Objetivos Generales y Específicos

The first reported detection of a Sy2 at Fermi -LAT energies was NGC 4945 which was noted in the 11-month Fermi -LAT catalogue (Abdo et al., 2010a). This object is at a redshift of z = 0.001908, exhibits strong starburst activity, was previously detected at soft γ-ray energies by the International Gamma-Ray Physics Laboratory (INTEGRAL) satellite (Petry et al., 2009), and is one of the brightest hard X-ray AGN (Itoh et al., 2008). Based on observations with the INTEGRAL (Beckmann et al., 2009) and Ginga (Iwasawa et al., 1993) satellites it is known to be a Compton-thick AGN. NGC 4945 is detected by the Fermi -LAT at a significance of 9.2σ above background; its high-energy spectrum is best described by a power law of index Γ = 2.31 ± 0.10.

Also in the 11-month Fermi -LAT catalogue, a source, 1FGL J0242.7+0007, was detected in the region of NGC 1068 with no obvious counterpart in radio or lower energy γ-rays. Lenain et al. (2010) analysed 1.6 years of data from the Fermi -LAT to investigate the origin of this emission and concluded that 1FGL J0242+0007 is indeed associated with NGC 1068, with a significance of 8.4 σ. The γ-ray spectrum detected is consistent with a power law of index Γ = 2.31 ± 0.13. NGC 1068 is the archetypal Sy2 galaxy: located at z = 0.003786, it is one of the closest Sy2s, and it is also one of the brightest. It exhibits both AGN and starburst activity in its central region; at infrared wavelengths, a circumnuclear starburst region at ∼ 1 kpc dominates the SED. High-energy observations of the core of NGC 1068 by Chandra have shown that the X-ray emission from the source originates in the narrow-line emitting region from a primarily photoionised plasma (Ogle et al., 2003).

As NGC 4945 and NGC 1068 both display starburst activity, it is important to determine whether the γ-ray emission originates from this activity or from the hosted AGN. No conclusion was reached as to the source of γ-ray emission in NGC 4945 in Abdo et al. (2010a). To determine the origin of the γ-ray emission from both NGC 4945 and NGC 1068, Lenain et al. (2010) attempted to use the available data to look for significant variability in the γ-ray emission, as such variability would not be expected if the emission was due to starburst activity. No variability could be detected, but this result was not statistically significant. The γ-ray luminosities of NGC 4945 and NGC 1068 are 2.0 × 1040 _{erg s}−1 _{and 1.7 ×10}41_{erg s}−1 _{respectively, which are comparable to the}

luminosities at these energies of ≈ 1040_{erg s}−1_{for the starburst galaxies NGC 253 and M82 (Abdo}

investigate this possibility, the supernova rates (RSN) and total gas masses (Mgas) of NGC 1068 and

NGC 4945 were compared with those of NGC 253, M82, the Large Magellanic Cloud (LMC) and the Milky Way, along with their infrared and radio luminosities. Models that attribute the emission of γ-rays at these energies to starburst behaviour, and hence to cosmic ray processes, depend on the product RSNMgas. The γ-ray luminosity and supernova rate of NGC 4945 are comparable to

those of NGC 253 and M82, so even though the object is a composite starburst/Sy2 galaxy its high-energy emission could be explained by starburst activity alone. In the case of NGC 1068, however, a more complex situation arises. The supernova rate in NGC 1068 (0.20 ± 0.08 yr−1) is the same as in M82 (0.2 ± 0.1 yr−1) and NGC 253 (0.2 ± 0.1 yr−1), but its radio and γ-ray luminosities are higher by a factor of ∼ 10. This would appear to support the hypothesis that the γ-ray emission in NGC 1068 probably originates from AGN activity rather than from starburst processes. Interestingly, when the product RSNMgasis plotted against γ-ray luminosity, Lγ, for the

previously mentioned galaxies, excluding NGC 1068, the relationship coefficient for a linear fit is as high as 0.95, rejecting the null hypothesis that there is no relationship between star formation and γ-ray luminosity in these objects with a probability of ∼ 99%. However, if NGC 1068 is included the probability that the null hypothesis is invalid drops to ∼ 62%, supporting the argument that γ-ray emission from this source is unlikely to originate from starburst activity (the plot is shown in Figure 4.3.1). Radio maps of NGC 1068 further support the conclusion of nonthermal emission within the object, as a structured jet can be detected on parsec and kiloparsec scales (Gallimore, Baum & O’Dea, 2004). In contrast, NGC 4945 shows extended emission consistent with the optical morphology of the edge-on galaxy, indicating likely starburst emission.

It is suggested by Lenain et al. (2010) that the high-energy emission from NGC 1068 could be due to a large, mildly relativistic zone of the wind-like outflow, at a few tens of parsecs from the core, which could emit GeV γ-rays by the external inverse-Compton process (EIC) discussed in Begelman & Sikora (1987). In this scenario, the infrared photons from the disc and from stellar emission are upscattered by high-energy electrons in the mildly relativistic zone of the outflow, resulting in the high-energy emission observed. At the distance from the core proposed for the relativistic zone, the infrared photon density is high enough to ensure significant emission while not being so high as to present high optical opacity from pair production. The model presented in Lenain et al. (2010) posits that the radio emission detected is due to synchrotron processes, while the γ-rays detected by the Fermi -LAT are interpreted as EIC emission with seed photons provided by thermal infrared emission from the accretion disc. The contribution from SSC processes is shown to be negligible in the SED of NGC 1068. The model as described has some trouble reproducing the hard X-ray spectrum observed with INTEGRAL, and it is proposed that this

Figure 4.3: Plot of SN rate multiplied by total gas mass against -ray luminosity for NGC 1068, NGC 4945, NGC 253, M 82, the LMC and the Milky Way. Taken from Lenain et al. (2010). originates from EIC processes on another population of leptons within the hot plasma located in the vicinity of the accretion disc. NGC 1068 is believed to have an accretion disc, because the soft X-ray spectrum, as measured by instruments such as XMM-Newton is dominated by thermal reflection emission (Kinkhabwala et al., 2002). Modelling the synchrotron and SSC emission from this component leads to the conclusion that the emission is negligible compared with that produced in the component responsible for the high-energy γ-ray emission. An alternative model (Lenain et al. 2010) posits that the hard X-ray emission observed with INTEGRAL and the high-energy γ-ray emission detected with Fermi originate from the same spacial component. This requires that the overall high-energy part of the SED be due to EIC processes from the same population of leptons and this would be tightly constrained by the available data; however, the particle energy distribution that results is unable to account for the SED in the radio domain.

The detection of both Sy1s and Sy2s at such high energies is of great interest to astronomers working in the TeV regime. The SEDs produced by Foschini et al. (2009) for the Fermi -detected Sy1s imply that two of the objects, PKS 1502+036 and 1H 0323+342, may emit at TeV energies, although the expected flux is relatively low. PKS 1502+036 is not a particularly promising prospect as it is situated at a redshift of z = 0.49, so any TeV emission is very likely to be strongly attenuated by the EBL. 1H 0323+342, however, is situated at a much lower redshift of z = 0.061 and so there is a much better chance of any very high energy emission being detectable.