Detailed radio to soft γ-ray studies of the 2005 outburst of the new X-ray transient XTE J1818-245

¹ ESAC, ISOC, Villañueva de la Cañada, Madrid, Spain e-mail: Marion.Cadolle@sciops.esa.int
² CEA-Saclay, DSM/IRFU/SAp, France
³ AIM UMR 7158, Paris, France
⁴ Departament d'Astronomia i Meteorologia and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (UB/IEEC), Martí i Franquès 1, 08028 Barcelona, Spain
⁵ Dr. Karl Remeis Sternwarte, Sternwartstr. 7, 96049 Bamberg, Germany
⁶ ECAP, Erwin-Rommel-Straße 1, 91058 Erlangen, Germany
⁷ INAF, Osservatorio Astronomico di Brera, Merate, Italy
⁸ Università degli Studi dell'Insubria, Como, Italy
⁹ Metsähovi Radio Observatory, Helsinki University of Technology TKK, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁰ APC-UMR 7164, Paris, France

Received: 4 August 2008
Accepted: 18 March 2009

Abstract

Context. XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, as first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft γ-rays with the INTEGRAL satellite, from 2005 February to September, during our INTEGRAL Target of Opportunity program dedicated to new X-ray novae and during Galactic Bulge observations.

Aims. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. The goals of our programme are to understand the physical processes close to the black hole and to study the possible connection with the jets that are observed in the radio.

Methods. We analysed radio, (N)IR, optical, X-ray and soft γ-ray observations. We constructed simultaneous broad-band X-ray spectra covering a major part of the outburst, which we fitted with physical models. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behaviour of the source.

Results. Spectral parameters were typical of the soft intermediate states and the high soft states of a black hole candidate. The source showed relatively small spectral variations in X-rays with considerable flux variation in radio. Spectral studies showed that the accretion disc cooled down from 0.64 to 0.27 keV in ~100 days and that the total flux decreased while the relative flux of the hot medium increased. Radio emission was detected several times, and, interestingly, five days after entering the HSS. Modeling the spectral energy distribution from the radio to the soft γ-rays reveals that the radio flares arise from several ejection events.

Conclusions. XTE J1818-245 probably belongs to the class of low-mass X-ray binaries and is likely a black hole candidate transient source that might be closer than the Galactic Bulge. The results from the data analysis trace the physical changes that took place in the system (disc, jet/corona) at a maximum bolometric luminosity of erg s^-1 (assuming a distance between 2.8–4.3 kpc) and they are discussed within the context of disc and jet models.