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Astron. Astrophys. 333, L5-L8 (1998)

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Letter to the Editor

The complex 0.1-100 keV X-ray spectrum of PKS 2155-304

P. Giommi 1, F. Fiore 1, 2, M. Guainazzi 1, M. Feroci 3, F. Frontera 4, 5, G. Ghisellini 6, P. Grandi 3, L. Maraschi 6, T. Mineo 7, S. Molendi 8, 1, A. Orr 9, S. Piraino 7, A. Segreto 7, G. Tagliaferri 6 and A. Treves 10

1 BeppoSAX Science Data Center, ASI, Via Corcolle 19, I-00131 Roma, Italy
2 Osservatorio Astronomico di Roma, Monteporzio Catone, Italy
3 Istituto di Astrofisica Spaziale, CNR, Via Enrico Fermi 21, I-00044 Frascati, Italy
4 Dipartimento di Fisica, Universitá di Ferrara, Via Paradiso 12, I-44100 Ferrara, Italy
5 Istituto TESRE, CNR, Via Gobetti 101, I-40129 Bologna, Italy
6 Osservatorio Astronomico di Brera, via Brera 28, I-20121 Milano, Italy
7 Istituto di Fisica Cosmica ed Applicazioni dell'Informatica, C.N.R. Via Ugo La Malfa 156, Palermo, Italy
8 Istituto di Fisica Cosmica, CNR, Via Bassini 15, I-20133 Milano, Italy
9 Space Science Department of ESA, ESTEC, Keplerlaan, Noordwijk, The Netherlands
10 Universita' di Milano, sede di Como, Via Lucini 3, I-22100 Como, Italy

Received 3 July 1997 / Accepted 24 July 1997


A long ([FORMULA] seconds) observation of the bright BL Lac object PKS 2155-304 has been carried out with the Narrow Field Instruments of the BeppoSAX satellite as part of the Science Verification Phase. The source was detected between 0.1 and about 100 keV at an intermediate intensity level compared to previous observations. The unique spectral coverage of BeppoSAX has allowed us to detect a number of spectral features. Between 0.1 and 10 keV the spectrum can be well described by a convex spectrum with (energy) slope gradually steepening from 1.1 to 1.6. At higher energies evidence for a sharp spectral hardening is found, while in the soft X-rays (0.1-1.0 keV) some evidence for an absorption feature was found. Indication for an emission line at 6.4 keV in the source rest frame is present. Repeated variability of [FORMULA] 20-30% around the mean flux is clearly detected on time scales of a few hours. From the symmetry and timescale of the observed variations we derive limits on the magnetic field and on the maximum energy of the emitting particles, implying that PKS 2155-304 should not be bright at TeV energies.

Key words: galaxies, active

Send offprint requests to: P. Giommi

© European Southern Observatory (ESO) 1998

Online publication: April 15, 1998