Gamma Ray-Burst

Discovered at the end of the 60s, Gamma-Ray Bursts (GRB) are still one of the most challenging puzzles for modern astrophysics. These phenomena are sudden huge flashes of hard X- (or soft gamma-)  rays, with durations ranging from a fraction of a second up to several hundreds of seconds and coming from random (and always different) directions in the sky. Despite their intensity over-shines that of all the other celestial sources put together and their rate of about 1 per day, the origin of GRBs remained completely obscure until the middle 90s. Indeed, at that epoch there was still a lively debate about the possibilities that they come from  a Galactic halo, from cosmological distances or even from the Oort cloud surrounding our solar system. In 1997, thanks to the measurements performed by the Italian-Dutch satellite for X-ray Astronomy, BeppoSAX,  based on instrumentation developed and operated by researchers at IASF –Bologna, it was possible to localize with unprecedented accuracy some GRBs, leading to the discovery of their optical counterparts and host galaxies and, eventually, to the determination of their cosmological distance scale  (from hundreds of millions up to several billions of light years), of their huge luminosity (which can be higher than a  billion of billions that of the Sun) and the connection of a fraction of at lest a fraction of these events with peculiar type of Supernovae (SNe). In addition to the revolutionary results by BeppoSAX, the researchers of IASF Bologna provided, and are still providing, a relevant contribution to the solution of this big mystery for modern science with research activities including systematic data analysis of X/gamma-ray data from present satellites, optical robotic and target-of-opportunity observations,  investigation of cosmological  use of GRBs, study of future GRB experiments.

The sky at high energies

Top figure: sources detected by INTEGRAL/IBIS and Swift/BAT in the band 20-100 keV, superimposed on the optical sky (date right requested, pending ok).
Bottom figure: zoomed view of a portion of the Galactic plane (see above rectangle) as seen by INTEGRAL.

Explosions of unthinkable dimensions occur at cosmological distances.
They release large amounts of energy, mostly through electromagnetic radiation which is invisible to the naked eye. By detecting such radiation, gamma-ray missions give us a view of the universe which is very different from that obtained by naked eyes. The gamma-rays are the most energetic form of electromagnetic radiation.
They carry large amounts of energy produced by the most catastrophic astronomical events, such as explosion of stars, collision of neutron stars, material accreted on black holes.
Moreover, the gamma rays are produced by relativistic particles trapped in magnetic fields trillions of times stronger
than the magnetic field of our planet earth. Many researchers working at the institute IASF-Bologna have significantly contributed to the exploration of the high energy sky. Their recent contribution is mainly based on observations performed by the INTEGRAL satellite which is the most sensitive gamma-ray observatory ever launched.
INTEGRAL is an ESA mission launched on October 2002. The research at the institute IASF Bologna is mainly focused on the study of supermassive black holes and binary systems hosting exotic compact objects (neutron stars, white dwarfs, stellar mass black holes) as well as on the discovery of new classes of gamma-ray sources never seen before.

AGN and supermassive black holes

The active galaxy Centaurus A, at different wavelengths (X-rays, Optical, and radio).

An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy.

Several active galaxies produce winds and/or jets of material that can reach relativistic speed and transport energy into the galaxy’s interstellar medium and/or cluster’s intergalactic medium.

The radiation from AGN is believed to be the result of accretion of mass by the supermassive black hole at the centre of the host galaxy. AGN are the most luminous persistent sources of radiation in the universe. As such, they can be used as a means of discovering distant objects, and their evolution as a function of cosmic time provides constraints on models of formation and evolution of galaxies.

Researchers at IASF-Bologna are studying AGN in an attempt to understand how accretion and ejection mechanisms do work, and how these impact the formation and

evolution of galaxies in the nearby and more distant universe.

Black holes

A black hole is a region of spacetime from which gravity prevents anything, including light, from escaping. This is why they are called “black”. Despite their name, black holes are known to astronomers as the brightest persistent sources in the universe. In fact, black holes can be inferred by their interaction with matter near black hole (but not within it). In particular, matter falling onto a black hole can form an accretion disk heated by friction up to great (million degrees) temperature, thereby emitting intense UV and X-rays. Part of this material is often also ejected out of the host galaxy itself in the form of winds and relativistic jets. In institutes like IASF-Bologna, astronomers design and build space telescopes sensitive to X-rays and gamma-rays, to study enigmatic objects such as black holes.

Galactic Compact Objects

Multiple stellar systems, that is two or more stars orbiting each other, are quite common in the Universe. Indeed, our “lonely” Sun is quite the exception rather than the rule. Because the more common multiple systems are formed by two stars, these are called binary systems. When one of the two stars exhausts its fuel (it does finish, sooner or later!) the gravitational pull is not balanced anymore by the radiation pressure, and the star implodes. As a result we have a very dense object, the density and dimension of which depend on the progenitor mass. The greater the progenitor mass, the denser the resulting object. The resulting density is so large (of the order of the nuclear matter and higher) that we call these object “compact object”.

It is easily understandable that binary systems hosting a compact object will give rise to “exotic” phenomena. This is due to the presence of super-strong magnetic and gravitational fields. For example, the gravitational pull of the compact object is so intense (billions of times than that on the Earth) that matter is snatched from the companion star.

Then a stream of matter forms, and this matter is channeled by the intense magnetic field onto the compact object surface, producing emission in the X-ray band.

Because X-ray radiation cannot (luckily!) reach the Earth surface, in order to detect it it is necessary to bring the instrumentation above our atmosphere. This is the reason why it is the space astrophysics that handles the study of these objects. Our research group analyses and interprets data coming from X-ray detectors aboard satellites, and complements them with observations performed at other wavelengths (optical, IR, UV, radio, etc). The final goal is to describe the physical condition and the processes that take place in these systems.

Terrestrial Gamma-ray Flashes (TGF)

Terrestrial Gamma-ray Flashes (TGF) are violent and very brief bursts of high-energy radiation (mainly X and gamma-rays, but also electrons) originated by intense electric fields in thunderstorms. AGILE is one of the three-only currently active satellites capable to detect this phenomenon from space, thanks to the Mini-Calorimeter instrument realized at IASF Bologna and originally designed to detect gamma-ray bursts of cosmic origin. Thanks to its peculiar characteristics, AGILE is giving significant contributions to the study of this mysterious phenomenon, specifically concerning its emission at the highest energies, demonstrating that science is always ready to undertake unexpected directions and that an instrument conceived to study the cosmos can indeed give important contributions to the study of our planet as well.