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Astron. Astrophys. 354, 513-521 (2000)

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2. Observations and data analysis

During Cycle 6 (October 15, 1996 to November 11, 1997) of its mission, CGRO was pointed continuously to the Virgo sky region for 7 weeks, beginning on December 10, 1996 and ending on January 28, 1997. The main target was the blazar 3C 279 which is located at ([FORMULA] = (12h56m11s, -5.8o47'21.5"), at a distance of [FORMULA]10.5o from 3C 273. The relevant observational parameters are given in Table 1.


Table 1. Continuous CGRO observations of 3C 273 during Cycle 6. The viewing period (VP) number in CGRO notation, the observational periods in calendar date and Truncated Julian Day (TJD), and the angular separation (Sep.) between 3C 273 and the pointing direction are given. The analysed time periods (VP-combinations) and their durations are given as well. For their explanation see text.

The spark-chamber telescope EGRET covers the energy range from [FORMULA]30 MeV to [FORMULA]30 GeV. The instrument and its calibration is described in detail by Thompson et al. (1993)and Esposito et al. (1999). The analysis of the EGRET data followed the standard EGRET procedure i.e. using count and exposure maps as well as predictions of the diffuse [FORMULA]-ray background (e.g. Hunter et al. 1997). The maps containing events with energies above 100 MeV were used for source detection and determination of the source position, and the ones of the 10 standard energy intervals for determination of the source spectrum by assuming a power law. The analysis applied the standard maximum-likelihood method (Mattox et al. 1996) and spatial selections ([FORMULA]25o off axis). Empirical flux corrections were applied for energies below 70 MeV.

The imaging Compton telescope COMPTEL covers the energy band [FORMULA]0.75 to [FORMULA]30 MeV. For a detailed description of the COMPTEL instrument see Sch"onfelder et al. (1993). The COMPTEL data have been analysed following the COMPTEL standard maximum-likelihood analysis procedures, which for point-source analyses including background generation are described in sufficient detail by Bloemen et al. (1994)and which derive quantitative source parameters like detection significances, fluxes, and flux errors. For consistency checks maximum-entropy images (see e.g. Strong et al. 1992) have been generated as well.

The fluxes of 3C 273 were determined by simultaneously fitting further sources or source candidates which are indicated by the maps (see Fig. 1). This approach leads iteratively to simultaneous flux determinations of several sources, including the generation of a background model which takes into account the possible presence of further sources. This analysis has been carried out for the four standard COMPTEL energy bands. The flux results given in Sect. 3 have been derived with point spread functions assuming an [FORMULA] power-law shape for the sources, which is approximately the correct shape for the MeV spectrum of 3C 273.

[FIGURE] Fig. 1. Virgo maps in the [FORMULA]-ray energy bands 3-10 MeV (left), 10-30 MeV (middle) as observed by COMPTEL, and [FORMULA]100 MeV (right) as provided by EGRET for the complete set of 7 weeks of continuous data (VPs 606 to 611.1). For COMPTEL the contour lines start at a detection significance of 3[FORMULA] ([FORMULA]-statistics for a known source) with a step of 0.5[FORMULA]. For EGRET the contour lines also start at a detection significance of 3[FORMULA], however with a step of 1[FORMULA]. The locations of the famous Virgo blazars 3C 273 (+) and 3C 279 ([FORMULA]) are indicated. 3C 273 is detected with 10.4[FORMULA] by EGRET. There is significant emission from 3C 273 in both COMPTEL maps. The COMPTEL 3-10 MeV map shows hints for additional sources.

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000