Title: INTEGRAL Observations of Abell 2142 and realted AGN
Proposal ID: 0120197
Subject category: Extragalactic Object
Principal investigator: Westmore
Institute: University of Southampton
Abstract
Coded masks cannot image diffuse emission on scales much greater than the point spread function of the
system. Rather than be a limitation, this often overlooked and poorly understood fact can be exploited as a
beneficial characteristic since it provides a natural way to filter and distinguish diffuse emission of
different scales, and point like sources. Taking advantage of this fact in general, and INTEGRAL in
particular, we intend to resolve outstanding questions on the origin and nature of the hard excess in the
galaxy cluster A2142.
Clusters of galaxies, such as A2142 have been seen to show emission from both diffuse
and point sources. The X-ray emission can be described by a thermal spectrum at low energies. Intriguingly,
an emission in excess of the hot gas' extrapolated thermal spectrum has been seen in A2142, by us, with
the balloon instrument HXR 81M, and with the satellite based instruments BATSE, and BeppoSAX. Each
observation showed a very different flux, with the difference between HXR and BeppoSAX being a factor of
10-20. The explanation of this discrepancy is intimately related to the question of the origin of the hard
excess. It is believed that the hard excess could be due to AGN associated with the cluster and/or inverse
Compton (IC) scattering of cosmic microwave background photons off of a population of hot electrons. The two
components are distinct in their extent. The AGN compnent will be point like whilst the IC will be diffuse;
the extent of the diffuse component is presently unknown, a nominal value was used to satisfy the PGT
requirements. The relative strength and distribution of each component is the remit of this proposal.
INTEGRAL is the first instrument capable of distinguishing these two components and so be able to
unambiguously ascertain the origin of the hard excess, the origin of the discrepancies between previous
measurements, and intriguingly, provide an estimate of the inter-cluster magnetic field.