Research Note

Search for the shortest variability at gamma rays in flat-spectrum radio quasars

L. Foschini¹, G. Ghisellini¹, F. Tavecchio¹, G. Bonnoli¹ and A. Stamerra^2,3

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
e-mail: luigi.foschini@brera.inaf.it
² Università di Siena, via Roma 56, 53100 Siena, Italy
³ INFN Pisa, via Buonarroti 2, 56127 Pisa, Italy

Received: 11 April 2011
Accepted: 26 April 2011

Abstract

We report about the search for short-term variability in the high-energy γ-ray energy band of three flat-spectrum radio quasars (3C 454.3, 3C 273, PKS B1222 + 216), whose flux at E > 100 MeV exceeded the value of 10^-5 ph cm^-2 s^-1 for at least one day. Although, the statistics was not yet sufficient to effectively measure the characteristic time scale, it allowed us to set tight upper limits on the observed doubling time scale (<2–3 h) – the smallest measured to date at MeV energies –, which can constrain the size of the γ-ray emitting region. The results obtained in the present work favor the hypothesis that γ rays are generated inside the broad-line region.