The sources of our Galaxy

This part regroups all the celestial objects that emits gamma rays that belongs to our galaxy, the Milky Way.

Our galaxy looked at in gamma rays

One of the biggest advantage when analyzing the sky using gamma rays is that regions generally obscured become directly observable. An example can be found looking at the center of the Galaxy. This region is deeply covered by interstellar dust at optical wavelengths, but the many sources that otherwise are hidden, become visible and bright at gamma rays. One of these sources is the massive black hole SgrA*, located at the very center of the Milky Way.

The image on the right is an animation combining optical and gamma-ray images of the central part of the Milky Way. The deep INTEGRAL IBIS/ISGRI hard X-ray image of the galactic bulge region has been colored in pink and superimposed onto the optical image of the same region. Credits: INTEGRAL: ISDC/A. Bodaghee/R. Walter; Optical: A. Mellinger.

At optical and infrared wavelengths, the stars, the diffuse gas and the dust dominate the emission in the Milky Way, although they are invisible at gamma rays. Most of the sources analyzed by INTEGRAL are neutron stars and black holes, end-products left after the supernova explosion of a huge star. Our galaxy must contain around one million black holes but only a few tens or so can be observed at a given time.

The image on the left gives a summary of our best current understanding of the evolution of stars, showing their birth, middle age and eventual demise. The lowest mass stars are shown at the bottom and the highest mass stars at the top. The latter end their life as supernova explosions often leaving behind a compact core that is either a black hole or a neutron star. Credits: NASA/CXC/M.Weiss.

The first source discovered by INTEGRAL

On 29 January 2003, INTEGRAL discovered the first member of a new class of black hole and neutron star X-ray binaries exhibiting a very high absorption, IGR J16318-4848. In this source, the inner neutron star or black hole is surrounded by a cocoon of extremely dense gas and dust which only let energetic X-rays and gamma rays passing through it and absorb all the other lower energy emissions. The size of the cocoon is comparable to the orbit of the Earth around the Sun. Due to the exceptional high absorption, IGR J16318-4848 and the other sources of the same class were mostly gone undetected with previous observatories and INTEGRAL was the first instrument revealing them.

The image on the right is an artist's impression of a strongly absorbed X-ray binary system. The supermassive companion star (on the right-hand side) ejects a lot of gas in the form of stellar wind. The compact black hole orbits the star and, due to its strong gravitational attraction, collects a lot of the gas. Some of it is funnelled and accelerated into a hot disc. This releases a large amount of energy in all spectral bands, from gamma rays through to visible and infrared. However, the remaining gas surrounding the black hole forms a thick cloud which blocks most of the radiation. Credits: ESA

Shortly after the discovery, IGR J16318-4848 was observed by other X-ray satellites (XMM-Newton, Chandra and Suzaku), as well as by optical and infrared telescopes. These combined observations showed that the companion star of IGR J16318-4848 is most probably a blue super-giant star that features a very dense stellar wind. The peculiar high absorption of the source is thought to be mainly related to such a strong wind.

The image on the left shows an artist's impression of a supergiant fast X-ray transient (SFXT), a rare class of binaries composed of a supergiant luminous star (in blue) and a compact object, such as a neutron star or a black hole. The curve at the bottom right shows how the intensity of the X-rays recorded by INTEGRAL changed within only about 2 hours on 17 September 2003 in one of these sources, called IGR J17544-2619. Credits: ESA

After IGR J16318-4848, INTEGRAL has discovered tens of similar sources that were then collectively termed "highly obscured high mass X-ray binaries". INTEGRAL has also discovered many sources where the companion star is a supergiant star and studied fast X-ray outbursts lasting only a couple of hours from such peculiar binary systems refered to as supergiant fast X-ray transients (SFXT).

The black hole Cygnus X-1

The source Cygnus X-1 (also abbreviated Cyg X-1) was chosen as a target for the first observation made by INTEGRAL after its launch, and is still often observed. Its name indicates that it is the first source of X-rays discovered in the Cygnus constellation, and indeed is the strongest X-ray emitting source from this region of the Milky Way as seen from Earth. It has been discovered already in 1964, and was the first source widely accepted to be a black-hole candidate.

The image on the right is a collection of images obtained by the four instruments on board INTEGRAL, superimposed on an artists impression of the system X-1. The artists impression represents the material being ripped from the giant blue star (left) spiraling into a disc around the black hole (right). The images from INTEGRAL are from left to right: the one obtained with the gamma ray imager (IBIS), the one from the Gamma spectrometer (SPI), from the optical camera (OMC) and the one obtained with the X-ray monitors (JEM-X). All of these tools observe simultaneously the same region in the sky to allow a detailed follow up of the spectral variations of the observed source. Copyright ESA. Illustration by the Integral team and ESA/ECF

Cygnus X-1 is inside the Milky Way, at a distance of approximately 2000 parsec (6000 light years) away from Earth. The source is a binary system, with a giant blue star 33 times heavier than the Sun, orbiting around an extremely dense and compact object, 15 times more massive than our Sun. Since a black hole captures all the matter and even the light in its vicinity, by definition it cannot be directly seen and identified, but the mass of the compact object in CygX-1 is largely too big to be a neutron star (neutron stars can only reach masses about 3 times bigger than the Sun).

The image on the left shows the region of the Milky Way where Cygnus X-1 is located. It only represents a very small part of the Cygnus constellation which is easily visible from Europe on a summer night. In this visible light picture the giant blue star forming the Cyg X-1 binary system together with the black hole is indicated by an arrow. Copyright ESA & Digitized sky survey. Image processing by ESA/ECF

The star and the black hole form a very close bond, with the star cycling around the black hole in only 5.6 days. The very strong gravitational field of the black hole strip away some of the star's material, that eventually form a disc around the compact object. The matter inside the disc cycles the faster the closer it gets to the central object, and finally falls inside it. Inside the disc, in the region close to the black hole, the physical conditions of the gas are extreme. The gas is heated by friction up to temperatures of millions of degrees, under these conditions the electrons are pulled out of the atoms, so that the whole gas is ionized and becomes a plasma. Before to fall into the black hole, the fast rotation of this electrically charged matter cause the formation of a huge electromagnetic field, together with the emission of X- and gamma-rays in two jets perpendicular to the disc. The gamma rays detected by INTEGRAL therefore tell us about what is happening in the extremely violent world close to the black hole, this violence being observed as rapid variations of the intensity of the light coming from Cygnus X-1.

The microquasar GRS 1915+105

One of the most fascinating sources of our galaxy is certainly the microquasar GRS 1915+105. It consists of a normal star with a mass comparable to that of our Sun that turns, in about a month, around a black hole ten times more massive than it.

The two celestial beings are very close. The gravitational pull from the black hole deforms the star in such a way that part of its atmosphere is attracted toward the black hole and form a disc. At the centre of this disc, part of the material is ejected through a pair of jets, which can be observed with the help of the modern radio telescopes, and part is indulged by the black hole and gives rise to X and gamma rays.

The image on the left is from an artists impression of the system. Created with BinSim by Robert Hynes. The central image was taken by the imager IBIS aboard INTEGRAL. Four detected sources are labeled. Credits: J. Rodriguez (ISDC). The figure on the right illustrates the temporal variations of the microquasar observed in the X-rays by INTEGRAL in October 2005. Credits: J. Rodriguez (CEA Saclay).

The astrophysicists associate the "dips" in the X-ray luminosity with the temporary disappearance of the internal regions in the disc that could have been swallowed by the black hole or ejected in the form of a jet. This scenario is also supported by the detection of the peaks in the source radio emission, interpreted as being due to the launch of material in the jet, occurring shortly after each X-ray dip.

The diffuse emissions from our Galaxy

The gamma-rays that are observed in the Milky Way not only can originate from distinct sources and stars, but can derive as well from the interstellar medium, a mixture of gas and dust grains that permeate the space between stars, and is often organized in clouds of various dimensions. The composition and distribution of the interstellar medium can be analyzed looking at its own emission, in analogy with what is done for stars. The study of this medium is particularly important to understand precisely our Galaxy.

Recent supernovae traced by Aluminum 26

The energy that is continuously released by stars is due to the burning of different chemical elements in the inner core of the stars themselves. When a star at the end of its life explodes, part of these elements are released in the space enriching the interstellar medium. The isotope Aluminum 26 (²⁶Al) is one of the chemical elements that can be found in the interstellar medium, and has the peculiarity of being radioactive. Nuclei of radioactive atoms spontaneously release protons and decay into other atoms, emitting light at a specific frequency, unique for each element.

The figure to the right is a shematic view of the radioactive decay of Aluminum 26 (Al) to Magnesium (Mg) as measured by INTEGRAL in gamma-rays. The background image of the Milky Way in visible light is overlaid with the Compton-GRO/COMPTEL false color map of Al 26 emission. The lower right inset shows the energy shift of the characteristic Al 26 emission line due to Galactic rotation. Credits: MPE Garching

In the case of the decay of Aluminum 26, gamma-ray photons are emitted at an energy of 1809 keV, with a lifetime of approximately one million years. All the signatures at 1809 keV therefore come from Aluminum 26 produced by stars less than a few millions of years ago, which is extremely recent compared to the age of our Galaxy and illustrates that the chemical elements are still in formation in the Milky Way. The observation of this emission in different regions of the Galaxy and the determination of the quantity of this element present today allows us to better understand the mechanics of nucleosynthesis which operates in these stars.

With INTEGRAL, astronomers have measured the amount of Aluminum 26 in the center of the Galaxy, and from this the rate of star formation and of supernova explosion in the Milky Way. The result is of typically two supernova explosions per century in the Milky Way (see ESA's Press Release).

Anti-electrons annihilation at the heart of the Milky Way

When an electron meets an anti-electron, also known as a positron, the two particles annihilate themselves and emit two photons at 511 keV. The detection of these photons is thus usually interpreted as a signature of the annihilation process.

The image on the right shows the glow of 511 keV gamma-rays at the center of our galaxy (scale is in degrees), as measured by INTEGRAL/SPI. This emission comes from the annihilation of 10 billion tons of anti-electrons per second at the center of our Galaxy.

The data available before the launch of INTEGRAL had revealed the presence of these photons at 511 keV at the centre of our Galaxy, but were unable to provide any other precise detail. The curiosity of astrophysicists was widened.

The observations of the Galactic center that INTEGRAL performed during the first few weeks of the mission already permitted to have a preliminary image of the repartition of the emission at 511 keV. This result was then greatly improved by the additional observations collected recently by INTEGRAL at this energy for most of the celestial sphere.

Beside confirming the presence of positrons at the centre of our Galaxy, INTEGRAL provided the very first hints to understand the origin of such particles in the middle of the interstellar space. The anti-matter could be produced by astrophysical sources like supernovae of type Ia and low-mass X-ray binaries. Many more exotic origins of the anti-matter signal observed by INTEGRAL have been proposed by physicists, in particular that they could be due to light dark matter, so the mystery remains.

Sources in other galaxies

INTEGRAL is also used to observe sources located outside the Milky Way, in other relatively close-by galaxies as well as at the far end of the known universe.

The quasar 3C 273

Till the 1950s many point-like radio-wave sources were lacking an optical counterpart identification, and were generally defined as quasi-stellar radio sources (i.e. "quasi-stars" that lead to "quasars"). It was only in the 60s that the idea that these could be extremely far objects was established. Indeed in 1962 one of the brightest objects of this class, the source 273 of the Third Cambridge Catalogue (3C 273), was measured receding at a velocity of 47,000 km/s, almost 16% of the speed of light, due to the expansion of the Universe and to its extreme distance - about 1000 million light-years - from us.

The image on the right compares images of the quasar 3C 273 taken in different ranges along the electromagnetic spectrum. It illustrates the difficulties of gamma-ray imaging with coded masks compared to techniques at radio (MERLIN), optical (Hubble) and X-ray (Chandra) wavelengths. INTEGRAL cannot resolve the jet of the quasar streching over about a million light-years, about 10 times the diameter of the Milky Way.

These objects are very energetic (emitting typically 10⁴⁰ Watt) and distant galaxies hosting a strongly interacting supermassive black hole in their center. These are the largest possible black-holes and typically have masses as huge as millions (or even billions) time the mass of the Sun. Around the black hole there is an accretion disk where the motion of the matter, before to fall into the black hole, gives rise to emissions in radio band, infrared rays, visible light, ultraviolet rays, X-ray and gamma rays.

The image on the left is from an INTEGRAL observation of the quasar 3C 273. It shows hard X-ray emission (at 20-40 keV) collected by the IBIS/ISGRI detector. The two other sources besides 3C 273 are nearby active galaxies also called Seyfert galaxies.

The bulk of the energy seems to be always emitted in the domain of gamma rays (at about 1 MeV) according to the continuous monitoring for more than 40 years by many observatories all over the world (see 3C 273's Database). These data show that its emission varies considerably in time, an important clue to understand the behavior and the emission processes in these objects.

The gamma-ray bursts

On 25 November 2002, INTEGRAL detected an extraordinary and unexpected "burst" of gamma rays coming from the region of the sky that it was observing. For about 20 seconds the imager IBIS and the spectrometer SPI on board INTEGRAL measured an unprecedented large flux of gamma rays, and for the first time such a mysterious occurrence could be studied by means of an imaging capable instrument.

Gamma-ray bursts (GRBs) are the most energetic events observed in the Universe, and for a few seconds they can become brighter than thousand millions of millions (10¹⁵) of stars similar to the Sun.

The figure on the right shows the first "snapshot" image of a GRB as observed by IBIS on 25 November 2002. The background image is an artist's representation of the explosion from which the gamma rays originate. Credits: ESA. Illustration by the INTEGRAL IBIS team and ESA/ECF.

The first GRB was discovered in the 1960's by an American military satellite which aim was to detect possible on-going Russian nuclear experiments. However, only at the end of the 1990s it was realized that these bursts do not originate in our solar system, and are not produced within our Galaxy, but have a cosmological origin and sometimes come from the far reaches of the Universe.

The image on the left shows an artist's impression of two neutron stars merging into a black-hole. Such events are probably responsible for the short gamma-ray bursts, lasting less than a few seconds. Credits: ESA 2002/Medialab.

Since the detection of the first GRB, a number of different theories have been developed to explain them. Presently, two theories are still being debated. For the GRBs characterized by a relatively long duration (~30 s on average), it was proposed that they might result from the explosion of a massive star at the end of its life. On the contrary, the GRBs with a shorter duration (0.3 s, on average) seem to be most likely produced during the collision between two neutron stars.

The graph on the right shows the evolution of the light intensity produced by the GRB of 25 November 2002 as observed by IBIS. The total duration of the burst was of about 20 s. The IBIS image of the event is shown in the insert. Here, the intensity of the gamma rays is represented with different colours from deep purple (less intense) to red (most intense). Credits: S. Mereghetti, IBIS team, IASF Milano.

A GRB occurs on average every two days. These events are totally unpredictable and appear randomly in the sky. However, the relatively large INTEGRAL field of view permits to observe, on average, one GRB per month. Even if the INTEGRAL payload was not specifically designed to support primarily a GRB mission, its wide energy band coverage and fine imaging capabilities makes this satellite well suited to provide important observational data on these mysterious explosions.

The graphic on the left shows a schematic view of the temporary happening of a GRB detected by INTEGRAL and the steps of the transmission towards the scientific community. Courtesy: S. Mereghetti & D. Gotz.

When a GRB is observed by INTEGRAL, the computers at the ISDC automatically calculate the position of the burst and send this information to all the other important research center through internet in less than 30 seconds. INTEGRAL is able to determine the position of the burst within a few arcmin. A precise and rapid determination of the position of the event is vital for the gamma ray science as it allows for subsequent "follow-up" observations in different energy bands of these phenomena with the robotic telescopes spread around the world. These telescopes are, indeed, automatically directed towards the region of the sky where the burst appeared every time an alert of a GRB is diffused via internet. Starting from the end of 2004, the GRBs observed by INTEGRAL are also followed-up by Swift, a NASA satellite mainly designed for the GRB science and capable to re-point the direction of the burst in the sky within about a minute.