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Astron. Astrophys. 354, 431-438 (2000)

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The concave X-ray spectrum of the blazar ON 231: the signature of intermediate BL Lacertae objects

G. Tagliaferri 1, G. Ghisellini 1, P. Giommi 2, L. Chiappetti 3, L. Maraschi 4, A. Celotti 5, M. Chiaberge 5, G. Fossati 6, E.Massaro 7, M. Maesano 7, F. Montagni 7, R. Nesci 7, G. Nucciarelli 8, E. Pian 9, C.M. Raiteri 10, F. Tavecchio 4, G. Tosti 8, A. Treves 11, M. Villata 10 and A. Wolter 4

1 Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
2 Beppo SAX Science Data Center, ASI, Via Corcolle 19, 00131 Roma, Italy
3 Istituto di Fisica Cosmica G.Occhialini, CNR, Via Bassini 15, 20133 Milano, Italy
4 Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
5 SISSA/ISAS, Via Beirut 2-4, 34014 Trieste, Italy
6 CASS, University of California at San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0424, USA
7 Istituto Astronomico, Università "La Sapienza", Via Lancisi 29, 00161 Roma, Italy
8 Osservatorio Astronomico, Università di Perugia, Via A. Pascoli, 06100 Perugia, Italy
9 ITeSRE/CNR, Via Gobetti 101, 40129 Bologna, Italy
10 Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
11 Istituto di Fisica, Università dell'Insubria, Via Lucini 3, 22100 Como, Italy

Received 20 September 1999 / Accepted 1 December 1999


ON 231 was observed with Bepppo SAX in May and June 1998, following an exceptional optical outburst which occured in April-May. We measured the X-ray spectrum from 0.1 up to 100 keV. In both occasions the spectrum had a concave shape, with a break detected at about 4 and 2.5 keV, respectively. We interpret the steeper component at energies below the break as due to synchrotron emission and the extremely flat component at energies above the break as due to inverse Compton emission. This is so far the best example in which both the synchrotron and the Inverse Compton component are detected simultaneously and with the same instruments in the X-ray spectrum of a blazar. We detect a decrease of the X-ray flux of about 30% below the break between the first and the second observation, and smaller variability amplitude between 4 and 10 keV. During the May observation we also detected a fast variability event with the flux below 4 keV increasing by about a factor of three in 5 hours. Above 4 keV no variability was detected. We discuss these results in the framework of synchrotron self-Compton models.

Key words: galaxies: BL Lacertae objects: general – galaxies: BL Lacertae objects: individual: ON 231 – X-rays: galaxies

Send offprint requests to: G. Tagliaferri (gtagliaf@merate.mi.astro.it)

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000