arXiv subjects

Martin Pohl

Publications and source records attributed to Martin Pohl.

130 records · Page 8Linked to original sources

arXiv1997

The pulsar contribution to the diffuse galactic gamma-ray emission

Here we investigate to what extent unresolved \gr pulsars contribute to the galactic diffuse emission, and further whether unresolved \gr pulsars can be made responsible for the excess of diffuse galactic emission above 1 GeV which has been observed by EGRET. Our analysis is based only on the properties of the six pulsars which have been identified in the EGRET data, and is independent of choice of a pulsar emission model. We find that pulsars contribute very little to the diffuse emission at lower energies, whereas above 1 GeV they can account for 25% of the observed intensity in selected regions for a reasonable number of directly observable \gr pulsars ($\sim$12). While the excess above 1 GeV \gr energy is observed at least up to six or eight degrees off the plane, the pulsar contribution would be negligible there. Thus pulsars do significantly contribute to the diffuse galactic $γ$-ray emission above 1 GeV, but they can not be made responsible for all the discrepancy between observed intensity and model predictions in this energy range.

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arXiv1997

Where Are We and Where Should We Go? Summary Reports of the Girona Conference Workshops

Four parallel workshops were organized at the 1996 Girona Conference, "Blazars, Black Holes, and Jets", with conference attendees free to choose which session they would like to attend. The chosen topics were ``Jet Models'', ``Multi-Frequency Support for High-Energy Observations'', ``Monitoring of Blazars'', and ``Host Galaxies and Environments''. While the individual sessions focused on their assigned topic, the goals of all the sessions were to identify points of agreement in the field and to set a course for future research. Following several hours of lively discussions, each session chair presented to the entire conference a summary of their own group's discussion. In this collective summary we have tried to reproduce the main opinions and ideas that were generated during the workshops.

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arXiv1996

A spectral study of gamma-ray emitting AGN

In this paper I investigate the $γ$-ray spectra of AGN by summing up the intensity and the power-law fit statistic of Quasars and OVV's and BL Lac's separately. The spectrum of the average AGN is softer than that of the extragalactic $γ$-ray background. It may be that BL Lac's, of which the average has a harder spectrum than Quasars, make up the bulk of the extragalactic background. We also find cut-offs both at low and at high energies in the spectra of Quasars and OVV's, however only at the time of $γ$-ray outbursts. While the cut-off at high energies may have something to do with opacity, the cut-off at low energies may be taken as indication that the $γ$-ray emission of Quasars is not a one component spectrum.

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arXiv1996

The galactic center arc as source of high-energy gamma-rays

We discuss the radio {\it arc} at the galactic center to be the counterpart of the high-energy \gr source 2EG J1746-2852. The observed \gr flux is constant within the statistical limits and the spectrum is very hard. The lack of variability makes it highly unlikely that any of the compact sources in the vicinity of the Galactic Center is the counterpart of 2EG J1746-2852. This includes the peculiar source Sgr A$^\ast$ at the very center of the Galaxy, which is often discussed to harbour a black hole of $10^6\, M_\odot$. Existing radio data on the {\it arc} support the view that its synchrotron emission originates from cooling, initially monoenergetic electrons which diffuse and convect from their sources to the outer extensions of the {\it arc}. If the source of high-energy electrons coincides with the sickle region (G0.18-0.04), as indicated by the radio data, then the ambient far- infrared photons can be up-scattered to \grs by inverse-Compton inter- action with the young high-energy electrons. We solve the continuity equation for the electrons including terms for diffusion, convection, monoenergetic injection, and the full energy loss. We show that both the flux and the spectrum of 2EG J1746-2852 can be well explained.

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