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Astro-Ph/0002366V1 18 Feb 2000 .Sch¨Onfelder, Vos@Mpe.Mpg.De V A&A manuscript no. (will be inserted by hand later) ASTRONOMY AND Your thesaurus codes are: ASTROPHYSICS (20; 13.09.2; 04.03.1; 04.19.1) 16.10.2018 The first COMPTEL source catalogue V. Sch¨onfelder1, K. Bennett4, J.J.Blom2, H.Bloemen2, W. Collmar1, A. Connors3, R.Diehl1, W. Hermsen2, A. Iyudin1, R.M. Kippen3, J. Kn¨odlseder5, L. Kuiper2, G.G. Lichti1, M. McConnell3, D. Morris3, R.Much4, U. Oberlack1, J. Ryan3, G. Stacy3, H. Steinle1, A. Strong1, R. Suleiman3, R. van Dijk4, M. Varendorff1, C. Winkler4, and O.R. Williams4 1 Max-Planck-Institut f¨ur extraterrestrische Physik, D–85740 Garching, Germany 2 SRON–Utrecht, Sorbonnelaan 2, NL–3584 CA Utrecht, The Netherlands 3 Space Science Center, University of New Hampshire, Durham NH 03824-3525, USA 4 Astrophysics Division, ESTEC, NL–2200 AG Noordwijk, The Netherlands 5 Centre d’Etude Spatiale des Rayonnements (CESR), BP 4346, F-31029 Toulouse Cedex, France Received 28 July 1999; Accepted 20 December 1999 Abstract. The imaging Compton telescope COMPTEL 1. Introduction aboard NASA’s Compton Gamma-Ray Observatory has opened the MeV gamma-ray band as a new window to as- COMPTEL has demonstrated that the sky is rich in phe- tronomy. COMPTEL provided the first complete all-sky nomena that can be studied at MeV energies. A variety of survey in the energy range 0.75 to 30 MeV. The catalogue, gamma-ray emitting objects are visible either in contin- presented here, is largely restricted to published results. It uum or line emission. Among the continuum sources are contains firm as well as marginal detections of continuum spin-down pulsars, stellar black-hole candidates, super- and line emitting sources and presents upper limits for var- nova remnants, interstellar clouds, nuclei of active galax- ious types of objects. The numbers of the most significant ies, gamma-ray bursters, and the Sun during solar flares. detections are 32 for steady sources and 31 for gamma- Line detections have been made in the light of the 1.809 26 44 ray bursters. Among the continuum sources, detected so MeV Al line, the 1.157 MeV Ti line, the 847 and 1238 far, are spin-down pulsars, stellar black-hole candidates, keV 56Co lines, and the neutron capture line at 2.223 MeV. supernova remnants, interstellar clouds, nuclei of active COMPTEL has also measured the diffuse inter- galaxies, gamma-ray bursters, and the Sun during solar stellar and cosmic gamma radiation, whose prop- flares. Line detections have been made in the light of the erties are described elsewhere (Strong et al. (1996), 26 44 1.809 MeV Al line, the 1.157 MeV Ti line, the 847 Strong et al. (1999), Bloemen et al. (1999a), 56 and 1238 keV Co lines, and the neutron capture line Kappadath et al. (1996), Weidenspointner et al. (1999)). at 2.223 MeV. For the identification of galactic sources, a For the identification of galactic sources a modelling of modelling of the diffuse galactic emission is essential. Such the diffuse galactic emission is essential. Such a modelling a modelling at this time does not yet exist at the required arXiv:astro-ph/0002366v1 18 Feb 2000 at this time does not yet exist at the required degree of degree of accuracy. Therefore, a second COMPTEL source accuracy. catalogue will be produced after a detailed and accurate This paper is restricted to all those sources that have modelling of the diffuse interstellar emission has become been definitely or marginally detected so far, and pro- possible. vides upper limits to the MeV flux from different types of objects. A second COMPTEL source catalogue will be Key words: Gamma rays: observations, Catalogs, Sur- produced after a detailed and accurate modelling of the veys diffuse interstellar emission has become possible. The Compton Observatory was launched on April 5, 1991 by the Space Shuttle Atlantis. During Phase-I of the Compton Observatory Program, a full-sky survey - the first ever in gamma-ray astronomy - was performed. Phase-I ended on November 17, 1992. Observations during the subsequent phases of the program resulted in deeper Send offprint requests to: exposures and complemented the survey. This source cat- V. Sch¨onfelder, [email protected] alogue is mainly restricted to the results from the first five 2 V. Sch¨onfelder et al.: The first COMPTEL source catalogue years of the mission (up to Phase IV/Cycle-5). In a few Our threshold for detection is a chance probability of cases, more recent results have been added. 99.7 %, corresponding to a 3σ Gaussian significance. The applicable statistics, i.e. the relevant trails, are discussed for each source in the original publication. 2. Instrument description and data analysis The sensitivity of COMPTEL is significantly deter- COMPTEL was designed to operate in the energy range mined by the instrumental background. A substantial 0.75 to 30 MeV. It has a large field-of-view of about 1 suppression is achieved by the combination of effec- steradian, and different sources within this field can be tive charged-particle shield detectors, time-of-flight mea- resolved, if they are more than about 3 to 5 degrees away surement techniques, pulse-shape discrimination, Earth- from each other (energy dependent). The resulting source horizon angle cuts and proper event selections in energy location accuracy is of the order of 1◦. The COMPTEL en- andϕ ¯-space. ergy resolution of 5 % to 10 % FWHM is an important fea- The application of the imaging techniques requires ture for gamma-ray line investigations. A detailed descrip- an accurate knowledge of the instrumental and cosmic tion of COMPTEL is given by Sch¨onfelder et al. (1993). COMPTEL background. A variety of background models COMPTEL consists of two detector assemblies, an up- has been investigated and is being used. In one method, per one of liquid scinitillator NE213, and a lower one of the background is derived from averaging high-latitude NaI (Tl). A gamma-ray is detected by a Compton collision observations. This assumes that the background has a in the upper detector and a subsequent interaction in the constant shape in the instrumental system in at least the lower detector. spatial coordinates (but not in Compton scatter angle) The arrival direction of a detected gamma ray is known for all observations, and it also assumes that the extra- to lie on a circle on the sky. The center of each circle is galactic source contribution is small and smeared out by the direction of the scattered gamma-ray, and the radius the averaging process (see Strong et al. (1999)). A second of the circle is determined by the energy losses E1 and E2 method derives the background from the data that are in both interactions. The detected photons are binned in being studied itself. This is accomplished by applying a a 3-dimensional (3-D) data space, consisting of the scatter low-pass filter to the 3-D data, which smooths the photon direction (defined by the two angular coordinates χ and distribution and eliminates (in the first approximation) ψ) and by the scatter angleϕ ¯ (derived from the measured the source signatures (e.g. Bloemen et al. (1994)). By ap- energy losses in both interactions). Each detected pho- plying iterations of this process the background estimate ton is represented by a single point in the 3-D dataspace. can be improved, further. All viewing periods have to be The signature of a point source with celestial coordinates handled separately to account for changes of the back- ◦ (χo, ψo) is a cone of 90 opening angle with its axis par- ground during the mission (Bloemen et al. (1999a)). For allel to theϕ ¯-axis. The apex of the cone is at (χo, ψo). line studies, we estimate the background below an under- Imaging with COMPTEL involves recognizing the cone lying cosmic gamma-ray line by averaging the count rate patterns in the 3-D dataspace. Two main techniques are from neighbouring energy intervals (Diehl et al. (1994)). applied: one is a maximum-entropy method that generates For sources within the Galactic plane the global dif- model-independent images (Strong et al. (1992)) and the fuse emission from the Galaxy is modelled by fitting a other one is a maximum-likelihood method that is used bremsstrahlung, and an inverse Compton component to to determine the statistical significance, flux and position the data. Also an isotropic component is added to these uncertainty of a source (de Boer et al. (1992)). fits to represent the cosmic gamma-ray background. The Significances are derived in this method from the quan- amplitude of each of these components is obtained as free tity -2 ln λ, where λ is the maximum likelihood ratio L(B) parameter from the fits. It has to be admitted, however / L(B+S); B represents the background model and S the (see above), that the modelling of the plane, at present, source model (or sky intensity model). In a point-source does not yet achieve the required degree of accuracy. 2 search, -2 ln λ formally obeys a χ3 distribution; in studies In addition to the normal double-scatter mode of op- 2 of a given source, χ1 applies. [This allows to translate a eration, two of the NaI crystals in the lower detector as- measured L value into a corresponding probability for it sembly of COMPTEL are also operated simultaneously as being a noise fluctuation, equivalent to a Gaussian σ de- burst detectors. These two modules are used to measure scription of significances. In studies of a ’known’ source, the time history and energy spectra of cosmic gamma-ray σ = √ 2lnλ applies.] bursts and solar flares.
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