Title: Identification of EGRET sources in the halo
Proposal ID: 0120019
Subject category: Compact Object
Principal investigator: Grenier
Institute: CEA Saclay
Abstract
A subset of the EGRET unidentified g-ray sources corresponds to a population of objects at a scale height of
~ 2 kpc above the Galactic disk or in the Galactic halo. Likely candidates for such old g-ray sources
include ms pulsars and red 5-blazars accreting from a low-mass star. It is expected that both types of
sources display nFn spectra culminating in the MeV band. Their very soft average spectral index of 2.5,
recorded above 100 MeV, and their position at high latitude, away from crowded regions, makes them
attractive targets to efficiently search for a counterpart source with IBIS and SPI. A positive detection
would shed light on their nature and on the high-energy activity of compact stars in their old age. The lack
of detection of a few of them would also bring an important constraint on their nature as a class. We
propose dedicated observations, 500 ks each, of 6 targets (i.e. 3 sources & 3 amalgamated couples of
sources) chosen for their potential flux and location outside the core program surveys. They are part of a
list of 20 sources, ranked in priority, for which counterpart searches are proposed whenever present in the
IBIS and SPI fields-of-view during other observations, i.e. up to 80 from the telescopes axis. Complementary
observations at various wavelengths and pulsar searches are planned for identification purposes and to
constrain the energy distribution.