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High-energy astrophysics has undergone a significant advance in recent years, mainly due to the data obtained by modern X-ray and gamma-ray observatories, revealing a variable and violent firmament at these high frequencies. Many of the celestial sources that emit the most energetic photons detected so far remain unidentified. Therefore, unveiling their nature and origin is today one of the frontiers of modern astrophysics.

The research group Fuentes de Alta Energía en la Galaxia (High-Energy Sources in the Galaxy) works within the field of multiwavelength astronomy, through observations of high-energy sources at radio, optical, infrared, and X-ray wavelengths. Thanks to the use of all these techniques, a global comprehension of the physical phenomena that take place in these celestial objects can be achieved (accretion disks around black holes and neutron stars, plasma ejections at relativistic velocities, synchrotron radiation, inverse Compton scattering, etc).

The direction toward the high-energy source cannot be accurately determined in most cases, which makes it difficult to identify the origin of the emission. Positional uncertainty ranges from several arcminutes to ~1°, depending on the instrument (for comparison purposes, the Moon has an apparent angular size of 30 arcminutes). Hence, observations at longer wavelengths are required, since the positional errors at them are tipically less than one arcsecond. If one or several sources are detected within the high-energy source error box in a different region of the electromagnetic spectrum, and if they also emit in a peculiar and distinctive way, they are immediately proposed to be counterpart candidates at lower energies. For instance, the presence of a clearly variable non-thermal radio source within the positional error box of the high-energy source is a sign that points to its accurate position. Once the counterpart at lower energies is identified, photometric and spectral analyses give us information about its physical nature and allow us to constrain the possible scenarios for the high-energy emission mechanism.

We obtain our observational data by means of a very wide range of instruments which are available to the astronomical community after the observation proposal is approved by the corresponding scientific committee. Some of the observatories that we used frequently are the radiointerferometer Very  Large Array (New Mexico, USA) and the optical/infrared telescopes located at the Calar Alto Astronomical Observatory (Almería, Spain) and at the Roque de los Muchachos Astronomical Observatory (La Palma, Canary Islands, Spain). In addition, our work is also based on the use and exploitation of archival data previously obtained by other researchers that are available to the community in order to be reanalysed with an aim different from that which motivated the observation in the first place. The archival databases of the European Southern Observatory (ESO), the National Radio Astronomy Observatory (NRAO), and the XMM-Newton X-ray observatory have been really useful in our research.