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

5. Discussion and conclusions

We have shown in the previous section that when electron-positron annihilation effects are taken into account in the bipolar magnetically dominated wind ejected by a Kerr-Newman black hole in isolation, a steep-spectrum gamma-ray source with no strong radio counterpart can be produced. The wind, which is tightly collimated into a jet, possibly experiences strong dissipation near the outer light cylinder (i.e. at cm from the black hole) due to plasma instabilities and shocks. These dissipative processes can load the jet with hot plasma. If the inner jet was to propagate with a bulk Lorentz factor of 5 and was viewed at 20 degrees off the axis, a Doppler factor of 2.5 would be attained as in Eq. (20). The jet is likely to wiggle either as a consequence of interactions with the enveloping medium or by firehose instabilities. If the jet were to wobble 5 degrees from its nominal value of , then the gamma-ray luminosity would vary by a factor of 9. Thus, extreme variability is expected in our model.

There are two main types of observations that could be performed in the immediate future to confirm our proposal of the nature of the source 3EG J1828+0142. VLBA observations of the sources in Table 2 could reveal some structure unresolved in the VLA images and show evidence of the twin jets and their end points in the ISM, if there is, indeed, a NP black hole which produces 3EG J1828+0142. On the other hand, observations with the gamma-ray spectrometer SPI and the imager IBIS in the forthcoming INTEGRAL mission should show evidence of the electron-positron annihilation line, which has a luminosity of erg s^-1 in the present model. The expected flux, ph cm^-2 s^-1, depends on some uncertain parameters like the width of the line and the Doppler factor. It is not surprising that COMPTEL instrument of the late Compton Gamma-Ray Observatory did not detect the source, since it would required at least two weeks of integration time and there is not yet a satisfactory model of the galactic gamma-ray background (Schönfelder et al. 2000). Notwithstanding, recent background modeling by Bloemen et al. (2000), including bremsstrahlung and inverse Compton components, shows a flux excess in the galactic disk at a few MeV that could be the effect of unidentified point sources similar to the one discussed here.

X-ray observations with the Chandra observatory also could be very useful in identifying the lower frequency counterpart of 3EG J1828+0142. The expected luminosity of the NP black hole in the Chandra energy range is erg s^-1, with a spectral index . It should appear as a point source positionally coincident with one of the nonthermal radio sources detected in the field.

The fact that the point-like radio sources found within the inner confidence contour of the gamma-ray source have, in most cases, a steeper spectral index than the canonical value of 0.5 expected from the outer jet can be explain by the contribution from the two lobes of radio emission formed at the points where the outer jets end, approximately 0.1 pc from the black hole. The emission from these lobes, which is not taken into account in the SED shown in Fig. 2, has been modeled by Punsly (1998b). The radio spectrum from these regions is expected to be steeper than the emission from the jet, with values of at 1 GHz (Punsly 1998b).

The recent variability analysis of the unidentified gamma-ray sources in the Third EGRET catalog carried out by Torres et al. (2000) shows that the most variable sources near the galactic plane tend to present steep indices . The model presented here is capable of explaining the association of such indices with high levels of variability and the absence of strong radio counterparts in a galactic source. The discovery of a nearby SNR, also reported in this work, provides additional support for the existence of a young black hole behind the source detected by EGRET.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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