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144 Chapter 5. Radio and optical observations sources. Although at first sight our target appears as a point-like source in Fig. 5.1, if we perform Gaussian fits to field objects with similar magnitudes, we find our target to have a Full Width at Half Maximum (FWHM) ∼ 15% greater, suggesting that it is in fact an extended optical object. In summary, the high spectral index and the extended optical emission indicate that this source is not a very promising REXB candidate. 1RXS J062148.1+174736. In the radio domain, this source displays an inverted or flat spectrum, which could account for a partially self-absorbed jet. On the other hand, the moderate variability observed, around 10%, could explain the higher non-simultaneous NVSS flux density measurement. In the optical domain, this source appears as a point-like object with magnitudes typical of low latitude highly absorbed sources. However, a careful inspection of the optical image, reveals that the FWHM of a Gaussian fit to our target source is ∼ 30% greater than those of other sources with similar magnitudes. This fact points to an extragalactic nature for this source. It is worth to mention that this source is the only one in our sample present in the second incremental release of the Two Micron All Sky Survey (2MASS) (Cutri et al. 2000). Its position, obtained from images taken in October 1997, is there listed as α = 6 h 21 m 47.749 s ± 0.009 s , δ = +17 ◦ 47 ′ 35.07 ′′ ± 0.13 ′′ , in very good agreement with our obtained coordinates. Finally, we would like to point out that Motch et al. (1998) list 3 stars within the X-ray error box as possible counterparts to the X-ray source. Our proposed optical counterpart on the basis of the radio position, would rule out any of these 3 stars as the optical counterpart of this X-ray source. Overall, it looks like an extragalactic object because of its optical extension. 1RXS J072259.5−073131. The radio counterpart presents an extended struc- ture towards the east, marginally present in the 3.6 and 6 cm maps, and visible as a one-sided jet in the 30 July 4 minute snapshot observation at 20 cm wavelength (see Fig. 5.3). Although the one-sided morphology is suggestive of an extragalactic nature for this source, specially if we take into account that we are mapping the source at arcsecond scales, we cannot rule out a possible galactic nature on the basis of the detected morphology. The source also shows a moderate degree of variability, displaying most of time a negative spectral index but also close to 0 in the July 22 observation. As can be seen in Fig. 5.2, the non-simultaneous NVSS flux density
5.5. Conclusions 145 measurement at 20 cm wavelength could be compatible with a non-thermal optically thin spectrum. However, simultaneous observations during July 30 show a flattening of the spectrum at higher wavelengths, which could be due to self-absorption in this energy range. The difference between the NVSS and our flux density measurement at 20 cm wavelength could be due to intrinsic variability and/or to the different VLA configurations used. Anyway, non-thermal synchrotron radiation remains as the most plausible interpretation for the radio emission of this source, specially at higher frequencies. The optical counterpart appears as a point-like object with the magnitudes of an absorbed object at low galactic latitudes. These properties are in good agreement with the expected ones for a microquasar, although the one-sided radio jet at arcsecond scales is most common in extragalactic objects. 1RXS J072418.3−071508. This source has been recently (March 2002) classified as a quasar in the SIMBAD database, and it is not any more a REXB candidate. It is listed as PMN J0724−0715 in the NED database, and is the source WGA J0724.3−0715 in Perlman et al. (1998), who reported a faint and quite broad Hα emission line (rest-frame Wλ = 30.3 ˚A, FWHM=4000 km s −1 ), and classified it as a flat spectrum radio quasar with z = 0.270. Nevertheless, we have reported here our observational results for this source, since it was a candidate when we performed the observations. In the radio it displays a low degree of variability of 3% at 3.6 cm and 4% at 6 cm, with a flat or even inverted spectrum. In the optical it appears as a complex double object, with a NW and a SE components, being the first one the counterpart to the radio source. Magnitudes in V , R and I typical of absorbed objects at low galactic latitudes were found. On the other hand, it appears catalogued with an X-ray extension of 40 ′′ in the RBSC. 5.5 Conclusions After the cross-identification of catalogs presented in Chapt. 4, we found 8 interesting sources to be studied (Group 1 sources). Here we have reported observations of all these sources except 1RXS J181119.4−275939 and 1RXS J185002.8−075833, that were not visible during our observations. We have studied the remaining 6 radio sources of Group 1, and found optical counterparts to all of them. We have obtained accurate positions at both radio and optical wavelengths, perfectly com-
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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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iv CONTENTS II A search for new mic
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vi CONTENTS
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viii Resum de la tesi d’Eddington
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x Resum de la tesi imprescindible p
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xii Resum de la tesi dels sistemes
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xiv Resum de la tesi un comportamen
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xvi Resum de la tesi MilliARC SEC 4
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xviii Resum de la tesi per poder de
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xx Resum de la tesi valors força s
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xxii Resum de la tesi 2. D’entre
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xxiv Resum de la tesi pano Alemán
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xxvi Resum de la tesi 3.3 Observaci
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xxviii Resum de la tesi habitual en
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xxxii BIBLIOGRAFIA
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2 Chapter 1. Introduction and backg
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10 Chapter 1. Introduction and back
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28 BIBLIOGRAPHY Lewin, W. H. G., va
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30 BIBLIOGRAPHY
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Chapter 2 Multiwavelength approach
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2.3. An interesting target in the s
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2.4. The radio counterpart: NVSS J1
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2.5. The optical/IR star: LS 5039 5
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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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