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22 Chapter 1. Introduction and background 1.2.5 Black hole states and radio emission Black holes mainly display two different kinds of behavior at X-ray energies. On one hand, when the total X-ray luminosity is found to be low, they display a spectrum significantly hard up to a few hundreds of keV. This is the so-called low/hard state. On the other hand, when they exhibit a high X-ray luminosity, their spectrum is substantially softer. This is the so-called high/soft state. The lower energy photons, responsible for the enhancement of X-ray luminosity and steepening of the spectrum in the high/soft state, are produced by a multicolor blackbody in the inner accretion disk, while the hard X-ray tail in the low/hard state is thought to be the result of a (inverse) Comptonizing corona above the disk. Apart from these two states, there is also the off state, when X-ray emission is almost or totally suppressed, and the hybrids very high and intermediate states, where both spectral characteristics (soft disk photons and hard tail) coexist. The accepted idea was that the accretion rate increased from the off state, through the low/hard state, the intermediate state up to the high/soft state (and eventually the very high state). However, this picture is not so clear now, because the same characteristics of a given spectral state (X-ray spectral and timing proper- ties) have been observed at very different X-ray luminosities (a traditional indicator of accretion rate). An important insight into the possible scenario related to the black hole states has been provided by radio observations. In this sense, long-term multiwavelength campaigns such as the one shown in Fig. 1.8 are very useful. There we can see two years of monitoring of GX 339−4 at radio, hard X-rays and soft X-rays. In the upper part of the figure it is marked the X-ray spectral state of the source along time. Starting in the low/hard state, a transition is observed into the high/soft state, which lasted around one year, followed by a transition back to the low/hard state and ending with an off state. The radio flux is clearly correlated with the hard X-rays in all spectral states. The correlation between the radio flux and the soft X-rays is more complicated, but can be shown to behave as follows: during the low/hard state radio emission is correlated with the soft X-ray photons, while during the high/soft state radio emission is suppressed. Faint radio emission is detected during the off state, compatible with it being a lower luminosity low/hard state. It is interesting to note that the spectral index of the radio emission is flat all the time, indicative of a compact and partially self-absorbed jet, except when
1.2. Microquasars 23 Low-hard state High-soft state L-H state Off state Figure 1.8: Two-year monitoring of the black hole candidate GX 339−4 at radio wavelengths (ATCA and MOST), hard X-rays (BATSE, 20–100 keV) and soft X-rays (RXTE ASM, 1.5–12 keV) (from Corbel et al. 2000). transitions between spectral states take place. Then, the radio emission is optically thin, probably as a result of major ejections of relativistic plasma clouds. After these observational results, it was proposed that compact radio jets may always be present during the low/hard states (as observed in other sources), while are suppressed in the high/soft states. The reason is not clear yet, but a coupling between the radio jet and the hard X-ray emitting corona is firmly established. A suggested possibility is that the corona is in fact the base of the radio jet. On the other hand, the synchrotron radiation mechanism, which is at work in the jet, has been seen up to infrared and possibly optical wavelengths. Moreover, it has been suggested that it could also generate photons up to X-ray energies in its optically thin part. If this is correct, then the jets would be extremely powerful, and certainly any model explaining accretion should account for their presence.
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UNIVERSITAT DE BARCELONA Departamen
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Per a la meva neboda Blanca, que va
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feina no presentada en aquesta tesi
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desfogar-nos conjuntament, per la s
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From the scientifical point of view
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Gràcies a la meva germana Nàdia,
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ii CONTENTS I Discovery and study o
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2.6. The X-ray counterpart: RX J182
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2.7. The γ-ray counterpart: 3EG J1
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2.7. The γ-ray counterpart: 3EG J1
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2.8. A proposed scenario to explain
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Figure 2.23: Scenario where the mul
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2.9. Conclusions 87 8. We have carr
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Bibliography Blandford, R. D., & K
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BIBLIOGRAPHY 91 Merck, M., Bertsch,
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Chapter 3 LS 5039 as a runaway micr
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3.1. Positions and proper motions 9
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3.1. Positions and proper motions 9
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3.3. The past trajectory of LS 5039
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3.4. SNR G016.8−01.1 101 DECLINAT
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3.4. SNR G016.8−01.1 103 Normaliz
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3.4. SNR G016.8−01.1 105 Galactic
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3.5. The H i surroundings of LS 503
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3.6. Discussion 109 experienced a h
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3.6. Discussion 111 At this point w
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3.7. Conclusions 113 6. Finally, th
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Bibliography Ankay, A., Kaper, L.,
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BIBLIOGRAPHY 117 Schaerer, D., de K
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Part II A search for new microquasa
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122 Chapter 4. The cross-identifica
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130 BIBLIOGRAPHY
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132 Chapter 5. Radio and optical ob
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DECLINATION (J2000) DECLINATION (J2
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148 BIBLIOGRAPHY
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150 Chapter 6. EVN and MERLIN obser
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166 BIBLIOGRAPHY
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General conclusions 171 Since each
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