Astron. Astrophys. 327, 61-71 (1997)

Optical-IUE observations of the gamma-ray loud BL Lacertae object S5 0716+714: data and interpretation ^*

G. Ghisellini ¹, M. Villata ², C.M. Raiteri ², S. Bosio ³, G. De Francesco ², G. Latini ³, M. Maesano ⁴, E. Massaro ⁴, F. Montagni ⁴, R. Nesci ⁴, G. Tosti ⁵, M. Fiorucci ⁵, E. Pian ⁶, L. Maraschi ⁷, A. Treves ⁸, A. Comastri ⁹ and M. Mignoli <sup>9

1</sup> Osservatorio Astronomico di Brera, via Bianchi, 46, I-22055 Merate, Italy
² Osservatorio Astronomico di Torino, I-10025 Pino Torinese, Italy
³ Università di Torino, via Giuria 1, Torino, Italy
⁴ Istituto Astronomico, Università di Roma, Unità GIFCO-CNR Roma I, via Lancisi 29, Roma, Italy
⁵ Osservatorio Astronomico, Università di Perugia, Perugia, Italy
⁶ Space Telescope Science Institute, Baltimore, USA
⁷ Osservatorio di Brera, via Brera, 28, Milano, Italy
⁸ International School for Advanced Studies, Trieste, Italy
⁹ Osservatorio Astronomico di Bologna, Italy

Received 31 October 1996 / Accepted 3 June 1997

Abstract

We monitored the BL Lac object S5 0716+714 in the optical band during the period November 1994 - April 1995, which includes the time of a -ray observation by the Energetic Gamma Ray Experiment Telescope (EGRET) on February 14-28, 1995. The light curves in the R and B bands show fast fluctuations superimposed on longer timescale variations. The color index correlates with intensity during the rapid flares (the spectrum is flatter when the flux is higher), but it is rather insensitive to the long term trends. Over the 5 month observational period the light curve shows an overall brightening of about 1 mag followed by a fast decline. The EGRET pointing covers part of the very bright phase () and the initial decline. An ultraviolet spectrum was also obtained with the International Ultraviolet Explorer (IUE) (1200-3000 Å) during the EGRET observations. The variability of the optical emission of S5 0716+714 by itself sets important constraints on the magnetic field strength and on the physical processes responsible for it. Interpreting the whole electromagnetic spectrum with synchrotron self Compton models leads to the prediction of a bright -ray state during the EGRET pointing. We discuss how the -ray data could be used as a diagnostic of the proposed models. ¹

Key words: BL Lac objects: general BL Lac objects: individual: S5 0716+714 gamma-rays: theory ultraviolet: galaxies

* Table 2 is only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html

Send offprint requests to: G. Ghisellini

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Contents

• 1. Introduction
• 2. The BL Lacertae object S5 0716+714
• 3. Observations and data analysis
  • 3.1. Optical
  • 3.2. Optical and ultraviolet spectra
    • 3.2.1. Optical spectrum
    • 3.2.2. Ultraviolet spectrum
• 4. Results of the optical monitoring
  • 4.1. The light curves
  • 4.2. Intranight variations
  • 4.3. Flux-spectral index correlation
• 5. Discussion
  • 5.1. Short time variations
  • 5.2. Homogeneous synchrotron self Compton models
  • 5.3. Inhomogeneous jet model
• 6. Conclusions
• References

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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