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

8 Chapter 1. Introduction and background
Table 1.2: Types and location of catalogued LMXBs.
Location Total XPs Bursters Z sources Atoll sources Transients REXBs
SMC 0 0 0 0 0 0 0
LMC 2 0 0 0 0 0 0
Galaxy 147 5 63 6 18 75 35
Total 149 5 63 6 18 75 35
including 5 X-ray transients. Approximately half of LMXBs are transients. REXBs
will be discussed in Sect. 1.1.3.
The galactic latitude distribution of the 147 galactic LMXBs has a mean of +0.4 ◦ ,
and a standard deviation of 12.2 ◦ , to be compared with the 4.6 ◦ value obtained
for HMXBs. It is clear the higher scatter in galactic latitude, compatible with
Population II sources. In fact, 13 LMXBs are located in globular clusters.
1.1.3 Radio emitting X-ray binaries
The first X-ray binary known to display radio emission was Sco X-1 in the late 1960’s.
Since then, many X-ray binaries have been detected at radio wavelengths with flux
densities ≥ 0.1–1 mJy. Moreover, the flux densities detected at cm radio wavelengths
are produced in small angular scales, and cannot be produced by thermal emission
mechanisms.
In this context, the most efficient known mechanism for production of intense
radio emission from astronomical sources is the synchrotron emission mechanism, in
which highly relativistic electrons interacting with magnetic fields produce intense
radio emission which tends to be linearly polarized. The observed radio emission can
be explained by assuming a spatial distribution of non-thermal relativistic electrons,
usually with a power-law energy distribution, interacting with magnetic fields.
Since some REXBs, like SS 433, were found to display elongated or jet-like
features, like in AGNs and quasars, it was proposed that flows of relativistic elec-
trons were ejected perpendicular to the accretion disk, and were responsible for
synchrotron radio emission in the presence of a magnetic field. Several models have
been proposed for the formation and collimation of the jets, including the presence