ray flux, with the photon index Γ = 2.47 ± 0.07 in L and 2.11 ± 0.08 in H. The change is confirmed by a systematic variation of the spectral energy distribution as well as a substantial reversal of the γ-ray signal in significance maps for low and high γ-ray energies. The X/γ-ray correlation indicates that the γ-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. We discuss possible locations of the γ-ray source. We also compare NGC 4945 with other starburst galaxies detected by LAT and we note similarities between those with active nuclei, e.g. unlikely high efficiencies of γ-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the γ-ray emission.

Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array

“…Similarly to NGC 1068, AGN-driven molecular winds and weak radio jets have been observed in NGC 4945 and Circinus [62,63,64,65]. These galaxies also show similarity of their Eddingtonscaled X-ray and gamma-ray luminosity with L 0.1−100GeV /L Edd ≃10 −4 and L 2−10keV /L Edd ≃10 −2 in all three sources [66,60], corroborating a correlation between the AGN activity and gamma-ray emission.…”

“…The Figure 4: Gamma-ray spectra of Seyfert galaxies (filled symbols) and starburst galaxies (open symbols) detected by Fermi-LAT and IACTs. The data points are from [9,46,25,47,60,10,67,72,73,69]. nearby ultra luminous infrared galaxy Arp 220 was observed by MAGIC and VERITAS but no significant excess over the background was found [70,71].…”

Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies

“…(1) Recent γ-ray observations (mostly from Fermi) have established strong constraints on ∼ GeV CRs in a number of nearby galaxies, complementing the classical Solarneighborhood constraints on inferred CR grammage, residence times, and energy density. Surprisingly, while the most dense starburst systems observed appear to be proton calorimeters, all "normal" ∼ L * and dwarf galaxies observed (the MW, Andromeda/M31, SMC, LMC, M33) have robust upper limits or detections indicating that at least ∼ 95 − 99% of the ∼ GeV CRs must escape without hadronic collisions, requiring large diffusivities (Lacki et al 2011;Tang et al 2014;Griffin et al 2016;Fu et al 2017;Wojaczyński & Niedźwiecki 2017;Wang & Fields 2018;Lopez et al 2018). (2) Analytic and numerical work explicitly following transport and scattering of CRs on "micro-scales" (e.g.…”

Constraints on Gamma-Ray and Neutrino Emission from NGC 1068 with the MAGIC Telescopes

“…The gamma-ray spectra of nearby starbursts and AGNs have been measured in the high-energy (HE, 0.1-100 GeV) band by Fermi-LAT (Ackermann et al 2012;Hayashida et al 2013;Tang et al 2014;Acero et al 2015;Lamastra et al 2016;Peng et al 2016;Ajello et al 2017;Wojaczyński & Niedźwiecki 2017). The starburst galaxies NGC 253 and M82 have also been detected in the very-high-energy (VHE, 0.1-100 TeV) band with Imaging Atmospheric Cherenkov Telescopes (IACTs; Acciari et al 2009;Acero et al 2009).…”