Simultaneous radio and X-ray observations of the low-mass X-ray binary GX 13+1

¹ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
² Center for Space Research, MIT, 70 Vassar Street, Cambridge, MA 02139, USA
³ School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, Scotland, UK
⁴ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
⁵ National Radio Astronomy Observatory, Socorro, NM 87801, Mexico
⁶ Deceased

Corresponding author: J. Homan, jeroen@space.mit.edu

Received: 2 September 2003
Accepted: 13 January 2004

Abstract

We present the results of two simultaneous X-ray/radio observations of the low-mass X-ray binary GX 13+1, performed in July/ August 1999 with the Rossi X-ray Timing Explorer and the Very Large Array. In X-rays the source was observed in two distinct spectral states; a soft state, which had a corresponding 6 cm flux density of ~0.25 mJy, and a hard state, which was much brighter at 1.3–7.2 mJy. For the radio bright observation we measured a delay between changes in the X-ray spectral hardness and the radio brightness of ~40 min, similar to what has been found in the micro-quasar GRS 1915+105. We compare our results with those of GRS 1915+105 and the atoll/Z-type neutron star X-ray binaries. Although it has some properties that do not match with either atoll or Z sources, GX 13+1 seems more similar to the Z sources.