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Astron. Astrophys. 324, 683-689 (1997)

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3. Propædeutics to the 44 Ti line analysis

3.1. A view to the Galaxy at 1.157 MeV

We first use background method (ii) in order to image the full celestial emission. Figure 1 shows likelihood maps of the Galactic plane in the narrow energy bands of Sect. 2.2. A striking feature of these maps is their relative emptiness, with only a few localised excesses. In contrast, COMPTEL all-sky images in the 1-3, 3-10, and 10-30 MeV ranges (see Schönfelder et al. 1996) display a ridge of emission along the Galactic plane, probably due to point-like sources and a diffuse component (e.g. Bloemen et al. 1994, Strong et al. 1994). Low statistics probably explains the difference: the extended emission of the Milky Way may remain below the sensitivity level achieved in narrow energy bands.

[FIGURE] Fig. 1. Maximum-likelihood maps of the Galactic plane in three energy bands: 0.89-1.07 MeV (bottom), 1.07-1.25 MeV (middle), 1.25-1.43 MeV (top). The contours start at [FORMULA] with steps of 3. The central map includes 44 Ti line emission at 1.157 MeV. Continuum sources are indicated by filled squares (Table 1). Stars show historical SNR s from the last millenium (Table 2).

Line studies require that the underlying continuum be suppressed. Background method (i) was developed to this aim and applied to the analysis of Galactic 26 Al emission at 1.809 MeV (Diehl et al. 1995). However the 44 Ti line is closer to the energy threshold of COMPTEL below 1 MeV, which makes more difficult an estimation of the continuum level from adjacent energies. We thus keep method (ii) and check any significant excess in the 1.07-1.25 MeV map of Fig. 1. Line emission can be assessed with respect to the neighbouring 0.89-1.07 and 1.25-1.43 MeV measurements. This choice ensures that we miss no potential 44 Ti source. Consistency checks between methods (i) and (ii) establish that systematic effects should not be in excess of 25%.

3.2. Continuum sources in the 1.157 MeV sky

A small number of excesses (Table 1) show up in all three maps of Fig. 1 and can be identified as continuum gamma-ray sources (see Schönfelder et al. 1996 for a review):


Table 1. Characteristics of the five continuum sources detected by COMPTEL in the 1.07-1.25 MeV band (Fig. 1). Fluxes are given in this band and two adjacent energy intervals together with their formal [FORMULA] uncertainty. An [FORMULA] power-law spectrum is assumed when deriving these values. Most detections are at the [FORMULA] significance level in all three bands.

  • the Crab is by far the most intense celestial source in the MeV regime (Much et al. 1995). It is detected with [FORMULA] of several hundreds in our narrow bands.
  • the X-ray binary/black-hole candidate Cygnus X-1 is the second strongest source in our maps (McConnell et al. 1994).
  • hosted by a peculiar elliptical galaxy, NGC 5128, Centaurus A is the closest active galactic nucleus detected by COMPTEL (Steinle et al. 1995).
  • a strong source shows up at [FORMULA], [FORMULA]. It coincides with a feature seen in the COS B data (Bloemen 1989) but is still unidentified.
  • emission is detected towards the Galactic Centre with a flux in the 1.07-1.25 MeV range which is larger than in adjacent energy bands. This excess may well belong to the extended ridge of Galactic diffuse emission.

At [FORMULA], there is evidence for an extended feature also visible in broad-band 1-3 MeV images.

We discard these continuum sources by including them in the background model with fluxes as given in Table 1. The feature towards the Galactic Centre deserves special attention. The band-to-band variations are within the [FORMULA] level and yield no evidence for significant 1.157 MeV line emission. From measurements in our neighbouring bands, we assign a 1.07-1.25 MeV flux of [FORMULA] to the underlying continuum.

We then repeat the maximum-likelihood analysis and build the residual likelihood map of Fig. 2.

[FIGURE] Fig. 2. Residual map of the Galaxy in the energy band 1.07-1.25 MeV. This likelihood map is similar to Fig. 1, except that the continuum sources of Table 1 were included in the background model, thus excluded from the search for 44 Ti sources. Contours start at [FORMULA] with steps of 3. Stars indicate historical SNR s from the second millenium (Table 2). Continuum emission may account for the elongated feature at [FORMULA] (Sect. 3.2). The excess close to the Crab seems consistent with the continuum source (active galactic nucleus) PKS 0528+134 at [FORMULA], [FORMULA] (Collmar et al. 1994).
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© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998