Low mass X-ray
binaries
Low mass X-ray binaries (LMXB) are binary stellar systems where a black hole or a neutron star are accreting mass, through the formation of a disk, from a “donor” star. The observed electromagnetic emission from these objects spans ten orders of magnitudes in energy and is the result of several physical components which change their contribution in time.
Top panels show an artistic sketch of a black hole X-ray binary. While X-ray (grey) and radio (blue) emitting regions are well defined (inner accretion flow and outer jet respectively), the O-IR emission could originate from both outer regions of the accretion inflow or the base of the jet.
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Lower panels show the typical spectral energy distribution expected for both jet and accretion inflow. The plots are adapted plots from Corbel & Fender 2002 (ApJ 573,1,L35), Migliari+2010 (ApJ 710, 1, 117) and Veledina+2013 (MNRAS 430 ,4, p.3196)
One of the most fascinating properties is the presence of collimated jets which are launched from the center of the system. Astrophysical jets are powerful streams of gas and energy which are often seen form when matter is falling onto compact stars with extreme gravity, such as neutron stars or black holes.
Due to the energy that they carry away, these “cosmic fountains” have a devastating impact on their nearby environment. As with our everyday garden fountains, the shape of the nozzle and the velocity of the fluid at the base of the stream determines the overall behaviour of the jet. However, due to the lack of observational constraints on the inner regions of astrophysical jets, we still do not fully comprehend how they are launched or how they are actually powered.
ACCRETING BLACK HOLES
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The aim of my research is to use strictly simultaneous, high time resolution observations in X-rays and at optical-infrared (O-IR) wavelengths to try to constrain the geometry and the physical properties of these powerful outflows. To do this I use some of the largest and most advanced ground and space telescopes available such as the Very Large Telescope in Chile, the XMM-Newton observatory or NICER
The study of the fast variability and lags between the X-ray (from the inflow) and O-IR emission (from the base of the jet) has led in the last years to the most stringent constrains on the physical parameters of jets, such as speed, height and launching radius. Furthermore novel approach permitted the development of new jet models which can reproduce the variability from these powerful outflows.
Left panel, view of the Very large telescope in Chile. Right panel, Artistic impression of XMM-Newton.
Artistic animation of the new physical scenario drawn thanks to the use of simultaneous high time resolution X-rays and O-IR observations. The mass fluctuations travel from the inflow (X-rays) to the jet (O-IR) in 0.1 s. The animation shows how the light pattern seen in the inner accretion flow is then repeated at the base of the jet with a delay.
ACCRETING NEUTRON STARS
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Years of studies have shown that also accreting neutron stars show common X-ray and radio phenomenology. Yet it is still not clear how the presence of a boundary layer or a strong magnetic fields affects the disc-jet coupling in these systems.
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Given this I have conducted the sub-second IR observations of these objects, leading to the discovery of non-thermal sub-second variability also in accreting neutron stars. This opens a new window in the study accretion flow and jets, as it permits to directly compare the properties of black holes and neutron stars.
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An example of the power of this method is shown in my Nature publication "A shared accretion instability for black holes and neutron stars". Thanks an impressive multi-wavelength campaign with 6 (both ground-based and space) observatories collecting high time resolution data simultaneously, we discovered for the first time the present of a rare accretion instability in a neutron star. This mechanism, although is a fundamental prediction of accretion disc theory, is still not fully understood, as it is very elusive and has been previously observed only in very bright black holes.
Artistic animation of the accretion instability. The disc gets cyclically depleted and rebuilt on short timescales, powering also strong jets. Neutron stars show a limit cycle that appears to be slower