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Astron. Astrophys. 364, 552-556 (2000)

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1. Introduction

During its lifetime, the EGRET instrument on board the Compton Gamma Ray Observatory detected 170 gamma-ray point sources which are not clearly identified with known objects at lower frequencies (Hartman et al. 1999). About half of these sources are concentrated near the galactic plane, suggesting that they have a relatively local origin (Romero et al. 1999a; Gehrels et al. 2000).

Different possible counterparts have been suggested for the galactic population of gamma-ray sources: supernova remnants (SNRs) in interaction with molecular or atomic clouds (e.g. Sturner & Dermer 1995; Esposito et al. 1996; Combi et al. 1998), massive stars with strong stellar winds (e.g. Romero et al. 1999a), young pulsars (e.g. Yadigaroglu & Romani 1997; Zhang et al. 2000), and isolated black holes (e.g. Dermer 1997; Punsly 1998a,b).

Recent variability analysis of the data in the Third EGRET catalog by Torres et al. (2000) show that many of the sources at low galactic latitudes display high levels of variability, confirming the results found by McLaughlin et al. (1996) for the sources contained in the Second EGRET catalog. The conjunction of steep gamma-ray spectral indices and strong variability in a few sources seems to suggest the existence of an entirely new population of high-energy sources in the Galaxy. One such source is 3EG J1828+0142, located at [FORMULA]. Hartman et al. (1999) suggest that it could be an Active Galactic Nucleus (AGN), although there is no strong radio blazar within the 95% confidence contour. The source presents a steep spectral index with a value [FORMULA] and variable gamma emission. If we introduce the variability index [FORMULA], where [FORMULA] is the fluctuation index of the gamma-ray source and [FORMULA] is the averaged fluctuation index of all known gamma-ray pulsars (which are usually considered as a non-variable population), we found that [FORMULA], a clear indication of strong variability.

In this paper, we propose that this source could be a magnetized black hole which originated from a relatively recent supernova explosion. We have used background filtering techniques to isolate the radio image of an extended SNR (which overlaps the gamma-ray signal) from the contaminating diffuse emission of the Galaxy, in radio observations at two frequencies. Both the variability and the steep spectral index of the gamma-ray radiation argue against the possibility that the compact object left after the explosion is a pulsar. We present a model for a magnetized black hole that can reproduce the observed high-energy spectrum and which predicts an intense electron-positron annihilation line that could be detected later by the SPI spectrometer or the IBIS imager of the INTEGRAL satellite.

In the next section we describe the data analysis technique. Then, we present our results, briefly discuss the probability of a chance association of the SNR and the gamma ray source, and outline the black hole model. Finally, we discuss the predictions that can test our proposal.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001