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

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 0.4-0.9 ×10 38 erg s −1 (assuming a distance between 2.8-4.3 kpc) and they are discussed within the context of disc and jet models.

Quiescent NIR and optical counterparts to candidate black hole X-ray binaries

“…Steeghs et al (2005a) observed it a few days after the outburst and detected an optical candidate counterpart with R ∼ 17.71 mag which was later confirmed by Zurita Heras et al (2011). We observed this source a month after the outburst discovery and detected a NIR counterpart with an apparent magnitude of Ks = 16.18 ± 0.02; this is consistent with the result from Cadolle Bel et al (2009). They observed XTE J1818-245 in the NIR two months after the outburst start and detected a counterpart with apparent magnitude K = 16.9 ± 0.2.…”

On different types of instabilities in black hole accretion discs: implications for X-ray binaries and active galactic nuclei

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Evidence of an irradiated accretion disc in XTE J1818−245★

“…They detected a new optical source with a magnitude of R ∼ 17.5 mag during their 1‐h observation that started on 2005 August 17 at 23:19 ut . Multiwavelength (from radio to soft γ‐rays) observations performed from August to September, searching for the nature of XTE J1818−245 and modelling its hard X‐rays, are extensively described in a companion paper (Cadolle Bel et al 2009, hereafter Paper I). This new X‐ray transient source is probably a low‐mass X‐ray binary and a black hole candidate, located closer to us than the Galactic Centre (Paper I).…”