Search

MAXI J1820+070 is a low-mass X-ray binary with a black hole (BH) as a compact object. This binary underwent an exceptionally bright X-ray outburst from 2018 March to October, showing evidence of a non-thermal particle population through its radio emission during this whole period. The combined results of 59.5 h of observations of the MAXI J1820+070 outburst with the H.E.S.S., MAGIC and VERITAS experiments at energies above 200 GeV are presented, together with Fermi-LAT data between 0.1 and 500 GeV, and multiwavelength observations from radio to X-rays. Gamma-ray emission is not detected from MAXI J1820+070, but the obtained upper limits and the multiwavelength data allow us to put meaningful constraints on the source properties under reasonable assumptions regarding the non-thermal particle population and the jet synchrotron spectrum. In particular, it is possible to show that, if a high-energy (HE) gamma-ray emitting region is present during the hard state of the source, its predicted flux should be at most a factor of 20 below the obtained Fermi-LAT upper limits, and closer to them for magnetic fields significantly below equipartition. During the state transitions, under the plausible assumption that electrons are accelerated up to ~500 GeV, the multiwavelength data and the gamma-ray upper limits lead consistently to the conclusion that a potential HE and very-HE gamma-ray emitting region should be located at a distance from the BH ranging between 1011 and 1013 cm. Similar outbursts from low-mass X-ray binaries might be detectable in the near future with upcoming instruments such as CTA., Peer Reviewed, Postprint (author's final draft)

In this work we analyse the features of the compressible subsonic turbulence which is produced in a controlled small volume. The goal is twofold. First, implementing a turbulent routine in the hydrodynamic code which is routinely used by the GAA (Astronomy and Astrophysics group, Barcelona, UPC) to perform simulations of stellar explosions. Second, analysing the quality of the results as a function of the different code parameters (resolution, viscosity, volume elements etc.) using standard techniques of signal analysis such as the Fourier Transform. Once the turbulent routine has been coupled to the hydrodynamic code, our main results match with those obtained by other investigators. According to our results, the most critical parameter governing the quality of the results seems the achieved resolution but sensitivities to other parameters are also explored.

The motivation of this thesis is to monitor the data of the 2018 outburst of the black hole candidate MAXI J1820+070. This project combines theoretical background about X-ray binaries with the experimental work done in the process of achieving optical magnitudes using a CCD camera: Telescopi Joan Oró ground based telescope. We present the optical light curves and colours of MAXI J1820+070 during the March-November 2018 outburst. After obtaining the magnitudes, using MAXI and Swift-UVOT as space observatories we extend our range to the X-ray band and study the different states and physical processes that occur in them. Finally, we give physical interpretation of all obtained data and characterize the spectral shape of the infrared, optical, UV and X-ray emission in the different components of MAXI J1820+070 outburst. We conclude that MAXI J1820+070 show the typical properties of a BHT: our results show the canonical BHT states, the disk irradiation changing in the soft state and the linear increase of the R-I colour indicating a cooling of the disk. We also give evidence that the optical emission in the soft state is bluer than in the hard.