The X/gamma-ray correlation in NGC 4945 and the nature of its gamma-ray source

Abstract

We report hints for the correlation between the X-ray and gamma-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 ~15 $\sigma$ in L and 14 $\sigma$ in H. However, the slope of the gamma-ray spectrum hardens with the increase of the X-ray flux, with the photon index $\Gamma$ = 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 gamma-ray signal in significance maps for low and high gamma-ray energies. The X/gamma-ray correlation indicates that the gamma-ray production is dominated by the active nucleus rather than by cosmic rays interacting with the interstellar medium. We discuss possible locations of the gamma-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 gamma-ray production in starburst scenario, which argues for a significant contribution of their active nuclei to the gamma-ray emission.

arXiv commentApJ in press

References in corpus (35)

• The Global Schmidt Law in Star Forming Galaxies
• Towards a unified model for black hole X-ray binary jets
• Modelling the behaviour of accretion flows in X-ray binaries
• Fermi Large Area Telescope First Source Catalog
• The Swift/BAT Hard X-ray Transient Monitor
• GeV Observations of Star-forming Galaxies with \textit{Fermi} LAT
• A connection between star formation activity and cosmic rays in the starburst galaxy M 82
• A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
• Detection of Gamma Rays From a Starburst Galaxy
• Detection of Gamma-Ray Emission from the Starburst Galaxies M82 and NGC 253 with the Large Area Telescope on Fermi
• The Distribution of H2O Maser Emission in the Nucleus of NGC 4945
• On The GeV & TeV Detections of the Starburst Galaxies M82 & NGC 253
• A giant radio flare from Cygnus X-3 with associated Gamma-ray emission
• Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
• BAT X-ray Survey - III: X-ray Spectra and Statistical Properties
• The variable hard X-ray emission of NGC4945 as observed by NuSTAR
• Seyfert 2 galaxies in the GeV band: jets and starburst
• Active Galactic Nuclei, Neutrinos, and Interacting Cosmic Rays in NGC 253 & NGC 1068
• Search for Gamma-ray Emission from X-ray Selected Seyfert Galaxies with Fermi-LAT
• Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT
• Structure of the Circumnuclear Region of Seyfert 2 Galaxies Revealed by RXTE Hard X-ray Observations of NGC 4945
• Discovery of GeV emission from the direction of the luminous infrared galaxy NGC 2146
• Simultaneous Chandra and RXTE observations of the nearby bright Seyfert 2 galaxy NGC4945
• Galactic outflow driven by the active nucleus and the origin of the gamma-ray emission in NGC 1068
• Stochastic Particle Acceleration in Turbulence Generated by the Magnetorotational Instability
• The Sub-parsec Scale Radio Properties of Southern Starburst Galaxies. II. Supernova Remnants, the Supernova Rate, and the Ionised Medium in the NGC 4945 Starburst
• Constraining Gamma-Ray Emission from Luminous Infrared Galaxies with Fermi-LAT; Tentative Detection of Arp 220
• Search for gamma-ray emission from star-forming galaxies with Fermi LAT
• The Radio-Gamma Correlation In Starburst Galaxies
• Absorption of high-energy gamma rays in Cygnus X-3
• Starburst galaxies as seen by gamma-ray telescopes
• Hard X-ray spectral variability of the brightest Seyfert AGN in the Swift/BAT sample
• Gamma-ray activity of Seyfert galaxies and constraints on hot accretion flows
• Gamma-ray emission from proton-proton interactions in hot accretion flows
• Spectral components in black hole X-ray binaries

Cited by in corpus (19)

• But What About... Cosmic Rays, Magnetic Fields, Conduction, & Viscosity in Galaxy Formation
• Testing Physical Models for Cosmic Ray Transport Coefficients on Galactic Scales: Self-Confinement and Extrinsic Turbulence at GeV Energies
• The $\gamma$-ray Emission of Star-Forming Galaxies
• Constraints on gamma-ray and neutrino emission from NGC 1068 with the MAGIC telescopes
• Particle acceleration in the superwinds of starburst galaxies
• Acceleration and Escape Processes of High-energy Particles in Turbulence inside Hot Accretion Flows
• First Predicted Cosmic Ray Spectra, Primary-to-Secondary Ratios, and Ionization Rates from MHD Galaxy Formation Simulations
• Standard Self-Confinement and Extrinsic Turbulence Models for Cosmic Ray Transport are Fundamentally Incompatible with Observations
• Cosmic rays and non-thermal emission in simulated galaxies. II. $\gamma$-ray maps, spectra and the far infrared-$\gamma$-ray relation
• Unveiling the origin of the gamma-ray emission in NGC 1068 with the Cherenkov Telescope Array
• Nonthermal processes in hot accretion flows onto supermassive black holes: An inhomogeneous model
• Cosmic rays from the nearby starburst galaxy NGC 253: the effect of a low luminosity active galactic nucleus
• Circinus Galaxy Revisted with 10 Years of Fermi-LAT Data
• A Simple Sub-Grid Model For Cosmic Ray Effects on Galactic Scales
• ArH+ and H2O+ absorption towards luminous galaxies
• A geomagnetic filter for the Fermi-LAT background
• Non-thermal emission from fall-back clouds in the Broad-Line Region of Active Galactic Nuclei
• Hunting Star-Forming Galaxies in the Gamma-Ray Domain
• Revisiting Disk Winds in Active Galactic Nuclei as an Origin of Cosmic Gamma-ray and Neutrino Backgrounds