📝 Original Info
- Title: Detailed Radio to Soft Gamma-ray Studies of the 2005 Outburst of the New X-ray Transient XTE J1818-245
- ArXiv ID: 0903.4714
- Date: 2015-05-13
- Authors: Researchers from original ArXiv paper
📝 Abstract
XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft gamma-rays with the INTEGRAL satellite, from 2005 February to September. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. We analyzed radio, (N)IR, optical, X-ray and soft gamma-ray observations and constructed simultaneous broad-band X-ray spectra. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behavior of the source. 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 gamma-rays reveals that the radio flares arise from several ejection events. XTE J1818-245 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 at a maximum bolometric luminosity of (0.4-0.9)e38 erg/s (assuming a distance between 2.8-4.3 kpc) and they are discussed within the context of disc and jet models.
💡 Deep Analysis
Deep Dive into Detailed Radio to Soft Gamma-ray Studies of the 2005 Outburst of the New X-ray Transient XTE J1818-245.
XTE J1818-245 is an X-ray nova that experienced an outburst in 2005, first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft gamma-rays with the INTEGRAL satellite, from 2005 February to September. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. We analyzed radio, (N)IR, optical, X-ray and soft gamma-ray observations and constructed simultaneous broad-band X-ray spectra. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behavior of the source. 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 t
📄 Full Content
X-ray Novae (XNe), also known as Soft X-ray Transients, are accreting Low-Mass X-ray Binaries (LMXB) that spend most of their time in a faint, quiescent state. They undergo large amplitude outbursts with rise times of only a few days or weeks, with typical recurrence periods of many years (Tanaka & Shibazaki 1996). The picture commonly accepted for an XN involves the transition from a low-mass accretion rate state to a high-mass accretion rate optically thick accretion flow, triggered by an accretion disc instability. The optically thick and geometrically thin accretion disc has a varying inner radius and temperature, emitting at typical X-ray energies of ∼1 keV. This region is Send offprint requests to: Dr. Cadolle Bel: Marion.Cadolle@sciops.esa.int probably surrounded by a hot corona, where soft X-ray photons originating in the disc undergo inverse Comptonization, emitting a power law spectrum up to γ-rays. A relativistic jet might be present, typically observed in the radio. These spectral characteristics are coupled to different levels of variability, to Quasi Periodic Oscillations (QPOs) observed in the power spectrum spectrum (e.g., Belloni et al. 2001;Belloni 2005a) and to changes in the radio. Depending on the relative strengths of each component and on how they vary, several spectral states have been identified: see, e.g., McClintock & Remillard (2006), Homan & Belloni (2005). In the second classification, the two main spectral states are the Low/Hard State (LHS), dominated by non-thermal emission and the High Soft State (HSS), dominated by emission from the accretion disc. In the LHS, the fast time variability is dominated by strong (∼30% fractional rms) Fig. 1. RXTE/ASM light curve of XTE J1818-245 during the 2005 outburst. The times of the observations are indicated for each instrument as well as the INTEGRAL revolutions (lasting ∼3 days between consecutive vertical dashed lines). Other observations lasting from 100 to 3000 s are indicated (see Table 1). band-limited noise. At times, low-frequency QPOs are present. In this state, flat-spectrum radio emission is observed, associated with compact jet ejection (Corbel et al. 2000(Corbel et al. , 2003;;Gallo et al. 2003Gallo et al. , 2006;;Fender et al. 2004). In the HSS, only weak power law noise is present in the power spectrum. No core radio emission is detected (see Fender et al. 1999Fender et al. , 2005: "quenching" of the jet). Further states have been identified as "Intermediate" based on the above-mentioned differences in the soft/hard Xray components, variability and radio emission: the HIMS (Hard InterMediate State) and SIMS (Soft InterMediate State). In the HIMS, the energy spectrum is softer than in the LHS, with evidence for a soft thermal disc component. The power spectra feature band-limited noise with characteristic frequency higher than the LHS and usually a rather strong 0.1-15 Hz type-C QPO (see, e.g., Casella et al. 2005). In the SIMS, the disc component dominates the flux. No strong band-limited noise is observed but transient type-A and type-B QPOs are seen (the frequency of which spans only a limited range). As in the HSS, no core radio emission is detected. XTE J1818-245 was discovered by the All-Sky Monitor (ASM) telescope on board the RXTE satellite on 2005 August 12. The ASM Hardness Ratio (hereafter, HR) indicated a very soft spectrum, often associated with a Black Hole (BH) as the compact object of the binary system (Levine et al. 2005). Follow-up RXTE/Proportional Counter Array (PCA) observations provided a refined position and no pulsations were detected (Markwardt et al. 2005). On August 16-17, the INTEGRAL Soft Gamma-Ray Imager (IBIS/ISGRI) observed the source for 12.6 ks during the Galactic Bulge (GB) program: Shaw et al. (2005) found a position in the ISGRI mosaic image consistent with (and with smaller uncertainties) than the one found by the RXTE instruments. Soon after, the optical counterpart was identified (Steeghs et al. 2005) and the Swift/X-ray Telescope, XRT, (Still et al. 2005) improved the X-ray position. Finally, Rupen et al. (2005) detected XTE J1818-245 with the Very Large Array (VLA) at the optical position: the flux densities at 4.9 GHz increased from ∼7 to 27 mJy between August 20-21.
Based on the above-mentioned characteristics, XTE J1818-245 was suggested to be a BH Candidate (BHC). Target of Opportunity (ToO) observations for XNe in outburst were triggered with INTEGRAL, associated with a large multi-wavelength campaign. We report here the results of the INTEGRAL observations of XTE J1818-245 together with Swift, RXTE and NIR/optical/radio data. We start with a description of the available data and of the analysis procedures employed in Sect. 2. Results are presented in Sect. 3, followed by the interpretations and discussions in Sect. 4. We summarize our conclusions in Sect. 5.
Table 1 summarizes the observations analysed in this paper, giving for each of them the instrument data available, en
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