In few words
My research interests covers several aspects of high energy Astrophysics, including:
- Origin of cosmic rays
- Particle acceleration at shocks
- Supernova remnants
- Balmer dominated shocks
- Low energy cosmic rays and star formation
- Bow shock pulsar wind nebulae
- Galactic winds
Origin of cosmic rays
The origin of cosmic rays (CR) is one of the great mistery of Astrophysics since their discovery, dated back to the 1914 by Victor Hess. After more than one century of active research, many aspects of CR physics still remain wrapped in the mists. I have been mostly attracted by the study of the Galactic component of CRs. In the most accepted scenario, the bulk of Galactic CRs are thought to be accelerate at collisionless shock produced by supernova remnants (SNR) expanding in the circumstellar medium. This paradigm has received much attention in recent years, but, in spite of big theoretical and experimental efforts, many aspects of this paradigm are still obscure and require further investigation.
The extragalactic component of CRs is even a deeper mistery. If one esclude top-down scenarios (requiring physics models beyond the Standard Model), extragalactic CRs can only be accelerated in very powerful sources like Active Galactic Nuclei (AGN) or Gamma Ray Bursts (GRB). At the moment neither the sources not the acceleration mechanism has been established jet.
CRs are also important because they play an active role in many processes ongoing in the Galaxy. The CR energy density measured close to Earth is of the order of 0.5 eV/cm^3. Noticeably the energy density of both the interstellar gas and the magnetic field are of the same order, implying that CRs can play an active role in the Galaxy evolution.
Unlike other areas of astrophysics, cosmic rays do not occupy a relevant role in mass culture, therefore remaining mainly a field for experts. Perhaps the only notable exception is constituted by the American comic book "Fantastic 4" created by Stan Lee and Jack Kirby in the sixties. In this novel a storm of cosmic rays hit four astronauts during a space mission and the subsequent mutations turn them into superheroes.
Particle acceleration at shocks
Shock waves arise when supersonic flows of plasma are faced with an obstacle, such as a planet or a star with a magnetic field, or when they encounter a slower moving flow. Because such kind of collisions are extremely frequent in the Universe, both in the Galactic and in the extragalactic context, the study of collisionless shock has an important role in the understanding of several astrophysical phenomena. The composition below shows a number of diverse astronomical sources where shocks have been detected.
Collisionless shock in magnetized plasma are believed to be one of the principal responsable for the production of non thermal particle population in the Universe by the so called Diffusive Shock Acceleration, a mechanism able to transfer energy from the bulk macroscopic motion of a plasma towards single charged particles.
Today Supernova Remnants (SNR) are thought to be the main sources of Galactic CRs. This idea is dated back to the early 30s, when Baade & Zwicky (1934) proposed that CRs were associated to Supernova (SN) based on an energetic argument: the energy density of CRs (about 3×10^40 erg/s) could easily be supplied by SN if roughly 10% of the explosion energy were turned into accelerated particles (this is assuming an energy release of 10^51 erg per SN event and a rate of about 1 explosion every 100 years in the Galaxy). A quantitative proposal for how this energy conversion would take place had to wait until the late 70s when a number of scientists independently suggested that the process of diffusive shock acceleration taking place at the blast wave launched by an SN explosion could provide the required conversion mechanism. Today this proposal has received many theoretical and observational confirmation. The main observational evidence is the presence of non thermal emission associated with the blast waves of SNRs, which ranges from the Radio up to high energy γ-rays.
The composite image on the right shows the SNR of Tycho observed in different wavelengths, from Radio to γ-rays.
In general, while the Radio and the X-ray emission clearly reveal the presence of electrons accelerated at least up to TeV energies, the origin of the γ-ray emission is still under debate as it could results either from leptonic or from hadronic processes. The present situation is such that both mechanisms seem to play a role, the relative importance depending on the ambient conditions of each single remnant (density of interstellar medium, strength and orientation of magnetic field, neutral fraction, etc.).
The difficulties to discriminate between different scenarios can be overcome using a multiwavelength study, where the emission at all wavelength are simultaneously taken into account in a single model. As an exemple, the plot on the right shows a model for the non thermal radiation of the Tycho's SNR where all non thermal radiation is considered. Such study reveals that the gamma ray emission is probably due to the decay of pion produced in hadronic collision and that the efficency of shock acceleration is around 10%, the exact amount predicted by the SNR paradigm [see Morlino & Caprioli (2014)].
Balmer dominated shocks
One of the possible diagnostic to study the physics of collisionless shock in astrophysical environments is through optical Balmer lines. Balmer dominated shocks are defined as shocks emitting strong Halpha lines. They are usually associated with supernova remnants shocks moving at speed above few hundreds km/s, but they have been detected also in association with pulsar wind nebulae. The mechanism responsible for the emission of such lines is the excitation of Hydrogen atoms from the interstellar medium by hot protons and electrons behind the shock.
The image on the right shows a small portion of the shock from the remnant SN 1006 observed in Hα with the Hubble Space Telescope. The high resolution of optical images allows one to study shocks with great spatial details, a possibility which cannot be achieved using other wavelengths, especially γ-rays.
The profile of Balmer lines carries several peaces of information on the physical conditions at the shock: temperature of electrons and protons, fraction of neutral particles, size of the shock layer, presence of a shock precursor, etc. All these information can be used to constrain the shock physics and the mechanism of particle acceleration. In particular, if CRs are efficiently accelerated, the shock structure is modified in several aspects (length of precursor, temperature of protons) and, as a consequence, the profile of the optical lines will be altered. Looking for such anomalous line profiles can help to understand to which efficiency shocks are able to accelerate non thermal particles and up to what energies.
Low energy cosmic rays and star formation
Cosmic rays are the only agent able to penetrate deep inside molecular clouds, because UV and X-ray photons are efficiently absorped by the external layer of the clouds. As a consequence, the amount of penetration of cosmic rays into molecular clouds regulates the ionization level of clouds and dense cores and thus affects their dynamical evolution and the process of star formation. In fact, inside the deep cores, the level of ionization determines the coupling between the gas and the magnetic field: the strongest is the coupling, the more difficult is the collapse of cores needet to form the proto-stars. For these reasons I am interested to investigate the proprerties of CR propagation close and inside clouds. The propagation regime of CRs inside a cloud is alo important understand the gamma-ray emission from clouds, which results from the decay of neutral pions produced in inelastic interactions of CRs in the dense gas.
Bow shock pulsar wind nebulae
Bow shock pulsar wind nebulae are formed when a pulsar escapes the SNR envelope and propagates through the cold interstellar medium. The highly supersonic motion of the pulsar produces a bow shock that sometime can be detected in Balmer lines. The picture on the right shows four bow shock nebulae observed in Hα. One can clearly see that the tale of the nebulae show a rapid transverse expansions behind the head of the nebula. Sometimes the expantion seems to have a periodic structures which reminds the development of some internal flow instabilities. Such structures could result from the interaction between the light relativistic wind produced by the pulsar and the neutral hydrogen able to penetrate inside the wind. Studying these objects can contribute to understand the dynamics of relativistic outflow, the properties of the pulsar as well as the properties of the interstellar medium where the pulsars propagate.
The possibility of a galaxy to launch winds has attracted attention for many different reasons. For instance, star formation is regulated by the amount of gas available, and winds affect the availability of such gas. In fact, galactic models that do not include feedback processes overpredict the amount of baryons and the star formation rate. Winds also pollute galactic halos with hot dilute plasma that may provide an important contribution to the number of baryons in the Universe, helping to solve the '''missing baryon problem'' at redshift z=0. Such gas might in fact have already been detected around the Milky Way in the form of a X-ray emitting plasma with temperature of several million degrees and through the absorption lines OVII and OVIII of X-ray emission from distant quasars. The hot gas around the Milky Way seems to extend up to ~100 kpc away from the Galactic plane with a total mass estimated to be ~10^10 Msol.
The figure on the right shows the magnetic field orientation of the galaxy NGC 4631, where the poloidal component of the field might suggest the presence of a galactic wind.
Galactic winds may be thermally-driven, namely powered by core-collapse SNe or momentum-driven, powered by starburst radiation. These two mechanisms of wind launching are thought to be at work in starburst galaxies and galaxies with active nuclei. On the other hand in a galaxy like the Milky Way, winds are unlikely to be due to such processes because thermal and radiation pressure gradients are expected to be too small. Remarkably, CRs can play an important role in launching winds because of the gradient that their pressure develops as a consequence of the gradual escape of CRs from the Galaxy. Moreover, if a wind is developed, it can affect the transport of cosmic rays in the Galaxy, by advecting them away from their sources.
The mutual feedback between CRs and galactic wind implies that one can develop possible diagnostics of the existence of winds in our own Galaxy, by directly looking at the features of the CR spectrum observed at the Earth.