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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint B/05


The INTEGRAL spectrometer SPI: performance of point-source data analysis

P. Dubath (1,2), J. Knödlseder (3), G. K. Skinner (3), P. Connell (4), I. Kreykenbohm (1,5), A. Strong (6), P. Sizun (7), D. Attie (7), S. Schanne (7), B. Corider (7), L. Bouchet (3), A. von Kienlin (6)

(1) INTEGRAL Science Data Centre, ch. d'Ecogia 16, 1290 Versoix, Switzerland
(2) Observatoire de Geneve, ch. des Maillettes 51, 1290 Sauverny, Switzerland
(3) CESR, 9, avenue du Colonel Roche, B. P. 4346, F-31028 Toulouse Cedex 4, France
(4) GACE - ICUMV, University of Valencia, Apdo 22085, E-46071 Valencia, Spain
(5) IAAT - Astronomie, Sand 1, 72976 Tübingen, Germany
(6) MPE, Giessenbachstr. 1, 85748 Garching, Germany
(7) Service d'Astrophysique, DSM/DAPNIA/SAp, CEA/Saclay, F-91 191 Gif-sur-Yvette, Cedex, France

To be published in: MNRAS

Abstract. The performance of the SPI point-source data analysis system is assessed using a combination of simulations and of observations gathered during the first year of INTEGRAL operations. External error estimates are derived by comparing source positions and fluxes obtained from independent analyses. When the source detection significance provided by the SPIROS imaging reconstruction program increases from ~10 to ~100, the errors decrease as the inverse of the detection significance, with values from ~10 to ~1 arcmin in positions, and from ~10% to ~1% in relative flux. These errors are dominated by Poisson counting noise. Our error estimates are consistent with those provided by the SPIROS program. With higher detection significance, the accuracy is ultimately limited to ~0.5 arcmin in position and of ~1% in relative flux by other types of errors. In these cases, SPIROS underestimates the true errors as it only takes into account the Poisson counting noise. At the low S/N end, SPIROS is also too optimistic, and the number of spurious detections is significantly higher than is expected from statistical noise fluctuations only. The analysis results do not depend significantly on the target off-axis angle, or on the number of pointings considered, provided that this number is larger than ~15. Realistic source confusion tests are carried out by adding simulated data to the Crab observation. Reliable flux values can be obtained for sources separated by angles as small as 0.5 degree if their positions are known and kept fixed in the data deconvolution process. However, when SPIROS is searching for best source positions, if two sources are separated by less than ~2 degrees, it may only find a single spurious source located between the real ones (with a flux equal to the sum of the fluxes). Finally, users of the SPIROS program can find a number of important recommendations in the conclusion.

Key words: Gamma rays: observations - Methods: data analysis - Instrumentation: miscellaneous

Preprint (460 kb PDF file including figures)


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