Wide band X-ray and optical observations of the BL Lac object 1ES 1959+650 in high state

¹ INAF-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
² ASI Science Data Center, Via Galileo Galilei, 00044 Frascati, Italy
³ Dipartimento di Fisica, Università La Sapienza, P.le Aldo Moro 2, 00185 Roma, Italy
⁴ Dipartimento di Fisica e Osservatorio Astronomico, Università di Perugia, Perugia, Italy

Corresponding author: G. Tagliaferri, tagliaferri@merate.mi.astro.it

Received: 4 July 2003
Accepted: 10 September 2003

Abstract

The blazar 1ES 1959+650 was observed twice by BeppoSAX in September 2001 simultaneously with optical observations. We report here the X-ray data together with the optical, R_C magnitude light curve since August 1995. The BeppoSAX observations were triggered by an active X-ray status of the source. The X-ray spectra are brighter than the previously published X-ray observations, although the source was in an even higher state a few months later, as monitored by the ASM onboard RossiXTE, when it was also detected to flare in the TeV band. Our X-ray spectra are well represented by a continuosly curved model up to 45 keV and are interpreted as synchrotron emission, with the peak moving to higher energies. This is also confirmed by the slope of the X-ray spectrum which is harder than in previous observations. Based on our optical and X-ray data, the synchrotron peak turns out to be in the range 0.1–0.7 keV. We compare our data with non-simultaneous radio to TeV data and model the spectral energy distribution with a homogeneous, one-zone synchrotron inverse Compton model. We derive physical parameters that are typical of low power High Energy peaked Blazars, characterised by a relatively large beaming factor, low luminosity and absence of external seed photons.