Universitat de - Departament d'Astronomia i Meteorologia ...

16 Chapter 1. Introduction and background
This ratio is always greater than 1, implying that we will detect the approaching
component brighter than the receding one (e.g., see Fig. 1.5). Solving for β cos θ in
Eq. 1.18 we obtain:
β cos θ = (Sa/Sr) 1/(k−α) − 1
(Sa/Sr) 1/(k−α) + 1
. (1.19)
As we have seen, at any given time da > dr. Therefore, if we only have a single
image of an ejection event, the receding component will be closer to the core than
the approaching one. Since the flux density decreases with increasing distance from
the core (due to adiabatic losses, and assuming no interaction with the interstellar
medium), the measured ratio Sa/Sr will be lower than the one that should be used
in the equation above, which is only valid at equal distances from the core. As a
consequence, in such a case Eq. 1.19 only allows us to obtain a lower limit for β cos θ.
Finally, it may also happen that we have a single image where only the approach-
ing component is detected, either because the relativistic deboosting of the receding
one is very strong or because of lack of sensitivity. In any case, we can use the fact
that we do not detect the counter-jet, and replace Sr with the 3σ level of the image
in Eq. 1.19. This will only provide a lower limit to β cos θ, expressed as
β cos θ > (Sa/3σ) 1/(k−α) − 1
(Sa/3σ) 1/(k−α) + 1
1.2.2 Quasars and microquasars
. (1.20)
After the introduction on accretion and the former explanation about special rel-
ativity effects, we can comment on some specific differences between quasars and
microquasars.
In the distant quasars radio emission was detected in the early times of radioas-
tronomy, because the jets in the detected quasars have small angles with respect to
the line of sight, and the flux densities of the approaching components are signifi-
cantly Doppler boosted. Hence, these objects appear in the sky as very bright radio
sources, showing one-sided jets in high resolution radio interferometric observations,
which usually reveal superluminal motions. The probability of having a quasar with
a jet pointing close to our line of sight is small, but provided enough number of
quasars, these objects are easy to detect, because their emission is persistent. As-
tronomers searched for the optical counterparts, and found objects that looked like