ISDC Data Centre for Astrophysics

Seyfert 2 galaxies in the GeV band: jets and starburst

We present Fermi/LAT data analysis of two unusual suspects in the GeV band, NGC 1068 and NGC 4945, which are both composite starburst/active galactic nuclei objects. We show that the GeV emission of NGC 4945 could be interpreted in terms of starburst activity, while the one of NGC 1068 should account for a dominant contribution from the central AGN.

Lenain et al. 2010, A&A, in press

ABSTRACT

The Fermi/LAT collaboration recently reported the detection of starburt galaxies in the high energy γ-ray domain, as well as radio-loud narrow-line Seyfert 1 objects. Motivated by the presence of sources close to the location of composite starburst/Seyfert 2 galaxies in the first year Fermi/LAT catalogue, we aim at studying high energy γ-ray emission from such objects, and at disentangling the emission of starburst and Seyfert activity. We analysed 1.6 years of Fermi/LAT data from NGC 1068 and NGC 4945, which count among the brightest Seyfert 2 galaxies. We search for potential variability of the high energy signal, and derive a spectrum of these sources. We also analyse public INTEGRAL IBIS/ISGRI data over the last seven years to derive their hard X-ray spectrum. We find an excess of high energy γ-rays of 8.3 σ and 9.2 σ for 1FGL J0242.7+0007 and 1FGL J1305.4-4928, which are found to be consistent with the position of the Seyfert 2 galaxies NGC 1068 and NGC 4945, respectively. The energy spectrum of the sources can be described by a power law with a photon index of Γ = 2.31 ± 0.13 and a flux of F_{100 MeV-100 GeV} =(8.60 ± 2.27) × 10^-12 erg cm^-2 s^-1 for NGC 1068, while for NGC 4945, we obtain a photon index of Γ = 2.31 ± 0.10 and a flux of F_{100 MeV-100 GeV} =(1.58 ± 0.32) × 10^-11 erg cm^-2 s^-1 . For both sources, we detect no significant variability nor any indication of a curvature of the spectrum. While the high energy emission of NGC 4945 is consistent with starburst activity, that of NGC 1068 is an order of magnitude above expectations, suggesting dominant emission from the active nucleus. We show that a leptonic scenario can account for the multi-wavelength spectral energy distribution of NGC 1068. High energy γ-ray emission is revealed for the first time in a Seyfert 2 galaxy. If this result is confirmed in other objects, new perspectives would be opened up into the GeV band, with the discovery of a new class of high energy γ-ray emitters.

Fermi gamma-ray image of NGC 1068
Test Statistic map of NGC 1068 between 100 MeV and 100 GeV. The green ellipses show the 68% and the 95% position errors from the 1FGL catalogue, the cyan and magenta circles show respectively the position error (at 68% and 95% CL) for the full data set with all the events accounted for, and for front events only. The white contours are taken from an optical image from the Digital Sky Survey, showing the extent of the Seyfert galaxy. The red boxed points denote the position of two quasars nearby NGC 1068.
Credits: J.-P. Lenain

Fermi gamma-ray image of NGC 4945
Same as Fig. 1 for NGC 4945.
Credits: J.-P. Lenain

Comparison of starburst properties
Relationship between SN rate, total gas mass and γ-ray luminosity of NGC 1068, NGC 4945, NGC 253, M 82, the LMC and the Milky Way.
Credits: J.-P. Lenain

SED of NGC 4945
Spectral energy distribution of NGC 4945, including the Fermi/LAT spectrum (black points). For clarity, we only show the INTEGRAL IBIS/ISGRI data in blue in the hard X-rays. The model for the EIC component from the accretion disc is shown in green. We show in red the data of NGC 253 as taken from the NED, with the Fermi/LAT spectrum published in Abdo et al. (2010) as well as the H.E.S.S. flux measurement from Acero et al. (2009), for comparison. The two objects have clearly very similar SEDs. The luminosity axis on the right is given for NGC 4945.
Credits: J.-P. Lenain

SED of NGC 1068
Spectral energy distribution of NGC 1068, including the Fermi/LAT spectrum. The black and red points are archival data from the NED, the red ones denote data taken from the central region of NGC 1068. For clarity, we only show the INTEGRAL IBIS/ISGRI data in blue in the hard X-rays. The EIC model for the outflow is shown in blue, and the corresponding SSC emission is shown in thin red and magenta lines for first and second order components, respectively. The thick red line shows the sum of the different emission components from the large outflow. The EIC component from the accretion disc is shown in green.
Credits: J.-P. Lenain