A short hard X-ray flare from the blazar NRAO 530 observed by INTEGRAL

¹ INAF/IASF-Bologna, via Gobetti 101, 40129 Bologna, Italy e-mail: foschini@iasfbo.inaf.it
² INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131, Trieste, Italy
³ INAF, Osservatorio Astronomico di Brera, via Bianchi 46, 23807, Merate, Italy
⁴ INAF, Osservatorio Astronomico di Torino, via Osservatorio 20, 10025, Pino Torinese, Italy
⁵ Osservatorio Astronomico, Università di Perugia, via B. Bonfigli, 06126 Perugia, Italy

Received: 22 November 2005
Accepted: 5 January 2006

Abstract

We report about a short flare from the blazar NRAO 530 occurred on 17 February 2004 and detected serendipitously by the IBIS/ISGRI detector on board INTEGRAL. In the  keV energy range, the source, that is otherwise below the detection limit, is detected at a level of ≈2  10^-10 erg cm^-2 s^-1 during a time interval of less than 2000 s, which is about a factor 2 above the detection threshold. At other wavelengths, only nearly-simultaneous radio data are available (1 observation at 2 cm on 11 February 2004), indicating a moderate increase of the polarization. This appears to be the shortest time variability episode ever detected in a high luminosity blazar at hard X-rays, unless the blazar is contaminated by the presence of an unknown unresolved rapidly varying source.