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Astron. Astrophys. 340, 35-46 (1998)

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The hidden X-ray Seyfert nucleus in 3C 273: Beppo SAX results

F. Haardt 1, G. Fossati 2, P. Grandi 3, A. Celotti 2, E. Pian 4, G. Ghisellini 5, A. Malizia 6, 7, L. Maraschi 5, W. Paciesas 8, C.M. Raiteri 9, G. Tagliaferri 5, A. Treves 10, C.M. Urry 11, M. Villata 9 and S. Wagner 12

1 Dipartimento di Fisica dell'Università di Milano, Milano, Italy
2 International School for Advanced Studies, SISSA/ISAS, Trieste, Italy
3 IAS, CNR, Frascati (RM), Italy
4 ITESRE, CNR, Bologna, Italy
5 Osservatorio Astronomico di Brera/Merate, Milano/Lecco, Italy
6 Department of Physics & Astronomy, University of Southampton, UK
7 Beppo SAX Science Data Center, Roma, Italy
8 University of Alabama in Huntsville, Huntsville, AL, USA
9 Osservatorio Astronomico di Torino, Pino Torinese (TO), Italy
10 Dipartimento di Fisica dell'Università di Como, Como, Italy
11 STScI, Baltimore, MD, USA
12 Landessternwarte, Heidelberg, Germany

Received 19 June 1998 / Accepted 3 September 1998


We present the results of 5 Beppo SAX AO1 Core Program observations of 3C 273 performed in Jan. 1997 and compare them in detail with data obtained during the satellite Science Verification Phase (SVP), in Jul. 1996 (Grandi et al. 1997).

3C 273 was about 15% brighter in the first 1997 observation than in the last one, and, on average, a factor 2 brighter than the SVP observation. A count rate variation in the 2-10 keV band of [FORMULA]% in [FORMULA] day was clearly detected during the last of the four pointings.

Power-law fits with Galactic absorption to all observations yield spectral indices in the range [FORMULA]. Though a power law is an acceptable representation of the data in the whole 0.1-200 keV range, there is indication of a steepening of the spectrum as the energy increases. Residuals with respect to a single power law suggest the presence of a weak fluorescence iron line in the MECS data. No other features are detectable. Our data therefore mark a difference with respect to the SVP data, where a steeper power law below 0.5 keV, an absorption feature at [FORMULA] keV, and a more prominent fluorescence iron line have been found.

The weakening of cold/warm matter signatures in our data with respect to the SVP ones may indicate that, at higher luminosities, the featureless continuum produced in a relativistic jet overwhelms any thermal and/or reprocessed radiation, while the two components were almost comparable during the lower state of Jul. 1996.

We quantitatively test this scenario, by considering an emission model which comprises the contribution from a thermal Seyfert-like nucleus and a non-thermal power-law component, and find that indeed the observed features in both the AO1 and SVP data are consistently reproduced by varying only the intensity of the non-thermal emission. Within this scenario, this radio-loud source shows evidence not only for thermal disk-like emission, but also substantial reprocessing of X-rays onto cold matter. There is no evidence of a direct correlation between the two components.

Key words: quasar: 3C 273 – X-rays: galaxies

Send offprint requests to: haardt@uni.mi.astro.it

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© European Southern Observatory (ESO) 1998

Online publication: November 3, 1998