Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (.pdfVIP

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Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (.pdf

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Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (

a r X i v : a s t r o - p h / 0 1 0 5 5 1 0 v 1 3 0 M a y 2 0 0 1 Astronomy Astrophysics manuscript no. (will be inserted by hand later) Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the Nearby Galaxy M 51 (NGC 5194) Yasushi Fukazawa1,3, Naoko Iyomoto2,3, Aya Kubota3, Yukari Matsumoto3, and Kazuo Makishima3 1 Department of Physical Sciences, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526 email:fukazawa@hirax6.hepl.hiroshima-u.ac.jp 2 Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229–8510 3 Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 Received 29 January 2001 / Accepted 27 April 2001 Abstract. We observed the nearby galaxy M 51 (NGC 5194) with BeppoSAX. The X-ray properties of the nucleus below 10 keV are almost the same as the ASCA results regarding the hard component and the neutral Fe Kα line, but the intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS data, we detected a bright hard X-ray emission component which dominates above 10 keV. The 10 – 100 keV flux and luminosity of this component are respectively 2×10?11 erg s?1 cm?2 and 2×1041 erg s?1. These are about 10 times higher than the extrapolation from the soft X-ray band, and similar to the flux observed with Ginga, which found a bright power law component in 2 – 20 keV band. Considering other wavelength properties and the X-ray luminosity, together with strong neutral Fe K line, the hard X-ray emission most likely arises from a low luminosity active nucleus, which is obscured with a column density of ～ 1024cm?2. This suggests that hidden low luminosity AGNs may well be present in other nearby galaxies. We interpret the discrepancy between Ginga and other X-ray satellites to be due to a large variability of absorption column density toward the line of sight over several years, suggesting that