Flux and spectral variations in the Circinus Galaxy

¹ Dipartimento di Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
² Osservatorio Astronomico di Roma, via dell'Osservatorio, 00044 Monteporzio Catone Italy
³ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA

Corresponding author: S. Bianchi, bianchi@fis.uniroma3.it

Received: 15 May 2002
Accepted: 24 September 2002

Abstract

We report on a dramatic flux (~ increase in the LECS and MECS band) and spectral variation between two BeppoSAX observations of the Circinus Galaxy performed almost three years apart. Through the analysis of all Chandra observations available in the archive, including a new DDT observation on May 2001, we show that a high flux state of an extremely variable Ultra Luminous X-ray source (CG X-1: Bauer et al. [CITE]), which is within the adopted BeppoSAX source extraction region of , is the most likely explanation for most of the observed variation. However, the presence of a high flux 6.7 keV line and the spectral variation of the PDS in the new BeppoSAX data could be partly due to intrinsic variation of the nucleus. Comparing the longest Chandra observation and the BeppoSAX one, we find that the long-term flux variability of CG X-1 is not accompanied by a significant spectral variability. We also re-analysed the Chandra HEG nuclear spectra and report on the presence of a Compton shoulder with a flux of about the line core, in agreement with theoretical expectations for Compton-thick matter.